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J. Phys. Chem. C 2007, 111, 3522-3527
Investigation of the Effect of Alkyl Chain Length on Charge Transfer at TiO2/Dye/ Electrolyte Interface Shogo N. Mori,*,†,§ Wataru Kubo,§ Taisuke Kanzaki,§ Naruhiko Masaki,‡ Yuji Wada,§ and Shozo Yanagida*,‡ Material and Life Science, Graduate School of Engineering, Osaka UniVersity, Suita, Osaka 565-0871 Japan, and Center for AdVanced Science and InnoVation, Osaka UniVersity, Suita, Osaka 565-0871 Japan ReceiVed: September 25, 2006; In Final Form: December 4, 2006
Dye-adsorbed nanoporous TiO2 films were prepared with Ru(H2dcbpy)(bpy)(NCS)2, Ru(H2dcbpy)(dmbpy)(NCS)2, and Ru(H2dcbpy)(dnbpy)(NCS)2, which have the same structure with different alkyl chain lengths of 0, 1, and 9 attached to a bipyridine ligand, and with conventional dyes, Ru(Bu4NHdcbpy)2(NCS)2, known as N719, and Ru(dcb)2(NCS)2, known as N3. The films were immersed in electrolytes containing I-/I3- redox couples, and lifetime of conduction band electrons, which were injected from the excited states of the dyes, was measured. When Li+ was employed as a counter charge of the redox couple, no noticeable difference was observed regardless of the difference of alkyl chain length of the dyes. Transient absorption measurements of dye cation also showed comparable dye cation lifetime, suggesting that the alkyl chain length of the ruthenium complexes has little influence on the charge transfers from TiO2 to I3- and from I- to dye cation. Electron lifetime was also measured for dye-sensitized solar cells (DSCs) using electrolytes containing tetrabuthylammonium or tetrahexylammonium cations instead of Li+. Except for N719, comparable electron lifetime was observed regardless of the species of the cations, indicating that the alkyl chain length of quaternary ammonium cations also did not influence the interfacial charge transfer. These results show that alkyl chain itself does not impede the approach of I-/I3- anions to the TiO2 surface, but the simultaneous control and concerted effect of the density of adsorbed dye and the size of cations are important to retard undesired interfacial charge transfers.
Introduction Electron transfer at semiconductor/dye/electrolyte interfaces has been applied for various photo-electrochemical devices. Dye-sensitized solar cell (DSC) is one of the applications which exploits photoelectrons injected from adsorbed dye to wideband gap semiconductor under light irradiation.1 The solar cells, which give energy conversion efficiency of ca. 10%, are comprised of Ru complex dye adsorbed nanoporous TiO2 electrode immersed in electrolytes containing I-/I3- redox couple.2 Figure 1 shows schematics describing the charge transfers in the DSCs. To utilize the photoelectrons for external loads, the electron transfer from the conduction band to dye cation (R1) must be slower than that from I- to dye cation (R2) and the electrons must diffuse faster than charge transfer between e-TiO2 and I3- (R3). Therefore, it is important to understand the mechanism and factors influencing the chargetransport rates of each process. Along this purpose, the time scale of these charge transfers has been measured by various groups. Transient absorption of dye cation reveals that R1 occurs between 10-9 to 10-3 seconds, depending on the electron density in the TiO2, while R2 can be completed in about 10-6 seconds when there is enough I-, for example, more than 20 mM in acetonitrile, existing in electrolytes,3 and appropriate cations are * To whom correspondence should be addressed. E-mail: yanagida@ mls.eng.osaka-u.ac.jp (S.Y.);
[email protected] (S.M.). † Current address: Department of Fine Materials Engineering, Faculty of Textile Science and Technology, Shinshu University, Ueda, 386-8567, Japan. ‡ Center for Advanced Science and Innovation, Osaka University. § Graduate School of Engineering, Osaka University.
