Acetonitrile Solution Effect on Ru N749 Dye Adsorption and Excitation

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Acetonitrile Solution Effect on Ru N749 Dye Adsorption and Excitation at TiO2 Anatase Interface Yoshitaka Tateyama,*,†,‡,§,|| Masato Sumita,† Yusuke Ootani,† Koharu Aikawa,†,⊥ Ryota Jono,# Liyuan Han,‡,▽ and Keitaro Sodeyama*,†,‡,|| †

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 § PRESTO, 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 ⊥ Department of Chemistry, Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan # School of Engineering, The University of Tokyo 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan ▽ Photovoltaic Materials Unit, NIMS, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan S Supporting Information *

ABSTRACT: We investigated stable structures and photoexcitation character of Ru N749 dye (black dye (BD)) adsorption to TiO2 anatase (101) interface immersed in bulk acetonitrile (AN) solution, a most representative electrode interface in dye-sensitized solar cells (DSCs). Density-functional-theory-based molecular dynamics (DFT-MD) with explicit solvent molecules was used to take into account the fluctuations of solvation shells and adsorbed molecules. We demonstrated that BD adsorption via deprotonated carboxylate two anchors (d2) is the most stable at the interface, while the one protonated carboxyl anchor (p1) has the average energy only slightly higher than the d2. This indicates that the p1 state can still coexist with the d2 under equilibrium. It is in contrast with the calculated large stability of the p1 in vacuo. Inhomogeneous charge distribution and anchor fluctuation enhanced by AN solution causes this d2 stabilization. The calculated projected densities of states and the photoabsorption spectra clearly show that the d2 state has larger driving force of the electron injection into the TiO2, whereas the photoabsorption in the wavelength region over 800 nm, a characteristic of BD sensitizer, is mainly attributed to the p1 state even in the AN solution. Consequently, the better performance of BD DSC can be understood in terms of the cosensitizer framework of the d2 and p1 states.

1. INTRODUCTION Dye-sensitized solar cells (DSCs) are being developed as lowcost photovoltaic systems with simple production processes of inexpensive materials compared with the conventional Si-based solar cells. Many DSC systems have been developed so far in laboratories and industries.1−6 Conventionally, DSCs with Ru(II) polypyridyl sensitizers such as N3, N719, and N749 dyes have shown the highest solar-to-electric power-conversion efficiencies.1,7−12 In particular, some of the present authors made a highest efficiency record of 11.4%, with the N749 dye [Ru(4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine)(NCS)3 ], socalled black dye (BD), and I−/I3- redox mediators in acetonitrile (CH3CN:AN) solution years ago.12 BD introduces one terpyridine instead of two bipyridine ligands in the N3 and N719 dyes to increase the absorption in the near-IR region up to 920 nm, which gives an increase in short-circuit current (Jsc). Later, Grätzel and coworkers proposed DSC systems involving porphyrin-based sensitizers with redox shuttles of Co complexes, which achieves the efficiency around 12%.13 This © XXXX American Chemical Society

value is attributed to significant increase in the open-circuit voltage (Voc) to 0.93 V mainly by use of the Co complex mediators with higher redox potential than the I−/I3− couple,14 although Jsc slightly decreases. Quite recently, organic− inorganic hybrid perovskites like CH3NH3PbX3 (X = I, Br, Cl), although it is not categorized in the DSC family now, showed the efficiencies over 15%, where Voc around 1 V is a major effect.15−17 Despite these achievements, fundamental properties and processes on the electrode−electrolyte interfaces have not been fully understood yet for both DSC and perovskite families. The interfacial structures and electronic states significantly affect both Jsc and Voc, leading to the total photoconversion efficiency of η = Jsc × Voc × FF/I0, where FF is the fill factor and I0 is the Special Issue: Michael Grätzel Festschrift Received: January 13, 2014 Revised: May 12, 2014

