Effects of Bulky Substituents of PushâPull Porphyrins on Photovoltaic

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Effects of Bulky Substituents of Push−Pull Porphyrins on Photovoltaic Properties of Dye-Sensitized Solar Cells Tomohiro Higashino,† Kyosuke Kawamoto,† Kenichi Sugiura,† Yamato Fujimori,† Yukihiro Tsuji,† Kei Kurotobi,‡ Seigo Ito,§ and Hiroshi Imahori*,†,‡ †

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan § Department of Materials and Synchrotron Radiation Engineering, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan ‡

S Supporting Information *

ABSTRACT: To evaluate the effects of substituent bulkiness around a porphyrin core on the photovoltaic properties of porphyrin-sensitized solar cells, long alkoxy groups were introduced at the meso-phenyl group (ZnPBAT-o-C8) and the anchoring group (ZnPBAT-o-C8Cn, n = 4, 8) of an asymmetrically substituted push−pull porphyrin with double electron-donating diarylamino groups and a single electron-withdrawing carboxyphenylethynyl anchoring group. The spectroscopic and electrochemical properties of ZnPBAT-o-C8 and ZnPBAT-o-C8Cn were found to be superior to those of a push−pull porphyrin reference (YD2-o-C8), demonstrating their excellent light-harvesting and redox properties for dye-sensitized solar cells. A power conversion efficiency (η) of the ZnPBAT-o-C8-sensitized solar cell (η = 9.1%) is higher than that of the YD2-o-C8sensitized solar cell (η = 8.6%) using iodine-based electrolyte due to the enhanced light-harvesting ability of ZnPBAT-o-C8. In contrast, the solar cells based on ZnPBAT-o-C8Cn, possessing the additional alkoxy chains in the anchoring group, revealed the lower η values of 7.3% (n = 4) and 7.0% (n = 8). Although ZnPBAT-o-C8Cn exhibited higher resistance at the TiO2−dye− electrolyte interface by virtue of the extra alkoxy chains, the reduced amount of the porphyrins on TiO2 by excessive addition of coadsorbent chenodeoxycholic acid (CDCA) for mitigating the aggregation on TiO2 resulted in the low η values. Meanwhile, the ZnPBAT-o-C8-sensitized solar cell showed the lower η value of 8.1% than the YD2-o-C8-sensitized solar cell (η = 9.8%) using cobalt-based electrolyte. The smaller η value of the ZnPBAT-o-C8-sensitized solar cell may be attributed to the insufficient blocking effect of the bulky substituents of ZnPBAT-o-C8 under the cobalt-based electrolyte conditions. Overall, the alkoxy chain length and substitution position around the porphyrin core are important factors to affect the cell performance. KEYWORDS: porphyrin, dye-sensitized solar cells, titanium oxide, redox shuttle, substituent bulkiness

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INTRODUCTION In view of the increasing demand for energy as well as serious concern for global warming due to greenhouse gases from fossil fuels, it is an urgent challenge to develop renewable energy resources. Solar energy has a highly technological potential for a sustainable energy supply.1 Conventional silicon-based solar cells have attained a power conversion efficiency (η) of 10− 20%, but their widespread use has been hampered on account of the high production cost. Dye-sensitized solar cells (DSSCs) have attracted considerable attention as a promising alternative to the silicon-based solar cells.2,3 Since the seminal work by Grätzel and co-workers,2 various ruthenium(II) bipyridyl complexes have been employed as the most effective TiO2 sensitizers.4−7 However, the high cost and scarcity of ruthenium are obstacles for practical applications. In this respect, organicbased dyes have been actively explored for highly efficient DSSCs.8−16 To achieve the goal, organic-based dyes should possess the following conditions: (i) high molar absorption coefficient in visible and near-infrared wavelengths, (ii) rapid electron transfer (ET) from the excited-state dye to a © XXXX American Chemical Society

conduction band (CB) of the TiO2 electrode, followed by ET from redox shuttle to the oxidized dye, and (iii) slow charge recombination (CR) between the injected electron and the oxidized dye as well as the electron and the oxidized redox shuttle in the electrolyte. So far, versatile organic-based dyes have been reported as potential sensitizers for DSSCs.17−27 Porphyrins are one of the most widely studied organic-based dyes for DSSCs owing to their extremely intense Soret band at 400−450 nm and intense Q bands at 550−600 nm.11−16 Porphyrins have several advantages in their optical, redox, and photophysical properties that can be fine-tuned by chemical modifications and metal insertion. Nevertheless, the DSSCs based on typical porphyrin sensitizers displayed lower photovoltaic properties than those with ruthenium(II) bipyridyl complexes because of insufficient light-collecting ability of the typical porphyrins at around 500 nm and beyond 600 nm. The Received: March 30, 2016 Accepted: May 26, 2016

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DOI: 10.1021/acsami.6b03806 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces expansion of a porphyrin π-conjugated system and/or the introduction of a push−pull structure are fascinating means to surmount this drawback and achieve a high η value of more than 10%.28−61 As a representative example, a push−pull porphyrin YD2 designed by Yeh et al. presented a high η value of 11% (Figure 1).50 In addition, the steric bulkiness of

electron-donating diarylamino groups, a single electron-withdrawing carboxyphenylethynyl anchoring group, and a single bulky 2,6-bis(alkoxy)phenyl group at the four meso-positions (Figure 1). To further increase the substituent bulkiness around the porphyrin core of ZnPBAT-o-C8, we also introduced alkoxy groups with a different chain length at 2,6-positions of the anchoring carboxyphenyl moiety (ZnPBAT-o-C8Cn: n = 4, 8). We examined the effect of the alkoxy substituents and substitution positions on the photovoltaic performances of ZnPBAT-o-C8- and ZnPBAT-o-C8Cn-based DSSCs. We also used YD2-o-C8 with one electron-donating diarylamino group, one electron-withdrawing carboxyphenylethynyl anchoring group, and two bulky 2,6-bis(alkoxy)phenyl groups at the four meso-positions as a reference to address the substituent effect.