Figure 1. The schematics view of the charge transfer in dye-sensitized solar cell.
chosen.4 Electron-diffusion coefficient (D) in the TiO2 ranges between 10-8 to 10-4 cm2/s, depending on also the electron density,5,6 and R3 appears inversely related with the value of D, ranging between 10-3 to 101 second.7,8 The slow and light intensity dependent electron diffusion has been modeled with intraband charge traps in the TiO2 electrode,9-11 and the long electron lifetime seems also to be caused by the traps.12 The high-energy conversion efficiency of DSCs has faster R2 than R1 and sufficient electron diffusion length in comparison to the thickness of the TiO2 electrodes.5
10.1021/jp066261y CCC: $37.00 © 2007 American Chemical Society Published on Web 02/07/2007
Effect of Alkyl Chain Length on Charge Transfer
J. Phys. Chem. C, Vol. 111, No. 8, 2007 3523
CHART 1: Chemical Structure of KD1 (Left), KD2 (Center), and Z907 (Right)
To increase the efficiency of DSCs, it has been desired to retard the charge recombination processes of R1 and R3, and several approaches have been proposed. One of them is growing thin insulating layer on the surface of TiO2, and the increases of the efficiency, because of the suppression of charge recombination, have been reported.13-15 Alternatively, we found that employing quaternary ammonium cations as a counter charge of I-/I3- redox couple suppressed R3, and the results were interpreted with the electric double layer, that is, the bulky ammonium cations hardly penetrate into the dye molecule layer and could not screen the negatively charged TiO2 within the size of the dye, resulting in less I3- concentrations in the vicinity of TiO2 surface.16 In view of alkyl chains in dye molecules, it has been reported that two long alkyl chains, instead of carboxyl groups, attached to a bipyridine ligand of N3 dye, impede the approaches of Ito dye caion, which also suggests that the chain could impede the I3- to TiO2 surface, resulting in elongation of the electron lifetime.17,18 The amphiphilic dye, known as Z907, has been often applied to nonorganic solvent-based hole conductor electrolytes, such as molten salts and hole-conducting molecules.18 Recently, influence of alkyl chain length of the amphihilic Ru dye with spiro-OMeTAD as a hole conductor on the electron lifetime and efficiency of DSCs was studied.19 The results showed that the increase of alkyl chain suppressed more effectively the charge recombination. On the other hand, no experimental result has been reported for the electron lifetime in DSCs using Ru dyes having different alkyl chain lengths, with organic solvent or molten salt electrolytes. To elucidate the role of alkyl chain on the charge transfer at TiO2/dye/liquid electrolyte interfaces, we prepared Ru complexes having three different alkyl chain lengths and measured conduction band electron lifetime, which is determined by the net effect of R1 and R3. Transient absorption due to the reduction of dye cation, which is by the processes of R1 or the combination of R1 and R2, was also measured. The measurements were also performed with two conventional Ru dyes, having different sizes, and with various electrolytes containing Li+, tetrabuthylammonium ion (TBA+), tetrahexylammonium ion (THA+), or 1-hexyl-3methylimidazolium ion (HMIm+) as counter charges of I-/I3- redox couple. We examined first the influence of alkyl chain in the dyes with an electrolyte containing only LiI and I2, second, the net effect of the alky chain and the sizes of electrolyte cations, and last, the net effect of the alkyl chain and molten salt electrolytes.
