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Structural Effects of the Donor Moiety on Reduction Kinetics of Oxidized Dye in Dye-Sensitized Solar Cells Jun-ichi Ogawa, Saurabh Agrawal, Nagatoshi Koumura, and Shogo Mori J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10432 • Publication Date (Web): 25 Jan 2016 Downloaded from http://pubs.acs.org on January 27, 2016

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The Journal of Physical Chemistry

Structural Effects of the Donor Moiety on Reduction Kinetics of Oxidized Dye in Dye-Sensitized Solar Cells

Jun-ichi Ogawa†, Saurabh Agrawal†, Nagatoshi Koumura ‡, and Shogo Mori†, §*

†

Division of Chemistry and Materials, Faculty of Textile Science and Technology, Shinshu University,

Ueda, Nagano 386-8567, Japan ‡

National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8565, Japan

§

Center for Energy and Environmental Science, Shinshu University, Ueda, Nagano 386-8567, Japan

*

Corresponding Author:

Shogo

Mori,

E-mail:

[email protected],

TEL:

+81-0268-21-5496

1 ACS Paragon Plus Environment

+81-0268-21-5818,

FAX:


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Abstract One of the major factors influencing the regeneration rate of the oxidized dye in dye-sensitized solar cells (DSSCs) is the energy difference (ΔG) between the levels of the dye’s HOMO and redox couple in the electrolyte. To investigate other factors that influence this process, we examined the effect of structural differences of donor moieties on the reduction rate of the oxidized dye of organic dyes that is composed of an acceptor unit, a π-conjugated linker unit and a donor unit, including carbazole dye (MK-1), triphenylamine dye (MK-88), and coumarin dye (MK-31). The DSSCs using MK-88 showed the fastest regeneration rate even though the ΔG was not the largest among the dye structures evaluated. The regeneration rates of all the dyes were enhanced by reducing the number of adsorbed dyes. Based on the results, we attribute the fast regeneration of MK-88 to the large collision cross section of the oxidized dye, that is, the increased reduction rate to the larger exposure of the HOMO of the dyes to the redox species. The effect of the exposed surface area on the reduction rate was as large as the free energy difference, suggesting a new design strategy for efficient sensitizers.


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1. Introduction The open circuit voltage (Voc) of dye-sensitized solar cells (DSSCs) is determined by the difference between the Fermi level of TiO2 and the redox potential of the redox couple in the electrolyte solution. Recently, the energy conversion efficiency of DSSCs was largely increased by replacing the I-/I3- redox couple with a cobalt (ΙΙ/ΙΙΙ) complex redox couple. The Voc increased over 1 V due to the high redox potential of the cobalt (ΙΙ/ΙΙΙ) complex redox couple compared with that of the I-/I3- redox couple.1 However, a high Voc is not obtained for many dyes when the redox couple is replaced with one that possesses a high redox potential. 2–4 One possible explanation for this result is the charge recombination of the injected electrons and the oxidized dyes. This recombination can happen more frequently when the oxidized dye's regeneration rate is low due to a small energy difference between the HOMO of the dye and the redox couple (ΔGreg). When the redox potential is shifted positively, the ΔGreg becomes smaller. The required energy difference for sufficient regeneration has been investigated in transient absorption studies for multiple dyes and redox couples. The required energy was shown to vary between 150 and 400 meV1,5,6 due to many other factors involved in the electron transfer kinetics. Therefore, the effective factors are important to identify. The factors that influence the reduction kinetics of the oxidized dye other than the ΔGreg have also been studied. Montanari et al. demonstrated that the regeneration rate was enhanced by an increased I- concentration in the



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electrolyte.7 Feldt et al. reported that the regeneration kinetics was also accelerated by an increased concentration of the cobalt complex, which served as the reductant in the electrolyte.8 In addition, the electronic coupling between the HOMO of the dye and the redox couple affected the regeneration rate.8,9 The regeneration rate decelerated when the distance between the HOMO and the redox couple was increased by the introduction of alkyl chain units, which acted as an obstacle. The recombination rate between the oxidized dye and the electrons injected from the semiconductor is controlled by electron transport in the semiconductor electrode, as explained by a continuous random walk model.10,11 The recombination rate was also changed by modifying the distance between the HOMO of the dye and the semiconductor surface.11,12 These results show that the recombination rate is affected by both electron transport and by the interfacial electron transfer between the oxidized dye and the semiconductor electrode.8 In this paper, we investigated the effect of the donor structure on the reduction kinetics of the oxidized dye in DSSCs employing I-/I3- redox couple. The area of a dye’s HOMO that is exposed to I- differs according to the structure of the donor, and the area is expected to influence the collision frequency between the oxidized dye and I-. We chose I-/I3- redox couple because the size of I- is small so that the effect of collision frequency would be more prominent than that with the large size redox couples. We employed three dyes that contained carbazole, triphenylamine or coumarin moieties. The structures of carbazole and coumarin are planar while


