ARTICLE pubs.acs.org/JPCC
Computational Spectroscopy Characterization of the Species Involved in Dye Oxidation and Regeneration Processes in Dye-Sensitized Solar Cells Maria Grazia Lobello, Simona Fantacci, and Filippo De Angelis* Istituto CNR di Scienze e Tecnologie Molecolari (ISTM-CNR), c/o Dipartimento di Chimica, Universita di Perugia, Via elce di Sotto 8, I-06213, Perugia, Italy
bS Supporting Information ABSTRACT:
We investigate the spectroscopic signature of oxidized dyes and of their complexes with I occurring in the oxidation/regeneration process in dye-sensitized solar cells. Following the transient absorption spectroscopy study by Clifford et al. [J. Phys. Chem. C, 2007, 111, 65616567], we report a DFT/TDDFT study on the geometrical, electronic, and optical properties of the neutral and cationic [RuII(dcbpy)2(X)2] complexes, where dcbpy = 4,40 -dicarboxy-2,20 -bipyridyl and X = NCS and CN, and of their adducts with I. We focus on the interaction between the oxidized dyes and iodide and perform DFT/TDDFT calculations using a continuum model of solvation on the complexes of interest, thus providing a picture of the electronic structure of the oxidized dyes and their adducts with I. We find a good agreement between the simulated and the experimental absorption spectra, including the absorption band in the IR region that is experimentally assigned to the [dye+:I] complexes. In line with Clifford et al., we find such band in the IR to overlap substantially with that of the dye cation for X = NCS, whereas it is readily identified for X = CN, for which the LMCT band of the cation has negligible intensity. Our results confirm the assignment of the IR band to a charge-transfer transition occurring within the [dye+:I] complex, corresponding to shift of electron density from the Ru-X HOMOs to the LUMO based on the I atom. We expect our study to provide the required theoretical basis for interpretation of further transient spectroscopic data of interest in the DSCs field.
1. INTRODUCTION Dye-sensitized solar cells (DSCs) are promising alternatives to conventional photovoltaics for the direct conversion of solar energy into electricity at low cost and with high efficiency.1 In DSCs, a dye sensitizer, adsorbed on the surface of a mesoporous nanostructured semiconductor film, usually made of titanium dioxide (TiO2), absorbs the solar radiation then transferring a photoexcited electron to the semiconductor conduction band. The concomitant charge hole that is created on the dye is transferred to a liquid electrolyte or to a solid substrate functioning as hole conductor.1,2 The dye is regenerated by electron donation from the electrolyte, usually a redox couple in an organic solvent. The iodide/triiodide (I/I3) redox couple has been demonstrated to be one of the most efficient redox systems to date,3 although it is now being rivaled by metal r 2011 American Chemical Society
complexes based on the Co(II)/Co(III)46 and Fe(II)/Fe(III) redox couples.7 In DSCs based on the most common (I/I3) redox couple, dye regeneration by I has to compete effectively with parasitic recombination processes of photoinjected electrons with the oxidized dye and electrolyte. The circuit is completed via electron migration through the external load and reduction of triiodide at the counter electrode.3 The Ru(II)(dcbpy)2NCS2 dye and its doubly deprotonated TBA salt, N3 and N719, respectively, have maintained a clear lead in DSC technology, with efficiencies exceeding 11%,8 although these metallorganic dyes are currently being challenged Received: May 27, 2011 Revised: August 9, 2011 Published: August 18, 2011 18863
dx.doi.org/10.1021/jp2049475 | J. Phys. Chem. C 2011, 115, 18863–18872
The Journal of Physical Chemistry C
ARTICLE
by fully organic dyes.911 Various factors have been shown to affect the competing oxidized dye regeneration/recombination dynamics in Ru(II)-dye-based DSCs, including distance dependence from the surface, the effect of surface blocking dye layers, and the driving force dependence of the process related to the dye oxidation potential.12,13 Interestingly, Clifford et al.14 reported a transient absorption study of dye regeneration kinetics by iodide for several Ru(II) bipyridyl dyes adsorbed onto nanocrystalline TiO2 films. Their data show that the regeneration process proceeds via the formation of an intermediate species as a specific complex between the Ru(III) oxidized dye cation and iodide.14 On the basis of spectroscopic evidence, the general mechanism for the regeneration reaction was proposed to be composed by two steps dyeþ þ I f ½dyeþ : I
ð1Þ
½dyeþ : I þ I f dye þ I2
ð2Þ
