nitrogen-containing heterocycles’ interaction with ru dye in dye-sensitized solar cells

Nitrogen-Containing Heterocycles’ Interaction with Ru Dye in Dye-Sensitized Solar Cells

Hitoshi Kusama,* Hideki Sugihara, and Kazuhiro SayamaEnergy Technology Research Institute, National Institute of AdVanced Industrial Science and Technology(AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

ReceiVed: August 27, 2009; ReVised Manuscript ReceiVed: October 21, 2009

Interactions of N-containing heterocycles such as pyrazole, imidazole, pyridine, pyrimidine, pyrazine, and4-t-butylpyridine (TBP) with the neutral and oxidized Ru(II)-polypyridyl dye (N719) have been studied usingdensity functional theory (DFT). All of the heterocycles formed two hydrogen bonds with N719; one via theN atom with a lone pair of heterocycles and the H atom of the carboxyl group for the dye ligand, and theother via the H atom adjacent to the N atom forming the other hydrogen bond and the O atom with a lonepair of carboxyl groups. Additionally, the heterocycles were examined as an additive in an I-/I3

- redoxelectrolyte solution of a dye-sensitized TiO2 solar cell. As the positive atomic charge on the S atom of theNCS ligand in the oxidized state decreased via interaction with the heterocycles, it became difficult forthe oxidized dye to interact with I-, which prevented the dye from regenerating and led to a decrease in theshort-circuit photocurrent density (Jsc) of the dye-sensitized solar cell. The DFT results also indicated that theheterocycles moved the energy of both HOMO and LUMO levels up, as well as raised the Fermi level ofTiO2. This produced lower electron injection efficiency from photoexcited dye into the TiO2 conduction bandand thereby decreased the Jsc value.

Introduction

Dye-sensitized solar cells (DSSCs) have been extensivelystudied due to their simple assembly and potential economicadvantages compared to conventional silicon solar cells, eversince Gratzel and co-worker reported a high photoelectricconversion efficiency of 7.9%.1 A typical DSSC has three maincomponents: (i) a sensitized photoanode, which is typically adye-sensitized nanocrystalline TiO2 film on a transparentfluorine-doped tin oxide (FTO) conducting glass, (ii) anelectrolyte solution containing I-/I3

- as a redox couple, and (iii)a cathode, which is a platinized (Pt) FTO conducting glass. TheDSSC energiessthose of the TiO2 conduction band, the HOMO/LUMO energies of the dye and the “redox energy” of the I-/I3

- couplesare usually discussed in terms of their relativepositions. For successful sensitization, the LUMO level of thedye must be higher in energy than the bottom of TiO2

conduction band, while the HOMO level must be lower thanthe redox energy to allow the dye to be regenerated from itsoxidized state. Open-circuit photovoltage (Voc) of a DSSC isdefined as the absolute value of the difference between thestandard reduction potential of the redox couple and the Fermilevel of the TiO2 photoelectrode. One of the reasons for asignificant advance in DSSC efficiency is the adoption of theRu(H2dcbpy)2(NCS)2 sensitizer (N3), where H2dcbpy is 2,2′-bipyridine-4,4′-carboxylic acid, adsorbed to nanocrystalline TiO2

films.2 This combination is crucial for further research anddevelopment of DSSCs.

Adding certain compounds to the electrolytic solution con-taining I-/I3

- as a redox couple can remarkably improve DSSCperformance. Some of the most effective and frequently usedadditives are nitrogen-containing heterocycles like pyridinederivatives.2-5 Generally, adding heterocycles enhances the Voc,the fill factor (ff), and the solar energy conversion efficiency

(η). Among the proposed mechanisms for Voc improvement, thedominant one is that heterocycles shift the semiconductor bandedge up by adsorbing onto the TiO2 surface.4-10 In fact, wedemonstrate a shift in the TiO2 Fermi level upon adsorption ofnitrogen-containing heterocycles on TiO2 anatase surface theo-retically.11

Pyridine derivatives, however, also reduce the Jsc of a DSSC,and there have been few reports about the mechanism of thisphenomenon. One plausible explanation is that the conductionband of TiO2 photoelectrode shifting up makes the driving forcefor the electron injection process from the LUMO of the dyeinto the conduction band of TiO2 decrease, thereby loweringthe injection efficiency and causing the low Jsc. Moreover, theJsc is also determined by the dye regeneration process whichtakes place at the nanocrystalline TiO2 electrode. The regenera-tion dynamics of N3 in I-/I3