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EXPERIMENTAL SECTION

Materials and General Procedures. Tetrahydrofuran and triethylamine were distilled from CaH2. Other solvents and chemicals were of reagent grade quality, purchased commercially, and used without further purification unless otherwise noted. Column chromatography, thin-layer chromatography (TLC), and size exclusion gel permeation chromatography (GPC) were conducted following the previously reported method.48 1H and 13C NMR spectra, highresolution mass spectra (HRMS), attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra, and electrical impedance spectra were obtained following the previously reported method.48 UV−vis absorption spectra of solutions and films were measured with a PerkinElmer Lambda 900 UV/vis/NIR spectrometer. Steady-state fluorescence spectra and fluorescence lifetimes (τf) were obtained by a HORIBA SPEX Fluoromax-3 or HORIBA Nanolog spectrofluorometer. Electrochemical measurements were made using an ALS 630a electrochemical analyzer.48 The X-ray photoelectron spectroscopy (XPS) data were acquired following the previously reported method.48 Density Functional Theory (DFT) Calculations. All calculations were carried out using the Gaussian 09 program.62 The structures of the porphyrins were optimized by using DFT with the restricted B3LYP (Becke’s three-parameter hybrid exchange functionals and the Lee−Yang−Parr correlation functional) level, employing a 6-31G(d) basis set. We confirmed that the optimized geometries were in stable points. Preparation of Dye-Sensitized TiO2 Cells. Preparation of TiO2 electrodes and sealed cells for photovoltaic measurements was conducted following the preceding literatures.47,63 Nanocrystal-like TiO2 particles (d = 20 nm, CCIC:PST23NR, JGC-CCIC for iodinebased electrolyte; d = 50 nm, CCIC:PST30NRD, JGC-CCIC for cobalt-based electrolyte) were employed as the bottom part of the photoanode, whereas submicrocrystal-like TiO2 particles (d = 400 nm, CCIC:PST400C, JGC-CCIC) were employed as the upper one. A TiO2 layer of 12 μm (d = 20 nm) for an iodine-based electrolyte or of 2 μm (d = 50 nm) for a cobalt-based electrolyte was coated onto an FTO glass by screen printing. Then, a TiO2 layer of 4 μm (d = 400 nm) was further formed on the top. These thicknesses were modulated by using YD2 for the iodine-based DSSC or YD2-o-C8 for the cobaltbased DSSC. The TiO2 electrode was soaked into an ethanol solution of 0.20 mM porphyrin at 25 °C for the iodine-based electrolyte or an ethanol/THF (v/v = 4/1) solution of 0.30 mM porphyrin at 35 °C for the cobalt-based electrolyte. The TiO2 electrode stained with porphyrin is denoted as TiO2/porphyrin. The porphyrin surface coverage on the TiO2 films (mol cm−2) was estimated by measuring the absorbance of the porphyrins that were detached from the porphyrin TiO2 film into THF/H2O (v/v = 1/1) solution containing 0.1 M NaOH. The sealed cell was assembled by using the dye-adsorbed TiO2 photoanode and a counter Pt electrode, which were contacted closely with a hot-melt ionomer film of Surlyn polymer gasket (DuPont, 50 μm for the iodine-based electrolyte and 25 μm for the cobalt-based electrolyte). The iodine-based electrolyte solution consisted of 1.0 M

Figure 1. Molecular structures of porphyrin sensitizers.

peripheral substituents is also a crucial factor in photovoltaic performances. Alkoxy-wrapped porphyrin sensitizers demonstrated higher photovoltaic performances than nonwrapped counterparts because the higher blocking effect of the long alkoxy chains effectively suppresses dye aggregation and inhibits CR with the redox shuttle in the electrolyte.51,53 Recently, Biroli and co-workers also reported the systematic work on the length and position on the meso-phenyl group of alkoxy chain.60,61 However, the effects of substituent bulkiness around the porphyrin core have not been fully addressed systematically. Recently, we found that ZnPBAT with double electrondonating diarylamino groups, a single electron-withdrawing carboxyphenylethynyl anchoring group, and a moderately bulky mesityl group at the four meso-positions exhibited an η value of 10.1%, which was higher than that of YD2 (η = 9.1%) under our optimized conditions (Figure 1).47 On the basis of the molecular structure of ZnPBAT, we expected that replacement of the mesityl group with a more sterically bulky group would provide the higher blocking effect which is more effective to inhibit the dye aggregation on TiO2 as well as suppress the CR between the injected electron in the CB of TiO2 and the oxidized redox shuttle in the electrolyte, leading to a likely increase in the η value. In this study, we designed a new asymmetrical push−pull porphyrin ZnPBAT-o-C8 with double B


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ACS Applied Materials & Interfaces Scheme 1. Synthesis of ZnPBAT-o-C8a

a

TIPS = triisopropylsilyl, dba = dibenzylideneacetone.

Scheme 2. Synthesis of ZnPBAT-o-C8Cna

a

NIS = N-iodosuccinimide.

1,3-dimethylimidazolium iodide, 0.03 M I2, 0.05 M LiI, 0.1 M guanidinium thiocyanate, and 0.50 M 4-tert-butylpyridine in an 85:15 acetonitrile/valeronitrile mixture, whereas the cobalt-based electrolyte solution comprised 0.25 M [Co(bpy)3]((CF3SO2)2N)2, 0.05 M [Co(bpy)3]((CF3SO2)2N)3, 0.1 M LiTFSI, and 0.5 M 4-tertbutylpyridine in acetonitrile.

lamino, one carboxylphenylethynyl, and one 2,6-bis(octyloxy)phenyl groups). First, 2,6-bis(octyloxy)phenyl-substituted zinc porphyrin 1 was prepared by using the corresponding dipyrromethanes.47,64 After demetalation by trifluoroacetic acid (TFA), free base analogue 2 was brominated by NBS. The cis-intermediate 3 was obtained by purification from a mixture of the corresponding cis- and trans-intermediates, followed by treatment with zinc acetate. Sonogashira coupling of 3 with triisopropylsilylacetylene afforded 4. The iodine(III)mediated substitution reaction of 4 with N,N-bis(4-methylphenyl)amine provided porphyrin 5 with two diarylamino

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RESULTS AND DISCUSSION Synthesis and Characterization. Scheme 1 illustrates the synthetic route of ZnPBAT-o-C8. The important synthetic challenge is how to introduce the three different substituents into the four meso-positions systematically (i.e., two diaryC


