Dye-sensitized solar cell based on an inclusion complex of a cyclic porphyrin dimer bearing four 4-pyridyl groups and fullerene C 60 †

Cyclic free-base porphyrin dimers ( H 4 -C 4 -CPD Py (TEO) and H 4 -Ptz-CPD Py (TEO) ) linked by butadiyne or phenothiazine bearing four 4-pyridyl groups and their inclusion complexes ( C 60 3 H 4 -C 4 -CPD Py (TEO) and C 60 3 H 4 -Ptz-CPD Py (TEO) ) with fullerene C 60 have been applied to dye-sensitized solar cells (DSSCs) as a new class of porphyrin dye sensitizers with pyridyl anchoring groups for attachment on a TiO 2 electrode. The FTIR spectra of the porphyrin dimers adsorbed on TiO 2 nanoparticles demonstrated that these porphyrin dimers are adsorbed on the TiO 2 surface through the formation of hydrogen bonding of pyridyl groups and/or pyridinium ions at Brønsted acid sites on the TiO 2 surface. The adsorption amount of the porphyrin dimers adsorbed on the TiO 2 electrode is 2.0 (cid:1) 10 17 molecules per cm 2 , that is, the adsorption amount of the porphyrin unit is 4.0 (cid:1) 10 17 cm (cid:3) 2 , which is higher than that of dye sensitizers with pyridyl groups reported so far. The photovoltaic performance of DSSCs based on phenothiazine-bridged cyclic porphyrin dimer H 4 -Ptz-CPD Py (TEO) is higher than that of DSSCs based on butadiyne-linked cyclic porphyrin dimer H 4 -C 4 -CPD Py (TEO) . Moreover, the photovoltaic performances of DSSCs based on cyclic free-base porphyrin dimers are higher than those of DSSCs based on their C 60 inclusion complexes C 60 3 H 4 -C 4 -CPD Py (TEO) and C 60 3 H 4 -Ptz-CPD Py (TEO) . On the basis of the electrochemical measurements (voltammetry and electrochemical impedance spectroscopy) and the transient absorption spectroscopy, the di ﬀ erences in the photovoltaic performances among these cyclic free-base porphyrin dimers are discussed from kinetic and thermodynamic considerations concerning the electron transfer processes in DSSCs.


Introduction
Dye-sensitized solar cells (DSSCs) employing dye-adsorbed TiO 2 electrodes are one of the most promising new renewable photovoltaic cells utilizing the sun as a free and inexhaustible energy source because of their interesting construction and operational principles, and low cost of production, since Grätzel and co-workers produced high-performance DSSCs based on a Ru-complex dye, which showed a solar energy-to-electricity conversion yield (h) of 11%. 1 To further improve the photovoltaic performance of DSSCs, many kinds of ruthenium (Ru) dyes, porphyrin dyes, phthalocyanine dyes and organic dyes bearing carboxyl groups as anchoring groups, which are adsorbed on the TiO 2 electrode through the bidentate bridging linkage between the carboxyl group of the dye and Brønsted acid sites (surface-bound hydroxyl groups, Ti-OH) on the TiO 2 surface, have been developed as dye sensitizers during the last two decades. [2][3][4][5][6][7][8][9] In particular, porphyrin dyes have been regarded as promising candidates for photosensitizers as a result of their strong Soret (400-500 nm) and moderate Q band (500-700 nm) absorption properties, as well as their electrochemical, photochemical and thermal stabilities. Much effort in molecular design and development of porphyrin dye sensitizers having carboxyl group have been made to further improve the photovoltaic performances of DSSCs so far. [10][11][12][13][14][15] Consequently, DSSCs based on donor-p-acceptor (D-p-A) porphyrin dyes bearing the diarylamino group as an electron donor and the benzothiadiazole-benzoic acid moiety as an electron acceptor, which exhibited good absorption features (bathochromic shi and broadening of the Soret and Q bands), have achieved h value of up to ca. 13%. 