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triphenylamine has a three-dimensional structure.

2. Experimental methods The DSSC consists of a photo-electrode, an electrolyte containing the redox couple, and platinum-sputtered FTO glass as the counter electrode. The photo-electrodes were prepared by depositing a paste containing TiO2 nanoparticles (PST-18NR, Dyesol) onto the FTO glass (SnO2:F, AGC Fabritech); the electrodes were then heated at 500°C for 30 min in air. The thicknesses of the electrodes were approximately 2 µm. After heating, the electrodes were immersed into dye solutions for 18 h at 25°C. The photo-electrodes were attached to platinum counter electrodes by a thermal adhesive film. The pores of the photo-electrodes were filled by injecting the electrolyte through two holes on the counter electrode. The holes were sealed with the thermal adhesive film and glass. The donor-π-conjugated linker-acceptor (D-π-A) type of metal-free organic dyes that were examined here employed carbazole (MK-1), triphenylamine (MK-88), and coumarin (MK-31) as donors. The structures of the dyes are shown in Figure 1, and their synthesis procedures have been described in the literature.13 The HOMO levels of the dyes were measured by differential pulse voltammetry. The values of MK-1, MK-88 and MK-31 were 1.1, 1.0 and 0.85 V vs NHE, respectively. The dyes were dissolved in mixed solvents

(Acetonitrile/Toluene/t-BuOH

=

1:1:1

for

MK-1

and

MK-88,

Acetonitrile/t-BuOH/EtOH = 2:2:1 for MK-31). The concentrations of the dye solutions were



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approximately 0.2 mM. The electrolytes were 0.3 M LiI, 0.05 M I2 and 0.6 M tBP in acetonitrile (Li electrolyte); 0.3 M tetrabutylammonium (TBA) iodide, 0.05 M I2 and 0.6 M tBP in acetonitrile (TBA electrolyte); 0.3 M LiClO4 and 0.6 M tBP in acetonitrile (Li inert electrolyte); and 0.3 M TBAClO4 and 0.6 M tBP in acetonitrile (TBA inert electrolyte). I-V measurements were taken under a solar simulator (Yamashita Denso, YSS-100A), and the light source was adjusted to match the Air Mass 1.5 spectrum and 1 sun (100 mW·cm-2). The light intensity was reduced with neutral density filters (ND filter). The IPCE spectrum was measured with a monochromator and a digital multimeter. Transient absorption measurements used a Nd:YAG laser (Spectra Physics, Quanta-Ray, FWHM = 7 ns, 532 nm, 10 Hz) as a pump laser and a diode laser (Coherent, LabLaser, 785 nm) as a probe laser. The repetition rate of the Nd:YAG laser pulse was controlled by a shutter attached to a function generator. The transient absorption kinetics of the cells using an electrolyte containing a redox couple was not strongly affected by the excitation frequency. With the inert electrolytes, the excitation frequency was decreased to the point that almost all of the injected electrons recombined with the oxidized dye before the following pulse was irradiated. The transient absorption signals were detected by a Si photo-detector (Model 1621, New Focus) with an amplifier (Voltage Amplifier DHPVA, FEMTO, DC mode, 10–40 dB), and the signals were averaged by an oscilloscope (Tektronix).


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Figure 1. Structures of the dyes. Carbazole dye, MK-1 (HOMO = 1.1 V vs NHE), triphenylamine dye, MK-88 (HOMO = 1.0 V vs NHE), and coumarin dye, MK-31 (HOMO = 0.85 V vs NHE).