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ACS Applied Materials & Interfaces

because of the asymmetric introduction of the two diarylamino moieties, as seen for the analogous comparison of ZnPBAT and YD2.47 On the other hand, the Soret bands of ZnPBAT-oC8Cn are different from that of ZnPBAT-o-C8. This implies that the electronic and/or geometric structures of ZnPBAT-oC8Cn are altered by introduction of the additional alkoxy groups on the carboxyphenyl moiety (vide infra). Overall, ZnPBAT-o-C8 and ZnPBAT-o-C8Cn possess the excellent light-collecting ability in the visible wavelength. The steadystate fluorescence spectra of the ZnPs were measured in ethanol (Figure S1), and the fluorescence maxima are listed in Table 1. In accordance with the trend of the absorption properties, the emission maximum is shifted toward longer wavelength in the order YD2-o-C8 < ZnPBAT-o-C8 ≈ ZnPBAT47 ≈ ZnPBAT-o-C8Cn. The τf values of the ZnPs were also measured by a time-correlated single-photon counting (TCSPC) technique. The decay curves of the fluorescence intensity in ethanol were fitted as single exponentials to give τf = 2.2 ns for ZnPBAT-o-C8, τf = 2.6 ns for ZnPBAT-o-C8C4, τf = 2.5 ns for ZnPBAT-o-C8C8, and τf = 1.8 ns for YD2-o-C8. These similar fluorescence features indicate that the additional alkoxy moiety has little impact on the excited state of the porphyrin sensitizers. From the intersection of the normalized absorption and fluorescence spectra, the zero-zero excitation energies (E0‑0) are determined to be 1.84 eV for ZnPBAT-o-C8, 1.83 eV for ZnPBAT-oC8C4, 1.83 eV for ZnPBAT-o-C8C8, 1.90 eV for YD2-o-C8, and 1.83 eV for ZnPBAT47 (Table 1). The first oxidation and reduction potentials (Eox and Ered) of the ZnPs were measured by using DPV in THF containing 0.1 M Bu4NPF6 as a supporting electrolyte (Table 1 and Figure S2). The Eox values of ZnPBAT-o-C8 (0.84 V vs NHE), YD2o-C8 (0.83 V vs NHE), and ZnPBAT (0.85 V vs NHE)47 are almost the same, whereas the Eox values of ZnPBAT-o-C8C4 (0.77 V vs NHE) and ZnPBAT-o-C8C8 (0.78 V vs NHE) are slightly smaller than that of ZnPBAT-o-C8, reflecting the electron-donating ability of the additional alkoxy moieties. In

moieties. Desilylation of 5 with TBAF and subsequent Sonogashira coupling with 4-iodobenzoic acid yielded ZnPBAT-o-C8. We attempted to replace the octyloxyl chains with the longer ones in ZnPBAT-o-C8, but it turned out to be a failure because of the more severe steric congestion around the porphyrin core. We also prepared bulkier porphyrin sensitizers with an alkoxy-substituted carboxyphenylethynyl anchoring group ZnPBAT-o-C8Cn (n = 4, 8) (Scheme 2). The alkoxy-substituted iodobenzoic acids 7 and 8 were prepared by iodination of 3,5-dihydroxybenzoic acid, followed by nucleophilic substitution reactions of 6. Then, deprotection of 5 and subsequent Sonogashira coupling with the corresponding aryl iodides 7 and 8 afforded ZnPBAT-o-C8C4 and ZnPBAT-oC8C8, respectively. YD2-o-C8 was prepared as a comparative compound by following the preceding literature.52 Optical and Redox Properties. The UV−visible absorption spectra of the zinc porphyrins (ZnPs) in ethanol are shown in Figure 2. Table 1 summarizes the peak positions and molar

Figure 2. UV/vis absorption spectra of ZnPBAT-o-C8 (green), ZnPBAT-o-C8C4 (blue), ZnPBAT-o-C8C8 (red), and YD2-o-C8 (black) in ethanol.

absorption coefficients (ε) of Soret and Q bands. The absorptions of ZnPBAT-o-C8 and ZnPBAT-o-C8Cn are broadened and red-shifted compared to that of YD2-o-C8

Table 1. Optical and Electrochemical Properties of Porphyrin Sensitizers λabsa/nm (ε/103 M−1 cm−1) ZnPBAT-o-C8

ZnPBAT-o-C8C4

ZnPBAT-o-C8C8

YD2-o-C8

ZnPBATg

435 466 598 657 466 602 662 460 603 663 446 584 643 433 460 596 661

(77.1) (79.1) (7.63) (18.7) (94.6) (7.00) (18.1) (89.9) (6.05) (17.3) (197) (11.8) (28.8) (75.0) (61.5) (7.03) (17.1)

λemb/nm

Eoxc/V

Eredc/V

E0‑0/V

Eox*d/V

ΔGinje/eV

ΔGregf/eV

690

0.84

−1.21

1.84

−1.00

−0.50

−0.44

695

0.77

−1.20

1.83

−1.06

−0.56

−0.37

693

0.78

−1.20

1.83

−1.05

−0.55

−0.38

663

0.83

−1.35

1.90

−1.07

−0.57

−0.43

697

0.85

−1.15

1.83

−0.98

−0.48

−0.45

a Wavelengths for Soret and Q bands maxima in ethanol. bWavelengths for emission maxima in ethanol by exciting at Soret wavelength. cDetermined by DPV (vs NHE). dDetermined by adding the E0‑0 value to the Eox one (vs NHE). eDriving forces for electron injection from the porphyrin singlet excited state (Eox*) to the CB of TiO2 (−0.5 V vs NHE). fDriving forces for the regeneration of porphyrin sensitizers by I−/I3− redox couple (+0.4 V vs NHE). gValues are taken from ref 47.

D


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ACS Applied Materials & Interfaces Table 2. Molecular Orbital Energy Levels of the ZnPsa LUMO+1 LUMO HOMO HOMO−1

ZnPBAT-o-C8

ZnPBAT-o-C8C4

ZnPBAT-o-C8C8

YD2-o-C8

ZnPBATc

−2.18 −2.46 (−3.23)b −4.65 (−5.28)b −4.97

−2.09 −2.35 (−3.24)b −4.57 (−5.21)b −4.90

−2.08 −2.33 (−3.24)b −4.56 (−5.22)b −4.90

−1.96 −2.30 (−3.09)b −4.62 (−5.27)b −5.00

−2.37 −2.64 (−3.29)b −4.79 (−5.29)b −5.09

a

Selected molecular orbital energy levels of porphyrins were estimated by DFT calculations with B3LYP/6-31G(d). The energies in eV are quoted with respect to the vacuum (1 hartree = 27.2116 eV). bThe energy levels of HOMO and LUMO were determined electrochemically, given that the absolute energy level of NHE is −4.44 eV under vacuum. cValues are taken from ref 47.