- electrolyte was studied by Durrantand co-workers.12 They found that the regeneration reactionproceeded via a transient intermediate complex [dye+ · iodide]formed by the reaction of photogenerated dye cation with oneiodide ion:

I2- then dismutates to yield iodide and triiodide:

Meanwhile, Lund and co-workers investigated the photosub-stitution reaction between cis-(NBu4)2 [Ru(Hdcbpy)2(NCS)2](N719), where NBu4 stands for (n-C4H9)4N+, and TBP.They identified one of the five degradation products, Ru-(H2dcbpy)(Hdcbpy)(NCS)(TBP).13 On the basis of these find-ings, Lin and co-workers concluded that the oxidized dye can

* To whom correspondence should be addressed. E-mail: [email protected] (H. Kusama).

dye+ + 2I- f [dye+ · I-] + I- f dye + I2- (1)

2I2- f I- + I3

- (2)

J. Phys. Chem. C 2009, 113, 20764–2077120764

10.1021/jp908270e CCC: $40.75 2009 American Chemical SocietyPublished on Web 11/10/2009

react not only with iodide but also with pyridine derivative suchas 2-methyl-4-propoxypyridine:5,14

They explained that reactions 1 and 3 presumably have thesame dynamic features and are competitive with each other.Reaction 3 decreases the dye cation concentration on the TiO2

anode, which suppresses reaction 1. With increasing concentra-tion of the pyridine derivative, reaction 1 may become moreand more difficult, leading to a decrease in Jsc. Although thereaction of oxidized dye with pyridine derivatives was suggestedby a cyclic voltammetry measurement,14 its structure andinteraction mechanism are currently unknown.

Herein we focus on the interaction of the N-containingheterocycles such as pyrazole, imidazole, pyridine, pyrimidine,pyrazine, as well as TBP with the N719 dye before and afterphotoexcitation, followed by ultrafast electron injection into theconduction band of the TiO2 semiconductor. The main tools inthis investigation are quantum chemical calculations at the DFTlevel, which are well suited for studies of large molecularsystems, such as N719. The results are compared to Jsc in aDSSC containing the heterocycles as additives in an electrolytesolution in order to elucidate the reduction mechanism of Jsc

by the heterocycles.

Experimental and Computational Details

Theoretical Calculations. Calculations were performed withDFT using the Gaussian 03W program.15 The geometries werefully optimized in vacuo at hybrid DFT levels by B3LYPfunctions, which combine Becke’s three-parameter exchangefunction (B3)16,17 with the correlation function of Lee, Yang,and Parr (LYP).18 For all of the systems, a LanL2DZ basis set,which corresponds to a Dunning/Huzinaga valence double-�basis (D95 V) for first-row elements19 and a Los Alamos ECPplus double-� basis for Na-La and Hf-Bi atoms20-22 was used.The energies were corrected for the zero-point vibrationalenergies (ZPE). The interaction energies were determined asthe difference in energy between the complex, on the one hand,and the sum of the isolated monomers (N719 and N-containingheterocycles), on the other. With this definition, a positiveinteraction energy corresponds to strong interaction. Thecounterpoise (CP) correction was incorporated from the basisset superposition error (BSSE) using the Boys and Bernardiprocedure in order to correct for the interaction energies,23,24

which were 2-4 kcal ·mol-1. Indeed, all of the complexes havenonimaginary frequency geometries. These findings confirm thatthe optimized structures correspond to real minima on thepotential energy surface. The Mulliken population analysis wasperformed on optimized structures to obtain the Mulliken atomiccharges. Natural bond orbital (NBO) analysis25,26 was alsoconducted on optimized geometries with the NBO 3.1 program27

included in the Gaussian program package in order to understandthe nature and magnitude of the intermolecular interactions.

Experimental Section. DSSC preparation and its photovoltaiccharacterization were conducted as previously described.28 Asandwich-type solar cell consisted of a nanocrystalline TiO2

photoelectrode (1.5 × 10-5 m thick) with N719 dye, a 2.5 ×10-5-m thick Lumirror spacer film, and a counter electrode,which was a Pt-sputtered FTO conducting glass. The electrolytesolution, which was composed of 0.5 mol ·dm-3 of N-containingheterocycle, 0.6 mol ·dm-3 of 1,2-dimethyl-3-propylimidazolium

iodide, 0.1 mol ·dm-3 of LiI, and 0.05 mol ·dm-3 of I2 inacetonitrile, was injected into the space between the twoelectrodes. The solar cell performance was then measured undersimulated solar light (AM 1.5, 100 mW · cm-2). Photocurrentwas measured using a digital source meter and a data acquisitionsystem. The apparent cell area of the TiO2 photoelectrode was0.25 cm2 (0.5 × 0.5 cm2).

