New molecular design of donor-π-acceptor dyes for dye-sensitized solar cells: control of molecular orientation and arrangement on TiO₂ surface

Abstract

A series of benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes OH1, OH2, OH4, OH7, and OH17 with carboxyl groups on different positions of a chromophore skeleton are synthesized and applied to dye-sensitized solar cells (DSSCs) as a new class of donor-π-acceptor (D-π-A) photosensitizers. In the dye OH1, a carboxyl group acts as not only the anchoring group for attachment on TiO₂ surface but also the electron acceptor. For OH2, OH4, and OH7, on the other hand, a carboxyl group is an anchoring group, but the electron acceptor is not a carboxyl group but a cyano group. The dye OH17 has two carboxyl groups, which are located at the same positions as those of OH1 and OH7. The absorption and fluorescence spectra and cyclic voltammograms of these fluorescent dyes resemble very well, showing a negligible influence of the position of carboxyl group on photophysical and electrochemical properties of these dyes. When these dyes are used in DSSCs, however, their photovoltaic performances differ considerably. In order to elucidate the difference in the DSSC performance among the five dyes, kinetics of the electron injection from the conduction band of TiO₂ to dye cation and to I₃⁻ ions in solution were studied by employing the transient absorption spectroscopy and the transient photovoltage technique, respectively. It is found that the charge recombination rate for OH2 is similar to that of OH1 and is much slower than those of OH4 and OH17, consistent with the high short-circuit photocurrent densities for OH1 and OH2. On the basis of the MO calculations (AM1 and INDO/S) and the charge recombination kinetics, the differences of the photovoltaic performances among the five dyes are discussed from the viewpoint of configuration of the dyes adsorbed on TiO₂ surface, and a new molecular design for D-π-A dye sensitizers based on a control of molecular orientation and arrangement of dye adsorbed on TiO₂ surface is proposed.

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Introduction

Since Grätzel and co-workers reported high solar cell performances for dye-sensitized solar cells (DSSCs) based on a Ru-complex dye in 1991,¹ DSSCs have received considerable attention because of high incident solar light-to-electricity conversion efficiency and low cost of production.^2–13 A diagrammatic illustration of the construction and the operational principle of typical DSSCs are shown in Fig. 1. To begin with, the dye sensitizer absorbs a photon (sun light) to generate a photoexcited-state of dye (S*) (1). Next, the photoexcited dye (S*) injects an electron to the conduction band (CB) (−0.5 V vs.NHE) of TiO₂ (2). The resultant oxidized dye (dye cation) is subsequently reduced back to its original neutral state by electron donation from the iodide ion I⁻ in the redox solution; this process is usually called dye regeneration or re-reduction (3). The injected electrons in CB move through the network of interconnected TiO₂ nanoparticles to arrive at the transparent conducting oxide (FTO) and then through the external circuit to the counter electrode (Pt-coated glass). The iodide ion I⁻ is regenerated by the reduction of triiodide ion I₃⁻ at the counter electrode through the donation of electrons from the external circuit (4) and then the electron flow cycle is completed. During this cycle, however, there are undesirable side processes: the electrons injected to the CB of TiO₂ may reduce either oxidized dye (recombination) (5) or I₃⁻ in solution (dark current) (6). These two processes (5) and (6) lower the short-circuit photocurrent density (J_sc) and the open-circuit photovoltage (V_oc) of DSSCs, respectively. The recombination process (5) competes with the re-reduction of the oxidized dyes by I⁻ (3). To generate greater J_sc and V_oc, therefore, the electron injection (2) and the regeneration (3) processes must be much faster than those of the recombination (5) and dark current (6).^2–7

Fig. 1 Diagrammatic illustration of (a) construction and (b) operational principle of DSSCs.