Figure 3. Selected molecular orbital diagrams for (a) ZnPBAT-o-C8, (b) ZnPBAT-o-C8C4, (c) ZnPBAT-o-C8C8, and (d) YD2-o-C8 obtained by DFT calculations with B3LYP/6-31G(d). To simplify the calculations, alkyl chains on the diarylamino groups are replaced with methyl groups.

contrast, the Ered values of ZnPBAT-o-C8 (−1.21 V vs NHE), ZnPBAT-o-C8C4 (−1.20 V vs NHE), ZnPBAT-o-C8C8 (−1.20 V vs NHE), and ZnPBAT (−1.15 V vs NHE)47 are roughly identical, but considerably shifted positively in comparison with that of YD2-o-C8 (−1.35 V vs NHE) (Table 1). These results reveal that the electron-donating character of the two diarylamino moieties at the meso-positions leads to the smaller HOMO−LUMO gap in ZnPBAT-o-C8 and ZnPBAT-o-C8Cn, achieving the excellent light-collecting ability. In fact, the trend of the electrochemical HOMO− LUMO gaps matches with that of the optical HOMO−LUMO gaps. On the basis of the spectroscopic and electrochemical measurements, driving forces for ET (ΔGinj) from the ZnP excited singlet state (1ZnP*) to the CB of TiO2 (−0.5 V vs NHE)47 and for reduction of the ZnP radical cation (ZnP+•) by the I−/I3− redox couple (+0.4 V vs NHE)47 (ΔGreg) are evaluated (Table 1). The ET processes are exothermic, and the driving forces satisfy the requirement for efficient ET. To get insight into the equilibrium geometry and electronic structures of the ZnPs, we performed DFT calculation at the level of B3LYP/6-31G(d).62 For all the ZnPs, the diarylamino and 2,6-bis(octyloxy)phenyl moieties at the meso-positions are almost perpendicular to the porphyrin plane because of the

steric hindrance (Figure S3). It is noteworthy that the carboxyphenyl moieties of ZnPBAT-o-C8Cn are twisted by ca. 15° relative to the porphyrin plane due to the steric hindrance of the additional alkoxy moieties, whereas those of ZnPBAT-o-C8 and YD2-o-C8 are almost planar. The absorption spectra of ZnPBAT-o-C8Cn probably reflect these structural features, which is consistent with the time-dependent DFT (TD-DFT) calculations based on these optimized geometries (Figure S4). The broad Soret bands of ZnPBATo-C8Cn consist of two transitions with large and small oscillator strengths. Contrary to this, the split Soret band of ZnPBAT-o-C8 is attributed to the corresponding two transitions with comparable oscillator strength. In fact, the TD-DFT calculations of ZnPBAT-o-C8 provided the different excitation energies that depend on the dihedral angles between the porphyrin and benzoic acid moieties (Figure S5). All the studied porphyrins largely possess two degenerate LUMOs (LUMO+1, LUMO) and two degenerate HOMOs (HOMO, HOMO−1) (Table 2), which is characteristic of typical porphyrins owing to their highly symmetric structure (D4h).47 The HOMO−LUMO gaps of the ZnPs are as follows: ZnPBAT (2.15 eV) 47 < ZnPBAT-o-C8 (2.19 eV) < ZnPBAT-o-C8C4 (2.22 eV) ≈ ZnPBAT-o-C8C8 (2.23 eV) E


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carboxylic acid around 1700 cm−1 (Figure S6). On the other hand, the FT-IR spectra of TiO2/ZnPBAT-o-C8 and TiO2/ ZnPBAT-o-C8Cn exhibit a disappearance of the ν(CO) band around 1700 cm−1 as well as a considerable increase in the symmetric carboxylate absorption, ν(COO−sym), around 1400 cm−1. These results support that the carboxylate binds to TiO2 with a symmetric bidentate mode.65 To obtain further insight into the adsorption state, we performed X-ray photoelectron spectroscopy (XPS) measurements for TiO2/ZnPBAT-o-C8 and TiO2/ZnPBAT-o-C8Cn (Figure S7). The O 1s photoelectron spectrum of TiO2/ZnPBAT-o-C8 exhibits three distinct oxygen peaks. The peak arising from the oxygen from the TiO2 surface is observable at 530.4 eV. The remaining peaks at 531.7 and 532.9 eV are assigned to two oxygen atoms of the carboxylate attached to the TiO2 surface and two oxygen atoms of the alkoxy moieties, respectively. Similar results were obtained for TiO2/ZnPBAT-o-C8Cn. These results are in good agreement with those obtained by the FT-IR spectra. Therefore, the alkoxy chains around the porphyrin core have no impact on the adsorption mode on TiO2. Photovoltaic Properties of DSSCs with Iodine-Based Electrolyte. To maximize the photovoltaic properties, we investigated the effects of soak time in ethanol as the soak solvent on the DSSC performance of the TiO2/ZnPBAT-o-C8, TiO2/ZnPBAT-o-C8C4, TiO2/ZnPBAT-o-C8C8, and TiO2/ YD2-o-C8 electrodes using iodine-based electrolyte (Figure 5).

< YD2-o-C8 (2.32 eV). This trend is in good agreement with those of the optical and electrochemical gaps for HOMO− LUMO. In the HOMO of all the ZnPs, electron density distributions are similar and mainly localized on the diarylamino moieties (Figure 3). Although the carboxyphenyl groups of ZnPBAT-o-C8Cn are twisted relative to the porphyrin plane, electron density distributions in the LUMOs of the ZnPs are also similar. These molecular orbital diagrams suggest high electron injection efficiencies (ϕinj) of ZnPBAT-o-C8 and ZnPBAT-o-C8Cn from the 1ZnP* to the CB of TiO2, as previously reported for YD2-o-C8 and ZnPBAT (vide infra).47 Adsorption Behavior. A TiO2 electrode was soaked into an ethanol solution of the porphyrin to give a porphyrinadsorbed TiO2 electrode for DSSCs. The porphyrin surface coverage (Γ) on the TiO2 electrode was estimated following the previously reported method.48 During the soak, ZnPBATo-C8 and YD2-o-C8 reach the saturated surface coverage on the TiO2 films in 4 h, while ZnPBAT-o-C8Cn reach the saturated surface coverage in 8 h (Figure 4). The slower

Figure 4. Soak time profile of the porphyrin surface coverage (Γ) for ZnPBAT-o-C8 (green), ZnPBAT-o-C8C4 (blue), ZnPBAT-o-C8C8 (red), and YD2-o-C8 (black) that adsorb on the TiO2 films without the scattering layer.