According to the reported kinetics,14 the intermediate formation of eq 1 is kinetically fast, whereas the subsequent reaction of the [dye+:I] intermediate with a second iodide species, eq 2, is slower and thus represents the rate-determining step of the overall regeneration process. The chemical nature of the possible short-lived [dye+:I] intermediate is in itself interesting: this species might be either a seven-coordination complex of Ru(III) with an I ligand directly bound to the metal center, or it might alternatively be a 1:1 [dye+:I] adduct based on either ion pairing or a specific π interaction between the I and the bipyridine ligand.14 In the last year, several experimental and computational studies have investigated the interactions between the oxidized dyes and iodide, providing insight into the mechanism of dye regeneration.1521 In particular, Tuikka et al.15 have recently characterized the adduct formed by the N3 complex and I2, revealing its structure by X-ray diffraction methods. Computational investigations by Privalov et al., Hu et al., and Schiffmann et al. have contributed to the understanding of the oxidized dye regeneration process.1619 Privalov et al.16 focused on the possible intermediates formed by the oxidized N3 dye and iodide, analyzing their structure and the related spin density distribution. They proposed four different interaction modes: (i) I binding to the S atom of the SCN group; (ii) direct interaction of I with the dcbpy ligand; (iii) I ligand exchange of the thiocyanate group; and (iv) I coordination to the Ru(III) center. They concluded that the outer sphere pathways involving (i) and (ii) hypotheses are compatible with dye regeneration, whereas inner sphere interactions (iii) and (iv), albeit plausible from a mechanistic point of view, might possibly imply the degradation of the dye or the decrease in the dye efficiency due to the formation of the RuI bond. Hu et al.17 characterized the thermodynamics of each step of dye regeneration process for N3 and for the analogue complex in which the NCS ligands have been substituted by Cl, comparing the regeneration processes occurring in the presence of three X/X3 redox couples, with X = Br, I, and At. Three local energy minima structures of [dye+: I] and [dye+:I2] with different interaction modes were individuated. Two structures refer to I(I2) attachment to both the carboxylic groups, cis and trans with respect to the NCS (Cl) group. The third structure shows the I(I2) located between the aromatic rings of dcpby ligands. The structure with I binding to the SCN ligand has been optimized and found very
Scheme 1. Chemical Structures of the Ru(dcbpy)2(NCS)2 (1), Left, and Ru(dcbpy)2(CN)2 (2), Right
close in energy to the other reported structures, although it was discarded based on the reported large structural distortions. Asaduzzaman et al. recently reported a study of the N3 dye interaction with iodide, including calculation of transition states for the regeneration process.19 Schiffmann et al.20 proposed a complete regeneration mechanism of the N3 dye, which leads via a barrier free pathway from the oxidized dye and I to the neutral dye and I3. Key to this process are the formation of stable [dye: I] and [dye:I2] intermediates that dissociate spontaneously upon the addition of further I or I2. The same authors discuss the role of several structural aspects of the solid/liquid interface. Furthermore, they find that atomistic properties of the electrolyte play an important role in shaping the distribution of ions near the solid/liquid interface, with the iodide/acetonitrile combination displaying a marked iodide concentration close to the dye. Kusama et al. reported on the effect of cations on the formation of complexes between Ru(II) dyes and iodide, finding that weakly coordination cations lead to higher dye/iodide interactions.21 Despite the fact that previous work has contributed to shedding some light onto the regeneration mechanism, a precise spectroscopic characterization of the intermediate species involved in such a complex process has not been achieved because of the inherently short-lived nature of the intermediates and to the superimposing transitions associated with the dye cation and with the possible [dye+:I] intermediate.14 As a matter of fact the steady-state absorption spectrum of the oxidized N3 dye has never been reported because of degradation of the dye cation, whereas the appearance of the signal related to the [dye+:I] intermediate was observed only for the N3 analogue in which the NCS ligands are replaced by CN groups.14 In the present Article, we thus report a theoretical investigation on the spectroscopic properties of the intermediate species involved in the dye regeneration process, comparing the simulated spectra with the available experimental data. In line with the work by Clifford et al.,14 we analyzed the