Results

Structures of Neutral and Cationic Dye. The starting pointof our study is the N719 complex in its ground singlet electronicstate, including two Na+ counterions which mimic the twoexperimental bulky two (n-C4H9)4N+ ones for computationalconvenience.29,30 The two protons are located on the carboxylicgroups in the cis position to the NCS ligands, as illustrated inFigure 1. After the electron injection into the TiO2 conductionband, the dye becomes a cation with a monopositive charge(1+) in the ground doublet electronic spin state. Table 1 liststhe main geometrical parameters of optimized dye structures.Compared to neutral N719 (N7190), Ru-N bonds of the

dye+ + pyridine derivative f [dye+ · pyridine derivative](3)

Figure 1. Optimized geometries of N7190 and N719+. White ) H;gray ) C; blue ) N; red ) O; purple ) Na; yellow ) S; and teal )Ru atoms.

TABLE 1: Main Optimized Geometrical Parameters ofN719

parameter N7190 N719+

Ru-NCS (Å) 2.057-2.057 1.999-1.999Ru-bpyH (Å) 2.061-2.061 2.085-2.085Ru-bpyNa (Å) 2.065-2.065 2.097-2.097N-C(NCS) (Å) 1.196-1.196 1.200-1.200C-S(NCS) (Å) 1.676-1.676 1.659-1.659O-H(carboxyl group) (Å) 0.984-0.984 0.984-0.984N(NCS)-Ru-N(NCS) (deg) 91.7 94.9N(bpyNa)-Ru-N(bpyH) (deg) 79.2-79.2 78.8-78.8N(bpyNa)-Ru-N(bpyNa) (deg.) 93.0 88.9N(bpyH)-Ru-N(bpyH) (deg.) 177.1 177.0

Nitrogen-Containing Heterocycles and Ru Dye J. Phys. Chem. C, Vol. 113, No. 48, 2009 20765

Ru-NCS binding and C-S bonds of the NCS ligands forcationic N719 (N719+) are 0.02-0.06 Å shorter, but Ru-Nbonds of the Ru-dcbpy binding are about 0.03 Å longer.Becoming cationic results in a 3° broadening of the N-Ru-Nbond angle for the Ru-NCS binding but also in a 4° narrowingof the N-Ru-N bond angle for the Ru-dcbpy in trans positionto the NCS ligands, that is, the COO-Na+ group side.

Interaction of N-Containing Heterocycles with the NeutralDye. Hydrogen bonds between the carboxyl group for the N719ligand and the H atom as well as the N atom with a lone pairof N-containing heterocycles (Scheme 1) seem to be a quiteplausible intermolecular interaction.31 In fact, no other optimumgeometries of dimer were obtained by DFT.

By complexation with N-containing heterocycles, the struc-tures of carboxyl group interacting with heterocycles changedramatically, although the main features of N7190, such as theRu-NCS distance are nearly identical (Table 1). Thus, Figure2 depicts only the hydrogen bond structures in the optimizedgeometries of N7190 ·heterocycle complexes. For pyrazole,imidazole, pyrimidine, and TBP, there are two conformersdenoted (a) and (b). It should also be noted that in the case ofN7190 ·pyrazole(a) there is a N-H · · ·O hydrogen bond insteadof C-H · · ·O one in Scheme 1. Table 2 lists the geometrichydrogen bond parameters for N7190 ·heterocycle complexes.

For all of the dimers, the intermolecular H · · ·N and H · · ·Odistances are smaller than the net van der Waals radii of thebinding atoms, 2.70 Å (H and N) and 2.60 Å (H and O),suggesting that all the tested N-containing heterocycles formintermolecular hydrogen bonds with the carboxyl group of theneutral dye. Compared to reference O-H bond length, 0.984Å, calculated for isolated N7190 in Table 1, complexationlengthens the O-H bond from 1.043 Å (pyrazole(b)) to 1.085Å (TBP), indicating that the O-H bonds are weakened in thecomplex structures. The bond elongation is also observed atthe N-H bond for N7190 ·pyrazole(a), but is not found in C-Hbonds for the other heterocycles, which suggests that theN-H · · ·O bond is preferable to the C-H · · ·O one. The shorterthe H · · ·B (B ) N, O) but the longer the A-H (A ) O, N, C),the stronger the hydrogen bond. Judging from A-H, H · · ·B,and A · · ·B bond distances, O-H · · ·N bonds are stronger thanC-H · · ·O ones. The O-H · · ·N bond angle is close to linearexcept for N7190 ·pyrazole(a), because in this case the N-H · · ·Ois consistent with medium strength hydrogen bonding31 unlikeC-H · · ·O.

