Effect of Cations on the Interactions of Ru Dye and Iodides in Dye

ARTICLE pubs.acs.org/JPCC

Effect of Cations on the Interactions of Ru Dye and Iodides in Dye-Sensitized Solar Cells: A Density Functional Theory Study Hitoshi Kusama,* Hideki Sugihara, and Kazuhiro Sayama Energy 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

bS Supporting Information ABSTRACT: A density functional theory (DFT) study was performed to elucidate the effect of counter cations [Hþ, Liþ, 1,2-dimethyl-3-propylimidazolium (DMPIþ), and tetrabutylammonium (TBAþ)] on the interactions between Ru(II)polypyridyl dye (N719) and iodide anions. The counter cations saturated the negative charge of the terminal carboxylic group. Symmetric bidentate binding with the oxygen atoms in the carboxylate group for the N719 ligand was observed with Liþ. DMPIþ and TBAþ formed two hydrogen bonds between each of the O atoms in the carboxylate group for the dye ligand and the H atoms of the alkyl groups. Cations drastically influenced the interaction between the dye and iodide ions via the S atom in the NCS ligand. For both monoiodide and diiodide ions, the interaction strength increased in the order TBAþ < DMPIþ < Liþ < Hþ, which corresponds to the short-circuit photocurrent density of a dye-sensitized solar cell. The results suggest that the stronger the interaction of oxidized dye with the iodide ions, the easier the dye regeneration after electron injection into the TiO2 conduction band proceeds by a I-/I3- redox couple, leading to a higher photocurrent in the cell.

’ INTRODUCTION Since O’Regan and Gr€atzel published a high solar energy conversion efficiency over 7% with Air Mass 1.5,1 dye-sensitized solar cells (DSSCs) based on the concept of photosensitization of wide band gap oxide semiconductors have attracted much interest. A basic DSSC consists of three fundamental components: (i) a sensitized photoanode, which is typically a Ru complex or organic dye-sensitized nanocrystalline TiO2 film on a transparent conductive oxide (TCO) glass, (ii) an electrolyte solution containing I-/I3- as a redox couple, and (iii) a cathode, which is platinized TCO glass. Illuminating a DSSC with light causes the ground state of dye to absorb a photon, and an electron is transferred to a higherenergy level. This leads to an excited state. Next, an electron is injected from the photoexcited dye into the TiO2 conduction band. The injected electron percolates through the porous TiO2 layer to the TCO glass, passing the external load to the cathode. Subsequently, the electron is transferred to I3- to yield I- at the cathode. Finally, I- reduces the oxidized dye to its original ground state. In this way, the device operates in a regenerative mode. One of the most popular dyes in DSSCs is [Ru(H2dcbpy)2(NCS)2] (N3) or TBA2[Ru(Hdcbpy)2(NCS)2] (N719, Scheme 1) where dcbpy is 4,40 -dicarboxy-2,20 -bipyridine.2 The photoexcited electron, which is localized on the polypyridyl ligand bonding to the TiO2 surface via the carboxyl group, can be efficiently injected into the TiO2 conduction band. After an electron is injected, the generated hole can be efficiently filled with an electron from r 2011 American Chemical Society

iodide redox couple. Because a critical component of a DSSC to determine the light harvesting efficiency is the sensitizing dye, great efforts have focused on understanding the operation mechanisms of the dye, including electron injection into the TiO2 conduction band as well as improving dye performance such as the spectral response in the red and infrared regions. Meanwhile, the light-to-electric energy conversion efficiency of DSSCs on the whole depends on not only the property of the dye but also the performances of the other component such as regeneration or rereduction3 of the oxidized dye by the I-/I3redox couple. Because the redox couple closes the light-conversion cycle, it is perhaps the second most important component of a DSSC.4-6 Nevertheless, the regeneration mechanism of oxidized dye has yet to be fully investigated. Clifford and co-workers have reported that the regeneration reaction of N3 dye proceeds in two steps:3 the process is initiated by binding of an iodide ion with the oxidized dye, which is after electron injection into TiO2, to form a transient intermediate complex [dyeþ 3 I-] dyeþ þ I - f ½dyeþ 3 I - ð1Þ A subsequent reaction of this complex with a second iodide ion affords a neutral dye and I2- via [dye 3 I2-] ½dyeþ 3 I - þ I - f ½dye 3 I2 - f dye þ I2 -

ð2Þ

Received: September 23, 2010 Revised: December 1, 2010 Published: January 7, 2011 2544

dx.doi.org/10.1021/jp109114b | J. Phys. Chem. C 2011, 115, 2544–2552

The Journal of Physical Chemistry C

ARTICLE

Scheme 1. Dye and Organic Cations Studied in This Paper

Then, I2- dissociates to yield iodide and triiodide ions 2I2 - f I - þ I3 -

ð3Þ

The formation of the first intermediate (1) is kinetically fast. However, the subsequent reaction of the intermediate with a second iodide species (2) is much slower and,hence, the rate-determining step of the overall regeneration reaction. Recently, Privalov and co-workers have identified plausible pathways of the two-step regeneration mechanism via such complexes using DFT calculations.5 In particular, they revealed that the process can be initiated by binding I- to the N3 dye cation via a sulfur atom of the NCS ligand, as described above. More recently, Schiffmann and co-workers have proposed an additional one-step pathway to regenerate the N3 dye cation by I2- via a transient intermediate complex [dyeþ 3 I2-]7 dyeþ þ I2 - f ½dyeþ 3 I2 - f dye þ I2