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dx.doi.org/10.1021/jp5004006 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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incident light power density.3−6 Jsc is governed by electron injection from dye (or solid electrolyte) to the electrode and the back electron transfer (BET) process as well as photoabsorbance of dye (or solid electrolyte). Interfacial dipole may affect the position of the Fermi energy in the electrode, which controls Voc. To make further improvement of both quantities, it is quite essential to elucidate structures of the dye adsorption and the electrolyte boundaries on the electrode as well as the consequent interfacial electronic and optical properties on the atomic and electronic scales. The adsorption structures of Ru dyes on the atomic scale have been proposed experimentally.18−25 Conventionally, the Ru dyes adsorb to the TiO2 anatase nanoparticle electrodes via carboxyl (−C(O)OH) and carboxylate (−COO−) groups. The vibrational spectroscopy experiments suggest that the multiple carboxylate anchors with bidentate bridging mode are most plausible for the N3, N719, and N749 dyes.18,19,23 The presence of the other modes like bidentate chelating and monodentate was also proposed.20−22 In this deprotonation mechanism, some protons originally attached to the carboxyl groups are transferred to the surface oxygen sites. Accordingly, the surface OH group is to be taken into account together. Because the hydrogen bonds (HBs) between the anchor and the surface may appear, the vibration frequencies of carbonyl CO and carboxylate COO− can be modulated. In fact, the presence of HBs with the protonated anchors was suggested for some Ru and organic dyes.24,25 Because this proton position and its dependence on the AN solution play a crucial role for the interfacial electronic states relevant to the Jsc and Voc,26,27 investigation of the anchoring mode and the number of anchors to the TiO2 interface immersed in the AN solution is still an issue of great importance. There are several theoretical calculations concerning the Ru dye adsorption to the (101) surface of anatase TiO2.28−39 Because of the computational cost, most works so far dealt with the adsorption on the vacuum surface or that with the polarizable continuum model (PCM) for the solvent effect. The latter cases usually used TiO2 clusters for the model surface.29−33,35,36 The calculations on the N3 and N719 adsorption usually suggest that the deprotonated two anchors are most probable and the bidentate bridging mode is dominant with the minor deprotonated monodentate.29−36 Regarding the BD adsorption, our previous study indicates stable single protonated carboxyl anchor to TiO2 anatase surface in vacuo.37 Although some works suggest the two deprotonated anchors,38,39 the single anchor still seems probable due to stiffness of the terpyridine. In fact, the stable single carboxyl anchor was proposed for several molecules in vacuo,40−42 whereas it is observed that coadsorption of AN may induce the proton transfer to the TiO2 surface.38 Hence, the anchor mode stability can be affected by the surrounding environment, and further investigation of the solution effect on the atomic scale is crucial.43−47 In this work, we investigated the effect of AN solution on the adsorption structures of dye and the resulting electronic states and photoexcitation. In particular, we focus on a typical system consisting of TiO2 anatase (101) surface, BD sensitizer, and counterion, immersed in the bulk AN solution. We carried out density functional-theory-based molecular dynamics (DFTMD) sampling with explicit solvent molecules for the interfacial structures and compare with the adsorption on the vacuum surface. Once the equilibrium trajectories were obtained, we examined the projected density of states of the interfaces by

DFT calculation as well as the photoabsorption spectra with time-dependent DFT (TDDFT). We then discussed the origin of the stability change of the protonated and deprotonated anchors in the bulk AN solution in terms of charge distribution and HB, photoabsorption depending on the anchor modes, and how to construct plausible electronic energy level alignment. Finally, we conclude that the deprotonated two anchors with the monodentate mode are more dominant for the BD adsorption under immersion of AN solution, whereas the protonated carboxyl anchor can still exist and be responsible for the photoabsorption of BD in the near-IR region. These results can give probable explanation of the better performance of the DSC with BD sensitizer.