Table 2 also lists the intermolecular bond energy, E, andoverall charge transfer (CT) taking place from the N-containingheterocycles to N7190, determined by the Mulliken populationand NBO analyses. N7190 · pyrazole(a), N7190 · imidazole,N7190 ·pyridine, and N7190 ·TBP are considered to be stronghydrogen bonds because they have energies over 15 kcal ·mol-1.31 The CT values differ according to analysis method,which might be attributed to the difference in charge placementbetween the two.32 However, the relative order among theheterocycles is similar.

Among the six N-containing heterocycles, TBP interactsmost strongly with N7190. Conformers N7190 · pyrazole(a),N7190 · imidazole(a) and N7190 · pyrimidine(b) are morefavorable complexes than N7190 · pyrazole(b), N7190 · imi-dazole(b) and N7190 · pyrimidine(a), respectively.

The bond length of the O-H · · ·N bond is a suitable geometricparameter for the evaluation of the energies of the complexes.The graphical correlations of E with O-H, H · · ·N, and O · · ·Nbond distances for all complexes except N7190 ·pyrazole(a) areshown in Figure 3. The longer the O-H bond, the higher the Evalue. The correlation coefficients between E and O-H bonddistance are 0.95. On the other hand, the shorter the H · · ·Nand the O · · ·N distances, the larger the E. The correlationcoefficients of E with H · · ·N, and O · · ·N distances are -0.97and -0.98. The E value is also closely correlated with CT, asrepresented in Figure 4. The correlation coefficients of E withCT determined by Mulliken population and NBO analyses are0.84 and 0.95, indicating that the greater the charge transfer,the higher the value of E.

Interaction of N-Containing Heterocycles with the Cat-ionic Dye. As with neutral state, N-containing heterocyclesinteract only with carboxyl group of cationic N719 via hydrogenbonding. Also similar to the neutral dye case, the main structuresof N719+ such as the N(NCS)-Ru-N(NCS) bond angle listedin Table 1 are virtually unchanged by interaction with theheterocycles. Figure 5 illustrates the optimized geometries ofN719+ ·heterocycle complexes by DFT calculations, and Table3 lists their geometric parameters, intermolecular bond energiesand overall charge transfers. Compared to reference O-H bondlength, 0.984 Å (Table 1), complexation lengthens the O-Hbond of the carboxyl group from 1.077 Å (pyrazole(b)) to 1.476Å (pyrazole(a)), indicating that the O-H bonds are weakenedin the complex structures. The bond elongation is also foundfor the N-H bond for N719+ ·pyrazole(a), but not in the C-H

SCHEME 1: Intermolecular Hydrogen Bond of aCarboxyl Group and a Pyridyl Group

Figure 2. Hydrogen bonds in the optimized geometries of N7190 ·N-containing heterocycles. White ) H; gray ) C; blue ) N; and red )O atoms.

20766 J. Phys. Chem. C, Vol. 113, No. 48, 2009 Kusama et al.

bond for other N-containing heterocycles. The intermolecularH · · ·N distances formed are less than the net van der Waalsradii of the binding atoms, 2.70 Å, showing that intermolecularhydrogen bonds via the N atom with a lone pair from theheterocycles is formed with the H atom of the carboxyl groupfor the N719+ ligand. Other than N719+ ·pyrazole(b), theresulting H · · ·O distances between the H atom of the hetero-cycles and the O atom of N719+ carbonyl group are also shorterthan the net van der Waals radii of the binding atoms, 2.60 Å.If we consider the A-H, H · · ·B, and A · · ·B bond lengths, theC-H · · ·O bonds are weaker than the O-H · · ·N ones inaccordance with the results for the neutral dye. The O-H · · ·Nbond angles are close to linear and larger than the C-H · · ·Oones. In the case of N719+ ·pyrazole(a), the N-H · · ·O bondhas comparable to the O-H · · ·N one. Although CT values aredifferent for Mulliken population and NBO analyses, the relativeorder among the heterocycles tested is very similar, andN719+ ·TBP(b) is highest but N719+ ·pyrimidine(a) is lowestin CT value.