ð4Þ

Then I2 reacts with I- to yield I3I2 þ I - f I3 -

ð5Þ

In this regeneration pathway, S 3 3 3 I bond formation is central. Privalov and co-workers have also provided theoretical support for the one-step regeneration pathway of organic dye cations.4 In a previous paper,8 we theoretically studied the effect of N-containing heterocycles, which are typically dissolved in the electrolyte solution with an I-/I3- redox couple to enhance DSSC performance,9 such as 4-t-butylpyridine (TBP) on the interactions of iodide ions with oxidized N3 dye via a two-step pathway. Our DFT results revealed that the dye simultaneously interacted with both the iodide ions via the S atom of the NCS ligand and the heterocycles forming two hydrogen bonds with the carboxyl group in the polypyridyl ligand. However, the interaction of the heterocycles weakened the interaction of the iodide ions in each step, which suppressed the regeneration of the oxidized dye after electron injection into the TiO2 conduction band by the I-/I3redox couple. Thus, the short-circuit photocurrent density (Jsc) of the DSSC was reduced. Because lithium iodide (LiI) and imidazolium iodide such as 1,2-dimethyl-3-propylimidazolium iodide (DMPII) as well as

iodine (I2) are generally used as iodide ion sources,9 cations like Liþ and DMPIþ in Scheme 1 and N-containing heterocycles as well as iodide anions are typically dissolved in the electrolyte solution of DSSCs. For N719 dye, the hydrophobic cations of TBAþ (Scheme 1), which increase the solubility of the dye in organic solvents for dye adsorption on the TiO2 film,10 also exist in the cell. Numerous studies have examined the influences of the cations on DSSC performances such as Jsc.11-21 However, cations have been discussed only in terms of interactions with TiO211,12,15,16 or iodide ions.15,18 Although the interaction of the dye with Liþ has been suggested by Hara and co-workers,22 its structure and interaction mechanism remain unknown. To the best of our knowledge, the interaction between dye and imidazolium has yet to be studied. Moreover, there have been only a few reports on the effect of cations on the regeneration reaction of oxidized dye.14 Cations may affect the interaction between the dye and the iodide ions, like N-containing heterocycles. Thus, their influence should be investigated for a clearer understanding of DSSCs as well as to improve performance of DSSCs. Herein, we focus on the intermolecular interactions of N719 dye with four cations, Hþ, Liþ, DMPIþ, and TBAþ, and their subsequent interactions with iodide ions. The main tools in this investigation are quantum chemical calculations at the DFT level, which are suitable for studies of large molecular systems like N719 dye. Additionally, to elucidate the effect of the cations on the regeneration of the oxidized dye by the I-/I3- redox couple, the results are compared to the DSSC performance containing cations in an I-/I3- electrolyte solution.

’ COMPUTATIONAL DETAILS DFT calculations were performed using the Gaussian 09 program at the Research Center for Computational Science, Okazaki, Japan, and with the Gaussian 09W program on personal computers.23 The geometries were fully optimized in the gas phase at the hybrid DFT levels by B3LYP functions, which combine Becke’s three-parameter exchange function (B3)24,25 with the correlation function of Lee, Yang, and Parr (LYP).26 A LanL2DZ basis set, which corresponds to a Dunning/Huzinaga valence double-ζ basis (D95 V) for first-row elements27 and a Los Alamos effective core potential (ECP) plus double-ζ basis for Na-La and Hf-Bi atoms,28-30 was used for all systems. The energies were corrected for the zero-point vibrational energies (ZPE), and the interaction energies were determined as the energy difference between the complex and the sum of the isolated monomers. Using this definition, a positive interaction energy corresponds to a strong interaction. All of the complexes have nonimaginary frequency geometries. These findings confirm that the optimized structures correspond to real minima on the potential energy surface. To understand the nature and magnitude of the intermolecular interactions, natural bond orbital (NBO) analysis31,32 was conducted on the optimized geometries with the NBO 3.1 program33 included in the Gaussian program package. In NBO analysis, the stabilization energy E(2) associated with ifj delocalization is explicitly estimated by the following equation Eð2Þ ¼ ΔEij ¼ qi

F 2 ði, jÞ εj - εi

ð6Þ

where qi is the ith donor orbital occupancy; εi and εj are the diagonal elements (orbital energies); and F(i, j) is the off-diagonal element associated with the NBO Fock matrix. Furthermore, Mulliken population analysis was performed on the optimized structures. 2545

dx.doi.org/10.1021/jp109114b |J. Phys. Chem. C 2011, 115, 2544–2552


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’ RESULTS AND DISCUSSION Effect of Cations on the Dye Structure. Our study began with the dye in its ground singlet electronic spin state with a total charge of 0. Two protonated carboxylate (carboxylic acid) groups were located on different bipyridine ligands trans to the NCS ligands.34,35 According to a recent report,35 the adsorption of N719 dye on the TiO2 surface occurs via two neighboring carboxylic acid/carboxylate groups linked to the same bipyridine ligand via a combination of hydrogen bonding and bidentate bridging. Thus, to investigate the effect of cations, we substituted three types of cations (Hþ, Liþ, and DMPIþ) for one of the TBAþ counterions (Scheme 1). For computational convenience, TBAþ at the carboxylate group participating the adsorption on the TiO2 was mimicked by Naþ.34,36 Table S1 in the Supporting Information lists the main geometrical parameters of the optimized dye structures. The tested cations have a negligible effect on the main features of N719, including the Ru-N bond distance. Figure 1 depicts the