2. CALCULATIONS We carried out DFT-MD sampling of the neutral system involving (6 × 2) anatase (101) surface slab with two layers consisting of (TiO2)96, BD (Ru(tpy)(COOH)3(NCS)3−) sensitizer, 100 AN solvent molecules, and one tetramethylammonium (CH3)4N+ counterion. The supercell has a monoclinic shape with the dimensions of 22.706 × 20.479 × 30.274 Å and α = 111.7°. The cell parameters a and b were determined from the experimental lattice constants of the TiO2 anatase (101) surface. We added the solution space over 20 Å in the c direction. Periodic boundary condition was used for the MD sampling. Experimentally, two protons of carboxyl groups of BD are replaced by two TBA cations to avoid the selfassembling of BDs.9 The present simulations, however, keep the protons to save the computational cost. The coverage of BD adsorption in the calculations corresponds to 3.57 × 10−7 mol/m2 concentration (1/24 per surface Ti sites), which is small enough to prevent the excessive interaction between the BDs and comparable to the real system.9 We adopted four different configurations of BD adsorption from the optimized structures for the TiO2 vacuum surface and stuffed AN solvent molecules in the empty space of the supercell to prepare the initial configurations. One state adsorbs via a protonated carboxyl anchor (−C(O)OH) labeled as p1 (Figure 1a), and another has two deprotonated carboxylate anchors (−COO−) labeled as d2 (Figure 1 b). The former was found to be the most stable on the vacuum TiO2 surface in our previous study,37 and the latter is a typical model for BD adsorption. We added the other two states with two protonated anchors (label p2) (Figure 1c) and one protonated and one deprotonated anchor (label p1d1) (Figure 1d). As a reference, we prepared the desorbed state labeled as D (Figure 1e) by putting the BD in the middle region of the bulk AN solution. Then, we carried out five individual DFT-MD samplings. Equilibrium of AN solvent is an important issue. Thus, we carried out DFT-MD simulation of liquid AN in advance and used an equilibrium geometry for the stuffing to construct the initial configurations. We also checked the radial distribution functions (RDFs) of the AN solvent in the calculated equilibrium trajectories and confirmed that the peak positions in the RDFs are in reasonable agreement with those of liquid AN. (See Figure S1 in the Supporting Information.) Here we adopted the supercell approach with the periodic boundary condition and plane-wave basis set to avoid any artifacts of cluster models for the surface and any dependence of the local basis sets for the delocalized nature of electrons on the surface. BLYP-type generalized gradient correction (GGA) functional was used in this stage.48,49 The cutoff energy of the basis set is 70 Ry and the Troullier−Martin-type pseudopoB



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Figure 2. DFT total energy distributions of the D, p1, d2, p2, and p1d1 states (see the text for the label definitions) in the equilibrium trajectories. The d2 state is found the most stable, and the next is the p1.

Table 1. Average and Standard Deviation of the Total Energy Distributions in the Calculated Equilibrium Trajectories of the TiO2 Anatase (101) − BD Sensitizer − AN Solution Systems Figure 1. Representative snapshots of the equilibrium trajectories of black dye (BD) adsorption to TiO2 anatase (101)/acetonitrile (AN) interfaces: (a) adsorption with one protonated carboxyl (−C( O)OH) anchor labeled as p1, (b) adsorption with two deprotonated (−COO) anchors labeled as d2, (c) adsorption with two protonated anchors (label p2), (d) adsorption with one protonated and one deprotonated anchors (label p1d1), and (e) the desorbed state labeled as D. Each trajectory was obtained with DFT-MD simulation with different initial configuration of BD, which was optimized on the vacuum surface, in advance. The white, dark-gray, purple, red, yellow, and light-gray spheres denote H, C, N, O, S, and Ti atoms, respectively. BD, TiO2 surface, and some adsorbed AN are shown with the ball-and-stick model, while the AN solvent molecules are displayed with the wireframe for visualization.

system

average energy (Hartree)

standard deviation (Hartree)

relative average energy (eV)

D p1 d2 p2 p1d1

−6183.053 −6183.071 −6183.072 −6183.049 −6183.029

0.040 0.043 0.036 0.050 0.041

0.00 −0.49 −0.52 +0.11 +0.65

energy is lower than the D by 0.52 eV. The p1 state has the average energy lower than the D by 0.49 eV, which is only slightly higher than the d2. Because the d2 and p1 states have quite small energy difference, we conclude that the d2 state is dominant at thermal equilibrium, but the p1 can coexist. Hereafter, we examine the d2, p1, and D states in detail. Note that the detail of the statistical error is described in the Supporting Information. In our previous study on BD adsorption to the TiO2 anatase surface in vacuo, the p1 type anchor is more stable than the deprotonated configurations by 1 to 1.5 eV.37 However, the presence of AN solvent significantly stabilizes the d2 state in this work, which is consistent with the d2 stability obtained in the previous PCM calculations.38,39 To understand the origin, we investigated the direct effect of the AN solution: AN adsorption to the TiO2 anatase surface and the AN solvation to the BD sensitizer under equilibrium. The RDFs of AN solvent from the surface Ti5c sites are shown in Figure 3a. The main feature of the NAN (N in AN molecule) adsorption is quite similar between the d2 and p1 states. Regarding the solvation shell, Figure 3b,c displays the RDFs from the oxygen atoms of the carboxyl/carboxylate groups in BD, where O and O″ correspond to the groups in the anchors and in the central solution region, respectively. These indicate a common feature to both d2 and p1 that the methyl groups of AN molecules (HAN) dominantly coordinate to the carboxyl/carboxylate groups, and the presence of surface pushes the AN solvent molecules, in particular, NAN, away from the anchors regions. Despite the several common features, we found some differences as well. In Figure 3a, the d2 state has slightly more AN adsorptions through the Ti5c-N bonds, where the AN coverages of the d2 and p1 states are estimated to be 0.44 and