adsorption profiles of ZnPBT-o-C8Cn can be ascribed to the steric hindrance and aggregation behavior caused by the additional alkoxy chains on the carboxyphenyl group (vide supra). The saturated Γ values of the ZnPs were determined as follows: Γ = 8.8 × 10−11 mol cm−2 for ZnPBAT-o-C8, Γ = 8.7 × 10−11 mol cm−2 for ZnPBAT-o-C8C4, and Γ = 8.7 × 10−11 mol cm−2 for ZnPBAT-o-C8C8, and Γ = 7.9 × 10−11 mol cm−2 for YD2-o-C8. The higher Γ value of ZnPBAT-o-C8 than that of YD2-o-C8 is consistent with the fewer same alkoxy chains of ZnPBAT-o-C8. On the other hand, the Γ values of ZnPBAT-oC8Cn and ZnPBAT-o-C8 are virtually the same, which is slightly smaller than that of ZnPBAT (Γ = 9.2 × 10−11 mol cm−2)47 due to replacement of the mesityl group with the bulkier 2,6-bis(alkoxy)phenyl group.47 Supposing that the porphyrin molecules are packed perpendicularly on a TiO2 electrode, the saturated Γ values are also calculated to be Γ = 11.1 × 10−11 mol cm−2 for ZnPBAT-o-C8, Γ = 10.5 × 10−11 mol cm−2 for ZnPBAT-o-C8C4, and Γ = 9.7 × 10−11 mol cm−2 for ZnPBAT-o-C8C8, and Γ = 8.8 × 10−11 mol cm−2 for YD2o-C8. While the slightly different adsorption behavior was observed, the similar calculated and experimental Γ values indicate that all the porphyrins yield well-packed porphyrin monolayers on TiO2. The FT-IR spectra of the ZnPBAT-o-C8 and ZnPBAT-oC8Cn solids display the distinctive band of ν(CO) of the

Figure 5. Plots of the η value as a function of soak time for DSSCs with TiO2/ZnPBAT-o-C8 (green), TiO2/ZnPBAT-o-C8C4 (blue), TiO2/ZnPBAT-o-C8C8 (red), and TiO2/YD2-o-C8 (black). The porphyrins were adsorbed on the TiO2 electrodes by soaking them into an ethanol solution of the ZnPs (0.2 mM) at 25 °C without coadsorbent.

The η value is expressed as the following equation: η = JSC × VOC × ff, where JSC is the short-circuit current, VOC is opencircuit voltage, and f f is the fill factor. The η value is increased with increasing the soak time to reach a top value in 6 h for YD2-o-C8 (η = 7.3%), 8 h for ZnPBAT-o-C8 (η = 7.6%), 5 h for ZnPBAT-o-C8C4 (η = 3.4%), and 5 h for ZnPBAT-oC8C8 (η = 3.2%) and then decreased to level off. The η values of DSSCs with ZnPBAT-o-C8C4 and ZnPBAT-o-C8C8 are much lower than those with YD2-o-C8 and ZnPBAT-o-C8. Since no coadsorbent chenodeoxycholic acid (CDCA) is used at this stage, aggregation behavior of ZnPBAT-o-C8C4 and ZnPBAT-o-C8C8 is much larger than that of YD2-o-C8 and ZnPBAT-o-C8. Then, we sensitized each porphyrin with various amounts of CDCA to suppress the porphyrin aggregation on TiO2 (Figure 6). The optimal η values are attained at different equivalents of CDCA (ZnPBAT-o-C8: η = 7.6% with 0 equiv of CDCA; ZnPBAT-o-C8C4: η = 6.7% with F


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Figure 6. Plots of the η value as a function of coadsorbent CDCA concentration in the soak solvent for DSSCs with TiO2/ZnPBAT-oC8 (green), TiO2/ZnPBAT-o-C8C4 (blue), TiO2/ZnPBAT-o-C8C8 (red), and TiO2/YD2-o-C8 (black). Note that maximal η values were obtained at different CDCA concentrations for the ZnPs (ZnPBAT-oC8: η = 7.6% with 0 equiv of CDCA; ZnPBAT-o-C8C4: η = 6.7% with 25 equiv of CDCA; ZnPBAT-o-C8C8: η = 6.5% with 15 equiv of CDCA; YD2-o-C8: η = 7.8% with 5 equiv of CDCA).

Figure 7. Photocurrent−voltage characteristics of the TiO2/ZnPBATo-C8 (green), TiO2/ZnPBAT-o-C8C4 (blue), TiO2/ZnPBAT-oC8C8 (red), and TiO2/YD2-o-C8 (black) cells under the maximal η conditions using iodine-based electrolyte.

the η value of the TiO2/ZnPBAT-o-C8 cell is lower than that of the TiO2/ZnPBAT cell (η = 10.1%). The reason is not clear at this stage, but the degree in the steric hindrance at the mesophenyl group would considerably change the porphyrin geometry on TiO2, affecting the ET kinetics and resultant photovoltaic performance.67,68 As seen during the optimization process, the η values of the TiO2/ZnPBAT-o-C8Cn cells are lower than those of the TiO2/ZnPBAT-o-C8 and TiO2/YD2o-C8 cells. These lower photovoltaic performances result from the strong aggregation tendency induced by the introduction of the alkoxy chains on the carboxyphenyl anchoring group (vide infra). The VOC value of the TiO2/ZnPBAT-o-C8C8 cell (0.706 V) is higher than that of TiO2/ZnPBAT-o-C8C4 (0.673 V). An analogous trend is seen for the VOC values of the TiO2/ ZnPBAT-o-C8 (0.737 V) and the TiO2/YD2-o-C8 (0.777 V) cells. These trends should be largely associated with the degree of the blocking effect provided by the number of the additional alkoxy groups as well as their chain length. In addition, the VOC values of the TiO2/ZnPBAT-o-C8Cn cells are lower than that of the TiO2/ZnPBAT-o-C8 cell. The twisted structures of ZnPBAT-o-C8Cn could affect the adsorption geometry on TiO2 and CR between the TiO2 and oxidized porphyrins, lowering the VOC values. The photocurrent action spectra (Figure 8a) parallel the absorption spectra of the corresponding porphyrins on TiO2 (Figure 8b). Indeed, the higher IPCE values of the TiO2/ ZnPBAT-o-C8 and TiO2/ZnPBAT-o-C8Cn cells than the TiO2/YD2-o-C8 cell at 680−800 nm match with the difference in the light-collecting property of the porphyrins (Figure 2). This also rationalizes at least partially the higher JSC and η values of the TiO2/ZnPBAT-o-C8 cell than the TiO2/YD2-oC8 cell. Although the IPCE spectra of the TiO2/ZnPBAT-oC8Cn cells also display photocurrent generation at up to 800 nm, the IPCE values in the range of 500−650 nm are lower than that of the TiO2/ZnPBAT-o-C8 cell. The IPCE is expressed by the following equation: IPCE = LHE × ϕinj × ηcol, where LHE is light-harvesting efficiency and ηcol is charge collection efficiency.47 The aggregation tendency of ZnPBATo-C8Cn on TiO2 may lower the ϕinj value because of selfquenching of the excited state. Nevertheless, the aggregation effect on the photovoltaic properties could be suppressed remarkably after sensitizing the TiO2 electrode in the porphyrin solution with the high ratio of CDCA. In fact, the maximum IPCE values of the TiO2/ZnPBAT-o-C8 and TiO2/ZnPBATo-C8Cn cells are rather comparable (Figure 8a), implying that the ϕinj and ηcol values are also comparable under the optimized