3. Results and Discussion I-V characteristics and IPCE spectra Table 1 shows the I-V characteristics of the DSSCs with the Li electrolyte. The light intensity from the solar simulator was attenuated with ND filters. The values of the short circuit current were almost proportional to the light intensity. Thus, under this condition, a high oxidized dye regeneration efficiency was expected. Figure 2 shows the IPCE spectra. At 532 nm, the DSSCs efficiently converted photons to electrons. On the other hand, at 785 nm, the DSSCs converted few or no photons to electrons.



Table 1. I-V characteristics of DSSCs.

Conditionsa MK-1 1 sun ND50 ND25 MK-88 1 sun ND50 ND25 MK-31 1 sun ND50 ND25

Voc/V 0.69 0.67 0.64 0.74 0.72 0.70 0.68 0.69 0.64

Isc/mAcm-2 7.3 3.7 1.9 7.0 3.6 1.9 8.5 4.4 2.3

FF 0.68 0.68 0.69 0.71 0.71 0.70 0.67 0.69 0.69

PCE/% 3.4 3.4 3.4 3.7 3.7 3.7 3.9 4.2 4.1

a: The DSSCs were measured under three different intensities using a solar simulator. “1 sun” denotes measurement under 100 mW·cm-2, while “ND50” and “ND25” denote an intensity of 1 sun passed through a neutral density filter with a transmittance equal to the specified number. The electrolytes were 0.3 M LiI, 0.05 M I2 and 0.6 M tBP in acetonitrile.

100 MK-1 MK-88 MK-31

80 IPCE / %

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60 40 20 0 300

400

500

600

700

800

wavelength / nm

Figure 2. IPCE spectra of DSSCs. The electrolytes were 0.3 M LiI, 0.05 M I2 and 0.6 M tBP in acetonitrile.


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Effect of the laser pump intensity on the transient absorption measurements Transient absorption was measured with various pump laser intensities. Figure 3a-c shows the transient absorption of the oxidized dyes (MK-1, MK-88 and MK-31, respectively) at 785 nm in the DSSCs with the Li electrolyte. Absorption spectra of the three oxidized dyes were measured, showing an absorption peak around 780 nm (data no shown). The spectra were similar to those measured with similar carbazole and coumarin dyes.14 Electrons in the TiO2 also absorb 785 nm lights. We checked also the effect on the transient absorption and found that the effect was less than 2 % so that the transient absorption can be assigned to the absorption of oxidized dyes. Table 2 shows the electron densities and the decay half-life determined by the transient absorption kinetics measurement of the DSSCs using MK-1, MK-88 or MK-31 with the Li electrolyte at various pump intensities. Figure 4a-b shows the transient absorption of the oxidized dye (MK-1 (a) and MK-31 (b)) in the cells without a redox couple. The intensity dependence differed according to the presence or absence of the redox couple. With the electrolyte containing the redox couple, the difference in the kinetics half-life between 11 and 200 µJ·cm-2 was 3-fold or less. At a low pulse intensity, the difference in the half-lives between 11 and 82 µJ·cm-2 was small. This result suggests that at a low pulse intensity the oxidized dyes were mostly reduced by I- in the electrolytes. For the electrolyte without the redox couple, the half-lives between 11 and 200 µJ·cm-2 differed by a factor of 10–20 or more. The recombination rates between the injected electrons and the oxidized dyes increased with increasing electron



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density in the TiO2 electrode. These results are consistent with a previous report and have been explained by electron diffusion-limited recombination.15


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1.2

1.2

(a)

(b)

MK-1

1.0

11 µJ / cm

2

21 µJ / cm 42 µJ / cm

0.4

82 µJ / cm

2 2 2

200 µJ / cm

0.2

ΔOD / arb.unit

ΔOD / arb.unit

0.8

0.6

11 µJ / cm

0.6

2

21 µJ / cm 42 µJ / cm

0.4

82 µJ / cm

2

2 2 2

200 µJ / cm

0.2

0.0

2

0.0

-0.2

-0.2 -6

10

1.2

MK-88

1.0

0.8

-5

10

-4

10 time / s

-3

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(c)

-2

-1

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-6

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10

-5

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-4

10 time / s

-3

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-2

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-1

10

MK-31

1.0 0.8 ΔOD / arb.unit

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11 µJ / cm

0.6

2

21 µJ / cm 42 µJ / cm

0.4

82 µJ / cm

2 2 2

200 µJ / cm

0.2

2

0.0 -0.2 -6

10

-5

10

-4

10 time / s

-3

10

-2

10

-1

10

Figure 3. Transient absorption of the oxidized dyes of MK-1 (a), MK-88 (b) or MK-31 (c) in DSSCs. The electrolytes were 0.3 M LiI, 0.05 M I2 and 0.6 M tBP in acetonitrile. The pulse intensities are shown in the figure.