tentials are used.50 For the sampling, we used Car−Parrinello type DFT-MD.51,52 After the thermal equilibrium is realized, we sampled the trajectories over 4 psec. A Nosé−Hoover thermostat was used with the temperature of 300 K for NVT ensemble.53 The time step is set to 0.12 fs (5 au), and 500 au is used for the fictitious mass for electrons in Car−Parrinello dynamics. Analysis of PDOS was carried out based on the sampled geometries in the equilibrium trajectories afterward. We also performed TDDFT linear response calculations of the BD sensitizer only extracted from the selected geometries using the Gaussian 09 package.54 The cluster boundary condition and CPCM (conductor-like polarizable continuum medium) is used to mimic the AN solution environment. The B3LYP hybrid functional is used for better comparison.55 To cover a wider range of wavelengths, we adopted 250 excited states.

3. RESULTS AND DISCUSSION We first examined the total energy distributions of the DFTMD sampling of the five individual states. The energy histograms after the equilibrations are shown in Figure 2. Clearly, p1 and d2 states are more stable than the D state. The p2 distribution is higher in energy than that of D state, while the p1d1 distribution is quite similar. These indicate that the two-anchor cases prefer deprotonation. The average energies listed in Table 1 show that the d2 state is the most stable, which seems consistent with the conventional scenario, and its average C



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Figure 4. Average structures and distances (in angstroms) of the characteristic atoms near the interface in (a) the p1 equilibrium state and (b) the d2.

has a shorter Ti5c−O bond, while stronger HB between H−O2c is realized in the p1 state. Compared with the optimized structures on the vacuum surface,37 the Ti5c−O in the d2 state is elongated by 0.1 to 0.2 Å mainly because of the inevitable deformation to form the two anchors. As shown in Figure 4b, the average distance between O and O’ in the individual anchors, 7.17 Å, is much larger than the p1 state (6.42 Å). Meanwhile, the average Ti5c−O and H− O2c distances of 2.26 and 1.50 Å in the p1 are shorter than those in vacuo, 2.36 and 1.62 Å, respectively. This implies that the protonated anchor is also stabilized by the presence of AN solution, probably through the bearing down effect of the bulk solvent. Thus, the Ti5c−O bonding itself does not seem to significantly contribute to the large stabilization of the d2 state in the AN solution. On the other hand, the formation of two HBs in the d2 can play a crucial role for the energy gain. The fluctuation and deformation of the COOH moieties of the dye is enhanced by the AN solvation, allowing the formation of the two HBs. Taking the stiffness of terpyridine into account, the two monodentate anchors that can keep the HBs are more probable in the BD than the bidentate bridging mode typical in the N3 and N719 with two bipyridines. The importance of HBs for the sensitizer stability also supports the fact that the protic solvents like water significantly reduce the durability of the DSC systems. These differences describe the resultant inhomogeneous charge and the structural change caused by the proton transfer. Conversely, we infer that the inhomogeneous charge distribution is facilitated by the methyl coordination of AN solvent to the dye and the NAN adsorption to the TiO2 surface, leading to proton transfer in the end and more attraction between the dye and the electrode. This can be a main reason for the large stabilization of the d2 state. In this respect, polar aprotic solvent with rather small size will play a similar role as the AN solution does. The electronic states of the d2 and p1 states are essential to understand the mechanisms of characteristic photoabsorption and Jsc increase in DSCs using the BD sensitizers. We calculated the projected density of states (PDOS) of sampled geometries in the equilibrium trajectories of the d2, p1, and D states and took the average differences in the Kohn−Sham orbital energies, which are listed in Table 2. Typical PDOSs showing the average features are displayed in Figure 5.