25 equiv of CDCA; ZnPBAT-o-C8C8: η = 6.5% with 15 equiv of CDCA; YD2-o-C8: η = 7.8% with 5 equiv of CDCA), depending on the difference in adsorption and aggregation behaviors of the ZnPs. The η values of the TiO2/ZnPBAT-oC8 and TiO2/YD2-o-C8 cells display little dependence on the CDCA concentration, which is in stark contrast with the moderate η-value dependence of the TiO2/ZnPBAT cell on the CDCA concentration (η = 8.6% with 10 equiv of CDCA).47 These support that the octyloxy moieties at the meso-phenyl group in ZnPBAT-o-C8 inhibit the dye aggregation. Counterintuitively, the η values of the TiO2/ZnPBAT-o-C8C4 and TiO2/ZnPBAT-o-C8C8 cells reveal intense dependence on the CDCA concentration irrespective of the presence of the octyloxy moieties at the meso-phenyl group as well as the anchoring group. There is a possibility that the additional alkoxy moieties in the anchoring group of ZnPBAT-o-C8Cn induce intermolecular alkoxy−alkoxy interactions in the solution as well as on the TiO2 surface. Accordingly, the intermolecular interactions between the twisted anchoring moieties with respect to the porphyrin planes in ZnPBAT-oC8Cn together with the slow adsorption process on the TiO2 surface may result in the unexpected high aggregation tendency on TiO2 (vide infra). Finally, the DSSCs were stored under dark conditions to enhance the photovoltaic properties, as we have already established (Figure S8).47,66 The highest η values are attainable after several days pass. The photocurrent−voltage curves of the DSSCs with TiO2/ZnPBAT-o-C8, TiO2/ ZnPBAT-o-C8Cn, and TiO2/YD2-o-C8 under the respective maximal η conditions are shown in Figure 7 (Table 3). The highest photovoltaic properties are increased in the order TiO2/ZnPBAT-o-C8C8 (JSC = 14.1 mA cm−2, VOC = 0.706 V, ff = 0.709, η = 7.0%) ≈ TiO2/ZnPBAT-o-C8C4 (JSC = 15.9 mA cm−2, VOC = 0.673 V, ff = 0.684, η = 7.3%) < TiO2/ YD2-o-C8 (JSC = 16.4 mA cm−2, VOC = 0.777 V, f f = 0.676, η = 8.6%) < TiO2/ZnPBAT-o-C8 (JSC = 18.6 mA cm−2, VOC = 0.737 V, f f = 0.665, η = 9.1%). It should be emphasized here that the η value of the TiO2/ZnPBAT-o-C8 cell is higher than that of the TiO2/YD2-o-C8 cell under our optimized conditions, supporting the advantage of the highly asymmetrical substitution in ZnPBAT-o-C8 where the two electron-donating diarylamino groups at the meso-positions are located with a cisconfiguration rather than a trans-configuration.47,49 However, G


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ACS Applied Materials & Interfaces Table 3. Photovoltaic Performance under the Optimized Conditions Using Iodine-Based Electrolytea JSC/mA cm−2 ZnPBAT-o-C8 ZnPBAT-o-C8C4 ZnPBAT-o-C8C8 YD2-o-C8 ZnPBATb

18.6 (19.0 ± 1.0) 15.9 (15.5 ± 0.9) 14.1 (14.1 ± 0.1) 16.4 (16.2 ± 0.2) 19.33 (18.61 ± 0.7)

VOC/V 0.737 (0.736 0.673 (0.689 0.706 (0.703 0.777 (0.779 0.719 (0.712

± 0.008) ± 0.016) ± 0.011) ± 0.003) ± 0.007)

η/%

ff 0.665 (0.649 0.684 (0.677 0.709 (0.704 0.676 (0.678 0.724 (0.734

9.1 (9.1 ± 7.3 (7.2 ± 7.0 (6.9 ± 8.6 (8.6 ± 10.1 (9.7 ±

± 0.03) ± 0.01) ± 0.01) ± 0.02) ± 0.01)

0.1) 0.3) 0.1) 0.1) 0.4)

Photovoltaic parameters exhibiting the highest η value. Parentheses denote average values from three independent cells. Values are taken from ref 47. a

b

Figure 8. (a) Photocurrent action spectra of the TiO2/ZnPBAT-o-C8 (green), TiO2/ZnPBAT-o-C8C4 (blue), TiO2/ZnPBAT-o-C8C8 (red), and TiO2/YD2-o-C8 (black) cells using iodine-based electrolyte. (b) Absorption spectra of the TiO2/ZnPBAT-o-C8 (green), TiO2/ZnPBAT-o-C8C4 (blue), TiO2/ZnPBAT-o-C8C8 (red), and TiO2/YD2-o-C8 (black) electrodes. The scattering TiO2 layers were not used for the TiO2 electrodes to obtain accurate absorption profile. The weak absorption at 800 nm can be assigned to ZnP+• on the TiO2.

for DSSCs with ZnPBAT-o-C8, ZnPBAT-o-C8Cn, and YD2o-C8 were conducted under standard AM 1.5 illumination and open-circuit conditions (Figure S10). The semicircle in the middle is associated with the ET process at the interface of TiO2−dye−electrolyte (Rp). A large Rp implies slow ET between the electrolyte and the TiO2, resulting in an increase in the VOC values. The differences in the Rp values for ZnPBAT-o-C8 (38.2 Ω) and YD2-o-C8 (40.2 Ω) as well as ZnPBAT-o-C8C4 (42.6 Ω) and ZnPBAT-o-C8C8 (54.5 Ω) agree well with those in the VOC values. The lower Rp values for ZnPBAT-o-C8 than YD2-o-C8 as well as ZnPBAT-o-C8C4 than ZnPBAT-o-C8C8 indicate the insufficient blocking effects on CR between the injected electron in the CB of TiO2 and the oxidized redox shuttle in the electrolyte solution. On the other hand, the Rp values for ZnPBAT-o-C8Cn are larger than those of ZnPBAT-o-C8 and YD2-o-C8, which contradicts with the difference in the VOC values. This suggests that the CR between the oxidized porphyrin and TiO2 rather than the CR between the oxidized redox shuttle and TiO 2 governs the V OC values.67,68 Nevertheless, the additional alkoxy chains at the anchoring group are likely to act as the blocking components of the porphyrin monolayer on TiO2 considering the large Rp values. The electron lifetimes (τe) were also estimated by the Bode plots of the DSSCs (Figure S11). The relationship between the τe value and the frequency of peak (f) in the Bode plot is represented as follows: τ ≈ 1/2πf.70 In this respect, the τe values for ZnPBAT-o-C8 (4.4 ms) and YD2-o-C8 (6.4 ms) are also consistent with the difference in the VOC values. Similarly, the τe value for ZnPBAT-o-C8C4 (6.4 ms) is shorter