Recently, much research has focused on the development of new metal-free organic dyes. In particular, donor–acceptor π-conjugated (D-π-A) dyes, with both the electron-donating (D) and electron-accepting (A) groups linked by a π-conjugated bridge exhibiting broad and intense absorption spectra, are proposed as one of the most promising organic dye sensitizers.^5–13 Most of the D-π-A photosensitizers have dialkylamine or diphenylamine moiety as electron donor, and carboxylic acid, cyanoacrylic acid or rhodanine-3-acetic acid moiety which acts as electron acceptor as well as anchoring group for attachment on TiO₂ surface. The carboxyl group enables a good electron communication between the dye and TiO₂ by forming a strong ester linkage with TiO₂ surface. The spectral features of the D-π-A dyes are associated with the intramolecular charge transfer (ICT) excitation from the donor to acceptor moiety of the dye, leading to efficient electron transfer from the excited dye through the acceptor moiety (carboxyl group) into the CB of TiO₂.

In our previous study, a series of novel benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes OH1–4 with carboxyl groups on different positions of a chromophore skeleton have been designed and synthesized as a new class of D-π-A photosensitizers (Scheme 1).¹⁴ In OH1, a carboxyl group acts as not only the anchoring group for attaching on TiO₂ surface but also the electron acceptor. For OH2, OH3, and OH4, on the other hand, a carboxyl group is an anchoring group, but the electron acceptor is not a carboxyl group but a cyano group. The photovoltaic performance of OH2 having a non-conjugated linkage between a carboxyl group and a chromophore is similar to that of OH1 and is higher than those of OH3 and OH4. In OH1, electrons can be injected efficiently from the carboxyl group to the CB of a TiO₂ electrode through the ester linkage with the TiO₂ surface. On the other hand, from the molecular structure of OH2, it was suggested that the cyano group acting as electron acceptor is located close to the TiO₂ surface. Consequently, dye OH2 can inject efficiently electrons from the cyano group to the CB of a TiO₂ electrode. In order to provide a further confirmation for this view, dyes OH5–7 with different lengths of non-conjugated alkyl chains containing a carboxyl group at the end position have been designed and synthesized (Scheme 1).¹⁵ It is found that in spite of the lengths of the alkyl chains, due to flexibility of alkyl chain, the cyano group of the dyes is located near the TiO₂ surface and thus a good electron communication between the dyes and the TiO₂ surface is established. It is concluded that a carboxyl group of D-π-A sensitizer is necessary not as an electron acceptor, but only as an anchoring group for attachment on the TiO₂ surface. This implies that the essential point for developing new and efficient D-π-A sensitizers for DSSCs is to design dye molecules capable of forming a strong interaction between the electron acceptor moiety of sensitizers and the TiO₂ surface.^14–17

Scheme 1 A series of benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes.

The aim of this paper is to investigate the effect of the charge recombination (5) and the dark current (6) on the photovoltaic performances of benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes OH1, OH2, OH4, OH7, and OH17 by using the transient absorption spectroscopy and the transient photovoltage technique. On the basis of the MO calculations (AM1 and INDO/S) and the kinetic information on the undesirable processes (5) and (6), the differences of photovoltaic performances among the five dyes are discussed by taking account of possible configurations of the dyes adsorbed on TiO₂ surface, and a new molecular design for D-π-A dye sensitizers based on a control of molecular orientation and arrangement of dye adsorbed on TiO₂ surface is proposed.