11d,e On the other hand, we have reported that a new type of D-p-A dye sensitizers bearing pyridyl group as electron-withdrawing anchoring group were predominantly adsorbed on the TiO 2 electrode through coordinate bonding between the pyridyl group of the dye and the Lewis acid site (exposed Ti n+ cations) on the TiO 2 surface. 16,17 It was demonstrated that the new-type of D-p-A dye sensitizers can inject electrons efficiently from the pyridyl group to the conduction band (CB) of the TiO 2 electrode through the coordinate bonding, rather than the bidentate bridging linkages of conventional D-p-A dye sensitizers bearing carboxyl group. Recently, Wang et al. reported that DSSCs based on D-p-A porphyrin dye bearing a pyridyl group reached solar energy-to-electricity conversion yield (h) of 3.96%. 18a On the other hand, Goutsolelos et al. reported the h value of 3.9% for DSSC based on porphyrin dye bearing four pyridyl groups. 18c However, the adsorption amounts (<5.0 Â 10 16 molecules per cm 2 ) of these porphyrin dye sensitizers bearing pyridyl group adsorbed on TiO 2 electrode are much lower than those of porphyrin dye sensitizers bearing carboxyl group, and thus the low dye loading leads to low lightharvesting efficiency (LHE) and poor surface coverage of the TiO 2 electrode, resulting in lowering of the photovoltaic performances of DSSCs. 18,19 More recently, Goutsolelos et al. have designed and synthesized "spider-shaped" porphyrin dye sensitizer bearing oligophenylenevinylene moieties, long dodecyloxy chains, and four pyridyl groups, which showed high dye loading value (1.9 Â 10 17 molecules per cm 2 ) due to an increase in the basicity of the pyridyl groups. As the results, the DSSC based on the "spider-shaped" porphyrin dye sensitizer reached the h value of 5.12%. 18e Recently, we have designed and prepared cyclic free-base porphyrin dimers (H 4 -C 4 -CPD Py (TEO) and H 4 -Ptz-CPD Py (TEO)) linked by butadiyne or phenothiazine bearing four 4-pyridyl groups and their inclusion complexes (C 60 3H 4 -C 4 -CPD Py (TEO) and C 60 3H 4 -Ptz-CPD Py (TEO)) with fullerene C 60 . 20 It was found that these porphyrin dimers have favorable photochemical and electrochemical properties for DSSC through the electrochemical measurements and the transient absorption spectroscopy. Moreover, as for phenothiazine derivative H 4 -Ptz-CPD Py (TEO), it would be expected that the phenothiazine unit possessing electron donating ability can provide a unidirectional ow of electrons toward the pyridyl anchoring group upon photoexcitation of the porphyrin, leading to the efficient electron injection from the photoexcited dye to the CB of TiO 2 electrode. Thus, in this work, to achieve high dye loading and high surface coverage of the TiO 2 electrode for DSSCs based on porphyrin dye sensitizers bearing pyridyl group, the cyclic free-base porphyrin dimers H 4 -C 4 -CPD Py (TEO) and H 4 -Ptz-CPD Py -(TEO) and their C 60 inclusion complexes C 60 3H 4 -C 4 -CPD Py -(TEO) and C 60 3H 4 -Ptz-CPD Py (TEO) have been applied to DSSCs as a new class of porphyrin dye sensitizers bearing pyridyl anchoring groups for attachment on TiO 2 electrode (Scheme 1). It was demonstrated that these porphyrin dimers are adsorbed on the TiO 2 surface through the formations of hydrogen bonding of pyridyl groups and/or pyridinium ion at Brønsted acid sites on the TiO 2 surface. Here we reveal the photovoltaic performances of DSSCs based on these porphyrin dimers from kinetic and thermodynamic consideration concerning the electron transfer processes in DSSCs, based on the electrochemical measurements (voltammetry and electrochemical impedance spectroscopy) and the transient absorption spectroscopy.