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Table 2. Electron densities and decay half-lives from the transient absorption measurements of DSSCsa using MK-1, MK-88 or MK-31 at various pump intensities. Pump intensity/µJ·cm-2 11 21 42 82 200

Electron density/cm3 6.5 × 1016 1.4 × 1017 2.8 × 1017 4.8 × 1017 8.7 × 1017

Half-life/µs 40 38 31 28 20

MK-88

11 21 42 82 200

6.7 × 1016 1.5 × 1017 2.7 × 1017 5.0 × 1017 8.9 × 1017

4.5 4.5 4.5 4.5 3.2

MK-31

11 21 42 82 200

7.6 × 1016 1.5 × 1017 2.5 × 1017 4.5 × 1017 7.3 × 1017

100 96 66 53 30

Dye

MK-1

a: Electrolytes were 0.3 M LiI, 0.05 M I2 and 0.6 M tBP in acetonitrile.


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1.2

1.2

(a)

(b)

MK-1

1.0

ΔOD / arb.unit

0.8

0.6 0.4

11 µJ·cm

-2

42 µJ·cm 0.2

MK-31

1.0

0.8 ΔOD / arb.unit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-2 -2

0.6 0.4

2

11 µJ / cm - 0.1 Hz 2

42 µJ / cm - 0.1 Hz

0.2

200 µJ·cm pump freq: 0.05 Hz

2

200 µJ / cm - 0.02 Hz

0.0

0.0

-0.2

-0.2 -6

10

-5

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10 time / s

-3

10

-2

10

-1

-6

10

10

-5

10

-4

10 time / s

-3

10

-2

10

-1

10

Figure 4. Transient absorptions for the oxidized dyes MK-1 (a) and MK-31 (b). The electrolytes were 0.3 M LiClO4 and 0.6 M tBP in acetonitrile. Three different pulse energies were used. The repetition rate of the pulsed laser used for MK-1 was 0.05 Hz, and that for MK-31 was 0.1 or 0.02 Hz.

Regeneration kinetics of the oxidized dye The reduction kinetics of the oxidized dyes was compared for the three dyes in DSSCs that used the Li electrolyte (Figure 5(a)). For the measurements, the pulse intensity was set to 42 µJ·cm-2. The half-lives of the regeneration of MK-1, MK-88 and MK-31 were 31 µs, 4.5 µs, and 66 µs, respectively. The ΔG between the HOMO of the dye and the I-/I3- redox potential were 0.66, 0.62, and 0.45 eV for MK-1, MK-88 and MK-31, respectively. The values for ΔG were calculated from an I-/I3- redox potential of 0.4 V vs NHE. The observed transients cannot be explained only by the difference in free energies. Figure 5(b) shows a comparison of the transient absorption kinetics of the oxidized dyes in the DSSCs using the TBA electrolyte. Almost the same kinetics between Li and TBA electrolytes was observed for MK-1 and MK-88. For 13 ACS Paragon Plus Environment


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MK-31, when the Li+ was replaced with TBA+, the kinetics was accelerated. However, this acceleration seems to be attributed to the desorption of the dyes from the TiO2 surface into the TBA electrolyte. When the photo-electrode was filled with the TBA electrolyte, a little portion of MK-31 dyes was dissolved by the electrolyte. Thus, more surface area of MK-31’s HOMO is exposed to iodide, resulting in a faster oxidized dye reduction; this is discussed in detail in a later section. The kinetics was similar between the Li and TBA electrolytes. As discussed below, this implies that only part of the HOMO on the donor moiety contributed to dye regeneration, while the rest of the HOMO spread away from the donor moiety to the anchor side of the dye did not contribute. In the case of the TBA electrolytes, the thickness of the electric double layer on the dye adsorbed semiconductor surface was suggested to be subject to change along with the ionic radius of the counter cation.16 When the TBA is employed, the TBA cations might not be able to penetrate the dye layer as well as iodide in the electrolytes. In other words, iodide was expected to hit only the end of the dye molecule. In the case of the Li electrolytes, Li caion should be able to penetrate into the dye layer and then I- is also expected to be in the dye layer. One possible reason that the anchor-side molecular orbitals did not contribute to the regeneration is that the blocking effect, provided by the densely packed dye layer, hindered the approach of Ito the side of dye molecules. These dyes have alkyl chains that cover the TiO2 surface and can prevent I3- from reaching the semiconductor surface.17 In this case, the dye molecules would


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block not only I3- but also I-. If the alkyl chains block the I-, the I- is unable to reach the HOMO located at the linker and anchor moiety site.