Results

The optical and electrochemical data, the HOMO and LUMO energy levels, and the DSSC performance parameters of OH1, OH2, OH4, OH7, and OH17 are summarized in Table 1. The absorption (λ_max^abs = 416–431 nm, ε = 21 100–26 100 dm³ mol⁻¹ cm⁻¹) and fluorescence spectra (λ_max^fl = 525–541 nm, Φ = 0.95–0.99) in 1,4-dioxane and cyclic voltammograms (E_ox = 0.28–0.34 V) of these dyes resemble very well, showing that the position of carboxyl group has a negligible influence on the photophysical and electrochemical properties of these dyes. On the other hand, the absorbance (0.8–0.9) and absorption peak wavelengths (410–430 nm) of adsorbed dyes on TiO₂ films are very similar to one another. The HOMO and LUMO energy levels of these dyes lie in the range of 0.86 to 0.91 V and of −1.63 to −1.75 V, respectively, indicating that all these dyes have similar HOMO and LUMO energy levels. Evidently, the LUMO energy levels for the five dyes are higher than the energy level of the CB of TiO₂ (−0.5 V), so that these dyes can efficiently inject electrons to the TiO₂ electrode. Furthermore, it can be presumed from the five dyes having similar HOMO energy levels that the rate of the dye regeneration by electron donation from I⁻ in solution is similar to one another.

Table 1 Optical and electrochemical data, HOMO and LUMO energy levels, and DSSC performance parameters of OH1, OH2, OH4, OH7, and OH17

Dye	λ abs/nm (ε/M^−1cm^−1)^a	λ em/nm (Φf)^b	E ox ^c/V	HOMO^d/V	LUMO^d/V	Molecules cm^−2^e	J sc ^f/mA cm^−2	V oc ^f/mV	ff ^f	η ^f (%)
a In 1,4-dioxane. b Fluorescence quantum yields (Φ) were determined by using a calibrated integrating sphere system (λex = 325 nm). c Oxidation potentials (Eox) were recorded in MeCN/Et4NClO4 (0.1 M) solution with a three-electrode system consisting of Ag/Ag^+ as a reference electrode, Pt plate as a working electrode, and Pt wire as a counter electrode. d Vs. normal hydrogen electrode (NHE). e Adsorption amount per unit area of TiO2 film. f The photocurrent–voltage characteristics were measured under a simulated solar light (AM 1.5, 61 mW cm^−2).
OH1	416 (25 700)	525 (0.97)	0.28	0.87	−1.71	6.8 × 10^16	2.12	508	0.57	1.00
OH2	425 (24 400)	537 (0.99)	0.34	0.91	−1.64	5.5 × 10^16	2.01	548	0.56	1.00
OH4	424 (22 200)	541 (0.97)	0.33	0.91	−1.63	6.6 × 10^16	1.50	470	0.58	0.67
OH7	431 (26 100)	533 (0.95)	0.30	0.86	−1.75	7.4 × 10^16	1.91	520	0.57	0.90
OH17	417 (21 100)	522 (0.97)	0.31	0.89	−1.65	6.5 × 10^16	1.30	488	0.59	0.61

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The MO calculations (AM1 and INDO/S) showed that the absorption bands of these dyes were mainly assigned to the transition from HOMO to LUMO, where HOMOs were mostly localized on the 3-dibutylamino-benzofuro[2,3-c]oxazolo[4,5-a]carbazole moiety for all these dyes, and LUMOs were localized mostly on the carboxyphenyl moiety for OH1 and OH17 and the cyanophenyl moiety for OH2, OH4, and OH7, suggesting a strong ICT nature upon illumination (Fig. 2).

Fig. 2 Calculated electron density changes accompanying the first electronic excitation of OH1, OH2, OH4, OH7, and OH17. The blue and red lobes signify decrease and increase in electron density accompanying the electronic transition, respectively. Their areas indicate the magnitude of the electron density change (light blue, green, blue and red balls correspond to hydrogen, carbon, nitrogen, and oxygen atoms, respectively).