Results and discussion
Photoabsorption properties of cyclic free-base porphyrin dimers and their inclusion complexes with fullerene C 60 The synthesis of H 4 -Ptz-CPD Py (TEO) has been reported elsewhere. 20e The synthetic pathway for H 4 -C 4 -CPD Py (TEO) is shown in Scheme 2. The inclusion complexes C 60 3H 4 -C 4 -CPD Py (TEO) and C 60 3H 4 -Ptz-CPD Py (TEO) with C 60 were prepared by slowly evaporating the mixed solution of the corresponding cyclic freebase porphyrin dimer in chloroform (0.1 mM, 5 mL) and C 60 in toluene (0.1 mM, 5 mL). The UV/vis absorption spectra of cyclic free-base porphyrin dimers H 4 -C 4 -CPD Py (TEO) and H 4 -C 4 -CPD Py (TEO) in benzonitrile are shown in Fig. 1 and their spectral data are summarized in Table 1. The porphyrin dimers H 4 -C 4 -CPD Py (TEO) and H 4 -Ptz-CPD Py (TEO) exhibit strong Soret band at around 420 nm and relatively weak Q band in the range 500-650 nm. For the C 60 inclusion complexes C 60 3H 4 -C 4 -CPD Py (TEO) and C 60 3H 4 -Ptz-CPD Py (TEO), it is difficult to obtain their exact absorption spectra because the 1 : 1 complex of H 4 -C 4 -CPD Py (TEO) or H 4 -Ptz-CPD Py (TEO) with C 60 is in dissociation equilibrium in solution of 10 À5 to 10 À6 M concentration which is suitable for the measurement of absorption spectra of porphyrins. In our previous work, however, we have demonstrated that upon addition of C 60 to the benzonitrile solution of the cyclic porphyrin dimers, their Soret bands were redshied with a decrease in intensity, whereas their Q bands were slightly redshied but increased in intensity. 20 The UV/vis absorption spectra of cyclic free-base porphyrin dimers and their C 60 inclusion complexes adsorbed on TiO 2 lm are shown in Fig. 2. It is worth mentioning here that the adsorption amount of the porphyrin dimers adsorbed on TiO 2 lm is 2.0 Â 10 17 molecules per cm 2 , that is, the adsorption amount of porphyrin unit is 4.0 Â 10 17 cm À2 , which is higher than those (<2.0 Â 10 17 molecules per cm 2 ) of porphyrin dye sensitizers and D-p-A dye sensitizer bearing pyridyl groups reported so far. 16,18 The Soret bands of C 60 3H 4 -C 4 -CPD Py (TEO) and C 60 3H 4 -Ptz-CPD Py (TEO) were redshied compared to those of H 4 -C 4 -CPD Py (TEO) and H 4 -Ptz-CPD Py (TEO), although there is little difference in the Q band between the cyclic free-base porphyrin dimers and their C 60 inclusion complexes. Thus, the UV/vis absorption spectra of these porphyrin dimers-adsorbed TiO 2 lms are in good agreement with those in benzonitrile.
Electrochemical and photochemical properties of cyclic freebase porphyrin dimers and their inclusion complexes with fullerene C 60 The electrochemical properties of cyclic free-base porphyrin dimers and their C 60 inclusion complexes were determined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) (see Fig. S7-S10 in ESI †). 20 The oxidation potential of the porphyrin unit in C 60 3H 4 -C 4 -CPD Py (TEO) (0.74 V vs. Fc/Fc + ) showed anodic shi by 0.03 V, compared with that of H 4 -C 4 -CPD Py (TEO) (0.71 V) ( Table 1). The oxidation potential of the porphyrin unit in C 60 3H 4 -Ptz-CPD Py (TEO) (0.75 V) also showed anodic shi by 0.03 V, compared with that of H 4 -Ptz-CPD Py -(TEO) (0.72 V). The reduction potentials corresponding to the reduction of the fullerene entity of C 60 3H 4 -C 4 -CPD Py (TEO) and C 60 3H 4 -Ptz-CPD Py (TEO) were observed at À0.94 V and À0.96 V, respectively, which is cathodically shied by 0.02 V and 0.04 V, respectively, compared to pristine C 60 (À0.92 V). 