When we compare neutral and cationic complexes, we findthat the H · · ·N bonds of cationic complexes are shorter than

their neutral counterparts, while the O · · ·H bonds are generallylonger than neutral ones. The E values of the cationic complexesare about 5 kcal ·mol-1 higher than the neutral ones. The CTvalues of the cationic complexes determined by the Mullikenpopulation analysis are larger than those of neutral complexes,while of the opposite is broadly true for the NBO analysis. Thisdisagreement might also be caused by the difference in chargeplacement between the two methods. Among the six N-containing heterocycles, TBP forms the most stable complexwith N719+, similar to the N7190 case. By comparing conform-ers, N719+ ·pyrazole(a), N719+ · imidazole(a), and N719+ ·py-rimidine(b) are preferable to N719+ ·pyrazole(b), N719+ · imid-azole(b), and N719+ ·pyrimidine(a), respectively, which is alsoconsistent with the results for the neutral state.

DSSC Performance. Table 4 lists the Jsc values of the DSSCwhen illuminated with 100 mW · cm-2 for additive concentrationof 0.5 mol ·dm-3. Jsc’s for the cells with N-containing hetero-cycles were lower than for the cell without an additive. Theaddition of imidazole to the electrolyte solution especiallyreduced Jscsby 40%.

TABLE 2: Selected Geometrical Parameters, Intermolecular Interaction Energies for Complexes (E), and Charge Transfer(CT) of N7190 ·N-Containing Heterocycle Complexes

A-H H · · ·B A · · ·B A-H · · ·B E Mulliken CT NBO CTspecies bond (Å) (Å) (Å) (deg) (kcal ·mol-1) (e) (e)

N7190 ·pyrazole(a) O-H · · ·N 1.074 1.506 2.568 169.1 20.49 0.1004 0.1040N-H · · ·O 1.032 (1.010)a 1.849 2.765 145.9

N7190 ·pyrazole(b) O-H · · ·N 1.043 1.579 2.620 176.2 14.09 0.1093 0.1063C-H · · ·O 1.081 (1.080)a 2.593 3.328 124.6

N7190 · imidazole(a) O-H · · ·N 1.078 1.492 2.566 174.2 18.50 0.1197 0.1377C-H · · ·O 1.080 (1.079)a 2.373 3.118 124.8

N7190 · imidazole(b) O-H · · ·N 1.069 1.508 2.576 176.5 17.45 0.1165 0.1317C-H · · ·O 1.079 (1.079)a 2.518 3.265 125.5

N7190 ·pyridine O-H · · ·N 1.072 1.513 2.585 178.3 16.65 0.1057 0.1328C-H · · ·O 1.087 (1.087)a 2.381 3.241 134.9

N7190 ·pyrimidine(a) O-H · · ·N 1.046 1.593 2.638 178.2 13.75 0.0763 0.1033C-H · · ·O 1.085 (1.085)a 2.418 3.255 132.9

N7190 ·pyrimidine(b) O-H · · ·N 1.049 1.584 2.632 176.4 14.55 0.0787 0.1040C-H · · ·O 1.087 (1.087)a 2.311 3.182 135.7

N7190 ·pyrazine O-H · · ·N 1.047 1.591 2.637 177.2 14.09 0.0777 0.1020C-H · · ·O 1.086 (1.086)a 2.345 3.203 134.7

N7190 ·TBP(a) O-H · · ·N 1.085 1.481 2.566 178.4 17.70 0.1161 0.1460C-H · · ·O 1.087 (1.087)a 2.402 3.255 134.3

N7190 ·TBP(b) O-H · · ·N 1.085 1.481 2.566 178.3 17.68 0.1165 0.1462C-H · · ·O 1.087 (1.088)a 2.400 3.253 134.3

a Distance for monomer.

Figure 3. Correlation between the intermolecular bond energy E andthe bond distance of the hydrogen bond for the N7190 ·N-containingheterocycles.

Figure 4. Correlation between the intermolecular bond energy E andthe overall charge transfer CT for the N7190 ·N-containing heterocycles.


Discussion

Theoretical studies of neutral and cationic complexes ofN719 ·N-containing heterocycles were conducted using com-putational models to identify the intermolecular interactionproperties. On the basis of the results that the dye does not bindwith TiO2 surface through all four carboxyl groups located onthe bipyridine ligand,33 we found that N-containing heterocyclesinteract with the free carboxyl group via two hydrogen bonds.These heterocycles were also examined as additives to theelectrolyte solution of a DSSC, causing a drastic decrease inthe value of Jsc.