wavelength regions. Without the light-scattering TiO2 layers, the absorptions of the TiO2/ZnPBAT-o-C8Cn electrodes at 500−650 nm and 700−750 nm are lower than that of the TiO2/ZnPBAT-o-C8 electrode (Figure 8b). Since, in the real cells, the light-scattering TiO2 layers are applied to the TiO2 electrodes, all the incident photons are captured by the porphyrins adsorbed on TiO2. However, for ZnPBAT-o-C8Cn, the amount of the porphyrins adsorbed on TiO2 is lower than that of ZnPBAT-o-C8 under the optimized conditions because of the large ratio of CDCA against the porphyrin on TiO2. In the case of ZnPBAT-o-C8Cn, the incident photons at the region where the molar absorption coefficients are low (i.e., 500−650 nm and 700−750 nm) tend to be absorbed by the porphyrins on the TiO2 film further away from the bottom of the FTO electrode. Accordingly, the injected electron in the CB of TiO2 would travel a longer distance through the network of the TiO2 nanoparticles to reach the FTO, thereby leading to an increase in significant CR and a decrease in ηcol in the specific wavelength regions. Consequently, the reduced amount of the porphyrins on TiO2 by the large addition of CDCA would lead to the low ηcol and the corresponding low IPCE values. The integrated IPCE values increase in the order TiO2/ZnPBAT-oC8C8 (13.9 mA cm−2) < TiO2/ZnPBAT-o-C8C4 (16.1 mA cm−2) < TiO2/YD2-o-C8 (16.7 mA cm−2) < TiO2/ZnPBAT-oC8 (18.4 mA cm−2), which are comparable with the JSC values (Table 3). We also employed electrical impedance spectroscopy (EIS) under the optimized cell conditions to understand the effect of alkoxy chains on the CR processes.47,69 The EIS Nyquist plots H


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is lower than that of the TiO2/YD2-o-C8 (JSC = 14.8 mA cm−2, VOC = 0.889 V, f f = 0.745, η = 9.8%). The photocurrent action spectra (Figure 10a) parallel the absorption spectra of the corresponding porphyrins on TiO2 (Figure 10b). The IPCE values of the TiO2/ZnPBAT-o-C8 cell at 680−800 nm are higher than those of the TiO2/YD2-o-C8 cell, whereas the IPCE values of the TiO2/ZnPBAT-o-C8 cell at 380−680 nm is lower than those of the TiO2/YD2-o-C8 cell. Overall, the net effect explains the higher JSC value of the TiO2/YD2-o-C8 cell than the TiO2/ZnPBAT-o-C8 cell. The EIS Nyquist plots for DSSCs with ZnPBAT-o-C8 and YD2-o-C8 were also performed under standard AM 1.5 illumination and open-circuit conditions (Figure S12). The difference in the Rp values (ZnPBAT-o-C8 (22.7 Ω) < YD2-oC8 (55.7 Ω)) is in parallel with that in the VOC values. The Bode plots exhibit the shorter τe value for ZnPBAT-o-C8 (0.57 ms) than that for YD2-o-C8 (1.7 ms), which is also consistent with the lower VOC value of the TiO2/ZnPBAT-o-C8 cell than that of the TiO2/YD2-o-C8 cell (Figure S13). Totally, the difference in the JSC and VOC values of the two TiO2 cells matches that in the η values. ZnPBAT-o-C8 possesses only two alkoxy chains in the one meso-phenyl group of the porphyrin core, which is a half number of the alkoxy chains in the two meso-phenyl groups of YD2-o-C8. The insufficient blocking effect in ZnPBAT-o-C8 would lead to a decrease in JSC and VOC of the cobalt electrolyte-based DSSC in comparison with YD2o-C8.

than that for ZnPBAT-o-C8C8 (9.3 ms), which is compatible with the difference in the VOC values of TiO2/ZnPBAT-oC8C4 and TiO2/ZnPBAT-o-C8C8 cells. Overall, the order in the Rp and τe values (ZnPBAT-o-C8 < ZnPBAT-o-C8C4 < ZnPBAT-o-C8C8) corresponds to the increasing number and length of alkoxy chains. Accordingly, additional alkoxy chains suppress the CR between the oxidized redox shuttle and TiO2, whereas the CR between the oxidized porphyrin and TiO2 would mainly affect the VOC values. Diau et al. reported the wrapping effect of alkoxy chains of meso-phenyl groups in push−pull ZnPs.51,53 They proposed that the long alkoxy chains improve the DSSC performance by creating dye insulation, reducing aggregation, elevating porphyrin LUMOs, and increasing the VOC value of the DSSC. Our results partially support their proposal, but imply that their position, number, and length around the porphyrin core have a large influence on the photovoltaic performance. In other words, the high photovoltaic performance will be achieved by the optimization of such parameters. Photovoltaic Properties of DSSCs with Cobalt-Based Electrolyte. We also performed similar DSSC experiments with ZnPBAT-o-C8 and YD2-o-C8 using the cobalt(II/III) redox couple to examine the effect of the redox couple. With increasing the soak time, the η value is raised to reach a top value in 1 h for ZnPBAT-o-C8 and 3 h for YD2-o-C8, and then decreased to level off (Figure S9). It is noteworthy that the maximal η values are 8.1% after 1 h sensitization for ZnPBATo-C8 and 9.8% after 3 h for YD2-o-C8. In contrast with iodine electrolyte-based DSSCs, the cell performance of the TiO2/ ZnPBAT-o-C8 and TiO2/YD2-o-C8 cells was not improved after both the CDCA and aging treatment. Figure 9 displays the photocurrent−voltage curves of the TiO2/ZnPBAT-o-C8 and TiO 2/YD2-o-C8 cells under the respective maximal η conditions (Table 4).