20e The small anodic shi of the oxidation potential of the porphyrin and the small cathodic shi of the reduction potential of C 60 compared with their reference compounds is indicative of the charge transfer interaction between the porphyrins and C 60 . The HOMO and LUMO energy levels were evaluated from the oxidation wave and the singlet excited energy of these porphyrin dimers (1.90 eV) based on the Q absorption band and uorescence band in benzonitrile (ca. 650 nm), respectively. The HOMO and LUMO energy levels of these porphyrin dimers was ca. À5.5 eV and ca. À3.6 eV, respectively (Table 1). Thus, this result shows that the HOMO energy levels are more positive than the I 3 À /I À redox potential (À4.9 eV), and thus this indicates that an efficient regeneration of the oxidized porphyrin dimers by electron transfer from the I 3 À /I À redox couple in the electrolyte is thermodynamically feasible. Evidently, the LUMO energy levels of these porphyrin dimers are higher than the energy level (E cb ) of the CB of TiO 2 (À4.0 eV), suggesting that an electron injection to the CB of TiO 2 is thermodynamically feasible (some researchers have proposed that an energy gap of over 0.2-0.3 eV is necessary for efficient electron injection). [2][3][4][5][6][7] Time-resolved absorption spectroscopy for cyclic free-base porphyrin dimers and their C 60 inclusion complexes was performed by femtosecond laser ash photolysis aer photoexcitation. 20 The photodynamics of these porphyrin dimers is summarized in Fig. 3. The decay of photoexcited state of C 60 3H 4 -C 4 -CPD Py (TEO) has two steps: the rst step has a lifetime of 18 ps, which corresponds to the disappearance of the singlet excited state of the 1 H 4 -C 4 -CPD Py (TEO)* (ca. À3.6 eV), that is, the 1 H 4 -C 4 -CPD Py (TEO)* undergoes intrasupramolecular electron transfer to give a completely charge-separated state C 60 c À -H 4 -C 4 -CPD Py (TEO)c + (ca. À3.7 eV). The C 60 c À -H 4 -C 4 -CPD Py (TEO)c + decays with a lifetime of 470 ps to the ground state. The decay of photoexcited state of C 60 3H 4 -Ptz-  CPD Py (TEO) also includes the charge-separated states C 60 c À -H 4 -Ptz-CPD Py (TEO)c + (ca. À3.8 eV), but has multiple steps to the ground state via the triplet charge separated state 3 (C 60 c À -H 4 -Ptzc + -CPD Py (TEO)) (ca. À4.3 eV) with a lifetime of 0.71 ms: the rst step has a lifetime of 20 ps, which is similar to that (18 ps) of C 60 3H 4 -C 4 -CPD Py (TEO). On the other hand, the energy diagrams for the photochemical events in these porphyrin dimes revealed that the energy levels of charge-separated states is very close to or lower than the E cb of the CB of TiO 2 electrode. This suggests that the electron injection from the C 60 c À in the charge-separated states to the CB of TiO 2 electrode is thermodynamically difficult.
FTIR spectra of cyclic free-base porphyrin dimers and their inclusion complexes with fullerene C 60 To elucidate the adsorption states of cyclic free-base porphyrin dimers and their C 60 inclusion complexes on TiO 2 nanoparticles, we measured the FTIR spectra of the porphyrin dimer powders and the porphyrin dimers adsorbed on TiO 2 nanoparticles (Fig. 4). For the powders of all the four porphyrin dimers H 4 -C 4 -CPD Py (TEO), H 4 -Ptz-CPD Py (TEO), C 60 3H 4 -C 4 -CPD Py (TEO), and C 60 3H 4 -Ptz-CPD Py (TEO), the C]N stretching band of pyridyl group was clearly observed at around 1590 cm À1 . Interestingly, when these porphyrin dimers were adsorbed on the TiO 2 surface, a new band appeared at around 1650 cm À1 , which indicates the formation of a pyridinium ion with Brønsted acid sites on TiO 2 surface. 21 In addition, the C]N stretching band at around 1590 cm À1 is shied by 3-5 cm À1 to higher wavenumber, that is, the resulting band can be assigned to the hydrogen-bonded pyridyl group to Brønsted acid sites on the TiO 2 surface. Consequently, these observations demonstrate that these porphyrin dimers are adsorbed on the TiO 2 surface through the formations of hydrogen bonding of pyridyl groups and/or pyridinium ion at Brønsted acid sites on the TiO 2 surface. Thus, the UV/vis absorption and the FTIR spectra of the porphyrin dimers adsorbed on TiO 2 lm indicate that high dye loading and high surface coverage of the TiO 2 electrode for DSSCs based on porphyrin dye sensitizers bearing pyridyl group are achieved by employing the cyclic porphyrin dimers bearing four pyridyl anchoring groups possessing the bonding ability to the two points on Brønsted acid sites on TiO 2 surface.