Why did the interaction between the dye and the heterocyclesreduce Jsc? As mentioned in the introduction, one hypothesis isthat the reaction of the oxidized dye and the heterocyclesdecrease the dye cation concentration on the TiO2 anode andsuppress the regeneration of the neutral dye by I-, resulting ina low Jsc.14 More recently, Privalov and co-workers have studiedthe interactions between I-/I3

- redox and N3 dye via DFTcalculations.34 They found that I- species interacted with cationicN3 via an NCS ligand forming an S-I bond. On the basis ofthis finding, the interaction of N-containing heterocycles withthe carboxyl group of dye ligand should prevent I- frominteracting with oxidized dye indirectly rather than directly atthe NCS ligand. However, as mentioned in the results sectionabove, the geometrical structure of the NCS ligand for N719+

was not influenced by the interaction with the heterocycles.Next, we consider the effects on the atomic charges of N719+

determined by Mulliken population and NBO analyses. Bothanalyses indicate that only the atomic charges on S atoms forthe NCS ligand as well as on the O atoms for O-H · · ·Nhydrogen bonding change considerably by interaction withN-containing heterocycles, even though there are sufficient

distances between the NCS ligands and the heterocycles.Although the values differ by method of analysis (see Table5), atomic charges on the S atom for horizontal NCS ligandreferring to N-containing heterocycles generally were lower thanthose for the vertical one achieved by complexation. As shownin Figure 6, the Jsc of the DSSC (Table 4) is positively correlatedwith the atomic charges on the S atom for the NCS ligand ofN719+, irrespective of its position and analysis method. Thecorrelations in Figure 6 suggest that the less positive the atomiccharge on the S atom for the NCS ligand of the oxidized dye,the more difficult the interaction of oxidized dye with I-, whichfurther suppresses the regeneration of the dye and lowers theJsc value.

We also consider whether the interaction between neutral dyeand N-containing heterocycles affects Jsc. De Angelis and co-workers has been reported that the adsorption of N719 throughtwo carboxyl groups on the TiO2 nanoparticle lowers its positionof LUMO level, resulting in a smaller HOMO-LUMO gap thanthat of isolated one via DFT calculations.35 It has been alsoreported that electrolyte solvent affected the molecular orbitalenergies of the dye such as HOMO and LUMO,36 and the rateof electron injection from photoexcited dye into the TiO2

conduction band,37 which could be ascribed to the interactiveability of the solvent.38 According to our findings, it may bepossible for N-containing heterocycles to have an effect on themolecular orbital energies of N719 dye because N-containingheterocycles actually interact with N719 via hydrogen bonding.

Figure 7 shows the energy level diagrams of N7190 andN7190 ·N-containing heterocycles. As expected, the HOMO andLUMO energy levels for N7190 changed drastically by interac-tion with the heterocycles. Comparison of the electronic energylevels for N7190 and N7190 ·heterocycle complexes in Figure7 shows that the interaction of N-containing heterocycles raisesthe energy level of both HOMO and LUMO by almost the sameamount, resulting in a HOMO-LUMO gap similar to that ofnoninteracting case, 1.59-1.60 eV. The HOMO energy levelwas moved from -4.43 to -4.19 eV (TBP), and that of theLUMO from -2.85 to -2.59 eV for the TBP interactions. Thustheir gap stayed almost constant, 1.60 eV, which was consistentwith the experimental results (see Supporting Information).Naively, this phenomenon should not to reduce Jsc values. Theraised HOMO energy level close to the I-/I3

- redox couplerestricts the acceptance of new electrons from the redox couple,while the raised LUMO energy level far from the TiO2

conduction band edge facilitates the effective electron injectioninto the TiO2. These trends should balance out for a steadyHOMO-LUMO gap and TiO2 conduction band edge, leadingto unchanged Jsc. However, as mentioned above,4-11 N-containing heterocycles shift the TiO2 band edge up. Therefore,the reduction mechanism of Jsc by N-containing heterocyclesis explained by a schematic representation of the energy levelsfor N7190 and N7190 · heterocycle complexes, as depicted inFigure 7. On the basis of the literature,7 the I-/I3