(Wako Pure Chemical), and all other solvents for synthesis were used as received. 4,4′-Dicarboxylicacid-2,2′-bipyridine was prepared from 4,4′-dimethyl-2,2′-bipyridine as described before.20 Ruthenium dyes, cis-dithiocyanate-N,N′-(4,4′-dicarboxylate-2,2′-bipyridine)-N,N′-(2,2′-bipyridine) ruthenium(II) (KD1) and cis-dithiocyanate-N,N′-(4,4′-dicarboxylate-2,2′-bipyridine)N,N′-(4,4′-dimethyl-2,2 ′-bipyridine) ruthenium(II) (KD2), were synthesized using a reported procedure18 except using microwave heating (reflux for 10 min by Discover BenchMate, CEM) instead of conventional heating (reflux 4 h). Further purification of the dyes was preformed on Sephadex LH-20 (Amersham Bioscience) column using DMF as an elution solvent. Chemical structures of KD1, KD2, and Z907 are shown in Chart 1. Characterization of dyes was carried out by 1H NMR in CD3OD.21 Preparation of Solar Cells. TiO2 electrodes were prepared from a colloidal suspension of nanosized TiO2 particles (P25, Nippon Aerosil, average size of 21 nm, and Nanoxide-T, Solaronix, average size of 18 nm) by a doctor blade technique. The electrodes were sintered at 450 and 550 °C for P25 and Nanoxide-T, respectively, for 30 min in air. The electrode having thickness of ca. 6 µm were used for DSCs, unless otherwise stated. For transient absorption measurement, electrodes having thickness of 2-4 µm were chosen. After the sintering, the electrodes were heated again to ca. 110 °C and were immersed into solutions containing 0.5-0.6 mM of Ru complex dyes over a night at room temperature. The dyes examined here were the above-synthesized dyes and Ru(H2dcbpy)(dnbpy)(NCS)2, known as Z907 (Solaronix), Ru(Bu4NHdcbpy)2(NCS)2, known as N719 (Solaronix), and Ru(H2dcb)2(NCS)2, known as N3 (Kojima chemical, Japan). Solvents of the solutions were acetonitrile (AN)/tBuOH (1:1.v) for N719, N3, and Z907, and DMF for KD1 and 2. The dye-adsorbed films were placed on a Pt sputtered fluorine doped tin oxide (FTO) substrate and were sealed with thermal adhesive films (HIMILLAN, Mitsui-DuPont Polychemical). 1-Hexy-3-methylimidazolium iodide was purchased from Shikoku Corp., Japan, and was washed with diethylether before usage. Electrolytes containing I-/I3- redox couples were introduced from a small drilled hole at the counter electrode. Efficiencies of the dye-sensitized solar cells were between 3 and 6% under AM1.5, 100 mW cm-2 irradiation (individual efficiencies are shown in Supporting Information). The amount of adsorbed dyes was measured from the absorption of the solutions, which were obtained by desorbing the dyes from the dye/TiO2 films with 0.1 M of NaOH aq for N719 and 0.1 M of NaOH in mixed solvents of water and methanol (1:9) for Z907.
Experimental Section
Measurements
Preparation of Dyes. Dichloro(p-cymene)ruthenium(II) dimmer (Aldrich), 4,4′-dimethyl-2,2′-bipyridine (Aldrich), 2,2′bipyridine (Wako Pure Chemical), ammonium thiocyanate
Diffusion coefficients and lifetimes were measured by stepped-light-induced transient measurements of photocurrent and voltage (SLIM-PCV), respectively.22 Solar cells were
3524 J. Phys. Chem. C, Vol. 111, No. 8, 2007
Mori et al.
Figure 2. Electron lifetime in TiO2 electrodes adsorbed by various Ru dyes having various alkyl chain lengths: C0, KD1; C1, KD2; C9, Z907.
irradiated by a diode laser (Lablaser, Coherent, 635 nm), and less than 10% of the laser intensity was stepped down by a function generator (FG-02, Toho-Technical research) or a personal computer through D/A converter (National Instruments). Short-circuit current and open-circuit voltage responses were recorded by digital oscilloscope (TDS 3052 Tektronix) through a current amplifier (SR570, Stanford Research Systems) or a differential amplifier (5307, NF Electronic Instruments), or these responses were directly stored on a digital multimeter (AD7461A, Advantest). Measurements were repeated at various light intensities controlled by a set of ND filters, and at least two identical samples were measured for each condition. Dye cation lifetime was measured by a transient absorption technique. Dye-adsorbed TiO2 electrodes were immersed by an electrolyte composed of 0.1 M of LiI in distilled propylene carbonate (PC) and were irradiated by a laser pulse (Flashpumped Nd:YAG, 532 nm, full width at half-maximum (fwhm) ) 7 ns, 10 Hz) at open-circuit conditions. The change of repetition rate to 1 Hz did not change the transients. Intensity of the pulse was ca. 0.4 mJ/cm2. Concentration of dye cation was monitored by a diode laser at 780 nm (Coherent, Lablaser), which probes ligand-to-metal charge transfer between Ru and NCS.23 The intensity of the laser was measured by a p-i-n photodiode (Hamamatsu) with ac-coupled amplifier (SA-230F5, NF Electronic Instruments) at a gain of 200 times. Transient measurements were typically repeated 128-256 times to average the signal. Results and Discussions Influence of Alkyl Chain Length in Ru Complex on Electron Lifetime. First of all, we examined the influence of alkyl chain length with electrolytes containing only 0.7 M LiI and 0.05 M I2 in acetonitrile. The efficiency of the DSCs was in the order of N719 > Z907 ≈ KD2 > KD1 ≈ N3. The order is mainly due to the different values of Voc, that is, N719 showed the highest open circuit voltage (Voc), while short circuit current (Jsc) was comparable among the examined dyes. Figure 2 shows the lifetime of photoinjected electrons in the DSCs at open-circuit conditions. Bottom axis shows short-circuit current, which was measured under the same laser intensity used for lifetime measurements. Regardless of the ligands of the Ru complexes examined herein, there was no distinguishable difference in the electron lifetime within the experimental error,
Figure 3. Transient absorption of various dyes adsorbed on TiO2 surface. Electrolyte was 0.1 M of LiI in PC. Dyes were excited by a 7-ns pulsed laser at 532 nm and were probed by a 780-nm diode laser.