1.2

1.2

(a)

Li electrolyte

1.0

(b)

TBA electrolyte

1.0 0.8 ΔOD / arb. unit

0.8 ΔOD / arb. unit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 0.4

MK-1 MK-88 MK-31

0.2 0.0

pump intensity: 42 µJ / cm

0.6 MK-1 MK-88 MK-31

0.4 0.2 0.0

2

pump intensity: 42 µJ / cm

-0.2

2

-0.2 -6

10

-5

10

-4

10 time / s

-3

10

-2

10

-1

-6

10

10

-5

10

-4

10 time / s

-3

10

-2

10

-1

10

Figure 5. Comparison of the transient absorption kinetics of the oxidized dye in DSSCs using different dyes. The dyes were MK-1, MK-88 and MK-31. (a) The electrolytes were 0.3 M LiI, 0.05 M I2 and 0.6 M tBP in acetonitrile. (b)The electrolytes were 0.3 M TBAI, 0.05 M I2 and 0.6 M tBP in acetonitrile. The pump intensity was 42 µJ·cm-2. We tested our hypothesis using DFT calculations. To do so, the dye molecules and the TiO2 nanoparticles were optimized using the B3LYP hybrid functional18 and 6-31G(d) basis set. The effects of the solvent were added using the conductor-like polarizable continuum model of solvation (C-PCM)19, as implemented in the Gaussian 09 software suite20. The optimized dye and TiO2 molecules were used to model the dye and TiO2 adducts. As the conjugated structures are considerably large and computationally demanding, we used the ADF code 21 to obtain reasonable geometries for the dye@TiO2 complexes. These calculations were performed using the generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) to account



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for the exchange–correlation effects22. The optimization results show that the MK-1 and MK-31 dyes are planar, whereas MK-88 consists of an out-of-plane TPA donor moiety (Figure 6). Furthermore, the HOMO of MK-88 is more localized over the TPA donor when compared with this level’s localization on the respective donor moieties of MK-1 and MK-31 dyes (Figure 6). A greater localization of the HOMO over the donor moiety provides the access required by interacting I- ions, which may therefore result in an enhanced regeneration rate of the oxidized dye.

Figure 6. Optimized conformations of MK-1, MK-88 and MK-31 dyes, along with the isodensity plots showing the HOMO levels.


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(a)

1.0 ΔOD / arb. unit

1.2

MK-1

0.8 0.6 0.4

0.4

0.0

0.0 -5

10

-4

-3

10

10

-2

10

Adsorption density Full 28%

0.6

0.2

-6

MK-88

0.8

0.2

10

(b)

1.0

Adsorption density Full 25% 5%

ΔOD / arb. unit

1.2

-1

-6

10

10

-5

10

time / s

(c)

1.2

MK-31

1.0

0.6 0.4

0.0 -5

-4

10

-1

10

0.4

0.0 10

-2

10

0.6

0.2

-6

-3

10

-3

10

-2

10

Low adsorption density MK-1 5% MK-88 28% MK-31 3%

0.8

0.2

10

(d)

1.0

Adsorption density Full 3%

0.8

-4

10

time / s

ΔOD / arb. unit

1.2

ΔOD / arb. unit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1

-6

10

10

time / s

-5

10

-4

10

-3

10

-2

10

-1

10

time / s

Figure 7. Transient absorption kinetics of the oxidized dye. (a) The adsorption density of MK-1 was controlled: full dye loading, dye loading reduced to 25%, and dye loading reduced to 5%. (b) The adsorption density of MK-88 was controlled: full dye loading, and dye loading reduced to 28%. (c) The adsorption density of MK-31 was controlled: full dye loading, and dye loading reduced to 3%. (d) The transient absorption kinetics at low adsorption density. The adsorption densities of MK-1, MK-88 and MK-31 were reduced to 5%, 28% and 3%, respectively. The electron densities were between 5 and 9 × 1016 cm-3. The electrolytes were 0.3 M LiI, 0.05 M I2 and 0.6 M tBP in acetonitrile. To examine whether the accessible surface area affects the regeneration kinetics, the transient absorption was measured under reduced dye loading conditions. When the dye loading