Photovoltaic performances of DSSCs based on cyclic free-base porphyrin dimers and their inclusion complexes with fullerene C 60 The DSSC was prepared using the dye-adsorbed TiO 2 electrode (9 mm), Pt-coated glass as a counter electrode, and an acetonitrile solution with iodine (0.05 M), lithium iodide (0.1 M), and 1,2-dimethyl-3-propylimidazolium iodide (0.6 M) as an electrolyte. The photocurrent-voltage (I-V) characteristics were measured under simulated solar light (AM 1.5, 100 mW cm À2 ). The incident photon-to-current conversion efficiency (IPCE) spectra and the I-V curves are shown in Fig. 5. The photovoltaic performance parameters are collected in Table 2. The IPCE values corresponding to the Soret band (29% at 424 nm) and the Q band (6% at 520 nm) for the phenothiazine-bridged cyclic   porphyrin dimer H 4 -Ptz-CPD Py (TEO) are higher than those (15% at 420 nm for the Soret band and 2% at 522 nm for the Q band) of the butadiyne-linked cyclic porphyrin dimer H 4 -C 4 -CPD Py -(TEO) (Fig. 5a). The I-V curves show that the short-circuit photocurrent density (J sc ) and h values of H 4 -Ptz-CPD Py (TEO) (1.40 mA cm À2 and 0.35%) are higher than those of H 4 -C 4 -CPD Py (TEO) (0.79 mA cm À2 and 0.19%) (Fig. 5b). The higher photovoltaic performance for DSSC based on H 4 -Ptz-CPD Py -(TEO) may be attributed to the efficient electron injection from the photoexcited dye ( 1 H 4 -Ptz-CPD Py (TEO)*) to the CB of TiO 2 electrode because the phenothiazine unit possessing electron donating ability can provide a unidirectional ow of electrons toward the pyridyl anchoring group upon photoexcitation of the porphyrin. Interestingly, the photovoltaic performances of DSSC based on the C 60 inclusion complexes C 60 3H 4 -C 4 -CPD Py (TEO) and C 60 3H 4 -Ptz-CPD Py (TEO) are lower than those of H 4 -C 4 -CPD Py (TEO) and H 4 -Ptz-CPD Py (TEO). The electron injection (z100 ps) from the photoexcited porphyrin to the CB of TiO 2 electrode is in kinetically competition with the intrasupramolecular electron transfer (18-20 ps), that is, the formation of charge-separated state C 60 c À -H 4 -C 4 -CPD Py (TEO)c + and C 60 c À -H 4 -Ptz-CPD Py (TEO)c + (Fig. 3). Therefore, the lower photovoltaic performances for DSSC based on the C 60 inclusion complexes would be attributed to the formation of charge-separated state, leading to low electron-injection efficiency from the photoexcited porphyrin to CB of TiO 2 electrode. On the other hand, the photovoltaic performance of DSSC based on C 60 3H 4 -Ptz-CPD Py (TEO) is slightly lower than that of C 60 3H 4 -C 4 -CPD Py (TEO), which may be attributed to the thermodynamically difficult electron-injection due to the lower triplet charge separated state 3 (C 60 c À -H 4 -Ptzc + -CPD Py (TEO)). Moreover, it is worth mentioning here that the open-circuit photovoltage (V oc ) values of the C 60 inclusion complexes C 60 3H 4 -C 4 -CPD Py (TEO) (315 mV) and C 60 3H 4 -Ptz-CPD Py (TEO) (304 mV) are lower than those of H 4 -C 4 -CPD Py (TEO) (376 mV) and H 4 -Ptz-CPD Py (TEO) (396 mV). Thus, electrochemical impedance spectroscopy (EIS) analysis was performed to study the electron recombination process in DSSCs based on these porphyrin dimers in the dark under a forward bias of À0.60 V with a frequency range of 10 mHz to 100 kHz. The large semicircle in the Nyquist plot (Fig. 6a), which corresponds to the midfrequency peaks in the Bode phase plots, represents the charge recombination between the injected electrons in TiO 2 and I 3 À ions in the electrolyte, that is, the charge-transfer resistances at the TiO 2 /dye/electrolyte interface. The Nyquist plots show that the resistance values of the large semicircle for   ions, leading to faster charge recombination and thus resulting in a lower V oc value. 3,6

Conclusions