Figure 3. Radial distribution functions (RDFs) of the N (labeled as NAN) and H (HAN) atoms of AN molecules (a) from the surface Ti5c sites, (b) the oxygen atoms in the BD carboxyl/carboxylate groups in the anchor (labeled as O), and (c) the O atoms of the carboxyl/ carboxylate groups in the central solution regions (labeled as O″) for the D, p1, and d2 states.

0.38, respectively, with respect to the available Ti5c sites in the present supercell. Note that the RDFs of Ti5c-HAN clearly show the presence of the AN solvent molecules facing the methyl moieties to the interface under equilibrium. Figure 3b shows that the d2 anchor more attracts the methyl moiety of the AN solvent, while even the protonated COOH group in the central solution region in the d2 state (Figure 3c) has larger repulsive interaction with the NAN moiety of AN solvent. These clearly indicate that more positive surface with more negative dye is realized simultaneously after the proton transfer in the d2 state. We point out that even the moiety away from the anchor region is significantly affected by the proton transfer, indicating rather long-range electrostatic modification within the BD sensitizer. We next examined the anchor regions of the d2 and p1 states, as shown in Figure 4. The average distances of the anchoring Ti5c−O bond, where Ti5c means five-fold coordinated Ti on the surface and O corresponds to oxygen in the carboxyl or carboxylate anchor group of BD, are 2.03 to 2.08 Å and 2.26 Å for the d2 and p1 states, respectively. On the other hand, they have the distance sets of (1.64 to 1.71, 1.02) Å and (1.05, 1.50) Å, respectively, for the O−H and H−O2c, where O2c is two-fold coordinated oxygen on the surface and H originally belongs to the BD sensitizer. Obviously, the d2 state D



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DFT hybrid functional, although it is sometimes overcorrected. For example, HSE06 functional gives ∼3.6 eV of the band gap for anatase TiO2.56,57 Therefore, we believe that the discussion at the BLYP level can still be valid for the relative relationship at least. The similar band gaps in all states here suggest that the interaction between the dye and the surface does not cause the midgap surface states. The gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied (LUMO) of the BD sensitizer are 0.86 eV for the D state and ∼1 eV for the p1 and d2. The HOMOs are rather triply degenerated and the mixtures of S nonbonding 3p and Ru 3d t2g orbitals. The LUMOs consist of π* orbitals of terpyridine ligand and slightly hybridize with empty Ti orbitals. These characters are consistent with the previous calculations.58,59 The gaps in the AN solution are larger than 0.64 eV obtained in our previous calculations of the gas phase..37 The AN solvation through the methyl coordination stabilizes the HOMO orbitals in the NCS moiety, which increases the HOMO−LUMO gap in a solvatochromic manner. The difference between the D state and the p1, d2 is mainly attributed to the structural difference of the terpyridine moiety controlling the π conjugation. The D state has rather perfect conjugation, while the adsorbed states have some geometrical limitations that cause the deformation of the terpyridine and the flexibility of COOH/COO− groups to keep the anchors, leading to less conjugation. Note that the gap of the d2 state should be larger than that of the p1 because of more localization of the terpyridine π* orbitals due to the symmetry breaking. In this respect, the present d2 gap seems more underestimated than the p1 at the BLYP level. We also found that the HOMO and LUMO energy positions are higher in the d2 states than in the p1 by ca. 0.2 eV in the AN solution. This is explained with more negative character of the deprotonated d2 state. In comparison, between the CBM and LUMO positions, both d2 and p1 states have higher LUMO, implying the driving force of the excited electron injection from the BD sensitizer to the TiO2 electrode. However, we emphasize that the BD photoabsorption consists of the forbidden and allowed peaks, both of which involve the LUMOs. Hence, the LUMO position cannot be an appropriate measure for the injection ability, and the photoexcitation energies with the intensity should be used instead. Therefore, we used photoabsorption spectra calculated with TDDFT method rather than the LUMOs in the PDOSs. Ru dyes usually have rather forbidden absorption within the singlet−singlet excitation at longer wavelength, which is responsible for the absorption edge. Hereafter, we refer the edge region around 800 nm as a forbidden excitation. The allowed peaks around 600 nm lead to primary absorption of the visible light. We carried out TDDFT-B3LYP calculations of the BD structures in the trajectories of the D, p1, and d2 states, taking ligand−metal and ligand−ligand excitation characters of the Ru dyes into account. Here we focus on the intrinsic spectra of the dye molecules for the characteristic absorption of the BD at longer wavelength because the charge-transfer excitation to the TiO2 surface does not seem to play a main role due to the common adsorption feature between BD and the conventional N719. The average photoabsorption spectra of the D, p1, and d2 equilibrium states are shown in Figure 6, and the selected peak energies are listed in Table 2. The p1 and D states have similar spectra because both keep all of the protons, while the d2 losing two protons shows rather