Figure 1. Optimized geometries of [cation-N719]0 species. White = H; lilac = Li; gray = C; blue =N; red = O; purple = Na; yellow = S; and teal = Ru atoms. Distances (normal letters) are given in angstroms and angles in degrees (italic letters).

carboxylate-cation binding in the optimized geometries, the O 3 3 3 H bond distance dose not differ from the other two bonds. For [Li-N719]0, symmetric bidentate binding is observed with a distance and angle of 1.929 Å and 71.7°, respectively. On the other hand, the organic cations in Scheme 1 bind with the oxygen atoms of the carboxylate group from the dye ligand via the H atoms of the alkyl group on the N atom. For [DMPI-N719]0, the intermolecular O 3 3 3 H distances are 2.091 Å for the methyl group at position 1 and 2.388 Å for the propyl group at position 3. These distances are smaller than the net van der Waals radii of the binding atoms, 2.720 Å,37 indicating the formation of intermolecular hydrogen bonds. The shorter the O 3 3 3 H but the longer the C-H distance, the stronger the hydrogen bonding.38 Thus, the methyl group forms stronger hydrogen bonds than the propyl group. Furthermore, both O 3 3 3 H-C angles are greater than 90° or somewhat more conservatively 110°, which also supports the formation of hydrogen bonding.38,39 Judging from the bond distances and angles, the observed intermolecular bonds in the [TBA-N719]0 species are also hydrogen bonds. Although the two H atoms have similar positions, these bonds differ in strength due to the additional interaction of the propyl group with the NCS ligand of the dye (Figure 1). The interaction of TBAþ via the terminal H atom with the S atom of the dye (dotted line in Figure 1) causes the Ru-N-C bond angle for the NCS ligand to narrow compared to the other cation species (Table S1, Supporting Information). The cation-N719 species were also optimized with a total charge of þ1 and a multiplicity of 2 after electron injection into the TiO2 conduction band. Oxidation causes the distances of Ru-bpycis and Ru-bpytrans bonds to lengthen but the distances of the Ru-NCS and the C-S bonds to shorten (Table 1). Compared to the neutral species (Table S1, Supporting Information), the oxidized species show smaller bond angles for N(bpytrans)-Ru-N(bpycis), N(bpytrans)-Ru-N(bpytrans), and N(bpycis)-Ru-N(bpycis) but larger angles for N-Ru-N for NCS ligands. The N-C distances and the N-C-S bond angles of NCS ligands remain the same after oxidation. As illustrated in Figure 2, except for the [H-N719] species, carboxylate-cation bonding drastically differs between the neutral and monopositive species. In the monopositive species, the

Table 1. Main Optimized Geometrical Parameters and Atomic Charges on the S Atom for the [Cation-N719]þ Species parameter Ru-bpycis (Å)a Ru-bpytrans (Å)a

[H-N719]þ

[Li-N719]þ

[DMPI-N719]þ

[TBA-N719]þ

2.091-2.087 2.098-2.096

2.091-2.090 2.096-2.096

2.092-2.092 2.095-2.095

2.092-2.092 2.094-2.095

Ru-NCS (Å)

1.997-1.996

1.997-1.996

1.996-1.998

1.996-1.997

N-C(NCS) (Å)

1.200-1.200

1.200-1.200

1.200-1.200

1.200-1.200 1.660-1.660

C-S (Å)

1.658-1.658

1.659-1.659

1.660-1.660

N(bpytrans)-Ru-N(bpycis) (deg)a

78.7-78.8

78.7-78.7

78.7-78.7

78.7-78.7

N(bpytrans)-Ru-N(bpytrans) (deg)a

89.5

89.5

89.2

89.3

N(bpycis)-Ru-N(bpycis) (deg)a

176.8

176.6

176.2

176.2

N(NCS)-Ru-N(NCS) (deg) Ru-N-C(NCS) (deg)

94.9 175.5-174.7

94.9 175.3-175.0

95.0 175.5-175.6

95.0 175.3-175.5

N-C-S (deg)

179.1-178.9

179.0-178.9

179.0-179.1

179.0-179.1

natural charge on S atom (e)b

-0.0029

-0.0135

-0.0275

-0.0302

Mulliken charge on S atom (e)c

0.0356

0.0265

0.0142

0.0121

a

Subscripts cis and trans refer to the NCS ligands. b Atomic charge on the S atom for the NCS ligand, which is denoted by the solid circle in Figure 2, determined by NBO analysis. c Atomic charge on the S atom for the NCS ligand, which is denoted by the solid circle in Figure 2, determined by Mulliken population analysis. 2546



Figure 2. Bonds between the carboxylate group of the dye ligand and the cation for the optimized [cation-N719]þ species. White = H; lilac = Li; gray = C; blue = N; red = O; yellow = S; and teal = Ru atoms. Distances (normal letters) are given in angstroms and angles in degrees (italic letters).