When the performances of DSSCs fabricated using these dyes as sensitizers were examined, however, large differences in the incident photon-to-current conversion efficiency (IPCE) spectra and the photocurrent–voltage (I–V) characteristics were observed (Table 1). The maximum IPCE value increased in the order of OH17 (31%@578 nm) < OH4 (34%@473 nm) < OH1 (46%@478 nm) < OH2 (58%@484 nm) = OH7 (58%@486 nm). The J_sc value for OH2 (2.01 mA cm⁻²) is close to that for OH7 (1.91 mA cm⁻²), and is similar to that for OH1 (2.12 mA cm⁻²) and higher than those of OH4 (1.5 0 mA cm⁻²) and OH17 (1.30 mA cm⁻²) with two carboxyl groups. The η values increase in the order of OH17 (0.61%) < OH4 (0.67%) < OH7 (0.90%) ≤ OH1 and OH2 (1.00%). The V_oc values for OH1, OH2, OH4, OH7, and OH17 are 508, 548, 470, 520, and 488 mV, respectively, depending largely on the sort of the dyes. Therefore, the photovoltaic performances of OH2 and OH7 are similar to that of OH1 and higher than those of OH4 and OH17. Here, it is worth mentioning that the J_sc and η values for OH7 with a longer alkyl chains containing a carboxyl group at the end position are slightly smaller than those for OH2.

The processes (5) and (6) affect J_sc and V_oc of DSSCs, respectively.^18–20 Thus, first of all, the transient absorption technique was applied to the dyes OH1, OH2, OH4, OH7, and OH17 to evaluate the rate of the charge recombination process (5). The absorption spectrum of dye cations on TiO₂ needed for the transient absorption spectroscopy was determined by the spectroelectrochemical measurements with the dye-adsorbed TiO₂ electrode in 0.1 M Et₄NClO₄/acetonitrile. A typical difference absorption spectrum for OH1 is depicted in Fig. 3, where the dye-adsorbed TiO₂ electrode is biased at 0.6 V corresponding to the oxidation potential of OH1 (E_ox = 0.58 V vs.Ag/AgCl) determined by cyclic voltammetry (CV). It is seen clearly that two absorption bands due to dye cation appear at around 550 and 800–1200 nm. Thus, the charge recombination kinetics was followed by monitoring the 850 nm band for the DSSC without a redox electrolyte. Fig. 4 shows the transient absorption traces for the five dyes. Significant differences in the charge recombination rate were observed for the five dyes. The recombination half-time (t_50%) increases in the order of OH17 (21 μs) < OH4 (195 μs) < OH2 (1.8 ms) ≤ OH1 (2.5 ms) < OH7 (9 ms), in accord with the increasing order of the maximum IPCE values.

Fig. 3 Difference absorption spectrum for OH1-adsorbed TiO₂ on FTO in 0.1 M Et₄NClO₄/acetonitrile measured at potentials between 0 and 0.6 V, where a dye-adsorbed TiO₂ on FTO as a working electrode, Pt wire as a counter electrode and Ag/AgCl as a reference electrode were used.

Fig. 4 Transient absorption data monitoring charge recombination of injected electrons to TiO₂ with OH1, OH2, OH4, OH7, and OH17 cations. The data were obtained by observing the decay of the 850 nm cation absorption for all five dye/TiO₂ film combinations, in the absence of a redox electrolyte.

The kinetics of the dark current (6) was studied by using the transient photovoltage technique. Fig. 5a shows a typical photovoltage transient for DSSC sensitized with OH1. The electron lifetimes (τ_e) of DSSCs based on OH1, OH2, OH4, and OH7 are plotted in Fig 5b against the V_oc values obtained under different intensities of light. The τ_e values for OH1, OH2, OH4, or OH7 measured at around 450 mV are 11.5, 10.2, 9.1, and 15.1 ms, respectively.

Fig. 5 (a) Transient photovoltage decay of DSSC sensitized with OH1 and (b) electron lifetimes (τ_e) of DSSCs based on OH1, OH2, OH4, or OH7 under corresponding V_oc values resulting from various light intensities.