In this work, to seek a direction in molecular design toward creating a new class of porphyrin dye sensitizers bearing pyridyl anchoring groups for achieving high dye loading and high surface coverage of the TiO 2 electrode for dye-sensitized solar cells (DSSCs), cyclic free-base porphyrin dimers (H 4 -C 4 -CPD Py -(TEO) and H 4 -Ptz-CPD Py (TEO)) linked by butadiyne or phenothiazine bearing 4-pyridyl groups and their C 60 inclusion complexes (C 60 3H 4 -C 4 -CPD Py (TEO) and C 60 3H 4 -Ptz-CPD Py -(TEO)) have been applied to DSSCs as the dye sensitizers. The Soret bands of C 60 3H 4 -C 4 -CPD Py (TEO) and C 60 3H 4 -Ptz-CPD Py (TEO) were redshied with a decrease in intensity, whereas their Q bands were slightly redshied but increased in intensity, compared to H 4 -C 4 -CPD Py (TEO) and H 4 -Ptz-CPD Py -(TEO). These porphyrin dimers are adsorbed on the TiO 2 surface through the formations of hydrogen bonding of pyridyl groups and/or pyridinium ion at Brønsted acid sites on the TiO 2 surface. It was found that the adsorption amount of the porphyrin dimers adsorbed on TiO 2 lm is 2.0 Â 10 17 molecules per cm 2 , that is, the adsorption amount of porphyrin unit is 4.0 Â 10 17 cm À2 , which is higher than those (<2.0 Â 10 17 molecules per cm 2 ) of porphyrin dye sensitizers and D-p-A dye sensitizer bearing pyridyl groups reported so far. The higher photovoltaic performance for DSSC based on H 4 -Ptz-CPD Py -(TEO) relative to H 4 -C 4 -CPD Py (TEO) may be attributed to the efficient electron injection from the photoexcited dye ( 1 H 4 -Ptz-CPD Py (TEO)*) to the CB of TiO 2 because the phenothiazine unit possessing electron donating ability can provide a unidirectional ow of electrons toward the pyridyl anchoring group upon photoexcitation of the porphyrin. On the other hand, the photovoltaic performances of DSSC based on C 60 3H 4 -C 4 -CPD Py (TEO) and C 60 3H 4 -Ptz-CPD Py (TEO) are lower than those of H 4 -C 4 -CPD Py (TEO) and H 4 -Ptz-CPD Py (TEO). The transient absorption spectroscopy and electrochemical measurements revealed that the lower photovoltaic performances for DSSC based on the C 60 inclusion complexes would be attributed to the formation of charge-separated state between the photoexcited porphyrin and C 60 , leading to low electron-injection efficiency from the photoexcited porphyrin to TiO 2 electrode. Consequently, this work provides that the cyclic porphyrin dimers with four pyridyl anchoring groups possessing bonding ability to the two points on Brønsted acid sites on TiO 2 surface, which can achieve the high dye loading and the high surface coverage of the TiO 2 electrode, would be expected to be one of the most promising classes of porphyrin dye sensitizers with pyridyl group.

Preparation of DSSCs
The TiO 2 paste (JGC Catalysts and Chemicals Ltd., PST-18NR) was deposited on a uorine-doped-tin-oxide (FTO) substrate by doctor-blading, and sintered for 50 min at 450 C. The 9 mm thick TiO 2 electrode was immersed into 0.1 mM porphyrin dimer solution in chloroform for 15 hours enough to adsorb the dye sensitizers. The DSSCs were fabricated by using the TiO 2 electrode (0.5 Â 0.5 cm 2 in photoactive area) thus prepared, Pt-coated glass as a counter electrode, and a solution of 0.05 M iodine, 0.1 M lithium iodide, and 0.6 M 1,2-dimethyl-3propylimidazolium iodide in acetonitrile as electrolyte. The photocurrent-voltage characteristics were measured using a potentiostat under a simulated solar light (AM 1.5, 100 mW cm À2 ). IPCE spectra were measured under monochromatic irradiation with a tungsten-halogen lamp and a monochromator. The amount of adsorbed dye on TiO 2 nanoparticles was determined form the calibration curve by absorption spectral measurement of the concentration change of the porphyrin dye solution before and aer adsorption. Absorption spectra of the dyes adsorbed on TiO 2 nanoparticles were recorded on the porphyrin dyes-adsorbed TiO 2 lm (thickness of 3 mm) in the transmission mode with a calibrated integrating sphere system. Electrochemical impedance spectroscopy (EIS) for DSSCs in the dark under a forward bias of À0.60 V with a frequency range of 10 mHz to 100 kHz was measured with a AMETEK Versa STAT 3.