O 3 3 3 Li bonds elongate, but the O 3 3 3 Li 3 3 3 O bond angle narrows. Additionally, the distance of the O 3 3 3 H bonds lengthens but is still shorter than the net van der Waals radii of the binding atoms for the [DMPI-N719]þ species. In contrast, the C-H bonds are not altered. One of the O 3 3 3 H bonds for [TBA-N719]þ also lengthens, but the other shortens. Moreover, one of the O 3 3 3 H-C bond angles widens, while the other narrows for both organic cation species by dye oxidation. However, the angles remain within van der Waals angular cutoff. Moreover, the interaction between the propyl group of TBA and the NCS ligand of the dye disappears. Effect of Cations on the Dye-I- Interaction. According to a previous study,5 the structure of N719 interacting with I- via the S atom of the NCS ligand, which is represented by eq 1, was optimized with a total charge of 0 and multiplicity of 2. Figure 3 represents the optimized geometries of the cation-N719-I species with select bond distances and angles. Table 2 lists the main geometrical parameters of the optimized structures. The intermolecular S 3 3 3 I distances are about 3.23 Å, which are stronger than the net van der Waals radii of the bonding atoms, 3.78 Å.37 The Ru-N distance for Ru-NCS bonding and the C-S distance of the NCS ligand interacting with the iodide ion are longer than the distances for the free NCS ligand, which are consistent with previous results.5 The slightly shorter N-C bond for the NCS ligand is caused by I- interaction. The C-S 3 3 3 I bond angles are over 90°. The interaction with I- also results in the narrowing of some of the N-C-S bond angles for the NCS ligand. Additionally, the angles for N-Ru-N and Ru-N-C narrow for Ru-NCS, but the angle broadens for N-Ru-N in Ru-bpy with trans coordination to the NCS ligand. Bonding between the carboxylate group for the dye ligand and the cations also varies with the I- interaction, except for [HN719-I]0. The O 3 3 3 H bond distances for the [DMPI-N719I]0 and [TBA-N719-I]0 species as well as the O 3 3 3 Li ones for [Li-N719-I]0 in Figure 3 are shorter than the [cation-N719]þ species in Figure 2. On the other hand, the lengths of the C-H bonds for the alkyl groups interacting with O atoms of the dye do not change upon I- interaction. The O 3 3 3 Li 3 3 3 O bond angle of [Li-N719-I]0 increases compared to [Li-N719]þ. For

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Figure 3. Optimized geometries of [cation-N719-I]0 species. White = H; lilac = Li; gray = C; blue = N; red = O; purple = Na; yellow = S; teal = Ru; and violet = I atoms. Distances (normal letters) are given in angstroms and angles in degrees (italic letters).

[DMPI-N719-I]0, the O 3 3 3 H-C bond angle for the methyl group widens, but that for the propyl group narrows with I- interaction. Angle narrowing also appears at one of the O 3 3 3 H-C bonds for the [TBA-N719-I]0 species. Table 2 also lists the intermolecular bond energy, E, of I- with the cation-N719 species and the atomic charges on the I atom determined by Mulliken population and NBO analyses. The E values are over 70 kcal mol-1, and increase in the order of TBAþ < DMPIþ < Liþ < Hþ. This order indicates the interaction stability between the oxidized dye and I-. Regardless of the analysis method, the atomic charge on the I atom becomes less negative in the order of DMPIþ < TBAþ < Liþ < Hþ and roughly corresponds to the order of the E value. The higher the E and the more positive the I atomic charge, the more stable the interaction between the oxidized dye and iodide ion. These results relate to the atomic charge on the S atom for the [cation-N719]þ species in Table 1. Among the four tested cations, the S atomic charge determined by both NBO and Mulliken population analyses becomes more positive in the same order as the E values of I- with the cation-N719 species. The more positive the atomic charge on the S atom for the NCS ligand of oxidized dye, the easier the dye interacts with I-.4,40 Effect of Cations on the Dye-I2 Interaction. Next, the [cation-N719-I]0 species interacting with a second I-, which is represented by eq 2, was optimized with a total charge of -1 and is depicted in Figure 4. Compared to [cation-N719-I]0, the intermolecular S 3 3 3 I distances for [cation-N719-I2]- are shorter regardless of the cation (Table 3). The distances of formed I-I bonds are about 3.07 Å, which is shorter than isolated I2- (3.46 Å). The Ru-N distances in the Ru-NCS are elongated regardless of whether the NCS ligand interacts with I-. The S 3 3 3 I-I bond angles are nearly linear, and the angles of C-S 3 3 3 I, N-C-S, and Ru-N-C bonds that interact with iodide ions drastically widen. The O 3 3 3 H distance for the methyl group elongates but remains within the van der Waals distance cutoff. However, the distance for the propyl group diminishes at the [DMPI-N719-I2]species. Additionally, the intermolecular bonds of O 3 3 3 Li for [Li-N719-I2]- and O 3 3 3 H for [TBA-N719-I2]- decrease in length. The O 3 3 3 H-C angle of the propyl group widens, 2547



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Table 2. Main Optimized Geometrical Parameters, Intermolecular Interaction Energies (E) of I-, and Atomic Charges on the I Atom for the [Cation-N719--I]0 Species [H-N719-I]0