which is ca. 50%. On the basis of the results from KD1, KD2, and Z907, alkyl chain length of the dyes influenced little the electron lifetime, at least up to C9. The results imply that the difference of Voc would be due to the degree of the shift of the conduction band edge potential, depending on the structure of the dyes. In DSCs, electrons in the TiO2 can transfer to dye cation or I3-. Thus, measured lifetime in Figure 2 is reciprocal of (1/τR3 + 1/τR1), where τR3 and τR1 are the lifetimes determined by R3 and R1, respectively.24 In the case of high concentration of Iwith N719, the vales of τR1 are practically infinite because the dye cations are reduced by I- faster than by the conduction band electrons. If the alkyl chain of Z907 impeded the approach of I- and I3-, τR1 would be shortened because of the long survived dye cation, and τR3 would be increased because of less density of I3- at the TiO2 surface. These simultaneous influences could result in the little influence on the measured electron lifetime. To examine the possibility, transient absorption measurement of dye cation was employed. Figure 3a shows the transients for KD1, KD2, and Z907 in an electrolyte containing 0.1 M of LiI in PC.25 The time giving the half of the maximum intensity was between 5 and 9 µs for both dyes.26 When there was no Iin the solvents, the halftime was longer than 100 µs, which corresponds to the time for the direct electron transfer from the conduction band to dye+, that is, the process of R1. The faster decay observed in Figure 3 was thus due to electron transfer from I- to dye cation. Comparable dye cation lifetimes of KD1, KD2, and Z907 showed that the alkyl chain did not prevent Ifrom approaching the dyes. Therefore, with the results of the lifetime in Figure 2, the alkyl chain attached to the bipyridine ligand did not impede the approach of I3-.