Ru(dcbpy)2(NCS)2 (1) and Ru(dcbpy)2(CN)2 (2) complexes (Scheme 1) for which a different spectroscopic behavior was found. The N3 complex 1 is the most widely employed dye in DSCs, and it represents a benchmark in DSC technology, whereas dye 2 has provided evidence of the formation of intermediate complex.14 Experimentally, it has been observed that in the presence of I the absorption spectra of both oxidized dyes show an intense absorption band with a maximum at ca. 800 nm. In this region, for dye 2, only this signal was recorded, whereas for 1 the intense band characteristic of the oxidized dye is also observed. To provide a rationale for these experimental observations, we investigated the spectroscopic properties of the oxidized dyes 1+ and 2+ and of the related dye/iodide intermediate complexes, 18864
dx.doi.org/10.1021/jp2049475 |J. Phys. Chem. C 2011, 115, 18863–18872
The Journal of Physical Chemistry C
ARTICLE
Table 1. Optimized Geometrical Parameters (angstroms and degrees) for Complex 1, 1+, 1+:I, and 1:Ia parameters
1
1+
1+:I
1:I
Table 2. Optimized Geometrical Parameters (angstroms and degrees) for Complexes 2, 2+, 2+:I, and 2:I parameters
2
2+
2+:I
2:I-
RuN3
2.08 (2.04)
2.02
2.03
2.06
RuC1
2.02
2.03
1.99
2.03
RuN4 RuN1
2.08 (2.04) 2.09 (2.10)
2.02 2.10
2.07 2.10
2.07 2.08
RuC2 RuN1
2.02 2.09
2.03 2.10
2.03 2.09
2.03 2.09
RuN2
2.07 (2.08)
2.10
2.09
2.07
RuN2
2.14
2.14
2.14
2.13
N3C1
1.18 (1.19)
1.19
1.18
1.19
C1N3
1.18
1.17
1.18
1.18
C1S1
1.64 (1.63)
1.62
1.62
1.63
N3I1
2.72
6.21
7.39
C1RuC2
90.3
90.9
90.8
90.1
N3RuN4
S1I1 91.7 (94.3)
95.2
91.5
91.9
C1RuN1
95.6
96.7
96.7
94.5
N1RuN3
95.6 (95.6)
95.7
96.2
95.2
N2RuN0 1
92.9
89.6
91.4
98.0
N2RuN0 1 N1RuN0 2
91.9 (88.4) 97.8 (97.5)
87.3 98.7
89.8 98.4
95.1 96.2
N1RuN0 2
98.3
99.1
97.7
96.0
3.30
a
Data of the lowest triplet state of the neutral compounds are reported in parentheses.
with reference to the different oxidation states of the dyes (1:Iand 2:I-, 1+:I and 2+:I) by means of density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. Previous DFT/TDDFT calculations on Ru(II) complexes have established the accuracy of the employed computational approach to describe the photophysics of this important class of compounds.8,2231 To the best of our knowledge, no excitedstate calculations on the oxidized dyes 1+ and 2+ have been reported, although very recently Yu et al. calculated the absorption spectrum of the oxidized dye for a related Ru(II) heteroleptic compound.32
2. COMPUTATIONAL DETAILS The molecular structure of dyes 1 and 2 in their neutral and oxidized forms has been optimized in water solution, considering an unrestricted doublet ground state for the oxidized dyes. For 1, we also optimized the geometry of the lowest triplet state. For benchmark purposes, we also investigated the [Ru(bpy)2(NCS)2]+ complex, with bpy = 2,20 bipyridine, for which the steady-state absorption spectrum of the dye cation has been experimentally characterized.33 For modeling of the [dye+:I] intermediate, a direct interaction between I and one of the NCS (CN) ligands has been considered because of the possible chemical affinity between I and NCS (CN),15 in agreement with the hypothesis of an outer-sphere pathway.16 The geometry of neutral [dye+:I] intermediates has been optimized, considering a doublet ground state. All calculations were performed using the Gaussian03 (G03) program package,34 employing the B3LYP35 exchange-correlation functional. Geometry optimizations were performed using a 3-21G* basis set.36 To check the adequacy of this computational setup in describing the geometry of the investigated systems, with particular reference to dye-I interactions, we also performed calculations in vacuo on the adduct formed by the N3 dye with two I2 molecules, thus mimicking the experimental results of ref 15. Solvation effects were included in the geometry optimizations by means of the conductor-like polarizable continuum model (C-PCM),37,38 as implemented in G03. To simplify geometry optimization in solution, we used water as a solvent. We separately checked the effect of solvation on the calculated spectral properties by performing an additional calculation in acetonitrile solution. Analysis of the electronic structure and TDDFT excited-state
calculations for all investigated complexes has been carried out in water solution using a larger DGDZVP basis set,39 which was shown to provide accurate absorption spectra for these systems.2427 The nonequilibrium version of C-PCM was employed for TDDFT calculations, as implemented in G03.40 The 3070 lowest transitions were computed on the investigated systems to simulate the absorption spectra. Transition energies and oscillator strengths have been interpolated by a Gaussian convolution with an σ of 0.17 eV.