- redox energy,the Fermi level of TiO2, and their gap were assumed to be -4.10eV (broken blue line) which remains constant upon the additionof the heterocycles,39,40 -3.30 eV (solid red line for N7190),and 0.80 eV, respectively. Consequently, because of the Fermilevel shifts by the heterocycles,11 the energy gaps between theLUMO of dye and Fermi level of TiO2 as well as the onebetween the HOMO of the dye and the I-/I3

- redox energydecrease drastically due to the interaction of the dye with theheterocycles. The Fermi level of TiO2 shifted from -3.30 to-2.70 eV, so the energy gap between it and the LUMOdecreased from 0.45 to 0.08 eV. The energy gap between I-/

Figure 5. Optimized geometries of N719+ ·N-containing heterocycles.White ) H; gray ) C; blue )N; red ) O; purple ) Na; yellow ) S;and teal ) Ru atoms.


I3- redox energy and HOMO level also decreased from 0.33 to

0.09 eV. It should be noted that N-containing heterocycleshifting greater TiO2 Fermi level also causes increased shiftsof both the HOMO and LUMO energy levels of the dye. Ofthe six heterocycles, imidazole shifted most not only Fermi level(by 0.60 eV) but also the HOMO and LUMO energy levels(by 0.23 eV) except for TBP (by 0.25 eV). In case of theinteraction with TiO2, the LUMO energy level and theHOMO-LUMO gap of N719 decreases, as mentioned above.35

The contribution to the dye of TiO2 may differ from that ofN-containing heterocycles.

Figure 8 indicates that there is good correlation between Jsc

of the DSSC in Table 4 and the energy gaps in Figure 7. Thegreater the LUMO-Fermi gap (broken line), the redox-HOMOgap (dotted line), and the sum of these gaps (solid line), thehigher the Jsc value. Thus, a decrease in the energy gap betweenthe LUMO of the sensitizing dye and the TiO2 Fermi leveldecreases the driving force of the electron injection process fromthe LUMO of dye into the conduction band of TiO2, leading tothe low injection efficiency and thus lower Jsc. At the same time,a decrease of the energy gap between HOMO of the sensitizingdye and the redox energy of the I-/I3

- couple makes it difficultfor the oxidized dye to accept new electrons from the redoxcouple, suppressing subsequent electron injection and thus alsothe Jsc value.

In summary, the reason why the addition of N-containingheterocycles to electrolyte solution decreases Jsc of a DSSC isrevealed in this DFT investigation to be the interactions betweenthe N719 dye and the heterocycles.

TABLE 3: Selected Geometrical Parameters, Intermolecular Interaction Energies for Complexes (E), and Charge Transfer(CT) of N719+ ·N-Containing Heterocycle Complexes

A-H H · · ·B A · · ·B A-H · · ·B E Mulliken CT NBO CTspecies bond (Å) (Å) (Å) (deg.) (kcal ·mol-1) (e) (e)

N719+ ·pyrazole(a) O-H · · ·N 1.476 1.111 2.549 159.9 24.94 0.2803 0.1458N-H · · ·O 1.067 (1.010)a 1.625 2.628 154.3

N719+ ·pyrazole(b) O-H · · ·N 1.077 1.482 2.559 176.6 19.42 0.1454 0.0735C-H · · ·O 1.080 (1.080)a 2.665 3.376 122.9

N719+ · imidazole(a) O-H · · ·N 1.193 1.301 2.491 174.3 25.94 0.2133 0.1166C-H · · ·O 1.080 (1.079)a 2.424 3.147 123.1

N719+ · imidazole(b) O-H · · ·N 1.140 1.369 2.508 176.7 24.78 0.1812 0.0991C-H · · ·O 1.079 (1.079)a 2.579 3.303 123.8

N719+ ·pyridine O-H · · ·N 1.175 1.329 2.504 178.5 23.49 0.1984 0.1102C-H · · ·O 1.087 (1.087)a 2.406 3.256 133.9

N719+ ·pyrimidine(a) O-H · · ·N 1.082 1.490 2.573 178.6 18.50 0.1176 0.0726C-H · · ·O 1.085 (1.085)a 2.457 3.283 131.9

N719+ ·pyrimidine(b) O-H · · ·N 1.090 1.472 2.561 177.6 18.99 0.1244 0.0755C-H · · ·O 1.087 (1.087)a 2.401 3.247 133.5

N719+ ·pyrazine O-H · · ·N 1.085 1.485 2.569 177.9 18.18 0.1201 0.0727C-H · · ·O 1.086 (1.086)a 2.416 3.254 132.9

N719+ ·TBP(a) O-H · · ·N 1.304 1.202 2.506 177.2 25.35 0.2805 0.1481C-H · · ·O 1.087 (1.087)a 2.355 3.216 134.9

N719+ ·TBP(b) O-H · · ·N 1.307 1.201 2.506 177.2 25.32 0.2822 0.1487C-H · · ·O 1.087 (1.088)a 2.352 3.214 135.0

a Distance for monomer.