[Li-N719-I]0

[DMPI-N719-I]0

[TBA-N719-I]0

Ru-bpycis (Å)a

2.084-2.066

2.082-2.074

2.081-2.082

2.080-2.082

Ru-bpytrans (Å)a Ru-NCS (Å)

2.057-2.062 2.058b-2.051

2.054-2.062 2.059b-2.052

2.050-2.062 2.060b-2.054

2.050-2.062 2.060b-2.054

N-C(NCS) (Å)

1.187b-1.197

1.187b-1.197

1.187b-1.196

1.187b-1.196

C-S (Å)

1.693 -1.674

1.694 -1.674

1.695 -1.676

1.695b-1.676

S 3 3 3 I (Å) N(bpytrans)-Ru-N(bpycis) (deg)a

3.231

3.230

3.229

3.228

79.2-79.3

79.3-79.3

79.4-79.1

79.4-79.1


93.8

93.8

93.5

93.6


176.2

176.1

175.7

175.7

N(NCS)-Ru-N(NCS) (deg) Ru-N-C(NCS) (deg)

91.9 168.8b-176.7

91.9 169.2b-177.6

91.8 170.0b-176.5

91.7 170.5b-177.4

N-C-S (deg)

176.4b-179.0

176.4b-179.1

176.5b-179.1

176.6b-179.1

C-S 3 3 3 I (deg) E (kcal 3 mol-1)

90.8

91.1

91.7

91.9

82.52

78.38

72.12

71.91

natural charge on I atom (e)c

-0.3605

-0.3660

-0.3740

-0.3727

Mulliken charge on I atom (e)d

-0.3347

-0.3413

-0.3505

-0.3498

parameter

a

b

b

b

Subscripts cis and trans refer to the NCS ligands. b Interacts with I-. c Determined by NBO analysis. d Determined by Mulliken population analysis.

Figure 4. Optimized geometries of [cation-N719-I2]- species. White = H; lilac = Li; gray = C; blue = N; red = O; purple = Na; yellow = S; teal = Ru; and violet = I atoms. Distances (normal letters) are given in angstroms and angles in degrees (italic letters).

while that of the methyl group narrows for [DMPI-N719-I2]-. A similar behavior is also observed for the [TBA-N719-I2]species, but the degree of change is much smaller. The E values for a second I- with cation-N719-I, and for I2with cations-N719 in Table 3 range from 11 to 24 kcal mol-1 and 83 to 106 kcal mol-1, respectively. The E values for both of these interactions increase in the order of TBAþ < DMPIþ < Liþ < Hþ, which corresponds to the finding of [cationN719-I]0 in Table 2. In addition, the bond distances for the intermolecular S 3 3 3 I and the intramolecular I-I decrease in this order. To confirm the reaction mechanism where the oxidized dye is regenerated by I2- via a one-step pathway through a transient intermediate complex [dyeþ 3 I2-] described by eq 4,7 we also geometrically optimized the [cation-N719-I2] species with a total charge of 0. The intermolecular S 3 3 3 I distances for all of the

[cation-N719-I2]0 species in Table 4 are about 3.00 Å, which is over 0.01 Å longer than [cation-N719-I2]- (Table 3) but less than the net van der Waals radii of the binding atoms (3.78 Å).37 This finding suggests that the one-step regeneration of oxidized N719 dye is performed by I2- regardless of cations. Contrary to the S 3 3 3 I bonds, I-I bond distances for [cation-N719-I2]0 are shorter than [cation-N719-I2]-. Figure 5 illustrates carboxylatecation bonding as well as that for S 3 3 3 I-I for the optimized [cation-N719-I2]0 species. The O 3 3 3 Li bond distances for [Li-N719-I2]0 are longer than [Li-N719-I2]-. The O 3 3 3 H bond for the methyl group of [DMPI-N719-I2]0 is shorter, but that for the propyl one is longer than [DMPI-N719-I2]-. At the same time, the O 3 3 3 H-C bond angle for the methyl group of [DMPI-N719-I2]0 is wider, but that for the propyl one is narrower. Similar phenomena are observed with the [TBAN719-I2]0 species. The relative order of the E values for I2among the cations in Table 4 agrees with the order for the [cationN719-I2]- species (Table 4), that is, TBAþ < DMPIþ < Liþ < Hþ. Table 5 lists the atomic charge on the I atom for the [cationN719-I2]- and [cation-N719-I2]0 species determined by NBO and Mulliken population analyses. The atomic charge for the [cation-N719-I2]- species becomes less negative in the order of TBAþ < DMPIþ < Liþ < Hþ regardless of the analysis method or position. For [cation-N719-I2]0, the atomic charge on the I atom interacting with the S atom becomes less negative in the same order for both analysis methods. As mentioned for the [cation-N719-I]0 species, the relative order of the atomic charge on the I atom and the E value (Tables 3 and 4) indicates the relative strength of the iodide interaction. The less negative the atomic charge of the I atom and the larger the E value of iodide ions for the [cation-N719-I2] species, the more preferable the interaction between the oxidized dye and the iodide anions. NBO Analysis of Hydrogen Bonding for DMPI Species. Figures 1-5 clearly demonstrate that hydrogen bonds are formed with the DMPI and TBA cations and the dye. In particular, the hydrogen bonds observed at the DMPI species have vastly different geometries for the dye-I2 interactions. To understand the nature and magnitude of the hydrogen bonding between 2548