3. RESULTS AND DISCUSSION 3.1. Molecular Structure of the Dyes in the Neutral and Oxidized Forms and of the [Dye+:I] Intermediates. As an
initial check of our computational setup, we performed calculations in vacuo on the adduct formed by the N3 dye with two I2 molecules, thus mimicking the experimental results of ref 15. The SI bonds, see the Supporting Information for the optimized structure, are calculated to be 2.97 Å, only slightly longer than the experimental values of 2.84 Å and in line with previous DFT results providing values in the 2.83 to 3.01 Å range. Main optimized geometrical parameters of the investigated 1 and 2 complexes and their oxidized forms are reported in Tables 1 and 2, whereas the optimized structures of the corresponding complexes with I (1+:I and 2+:I) are reported, along with atom labels, in Figure 1. Our optimized structures in solution for 1+:I and 2+:I show the I atom mainly interacting with the NCS and CN groups, without substantial distortions of the complex skeleton. Similar structures were computed in vacuo in refs 16 and 17, although in ref 17, these structures showed a strong structural distortion that is not found in our calculations in solution. Going from 1 to 1+, the most noticeable geometrical change is the 0.06 Å shortening of the Ru-NCS bond distances, which is related to the removal of one electron from the HOMO of the neutral 1 complex of antibonding character with respect to the RuN bond; see the electronic structure below. Similar variations in geometrical parameters are calculated for the oxidized dye 1 and for the neutral dye in its triplet state; see Table 1. Upon oxidation of complex 2, no sizable geometrical changes are observed, consistent with the removal of one electron from almost pure t2g Ru states of non bonding character; see below. Binding of I to the NCS group of the cationic complex 1+ introduces a shortening of the Ru-NCS bond directly involved in the interaction with I, bringing this distance close to that found for the neutral species 1. The I atom shows a slightly different orientation in the 1+:I and 2+:I complexes: because of the 18865
dx.doi.org/10.1021/jp2049475 |J. Phys. Chem. C 2011, 115, 18863–18872
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Optimized molecular structures of the 1+:I and 2+:I complexes.
Figure 2. Schematic representation of the energy level alignment of the molecular orbitals for complex 1 in its neutral (left) and oxidized (right) states. Also shown are isodensity plots of relevant molecular orbitals.
shorter CN ligand, in 2+:I, the I atom lies closer to the bipyridine ligand than in 1+:I (4.8 vs 6.6 Å distance to the closest bipyridine nitrogen). The SI (NCS) and NI (CN) distances of 1+:I and 2+:I are calculated to be 3.30 and 2.72 Å, consistent with a stronger interaction between 2+ and I than 1+. In the complexes between the neutral dyes and I, the SI and CI distances are 7.39 and 6.21 Å, suggestive of a very weak, if not vanishing, interaction. 3.2. Neutral and Oxidized Dyes: Electronic Structure and Absorption Spectra. As from previous investigations,22 the HOMOs of 1 can be divided in two sets of quasi-degenerate orbitals of mixed Ru t2g and NCS character (HOMO/HOMO-2 and HOMO-4/HOMO-6), whereas the HOMO-3 is entirely localized on the NCS ligands; see Figure 2. The percentage of the metal states in the HOMOs ranges between 51% for the HOMO
and 40% for the HOMO-6. The first LUMOs are bipyridine π* orbitals with contributions coming from the COOH groups; see Figure 2. For 2, we computed quite a different electronic structure, with the HOMOHOMO-2 having Ru t2g/CN π character, with metal contributions going from 80 to 77%; see Figure 3. For 2, the degenerate couple HOMO-3/HOMO-4 are π bonding orbitals on the bipyridyl rings, and the HOMO-5 is the π-bonding orbital localized on the CN ligands. Analogously to 1, the first LUMOs of 2 have bipyridyne π* character. In line with previous calculations41 and with the electrochemistry of these systems,42 the HOMO of 2 is calculated to lie 0.38 eV below that of 1, consistent with the more positive oxidation potential measured for 2.42 This different ligand character in the HOMOs suggests that the two complexes might show different spectral properties upon oxidation. 18866
dx.doi.org/10.1021/jp2049475 |J. Phys. Chem. C 2011, 115, 18863–18872
The Journal of Physical Chemistry C
ARTICLE
Figure 3. Schematic representation of the energy level alignment of the molecular orbitals for complex 2 in its neutral (left) and oxidized (right) states. Also shown are isodensity plots of relevant molecular orbitals.