TABLE 4: Short Circuit Photocurrent Density (Jsc) andOpen-Circuit Photovoltage (Voc) in a DSSC usingN-Containing Heterocycles as an Electrolyte SolutionAdditive

additive Jsc (mA · cm-2) Voc (V)

none 16.5 0.62pyrazole 15.6 0.67imidazole 9.9 0.85pyridine 14.7 0.73pyrimidine 15.2 0.67pyrazine 15.4 0.67TBP 14.2 0.77

TABLE 5: Atomic Charge on the S Atom for the NCSLigand Determined by Mulliken Population and NBOAnalyses (in e)

Mulliken NBO

species horizontala verticalb horizontala verticalb

N719+ 0.0200 0.0200 0.1039 0.1039N719+ ·pyrazole(a) 0.0074 0.0068 0.0902 0.0908N719+ ·pyrazole(b) 0.0074 0.0078 0.0899 0.0932N719+ · imidazole(a) 0.0058 0.0056 0.0886 0.0899N719+ · imidazole(b) 0.0044 0.0066 0.0874 0.0910N719+ ·pyridine 0.0052 0.0074 0.0880 0.0921N719+ ·pyrimidine(a) 0.0084 0.0109 0.0909 0.0951N719+ ·pyrimidine(b) 0.0107 0.0107 0.0936 0.0945N719+ ·pyrazine 0.0106 0.0113 0.0931 0.0952N719+ ·TBP(a) 0.0019 0.0051 0.0853 0.0900N719+ ·TBP(b) 0.0016 0.0046 0.0850 0.0895

a Horizontal NCS ligand refers to N-containing heterocycles asdenoted by h in Figure 3. b Vertical NCS ligand refers toN-containing heterocycles as denoted by V in Figure 3.

Figure 6. Correlation between the Jsc of the DSSC and the atomiccharge on the S atom for the NCS ligand of N719+.


Conclusions

Interactions of six different N-containing heterocycles withneutral and cationic N719 dye were investigated by a DFTmethod with a full geometric optimization. Hydrogen bondsbetween the H atom of the carboxyl group for the N719 ligandand the N atom with a lone pair of N-containing heterocycles,and between the O atom with a lone pair of carboxyl groupsfor the N719 ligand and the H atom adjacent to the N atomforming the other hydrogen bond are formed on both neutraland cationic complexes. The H · · ·N hydrogen bonds are strongerthan the O · · ·H ones. Among the six heterocycles, TBP hadthe strongest interaction. The interaction of N-containingheterocycles reduced the atomic charges on the S atoms of theNCS ligands for cationic dye. Comparing this finding of theDFT analysis to experimentally measured dye-sensitized TiO2

solar cell performance, Jsc was observed to increase with theatomic charge on the S atom of the NCS ligand. The lesspositive the atomic charge on the S atom for the NCS ligand ofoxidized N719, the more difficult the interaction of oxidizeddye with I-, which suppresses further dye regeneration and

lowers Jsc. In addition, the interaction with N-containingheterocycles raises the energy level of both HOMO and LUMO,leaving the HOMO-LUMO gap constant. Combined with theFermi level shift of the TiO2 due to adsorption of the N-containing heterocycles on the TiO2 surface, the energy gapsbetween the LUMO of the dye and the Fermi level of TiO2,and between the HOMO of N719 and the redox energy of I-/I3- couple are both decreased by the interaction of N-containing

heterocycles via hydrogen bonds. The decreased energy gapbetween the LUMO and the Fermi level causes a decrease inthe driving force for electron injection from the LUMO of thedye into the conduction band of TiO2, leading to the lowinjection efficiency and low Jsc. At the same time, the reducedenergy gap between HOMO of the sensitizing dye and redoxenergy of the I-/I3

- couple makes it difficult for the oxidizeddye to accept new electrons from the redox couple, resulting insuppressed subsequent electron injection and consequently alsoa lower Jsc.

Supporting Information Available: Absorption spectra ofN719 dye. This material is available free of charge via theInternet at http://pubs.acs.org.

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JP908270E


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