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Table 3. Main Optimized Geometrical Parameters and Intermolecular Interaction Energies (E) for the [Cation-N719-I2]Species [H-N719-I2]-

[Li-N719-I2]-

[DMPI-N719-I2]-

[TBA-N719-I2]-

Ru-bpycis (Å)a

2.079-2.076

2.079-2.078

2.078-2.085

2.079-2.082

Ru-bpytrans (Å)a Ru-NCS (Å)

2.034-2.057 2.074b-2.071

2.035-2.051 2.077b-2.075

2.037-2.045 2.078b-2.077

2.038-2.045 2.081b-2.077

N-C(NCS) (Å)

1.184b-1.194

1.184b-1.194

1.185b-1.194

1.185b-1.194

C-S (Å)

1.708 -1.683

1.708 -1.683

1.708 -1.684

1.707b-1.685

S 3 3 3 I (Å) I-I (Å)

2.965

2.964

2.968

2.983

3.062

3.066

3.078

3.086


79.5-79.3

79.5-79.4

79.5-79.4

79.5-79.3


92.2

92.4

92.9

92.4

N(bpycis)-Ru-N(bpycis) (deg)a N(NCS)-Ru-N(NCS) (deg)

173.8 89.7

173.8 89.5

174.0 89.3

174.4 89.5

Ru-N-C(NCS) (deg)

177.6b-177.1

178.4b-177.3

179.9b-177.7

179.0b-177.3

N-C-S (deg)

179.8 -179.2

179.9 -179.2

179.2 -179.2

179.2b-179.2

C-S 3 3 3 I (deg) S 3 3 3 I-I (deg) E(second I-) (kcal mol-1)

100.6

101.5

103.3

103.7

179.1

178.5

177.7

177.4

23.08

18.03

12.11

11.63

E(I2-) (kcal mol-1)

105.60

96.40

84.23

83.54

parameter

a

b

b

b

b

b

b

Subscripts cis and trans refer to the NCS ligands. b Interacts with I-.

Table 4. Main Optimized Geometrical Parameters and Intermolecular Interaction Energies (E) for the [Cation-N719-I2]0 Species [H-N719-I2]0

[Li-N719-I2]0

[DMPI-N719-I2]0

[TBA-N719-I2]0

Ru-bpycis (Å)a

2.082-2.068

2.080-2.075

2.078-2.084

2.078-2.084

Ru-bpytrans (Å)a

2.055-2.067

2.052-2.067

2.047-2.069

2.048-2.069

Ru-NCS (Å)

2.068b-2.050

2.069b-2.050

2.070b-2.051

2.070b-2.052

N-C(NCS) (Å)

1.185b-1.197

1.185b-1.197

1.185b-1.196

1.185b-1.196

C-S (Å) S 3 3 3 I (Å)

1.703 -1.673 3.010

1.704 -1.674 3.004

1.705 -1.675 2.995

1.706b-1.676 2.997

I-I (Å)

3.030

3.033

3.038

3.037


79.2-79.3

79.3-79.2

79.4-79.1

79.4-79.1


93.0

93.0

92.9

92.9


176.2

176.0

175.7

175.7

N(NCS)-Ru-N(NCS) (deg)

91.8

91.8

91.8

91.7

Ru-N-C(NCS) (deg)

169.1b-176.9

169.4b-177.9

170.0b-178.1

170.4b-178.1

N-C-S (deg) C-S 3 3 3 I (deg)

176.3 -179.0 91.1

176.4 -179.1 91.6

176.7 -179.2 92.7

176.6b-179.1 92.5

S 3 3 3 I-I (deg) E (kcal 3 mol-1)

172.9

173.3

174.3

174.1

77.78

73.85

67.71

67.65

parameter

a

b

b

b

b

b

b

Subscripts cis and trans refer to the NCS ligands. b Interacts with I-.

the O atom of the carboxylate group and the H-C of the alkyl group, we performed NBO analysis on the DMPI complexes. In the NBO analysis, the importance of the hyperconjugation interaction and electron density transfer from the lone electron pairs of the Y atom to the X-H antibonding orbital in the XH 3 3 3 Y system is well documented.32 In general, such an interaction leads to an increase in the occupancy of the X-H antibonding orbital. As listed in Table 6, the occupancies of the σ*(1)C-H antibonds increase upon complexation. The increased occupancy in the X-H antibonding orbital weakens the X-H bond, resulting in elongation. Actually, the C-H bond lengths for DMPI alkyl groups involving hydrogen bonds are generally longer than the length of the monomer (1.093 Å for the methyl group and 1.095 Å

for the propyl group) (see Figures 1-5). The lone pair, LP, of the O atom at the carboxylate group for the dye participates as a donor, and the σ*(1)C-H antibond of DMPI acts as an acceptor in a strong intermolecular charge transfer interaction, LP(n)Of σ*(1)C-H. This interaction can be considered a measure of the relative stability of these hydrogen bondings. The value of the charge-transfer energy, E(2), which corresponds to the interaction for the methyl group in the [DMPI-N719]0 species, is larger than that for the propyl group. Therefore, in the case of [DMPIN719]0, the extent of change in both the σ*(1)C-H antibond occupancy and the H atomic charge of the methyl group is larger than that of the propyl group by complexation.39 This observation is consistent with the lengthening of the C-H bond as well 2549