Effect of Alkyl Chain Length on Charge Transfer The halftimes of the transient absorption for N719 and N3 were about half of those for KD1, KD2, and Z907 (shown in Figure 3b). Wang et al. has also reported that the transient absorption of Z907 cation took much longer time to decay than that of N719 cation,18 and the longer dye cation lifetime was interpreted with the retardation of the approach of I- to dye cation by the alkyl chain. Our results suggest that the difference between a group of KD1, 2, and Z907 (group A) and a group of N719 and N3 (group B) was not related to the block of Iapproaches.27 Other factors influencing the dye cation reduction rate could be the difference of free energy and the physical distance between I- and highest occupied molecular orbital (HOMO) orbital of the dyes. The redox potentials of the dyes were 1.06, 0.91, and 0.83 V versus NHE for N3, N719, and Z907, respectivly.28-30 Such a degree of potential difference is probably not able to account for the difference of the electrontransfer rates.31 Concerning the distance, the HOMO of all the dyes examined here is probably located on the NCS ligands.31,32 In the case of group B, the NCS ligands are located far from the surface of TiO2 because two bipyridine ligands are attached to the TiO2. On the other hand, Z907 has one anchoring ligand, and thus, the NCS ligands can be located close to the surface (see Supporting Information). Such orientation could increase the charge-transfer rate from the conduction band to dye cation and decrease the rate from I- to the dye cation. Another speculation here is the difference of reorganization energy, which could be caused by different anchoring types of the dyes. For group A dyes, the dyes are likely to be anchored by the two carboxylic acids attached to one of the bipyridine ligands, while for group B dyes, the dyes are anchored though two carboxylic acids from two bipyridine ligands. In the case of N3 dye, the reorganization energy is reduced significantly when the dyes are adsorbed on the surface TiO2.33 The proposed explanations of the reduction were due to the accessibility of electrolyte around the dyes and due to the change of rigidity of the dyes. Since the different anchoring types of group A and group B would change both the accessibility and rigidity, our results seem to be consistent with the proposed models. Effect of Alky Chain Length of Quaternary Ammonium Cation. Alkyl chain in the Ru dyes may increase the electron lifetime by corporation with bulky cations when they are employed instead of small Li+ in electrolyte. Figure 4 shows the electron lifetime in TiO2 when TBA+ was employed as a counter charge of I-/I3-. The order of the DSCs’ efficiency was the same as with the case of Li+. As it was reported, TiO2 anchored with N719 showed the increased electron lifetime in comparison to the case of Li+. Except for the N719, the rest of the dyes showed no noticeable difference between Li+ and TBA+. In view of current-voltage (I-V) characteristics, employment of TBA+ increased the Voc with the decrease of Jsc for all the dyes examined here. The increase of Voc is mainly due to the shift of conduction band edge potential. Except for the case of N719 with TBAI, the electron lifetimes were comparable, while the values of Voc vary from 0.42 to 0.68 V. The results suggest that at least 0.26 V difference in the potential has little influence on the electron lifetime, which is consistent with the fact that little difference in electron lifetime was seen although the HOMO potentials of N3, N719, and Z907 differ by 0.23 V. For the case of Z907, we checked also the case of THA+ and found that it did not increase the lifetime while significant increase was seen with N719. For the case of N719 with TBA+, the reduction of the amount of adsorbed dye decreases the electron lifetime to the comparable value with the case of Li+.16 For the case of Z907 with TBA+, there was
J. Phys. Chem. C, Vol. 111, No. 8, 2007 3525
Figure 4. Electron lifetime in TiO2 electrodes adsorbed by various Ru dye having different alkyl chains, immersed in electrolyte containing quaternary ammonium cation.
a possibility that the amount of adsorbed Z907 was less than that of N719. To check the possibility, the amount of adsorbed N719 and Z907 dye was measured, and a comparable amount of dyes, about 10-10 mol/cm2,34 corresponding to 0.6 dye per 1 nm2, was observed. In the case of N3, the projected area of the dye is estimated to be 1 nm2,35 corresponding to 40% of TiO2 surface not covered by the dye. In the case of N719, two TBA+ replace H+ of carboxylic acid in N3 dye, and thus, a larger portion of the surface area must be covered by N719. For the case of Z907, long alkyl chains are attached to one of the bipyridine ligands, meaning that the dye is anchored by two carboxylic acids attached to the other bipyridine. This means that the alkyl chains would direct out of the TiO2 surface, suggesting that dye coverage area by Z907 is less than by N719 (see Supporting Information for graphical illustration), causing less sensitivity to the size of cations in the electrolyte. However, the orientation of the adsorbed dye could differ depending on the TiO2 surface conditions, that is, Z907 might work better with other TiO2 particles. To check the possibility, we prepared electrodes from two different TiO2 particles, P25 and NanoxideT, where P25 consists of anatase and rutile while Nanoxide-T consists of only anatase. With adsorbing Z907 and using TBA+, no difference was observed between these two particles. On the basis of these results, we concluded that the important factor to retard charge recombination is not the length of alkyl chain itself but the coverage of the TiO2 surface by adsorbed dye,
3526 J. Phys. Chem. C, Vol. 111, No. 8, 2007
Figure 5. Electron lifetime in DSCs using two different dyes Z907 and N719 with electrolytes consisting of 1 M of HMImI and 0.1 M of I2 in MAN.