Table 2. Kohn Sham Orbital Energies (in eV) with Respect to the Valence Band Maximum (VBM) of the TiO2 Surface at the BLYP Level and the Excitation Energies (in eV) Obtained in the TDDFT Calculations at the B3LYP Level, Averaged over the Sampled Geometries in the Equilibrium Trajectories of the D, p1, and d2 Statesa CBM of TiO2 HOMO of BD LUMO of BD HOMO−LUMO gap of BD excitation energy (forbidden) excitation energies (allowed)

D

p1

d2

2.09 1.58 2.44 0.86 1.53 2.48

2.09 1.37 2.41 1.04 1.49 2.43

2.09 1.59 2.59 1.00 1.63 2.20, 2.39

a

Photoexcitation with weak intensity around the absorption edge is labeled as “forbidden”, while the peaks with strong intensities in the central region of the visible light are referred as ’allowed’ in this work.

Figure 5. Projected densities of states (PDOSs) of the representative snapshots in the equilibrium trajectories of the D, p1, and d2 states. In addition to the total DOS, PDOSs of Ti bulk, O bulk, NAN, as well as SNCS and Ru consisting of the HOMOs and Ctpy constructing the LUMOs are also shown.

The calculated band gaps between the valence band maximum (VBM) and conduction band minimum (CBM) of TiO2 anatase surfaces are 2.09 eV for all states. The values are similar to those of TiO2 anatase bulk and vacuum surfaces calculated with pure GGA functionals.37 Thus, the AN adsorption affects only the absolute positions of the valence and conduction bands, not the band gap. It is obvious that there is a large underestimation of the gap compared with the experimental value of 3.2 eV. This can be remedied by use of a E



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Figure 7. Schematic picture of the alignment between the ground state and the TDDFT excitation energies of the BD dye (d2 and p1 states) and the projected density of states of the TiO2/BD/AN interfaces calculated with DFT method. As proposed in the previous studies,36,60 this scheme is crucial for the discussion of the electron injection from the forbidden and allowed electronic excitations.

edges, corresponding to the forbidden peaks, of the d2 and p1 are estimated to be 3.2 and 2.9 eV, respectively, with respect to the VBM. On the other hand, the allowed peaks of both states are located between 3.4 and 3.5 eV. Because the energy underestimation still seems to exist in the calculations, we cannot directly compare these calculated values with the experimental gap of 3.2 eV for anatase TiO2. Yet, we can conclude that the absorption edge at longer wavelength just matches with the CBM of anatase TiO2 and thus the electron injection efficiency seems rather small, whereas the allowed photoabsorption peaks are located sufficiently above the CBM. The present calculations demonstrated that the d2 state surely has large driving forces for the electron injection, as expected in the conventional scenario. In fact, such a hot-injection mechanism was suggested recently as well.61 The current results also indicate that the structural fluctuation inducing the energy variation certainly affects the control of excitation and electron injection. This work demonstrates that AN solvation to the dye and AN adsorption to the TiO2 surface both play essential roles for the anchor stability of the dye adsorption and the dye excitation through the protonation/deprotonation. The present results indicate that small polar aprotic solvent molecule with a certain dipole seems ideal. AN is likely to be a best solvent, while propylene carbonate (PC) with sufficient potential window and boiling as well as melting points may be another possibility. Regarding the Jsc increase, intrinsically more negative dye with lower electron affinity is needed for efficient use of the light at longer wavelength. On the other hand, the coexistence of different configurations of dyes can contribute to panchromatic absorption, which is already used in the cosensitizer strategy.