ARTICLE

as the closer O 3 3 3 H distance for the methyl group in Figure 1. Similar behavior appears with the [DMPI-N719]þ (Figure 2) and [DMPI-N719-I]0 (Figure 3) species. On the contrary, for [DMPI-N719-I2]- and [DMPI-N719-I2]0, the methyl groups form weaker hydrogen bonds than propyl groups because the methyl groups exhibit fewer changes in the σ*(1)C-H

antibond occupancy and atomic charge of the H atom interacting with the O atom as well as lower E(2) values for LP(n)Of σ*(1)C-H. This finding also corresponds with the less elongated C-H bond and longer O 3 3 3 H bond for the methyl group than the propyl one (Figures 4 and 5). Eventually, NBO analysis demonstrates that the methyl group at position 1 of DMPIþ forms a more preferable hydrogen bond with the carboxylate O atom of the dye than the propyl group at position 3 for [DMPI-N719]0, [DMPI-N719]þ, and [DMPIN719-I]0 but forms a less preferable bond for [DMPI-N719I2]- and [DMPI-N719-I2]0. Implications on the DSSC Performance. The DFT studies of cation-N719 and cation-N719-iodide species clarified that the cations interacting with the carboxylate group at the bipyridyl ligand of N719 dye affect the interaction between the oxidized dye and the iodide ions and that the dye-iodide interaction strength increases in the order of TBAþ < DMPIþ < Liþ < Hþ for both monoiodide or diiodide ions. These results reveal that the ease of dye regeneration by I-/I3- redox couple in the electrolyte solution of DSSC increases in the order of TBAþ < DMPIþ < Liþ < Hþ regardless if via a one- or two-step regeneration mechanism by the redox couple. According to a previous study on cations in the electrolyte solution by Nakade and co-workers,17 the Jsc value of DSSC using N719 dye and I-/I3- redox couple increases in the order of TBAþ (6.8 mA cm-2) < DMPIþ (8.5 mA cm-2) < Liþ (13.4 mA cm-2). Additionally, Zhang and co-workers have reported that the electrolyte solution of LiI shows a higher Jsc of DSSC with

Figure 5. Optimized geometries of [cation-N719-I2]0 species. White = H; lilac = Li; gray = C; blue = N; red = O; purple = Na; yellow = S; teal = Ru; and violet = I atoms. Distances (normal letters) are given in angstroms and angles in degrees (italic letters).

Table 5. Atomic Charge on the I Atom for the [Cation-N719-I2] Species Determined by NBO and Mulliken Population Analyses (in e) NBO first Ia

second Ib

I2

first Ia

second Ib

I2

-

[H-N719-I2]

-0.0663

-0.3439

-0.4102

-0.0598

-0.3402

-0.4000

[Li-N719-I2]-

-0.0673

-0.3497

-0.4170

-0.0608

-0.3467

-0.4075

[DMPI-N719-I2][TBA-N719-I2]-

-0.0738 -0.0836

-0.3632 -0.3674

-0.4370 -0.4510

-0.0674 -0.0767

-0.3613 -0.3658

-0.4287 -0.4425

[H-N719-I2]0

-0.0644

-0.2910

-0.3554

-0.0570

-0.2844

-0.3414

[Li-N719-I2]0

-0.0649

-0.2973

-0.3622

-0.0579

-0.2912

-0.3491

[DMPI-N719-I2]0

-0.0651

-0.3070

-0.3721

-0.0585

-0.3018

-0.3603

[TBA-N719-I2]0

-0.0654

-0.3049

-0.3703

-0.0589

-0.2998

-0.3587

species

a

Mulliken

I atom interacts with the S atom of NCS ligand. b I atom interacts with the first I.

Table 6. Selected Second-Order Perturbation Energy, E(2) (in kcal mol-1), Occupancy Data of NBOs (in e), and Atomic Charge (in e) for DMPI Species DMPIþ parameter

[DMPI-N719]0

methyl propyl methyl

propyl

methyl

0.48

2.02

0.54

2.64

0.68

0.82

1.20

0.08

1.86

0.06

0.48

3.22

0.62

4.50

0.74

1.10

5.24

1.72

3.28

0.0071 0.0138 0.0210

0.0178

0.0162

0.0174

0.0195

0.0180

0.0126

0.0285

0.0133

0.0226

0.0139

0.0040

0.0091

0.0036

0.0124

0.0042

0.0055

0.0147

0.0062

0.0088

0.2431 0.2328 0.2836

0.2619

0.2757

0.2589

0.2828

0.2643

0.2716

0.2818

0.2740

0.2763

0.0405

0.0291

0.0326

0.0261

0.0397

0.0315

0.0285

0.0490

0.0309

0.0436

E(2) for LP(3)Ofσ*(1)C-Ha

0.17

sum of E(2)b

5.27

Δσ*(1)C-Hc atomic charge on H atom Δ(atomic charge on H)d

methyl

methyl

2.47

σ*(1)C-H occupancy

[DMPI-N719-I2]0

propyl

2.63

for LP(1)Ofσ*(1)C-H

[DMPI-N719-I2]-

methyl

E(2) for LP(2)Ofσ*(1)C-Ha

E

[DMPI-N719-I]0

propyl

a

(2)

[DMPI-N719]þ

propyl

propyl

2.18

1.51

1.97

3.06

0.10

1.31

0.28

0.11

a Number in parentheses denotes the specific lone pair, LP, and the specific valence antibond, σ*. b Sum of E(2) for LP(n)Ofσ*(1)C-H. c Change in the occupancy of the σ*(1)C-H orbital. d Change in the atomic charge on the H atom.