Table 3. Comparison between Calculated and Experimental Data (in Parentheses) Absorption Bands (nanometers) for Complexes 13
1 2 3
I
II
III
564 (534)
372 (396)
313 (313)
494
399
312
(493)
(365)
(310)
486
347
(520)
(372)
(286)
To gain insight into the changes of the electronic structure of both dyes upon oxidation, we thus analyzed the energy and character of the frontier molecular orbitals of 1+ and 2+. The electronic structure of the investigated cationic complexes is compared with that of the corresponding neutral complexes in Figures 2 and 3, whereas a summary of all molecular orbitals energies is reported in the Supporting Information. In the oxidized 1+ dye, all molecular orbitals maintain a similar character as in 1 but are stabilized in energy with respect to those of the neutral species due to the positive charge. For the α manifold, we still individuate the Ru-NCS HOMOs, with the HOMO-3 entirely localized on the NCS ligands, whereas the first LUMOs are the substituted bipyridine π* orbitals. The removal of one electron from 1 leads to the LUMO of the β manifold to show Ru-NCS character; the HOMO/HOMO-1 is a degenerate couple of mixed Ru and NCS character and the HOMO-2, similar to the HOMO-3 of the α manifold, gains a 15% contribution from the Ru t2g states. For 2+, we computed a different electronic structure with respect to the corresponding neutral species, with the highest occupied α orbitals being the π states of bipyridine ligands,
whereas the mixed Ru-CN orbitals are the HOMO-2/HOMO-4 and show reduced metal percentages with respect to the neutral dye ranging from 45 to 60%. In line with the electron density distribution of the neutral dyes, the β HOMO/HOMO-1 of 2+ is largely localized on the metal center. The β HOMO-2 resembles the HOMO-3 of the neutral dye, a bipyridine π orbital with a 15% contribution from the Ru t2g states. Similarly to 1+, the β LUMO has a mixed Ru-CN character, albeit in this case the metal contribution is larger than in 1+. Comparing the electronic structure of 1+ and 2+, we notice that in the latter the HOMOLUMO gap in the β manifold is 2.34 eV, whereas in 1+ a 1.64 eV HOMOLUMO gap is computed. The different nature and energy of the HOMOs and LUMOs formed upon oxidation of the two dyes has relevant implications for the different observed spectroscopic behavior. Transient absorption kinetics for monitoring charge recombination in nanocrystalline TiO2 films sensitized with 1 and 2 shows quite different behavior for the two apparently similar dyes.14 The transient signal for 1+ in the absence of iodide is approximately an order of magnitude larger than that of 2+.14 Following the spectral assignments proposed by Tachibana et al. for these transients, 1+ shows a broad absorption band at ca. 800 nm, with a ground-state bleach centered at ∼540 nm.14,4345 As previously mentioned, the steady-state absorption spectrum of N3 cation 1+ could not be directly measured because the complex is prone to decomposition of NCS ligands upon reaction with chemical oxidants.33 The absorption spectrum of the uncarboxylated analogue [Ru(bpy)2(NCS)2]+ has been recorded in acetonitrile solution by the addition of a Ce(IV) salt to the neutral dye solution.33 The [Ru(bpy)2(NCS)2] complex, hereafter 3, is endowed with a lower oxidation potential than 1 and is therefore more stable with respect to decomposition of the NCS groups.33 The absorption spectrum of 3 is rather similar to that of 1, apart from a