Discussion

On the basis of the ICT characteristics revealed by the MO calculations and the kinetics of the processes (5) and (6) estimated by the transient absorption spectroscopy and the transient photovoltage technique, the differences of photovoltaic performances among the dyes OH1, OH2, OH4, OH7, and OH17 will be discussed below from the viewpoint of configurations of the dyes adsorbed on TiO₂ surface. Based on the ICT characteristics and the t_50% values, possible configurations of the dye molecules adsorbed on TiO₂ surface may be inferred as shown in Fig. 6. The short t_50% for OH4 (195 μs) demonstrates that the cationic charge on the dialkylamino moiety in the photoexcited state is located in close proximity to the TiO₂ surface because of the rigid double ester linkages with TiO₂ surface for OH4, which results in the rapid recombination of electrons injected to the CB of TiO₂ with dye cation. In this connection, the rigid double ester linkages with TiO₂ surface prevent the cyano group of the dye from approaching the TiO₂ surface, so that the intramolecular charge transfer from the dialkylamino moiety to the carboxylphenyl moiety for OH4 accompanying photoexcitation flows in the opposite direction of TiO₂ surface, which leads to reduction in electron-injection yield from the excited dye to the CB of TiO₂. Thus, the rapid charge recombination and the low electron injection-yield may explain smaller IPCE and J_sc values for OH4.

Fig. 6 Plausible configurations of OH1, OH2, OH4, OH7, and OH17 on TiO₂ surface. The green and blue arrows show electron injection from the dye molecule into TiO₂ and charge recombination of injected electrons to TiO₂ with dye molecule, respectively.

In OH1, on the other hand, the intramolecular charge transfer takes place from the dialkylamino moiety to the carboxylphenyl moiety upon photoexcitation and the large t_50% value of OH1 (2.5 ms) is comparable to those of the traditional D-π-A dye sensitizers reported earlier.¹⁹ Thus, as shown in Fig. 6, electrons in OH1 will be efficiently injected from the dye through carboxyl group to the CB of TiO₂, leading to the large IPCE and J_sc values for the DSSC based on OH1.

It has been already suggested that the phenylcyano group acting as electron acceptor in OH2 and OH7 is located close to the TiO₂ surface.^14,15 Therefore, the dyes OH2 and OH7 can efficiently inject electrons into the TiO₂ electrode through the cyano group (Fig. 6). In view of the ICT characteristics of OH2 and OH7, the increased distance between the cationic charge on the dialkylamino moiety in the photoexcited state and the TiO₂ surface explains the large t_50% values of OH2 (1.8 ms) and OH7 (9 ms). Thus, the high electron-injection yield and the retardation of the charge recombination for OH2 and OH7 lead to their larger J_sc values. As for the reason for the large t_50% value of OH7, we presume that the longer alkyl chain containing a carboxyl group at the end position of OH7 causes the retardation of the charge recombination because of the increased separation of the cationic charge on the dialkylamino moiety of the dye from the TiO₂ surface.

On the other hand, the dye OH17 with two carboxyl groups can interact strongly with TiO₂ surface through double ester linkages, so that electrons will be injected from the dye to TiO₂ through carboxyl group in carboxyphenyl moiety, similarly to the case of OH1. However, the J_sc or IPCE value of OH17 is smaller than that of OH1. Tian and co-workers have also reported that stronger adsorption of the dye with two carboxyl groups on the TiO₂ surface could facilitate charge recombination of the injected electron and oxidized dye.²¹ In view of this and the smallest t_50% value for OH17 (21 μs), the strong interaction with TiO₂ surface for OH17 may enhance the charge recombination between the injected electron and the excited dye (dye cation), which was faster than for comparable dyes that have one carboxyl group because of close approach of the oxidized OH17 to TiO₂ surface. This may be a sound reason for the low photovoltaic performance of OH17.