2550


The Journal of Physical Chemistry C N719 than using DMPII.19 Furthermore, Pelet and co-workers have reported the results of laser flash photolysis experiments; using Liþ regenerates the oxidized dye more than 10 times faster than employing TBAþ.14 Although imidazolium had a different alkyl group, Park and co-workers have found that the Jsc for a LiIcontaining cell is about 25% higher than a cell using 1,2-dimethyl3-hexyl imidazolium iodide.13 Nazeerruddin and co-workers have studied the influence of protons on DSSC performance.20 The Jsc of the cell increases with the number of Hþ ions but decreases with the number of TBAþ for [Ru(dcbpy)2(NCS)2] dye, where N3 has four protons, whereas N719 has two protons and two TBAþ, indicating the Jsc value of TBAþ < Hþ. They hypothesized that one reason for the effect of TBAþ on the Jsc could be that the I-/I3- redox couple slowly regenerates the dye due to a decreased driving force between the dye and the redox couple. Herein, the driving force seems to be the interaction strength between the dye and the redox couple. In fact, the E value for iodide with N719 in this study is lower than the value with N3 reported in a previous paper.8 Thus, our results theoretically support the hypothesis suggested by Nazeerruddin and co-workers. Various mechanisms have been proposed to describe the Jsc of DSSC with different cations. Kelly and co-workers have investigated the influence of cations on the electron injection yields from photoexcited ruthenium polypyridyl complexes into the TiO2 conduction band using nanosecond laser experiments.12 They found that increasing the cation concentration increases the oxidized sensitizer concentration and attributed this influence to a positive shift in TiO2 electron acceptor states. This positive shift results in favorable energetics for electron injection due to a larger potential difference between the conduction band edge of the TiO2 and the LUMO level of the dye, leading to the higher Jsc. Kambe and co-workers have reported the effect of cations on the electron transport properties of nanocrystalline TiO2 films by transient photocurrent experiments.21 They observed that the significant increase in the diffusion coefficient for electron transport in TiO2 at high concentrations of Liþ, which afford a larger Jsc, might be caused by the formation of effective trap sites in TiO2. As mentioned in the Introduction, the key factor in these studies is the adsorption of cations on the TiO2 surface or the interaction of cations with TiO2. However, as confirmed in this study, cations can simultaneously interact with a dye molecule. Therefore, our results strongly suggest that the interaction of cations with the dye, which affects the regeneration of the oxidized dye by I-/I3- redox couple, is one of the reasons why cations significantly affect the Jsc value of DSSC. Although the maximum value of 10.4% was announced in 2005,41 the authorized solar energy conversion efficiency of DSSC has not improved in the last five years.42,43 One reason is because a DSSC consists of many components, and currently most experimental studies separately examine each field. However, integrating each individually optimized constituent does not necessarily enhance cell performance. Hence, our results show that to improve DSSC performance not only each component of the DSSC but also their interactions such as those between the cation, dye, and iodide anion must be considered.

’ CONCLUSIONS A DFT method with full geometric optimization confirmed the interactions of I- ions with neutral and oxidized N719 dye

ARTICLE

binding to four different cations (Hþ, Liþ, DMPIþ, and TBAþ). In the absence of I-, Liþ interacts with the O atoms of the carboxylate group at the N719 bipyridyl ligand via a symmetric bidentate binding. In contrast, DMPIþ and TBAþ form intermolecular hydrogen bonds between their H atoms in the alkyl groups and the carboxylic O atoms of the dye ligand. For a twostep reaction via a transient intermediate complex [dye 3 I2-], the first I- bonds to the S atom of the NCS ligand of the oxidized dye, and the bonds between the dye and the cations have some geometrical variations. Then the second I- bonds to the first Iwhich forms an I-S bond. Although hydrogen bonding occurs between the dye and DMPIþ and TBAþ, the relative strength of the hydrogen bonds between the methyl and the propyl group for DMPIþ is inverted. These behaviors also appear in the one-step pathway via the transient intermediate complex [dyeþ 3 I2-]. Regardless of whether the oxidized dye interacts with I- ions in a one-step or two-step mechanism, the cations have remarkable effects on the interactions of dye with iodide ions. The interaction strength, which is determined by the interaction energy and atomic charge of the I atom, increases in the order of TBAþ < DMPIþ < Liþ < Hþ. This order corresponds to the relative Jsc order of the DSSC experimentally measured. The stronger the interaction of the oxidized dye with I- via the S atom of its NCS ligand, the easier dye regeneration after electron injection into the TiO2 conduction band proceeds by I-/I3- redox couple through the [dye 3 I2-] and/or [dyeþ 3 I2-], which leads to a larger Jsc value of DSSC. Our results show that not only the component of the DSSC but also their interactions need to be considered in developing DSSC performance.

’ ASSOCIATED CONTENT

bS

Supporting Information. Main optimized geometrical parameters of the [cation-N719]0 species. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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Effect of Cations on the Interactions of Ru Dye and Iodides in Dye

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