Figure 6. Average photoabsorption spectra of the sampled geometries of the D, p1, and d2 equilibrium states, calculated with the TDDFT technique at the B3LYP level.

different spectra with weaker intensity. Hence, the difference of the adsorption structure and the protonation affects the photoabsorption spectra to a certain extent. Here we found that the p1 state has the absorption edge at a longer wavelength around 850 nm than the d2 with the edge around 750 nm. It is consistent with the spectra in our previous study for the adsorption in vacuo.37 This indicates that photoabsorption at longer wavelength up to ca. 900 nm, a characteristic of BD sensitizer, is mainly attributed to the p1 state. Therefore, coexistence of the p1 state is quite crucial for the performance of DSC with BD. We also point out that the allowed peaks between 500 and 700 nm are likely to be more responsible for the energies of the excited electrons in the dye. The calculated spectra show the complementary relationships between the d2 and p1 spectra, leading to panchromatic absorption. This may also contribute to the BD performance. The alignment between the electronic states of BD and the bands of TiO2 anatase surface is significant to understand the interfacial electron transfer. However, the PDOSs at DFT pure GGA level cannot be used for the direct estimation because of the gap underestimation previously described. We thus adopted the combination strategy of the PDOSs mainly for the TiO2 surface and AN solution, and the photoabsorption spectra for the excited states of the BD sensitizer, which was first proposed by De Angelis et al.36,60 The schematic picture is shown in Figure 7. This has an advantage that one can take into account the forbidden and allowed excitations with the intensity, as previously discussed. Following ref 36, we used the HOMO level of BD sensitizer for the ground-state energy reference of the photoabsorption. Referring to the VBM of TiO2 surface, BD HOMO in the d2 state is higher in energy than the p1 by 0.25 eV. The absorption

4. CONCLUSIONS We investigated structural stability and electronic excitation properties of Ru N749 dye (BD) adsorption to TiO2 anatase (101) surface immersed in AN solution, a most representative electrode interface in DSCs. DFT-MD with explicit solvent molecules was used to take into account the fluctuations of solvation shell and adsorbed molecules. We demonstrated that F



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BD adsorption via deprotonated carboxylate two anchors (d2) is the most stable at the interface, while the state with protonated carboxyl one anchor (p1) has the average energy only slightly higher than the d2 by ca. 0.05 eV. This indicates that the p1 state can still coexist with the stable d2 under thermal equilibrium. Compared with the large stability of the p1 state on the vacuum TiO2 surface, the presence of the AN solution induces noticeable stabilization of the d2. The analysis of the equilibrium trajectories and the average structures indicates that this d2 stabilization is mainly attributed to the inhomogeneous charge distribution and the anchor fluctuation (rotation) enhanced by the AN solution. Both d2 and p1 states have the monodentate anchoring mode with HBs between the carboxyl/carboxylate groups and the surface, implying that the HBs play a crucial role for the BD case with rather stiff terpyridine. The resultant mechanism obtained here can be applied for the search of other electrolyte solvents. The combination of projected densities of states at DFTBLYP level and photoabsorption spectra at TDDFT-B3LYP level indicates that the d2 state has larger driving force of the electron injection into the TiO2 electrode, consistent with the conventional scenario. On the other hand, the photoabsorption of wavelength region over 800 nm, a characteristic of BD sensitizer, is mainly attributed to the p1 state even in the AN solution. These results indicate that the TiO2 electrode − BD sensitizer − AN solution interface may realize coexistence of the dominant d2 states with the p1, leading to enhancement of the photoabsorption at longer wavelength and the increase in electron injection to the electrode, simultaneously. This mechanism can be regarded as a sort of the cosensitizer framework.

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ASSOCIATED CONTENT

RDFs of the AN solution in the present DFT-MD sampling, the adsorption preference depending on the slab thickness, the detailed energy sampling data with error bars. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.T.). *E-mail: [email protected] (K.S.). Notes

The authors declare no competing financial interest.

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REFERENCES

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ACKNOWLEDGMENTS

Y.T. and K.S. were partially 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, The University of Tokyo, Kyushu University, as well as the K computer at the RIKEN AICS through the HPCI Systems Research Project (Proposal Number hp130021). G



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