that is, a much larger projected area of N719 can cover TiO2 surface more than Z907 at the matched density of adsorbed dye, and the choice of counter cation, whose structure is sufficiently large so that the adsorbed dye can block the penetration of the cation into the dye molecule layer. Influence of Alkyl Chain of Ru Dyes in Molten Salt Electrolytes. Z907 has been frequently applied with molten salt electrolytes. This is because both the amphiphilic dye and the molten salts are superior in terms of long-term stability of DSCs. In addition, with molten salt electrolytes, Z907 has shown better energy conversion efficiency than N719,28 implying a possibility that the alkyl chain might work well with molten salts. Here, we prepared DSCs with N719 and Z907 with molten salts consisting only of HMImI and 1 M of I2 (denoted as E1) and 1 M of HMImI and 0.1 M of I2 in methoxyacetonitrile (denoted as E2). For both electrolytes, N719 showed higher efficiency, which is different from the previous report.28 This will be discussed later. Figure 5 shows the electron lifetime of the DSCs using E2. N719 showed a longer electron lifetime than Z907. The trend was the same for E1 (Supporting Information). In view of the size of imidazolium cation, organic liquid electrolytes containing DMPIm+ showed comparable electron lifetime with Li+, but HMIm+ showed a longer lifetime in the case of N719. The results can be interpreted similarly with the case of quaternary ammonium cation, that is, when the size of cation is sufficiency large, relative to the size of uncovered TiO2 between the dyes, charge recombination is suppressed. The size of the cations was estimated by calculations using MOPAC/ PM5 including water solvation effects by COSMO in CAChe ver 6.1, and the longest distance between two atoms in the molecule was 6.1, 7.3, 10.1, 11.7, amd 15.0 Å for DMPIm+, tetra-propylammonium (TPA+), TBA+, HMIm+, and THA+, respectively. TPA+ has also induced the increase of the electron lifetime when N719 was employed.36 On the basis of this, there should be a critical value between 6.1 and 7.3 Å for the size of cations to increase the electron lifetime in DSCs with N719. As it was discussed in a previous section, an N719 dye is adsorbed over the TiO2 surface area of 1.6 nm2. This means that only a distance of a few Å is exposed between the dyes, which is consistent with the above interpretation using the size of cations. Implications to the Performance of Dye-Sensitized Solar Cells. On the basis of the above experimental results, the alkyl chain attached to dyes or cations in electrolyte itself did not
Mori et al. influence the I-/I3- approach to dye molecules in organic solvents, and the lifetime measurement with quaternary ammonium cations and imidazolium cations showed that the density of adsorbed dye molecules and the size of cations are important factors reducing the interfacial charge transfer between conduction band electron and I3- in the electrolyte. This suggests that the increase of the amount of adsorption for other dyes by, for example, an increase of temperature during the process of dye adsorption or HCl treatment37 may result in the decrease of the charge-transfer rate for R3. However, this approach might induce aggregation of dyes and result in the decrease of Jsc. A remedy of the dilemma is to design dye so that the dyes can cover the surface of TiO2 without aggregation, or coadsorbents should be sought so that the material can suppress the aggregation and cover the exposed TiO2 surface between dye molecules. Difficulty of this approach, however, is that some coadsorbent might accelerate the recombination.38 Schmidt-Mende et al. reported that the increase of alkyl chain length of Ru dyes reduces the interfacial charge transfer at TiO2/ dye/solid-state hole conductor.19 For this case, the holeconducting molecule is much larger than the size of I-/I3- so that the hole conductor is probably not able to penetrate into the dye molecule layer. Then, the increase of alkyl chain increases the distance between TiO2 surface and the hole conductor, as long as the mechanical strength of the chain can stand. On the other hand, for the case of liquid electrolyte, alkyl chain could swing and I-/I3- anion can move freely between the chains. Otherwise, the alkyl chain should be so dense that the chains become aggregated strongly