It was found from the transient photovoltage measurements that the τ_e values of DSSCs based on OH1, OH2, OH4, and OH7 are similar to one another. Ko et al. have demonstrated that the dark current (6) is effectively controllable by the modification of dye molecular structure: the introduction of hydrophobic alkyl chains and bulky substituents to dye molecular structure discourages the process (6), leading to the increased V_oc.^20c,d,f Judging from the close τ_e values for OH1, OH2, OH4, OH7, and OH17, however, the differences in V_oc among the five dyes are not ascribable to the process (6). Meanwhile, it has been reported that the dipole of a dye adsorbed on TiO₂ affects the electronic state of TiO₂ and the Fermi level of TiO₂ can be changed by adsorption of dyes having permanent dipoles.²² The directions of dipole moments of OH1, OH2, OH4, OH7, and OH17 in the adsorption state are different because of different positions of carboxyl groups on a chromophore skeleton and the number of carboxyl groups. Thus, it may be reasonable to conclude that the difference in V_oc among the five dyes arises not from the process (6), but from the difference in the direction of the dipole moments of the respective dyes.

Consequently, it is revealed that the differences in photovoltaic performance among the D-π-A dye sensitizers OH1, OH2, OH4, OH7, and OH17 with carboxyl groups on different positions of a chromophore skeleton are ascribable not only to electron-injection yield from the excited dye to the CB of TiO₂, but also to the recombination process between the electrons injected to the CB of TiO₂ and the excited dye. The high electron-injection yield and the retardation of the charge recombination process (5) for OH2 and OH7, which are functionally-separated into an anchoring group for attachment on TiO₂ surface and an electron acceptor moiety, are responsible for the high J_sc comparable to that of the D-π-A dye sensitizer OH1.

Conclusions

As a new class of donor-π-acceptor (D-π-A) dye sensitizers, a series of benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes OH1, OH2, OH4, OH7, and OH17 with carboxyl groups on different positions of a chromophore skeleton were examined as photosensitizers in DSSCs. In order to elucidate the difference of the DSSC performances among the five dyes from the viewpoint of configuration of the dyes adsorbed on TiO₂ surface, kinetic studies on the reactions of electrons injected to the CB of TiO₂ with dye cation and with I₃⁻ ions in solution were performed by employing the transient absorption spectroscopy and the transient photovoltage technique. Combining the results with the MO calculations explains that the high electron-injection yield and the retardation of the charge recombination for new type of D-π-A dye sensitizers OH2 and OH7 functionally-separated into an anchoring group and an electron acceptor moiety are responsible for the large J_sc values of OH2 and OH7 comparable to that of OH1 which is a traditional D-π-A dye sensitizer. We propose that, for developing high performance DSSCs, it is necessary to create a novel molecular design of organic dye sensitizers capable of controlling not only the photophysical and electrochemical properties of the dyes themselves but also the molecular orientation and arrangement of the dyes on TiO₂ surface, which will be made possible by the exquisite molecular design and synthetic strategy of a new type of D-π-A dye sensitizers functionally separated into an anchoring group and an electron acceptor moiety.

Experimental

Absorption spectra were taken on a Shimadzu UV-3150 spectrophotometer and fluorescence spectra were measured with a Hitachi F-4500 spectrophotometer. The fluorescence quantum yields (Φ) were determined by a Hamamatsu C9920-01 equipped with CCD by using a calibrated integrating sphere system (λ_ex = 325 nm). Cyclic voltammograms (CVs) were recorded in MeCN/Et₄NClO₄ (0.1 M) solution with a three-electrode system consisting of Ag/Ag⁺ as a reference electrode, Pt plate as a working electrode, and Pt wire as a counter electrode, by using a Hokuto Denko HAB-151 potentiostat equipped with a functional generator. Spectroelectrochemical measurements were carried out with a Shimadzu UV-3150 spectrophotometer, where a dye-adsorbed TiO₂ on FTO was used as a working electrode in 0.1 M Et₄NClO₄/acetonitrile with Pt wire as a counter and Ag/AgCl as a reference electrode.