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was reduced, a larger surface of the HOMO was exposed to the iodide, and the regeneration rate for the dyes was expected to follow the order of the HOMO levels. Figure 7 shows the transient absorptions for MK-1 (a), MK-88 (b) and MK-31 (c) in DSSCs with different amounts of each dye. Compared with the fully adsorbed DSSCs, the adsorption densities were reduced to 25% and 5% for MK-1, to 28% for MK-88, and to 3% for MK-31. When the dye loading of MK-1 was reduced to 25%, the regeneration rate was 6 times greater. On the other hand, the regeneration rate of MK-88 was only accelerated by a factor of 2 when the dye loading was reduced to 28%. Among the dyes examined, the regeneration rates became close for the DSSCs using MK-1 and MK-88 (Figure 7(d)). These results indicate that the sterically bulky donor present on MK-88 possesses a region of the HOMO that is more accessible to I- under conditions of full adsorption. To further understand this observation, we modeled and optimized MK-1@TiO2, MK-88@TiO2 and MK-31@TiO2 complexes (Figure 8). To explore the possible packing scheme, we attempted to model the closest possible arrangement of two dye molecules on a single TiO2 nanoparticle. Figure 9 shows possible arrangements for each of the dye types on TiO2. Owing to the planar conformations of MK-1 and MK-31 dyes, slide stacking (scheme A) is feasible. For MK-88, the presence of an out-of-plane TPA donor hinders scheme A, and the dye adsorbs in a sidewise sliding pattern (scheme B). This effect results in a zigzag arrangement of the MK-88 dye molecules on TiO2. This orientation may aide in rapid regeneration because of


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the exposed HOMO of the dye. However, in the case of MK-1 and MK-31, the dye molecules may stack tightly, which limits the accessibility of I- to the HOMO.

Figure 8. Front and side views of the optimized geometries for MK-1@TiO2, MK-88@TiO2, and MK-31@TiO2 complexes.

Figure 9. Adsorption schemes for MK-1, MK-88 and MK-31 dimeric configurations adsorbed on a (TiO2)34 nanoparticle. Here, the dye arrangement scheme (A) involves the P1 and P2 positions, while scheme (B) involves the P2 and P4 positions.

When the dye loading of MK-1 and MK-31 was reduced to less than 10%, the regeneration rate of the DSSC using MK-31 was significantly accelerated to 6-fold above that of MK-1. The rate



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for MK-31 changed by more than 100-fold. The fastest reduction of MK-31 under low dye loading conditions cannot be explained by the difference in free energy and the area of the HOMO that is exposed to iodide. A previous study reported that the partial charge of the coumarin moiety would attract I3- through attracting Li+ in the electrolyte.17 Similarly, it is possible that the partial charge of the coumarin moiety in MK-31 may attract I-. However, for the DSSC under reduced dye loading conditions with the TBA electrolyte, we found that the fastest reduction rate was from MK-31 (data not shown). Thus, the partial charge effect cannot adequately explain the results either. This puzzle is currently under investigation. Conclusions The effects of the donor moiety structure on oxidized dye reduction kinetics were evaluated in DSSCs with D-π-A types of metal-free organic dyes. The regeneration kinetics was affected by the structural differences between the donors. The accessible surface area effect was stronger than the 200 meV free energy difference between the HOMO of the dyes and the redox potential. When the HOMO of each adsorbed dye was localized close to the TiO2 surface, the surface area of the HOMO that was accessible to I- decreased; thus, the regeneration rate was decreased. Therefore, the introduction of a sterically bulky donor moiety into the end of a dye can force a portion of the HOMO to face the bulk electrolyte and is a strategy for obtaining high regeneration efficiency.


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AUTHOR INFORMATION Corresponding Author *E-mail

[email protected];

Tel (+81)268-21-5818

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by New Energy and Industrial Technology Development Organization (NEDO) of Japan.

References

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Supporting Information The complete author list for ref 20. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision B.01, Gaussian, Inc., Wallingford CT, 2010.


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