because of hydrophobic interaction in the ionic media. In comparison between N719 and Z907, we could not observe the advantage of Z907 in view of electron lifetime and energy conversion efficiency, among the cations and TiO2 particles examined here. Since the packed density of adsorbed dyes is the important factor, the result observed here might differ when other TiO2 was used. The influence of surface conditions, for example, different crystal face of TiO2, on the conformation of adsorbed dye and their effects on the efficiency of DSCs have been discussed.39,40 Because the dye groups A and B have different distances between the two anchoring carboxyl groups, adsorbed density and conformation of the dyes may depend on the condition of the surface. Superior performance of Z907 observed in literature might be caused by compatible TiO2 surface condition with Z907, implying that the modification of TiO2 surface structure may be another option to increase the electron lifetime and, consequently, the efficiency of the DSCs. Among the dye group B, KD2 and Z907 showed higher energy conversion efficiency than KD1, mainly because of the difference of Voc. However, comparable efficiencies between KD2 and Z907 suggest that the alkyl chain length up to C9 has little influence on the efficiency. Conclusions We studied the influence of alkyl chain length of Ru complex dyes on interfacial charge transfer at the TiO2/dye/electrolyte. It was found that, up to C ) 9, the alkyl chain length itself did not impede the approach of I-/I3- anions to the TiO2/dye surface, and thus no noticeable difference on conduction band electron lifetime and dye cation lifetime was observed. Except for N719 Ru dye, retardation of charge transfer from the conduction band to I3- was not seen with quaternary ammonium cation and HMIm+, suggesting that simultaneous control of the structure of dyes, density of adsorbed dyes, and the species of cations as the counter charges of I-/I3- is a critical factor to
Effect of Alkyl Chain Length on Charge Transfer have in directional electron transfer. In the case of N719, increases of electron lifetime were seen with the cation having the size of more than 7.3 Å. Acknowledgment. We thank Mr. H. Tsutsumi and K. Nakamura for their help on measurements. Supporting Information Available: I-V characteristics of DSCs examined in this paper, electron diffusion coefficients in the DSC using N719 and Z907, electron lifetime in DSCs using only molten salts, and transient absorption data with TBAI. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Gra¨tzel, M. J. Photochem. Photobiol., A 2004, 164, 3. (3) Montanari, I.; Nelson, J.; Durrant, J. R. J. Phys. Chem. B 2002, 106, 12203. (4) Pelet, S.; Moser, J. E.; Gra¨tzel, M. J. Phys. Chem. B 2000, 104, 1791. (5) Fisher, A. C.; Peter, L. M.; Ponomarev, E. A.; Walker, A. B.; Wijayantha, K. G. U. J. Phys. Chem. B 2000, 104, 949. (6) Kopidakis, N.; Schiff, E. A.; Park, N. G.; Van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2000, 104, 3930. (7) Nakade, S.; Saito, Y.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 8607. (8) Kopidakis, N.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2003, 107, 11307. (9) Nelson, J. Phys. ReV. B 1999, 59, 15374. (10) Van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2001, 105, 11194. (11) Peter, L. M.; Walker, A. B.; Boschloo, G.; Hagfeldt, A. J. Phys. Chem. B 2006, 110, 13694. (12) Bisquert, J.; Zaban, A.; Greenshtein, M.; Mora-Sero, I. J. Am. Chem. Soc. 2004, 126, 13550. (13) Kay, A.; Gra¨tzel, M. Chem. Mater. 2002, 14, 2930. (14) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. J. Am. Chem. Soc. 2003, 125, 475. (15) Yum, J.-H.; Nakade, S.; Kim, D.-Y.; Yanagida, S. J. Phys. Chem. B 2006, 110, 3215. (16) Nakade, S.; Kanzaki, T.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2005, 109, 3480. (17) Wang, P.; Klein, C.; Moser, J. E.; Humphry-Baker, R.; Cevey-Ha, N. L.; Charvet, R.; Comte, P.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Phys. Chem. B 2004, 108, 17553. (18) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Gra¨tzel, M.; Sekiguchi, T. Nat. Mater. 2003, 2, 402. (19) Schmidt-Mende, L.; Kroeze, J. E.; Durrant, J. R.; Nazeeruddin, M. K.; Gra¨tzel, M. Nano Lett. 2005, 5, 1315. (20) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382.
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