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CONCLUSION We have successfully prepared new asymmetrical push−pull porphyrins ZnPBAT-o-C8 and ZnPBAT-o-C8Cn (n = 4, 8) to address the effects of substituent bulkiness around the porphyrin core on the photovoltaic properties. The ZnPBATo-C8-sensitized solar cell exhibited the higher η value of 9.1% than that of the YD2-o-C8-sensitized solar cell (η = 8.6%) under the optimized conditions using iodine-based electrolyte. On the other hand, the solar cells based on ZnPBAT-o-C8Cn, possessing the additional alkoxy chains on the anchoring group, revealed the lower η values of 7.3% and 7.0%. Although ZnPBAT-o-C8Cn exhibited the higher resistance to CR between the electron in the TiO2 and oxidized redox shuttle at the TiO2−dye−electrolyte interface, the unexpected aggregation tendency on TiO2 induced by the additional alkoxy chains resulted in the smaller amount of the porphyrins on TiO2 by the excess use of CDCA, lowering the ηcol and η values. Meanwhile, the ZnPBAT-o-C8-sensitized solar cell revealed the lower η value of 8.1% than that of the YD2-o-C8sensitized solar cell (η = 9.8%) using cobalt-based electrolyte. The smaller η value of the ZnPBAT-o-C8-sensitized solar cell using the cobalt-based electrolyte may be explained by the insufficient blocking effect of the bulky substituents of ZnPBAT-o-C8 under the cobalt-based electrolyte conditions. These results imply that the position, number, and length of

Figure 9. Photocurrent−voltage curves of the TiO2/ZnPBAT-o-C8 (green) and TiO2/YD2-o-C8 (black) cells under the respective maximal η conditions using cobalt-based electrolyte solution.

The optimized DSSC performance of the TiO2/ZnPBAT-oC8 (JSC = 13.2 mA cm−2, VOC = 0.829 V, f f = 0.746, η = 8.1%)

Table 4. Photovoltaic Parameters under the Optimized Conditions with Cobalt-Based Electrolytea ZnPBAT-o-C8 YD2-o-C8

a

JSC/mA cm−2

VOC/V

ff

η/%

13.2 (12.8 ± 0.6) 14.8 (14.7 ± 0.1)

0.829 (0.805 ± 0.020) 0.889 (0.891 ± 0.004)

0.746 (0.748 ± 0.006) 0.745 (0.743 ± 0.006)

8.1 (7.7 ± 0.4) 9.8 (9.7 ± 0.1)

Photovoltaic parameters exhibiting the highest η value. Parentheses denote average values from three independent cells. I


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Figure 10. (a) Photocurrent action spectra of the TiO2/ZnPBAT-o-C8 (green) and TiO2/YD2-o-C8 (black) cells using cobalt-based electrolyte solutions. (b) Absorption spectra of the TiO2/ZnPBAT-o-C8 (green) and TiO2/YD2-o-C8 (black) electrodes for the cobalt electrolyte-based DSSCs. The scattering TiO2 layers were not used for the TiO2 electrodes to obtain accurate absorbance profile. The weak absorption at 800 mn can be assigned to ZnP+• on the TiO2. Zakeeruddin, S. M.; Grätzel, M. Highly Efficient Light-Harvesting Ruthenium Sensitizer for Thin-Film Dye-Sensitized Solar Cells. ACS Nano 2009, 3, 3103−3109. (7) Han, L.; Islam, A.; Chen, H.; Malapaka, C.; Chiranjeevi, B.; Zhang, S.; Yang, X.; Yanagida, M. High-Efficiency Dye-Sensitized Solar Cell with a Novel Co-Adsorbent. Energy Environ. Sci. 2012, 5, 6057− 6060. (8) Mishra, A.; Fischer, M. K. R.; Bäuerle, P. Metal-Free Organic Dyes for Dye-Sensitized Solar Cells: From Structure: Property Relationships to Design Rules. Angew. Chem., Int. Ed. 2009, 48, 2474−2499. (9) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. DyeSensitized Solar Cells. Chem. Rev. 2010, 110, 6595−6663. (10) Clifford, J. N.; Martínez-Ferrero, E.; Viterisi, A.; Palomares, E. Sensitizer Molecular Structure-Device Efficiency Relationship in Dye Sensitized Solar Cells. Chem. Soc. Rev. 2011, 40, 1635−1646. (11) Imahori, H.; Umeyama, T.; Ito, S. Large Pi-Aromatic Molecules as Potential Sensitizers for Highly Efficient Dye-Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1809−1818. (12) Martínez-Díaz, M. V.; de la Torre, G.; Torres, T. Lighting Porphyrins and Phthalocyanines for Molecular Photovoltaics. Chem. Commun. 2010, 46, 7090−7108. (13) Imahori, H.; Umeyama, T.; Kurotobi, K.; Takano, Y. SelfAssembling Porphyrins and Phthalocyanines for Photoinduced Charge Separation and Charge Transport. Chem. Commun. 2012, 48, 4032− 4045. (14) Li, L.-L.; Diau, E. W.-G. Porphyrin-Sensitized Solar Cells. Chem. Soc. Rev. 2013, 42, 291−304. (15) Urbani, M.; Grätzel, M.; Nazeeruddin, M. K.; Torres, T. MesoSubstituted Porphyrins for Dye-Sensitized Solar Cells. Chem. Rev. 2014, 114, 12330−12396. (16) Higashino, T.; Imahori, H. Porphyrins as Excellent Dyes for Dye-Sensitized Solar Cells: Recent Developments and Insights. Dalton Trans. 2015, 44, 448−463. (17) Wang, Z.-S.; Koumura, N.; Cui, Y.; Takahashi, M.; Sekiguchi, H.; Mori, A.; Kubo, T.; Furube, A.; Hara, K. HexylthiopheneFunctionalized Carbazole Dyes for Efficient Molecular Photovoltaics: Tuning of Solar-Cell Performance by Structural Modification. Chem. Mater. 2008, 20, 3993−4003. (18) Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. High Efficiency of Dye-Sensitized Solar Cells Based on Metal-Free Indoline Dyes. J. Am. Chem. Soc. 2004, 126, 12218−12219. (19) Choi, H.; Baik, C.; Kang, S. O.; Ko, J.; Kang, M.-S.; Nazeeruddin, M. K.; Grätzel, M. Highly Efficient and Thermally Stable Organic Sensitizers for Solvent-Free Dye-Sensitized Solar Cells. Angew. Chem., Int. Ed. 2008, 47, 327−330. (20) Zeng, W.; Cao, Y.; Bai, Y.; Wang, Y.; Shi, Y.; Zhang, M.; Wang, F.; Pan, C.; Wang, P. Efficient Dye-Sensitized Solar Cells with an Organic Photosensitizer Featuring Orderly Conjugated Ethylenediox-

bulky substituents around the porphyrin core have a large influence on the photovoltaic performance. Therefore, further improvement in the cell performance may be achieved by rational design in which bulky substituents are introduced into suitable positions of push−pull porphyrins.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b03806. Synthesis, fluorescence spectra, DPV curves, results of DFT calculations, FT-IR spectra, XPS spectra, and EIS Nyquist and Bode plots (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA, JST) and JSPS KAKENHI Grant number 25220801.

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REFERENCES

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