Computational methods

The semi-empirical calculations were carried out with the WinMOPAC Ver. 3.9 package (Fujitsu, Chiba, Japan). Geometry calculations in the ground state were made using the AM1 method.²³ All geometries were completely optimized (keyword PRECISE) by the eigenvector following routine (keyword EF). Experimental absorption spectra of the five compounds were compared with their absorption data by the semi-empirical method INDO/S (intermediate neglect of differential overlap/spectroscopic).²⁴ All INDO/S calculations were performed using a single excitation full SCF/CI (self-consistent field/configuration interaction), which includes the configuration with one electron excited from any occupied orbital to any unoccupied orbital, where 225 configurations were considered [keyword CI (15 15)].

Preparation of dye-sensitized solar cells

The TiO₂ electrodes used for dye-sensitized solar cells were prepared as follows. Into a powder (1.30 g) of TiO₂ (P-25, Φ = 30–40 nm) in a mortar was added water (1.9 mL) in six portions with well stirring. Then, three drops of 12 M nitric acid and polyethylene glycol (80 mg) were successively added, and the mixture was well kneaded to a smooth paste. It was then applied on a fluorine-doped-tin-oxide (FTO) substrate and sintered for 30 min at 500 °C. The 7 μm thick TiO₂ electrode (0.5 × 0.5 cm² in photoactive area) was immersed into a 0.3 mM tetrahydrofuran solution of the dye for hours enough to adsorb the photosensitizer. The DSSCs were fabricated by using the dye-adsorbed TiO₂ electrode thus prepared, Pt-coated glass as a counter electrode, and a solution of 0.05 M iodine, 0.1 M lithium iodide, and 1,2-dimethyl-3-n-propylimidazolium iodide in acetonitrile as electrolyte. The photocurrent–voltage characteristics were measured using a potentiostat under a simulated solar light (AM 1.5, 61 mW cm⁻²). IPCE spectra were measured under monochromatic irradiation with a tungsten–halogen lamp and a monochromator. The dye-coated film was immersed in a mixture of THF solvent, DMSF and 1 M NaOH aq (5 ∶ 4 ∶ 1), which was used to determine the amount of dye molecule adsorbed onto the film by measuring the absorbance. The quantification of each dye was made based on the λ_max and the molar extinction coefficient of each dye in the above solution. Absorption spectra of the dye-adsorbed TiO₂ films adsorbed by dyes were measured in the diffuse-reflection mode by a Shimadzu UV-3150 spectrophotometer with a calibrated integrating sphere system ISR-3100.

Transient absorption spectroscopy

The transient absorption measurements were performed by the conventional pump and probe technique. The dyes adsorbed on TiO₂ film were excited at 480 nm for all five dyes with pulses from a nitrogen laser pumped dye laser (1 ns pulse duration, 6 Hz repetition, ∼44 μJ cm⁻² at 480 nm). The resulting photoinduced change in optical density was monitored by employing the probe light of 850 nm from a light emitting diode corresponding to dye cation absorption determined by in situ spectroelectrochemical measurement of dye adsorbed on TiO₂ film, a Hamamatsu Photonics C8366 PIN photodiode as a photosensor, and an IWATSU DS-4372 oscilloscope.

Transient photovoltage technique

Transient photovoltage measurements in the DSSCs employed a 100 μs exciting pulse generated by a ring of light emitting diode (λ = 475 nm), which is controlled by a pulse motor. The pulse was incident on the TiO₂ side of the device and its intensity was controlled to keep the modulation of the voltage below 5 mV. White bias light impinging the cell from the same direction was also supplied by halogen lamp (150 W). The transient photovoltage decay was obtained from an oscilloscope (IWATSU DS-4372) with an RC circuit (τ = 2.2 s) through an NF 5307 voltage amplifier.

Acknowledgements

This work was supported by Grants-in-Aid for Scientific Research (B) (19350094) and for Young Scientist (B) (22750179) from the Japan Society for the Promotion of Science (JSPS), by General Sekiyu Research & Development Encouragement & Assistance Foundation, and by the Kurata Memorial Hitachi Science and Technology Foundation.

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