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J. Phys. Chem. C 2008, 112, 15576–15585
Naphthyl-Fused π-Elongated Porphyrins for Dye-Sensitized TiO2 Cells Shinya Hayashi,† Masanobu Tanaka,‡ Hironobu Hayashi,† Seunghun Eu,† Tomokazu Umeyama,† Yoshihiro Matano,† Yasuyuki Araki,§ and Hiroshi Imahori*,†,|,⊥ Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, Kyoto UniVersity International InnoVation Center, Nishikyo-ku, Kyoto 615-8520, Japan, Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira 2-1-1, Aoba-ku, Sendai, 980-8577, Japan, Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, and Fukui Institute for Fundamental Chemistry, Kyoto UniVersity, 34-4, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan ReceiVed: June 11, 2008; ReVised Manuscript ReceiVed: July 25, 2008
Novel unsymmetrically π-elongated porphyrins, in which the naphthyl moiety is fused to the porphyrin core at the naphthyl bridge with a carboxyl group (fused-Zn-1) or at the opposite side of the phenyl bridge with a carboxyl group (fused-Zn-2), have been synthesized to improve the light-harvesting abilities in porphyrinsensitized solar cells. As the results of π-elongation with low symmetry, Soret and Q bands of fused-Zn-1 and fused-Zn-2 were red-shifted and broadened, and the intensity of Q-band relative to that of Soret band was enhanced. The fused-Zn-1 and fused-Zn-2-sensitized TiO2 solar cells showed the power conversion efficiencies (η) of 4.1% and 1.1%, respectively, under standard AM 1.5 conditions. The η value of the fusedZn-1 cell was improved by 50% compared to the reference cell using unfused porphyrin (Zn-1). The fusedZn-1-sensitized cell revealed high IPCE (incident photon-to-current efficiency) values of up to 55%, extending the response of photocurrent generation close to 800 nm. Thus, the improved photocurrent generation of the fused-Zn-1-sensitized cell relative to the Zn-1-sensitized reference cell is responsible for the remarkable difference in the η values. The η value of the fused-Zn-2 cell was much lower than that of the fused-Zn-1 cell. DFT calculations disclosed that there are significant electron densities on the carboxyl group in the LUMO of fused-Zn-1, whereas there are little electron densities on the carboxyl group in the LUMO of fused-Zn-2. Accordingly, the larger electronic coupling between the porphyrin and the TiO2 surface in the fused-Zn-1-sensitized cell may be responsible for the high cell performance, due to the efficient electron injection from the porphyrin excited singlet state to the conduction band of the TiO2 electrodes. To further improve the cell performance, 5-(4-carboxylphenyl)-10,15,20-tetrakis-(2,4,6-trimethylphenyl)porphyrinatozinc(II) (Zn-3), possessing different light-harvesting properties, was coadsorbed with fused-Zn-1 onto an TiO2 electrode. Under the optimized conditions, the cosensitized cell yielded maximal IPCE value of 86%, short circuit photocurrent density of 11.7 mA cm-2, open-circuit voltage of 0.67 V, fill factor of 0.64, and η of 5.0% under standard AM 1.5 conditions. Introduction Since the first report on dye-sensitized solar cells (DSSC) with a porous, nanocrystalline TiO2 electrode sensitized with an Ru polypyridyl complex by Gra¨tzel and co-workers,1 much effort has been devoted to understanding the photocurrent generation mechanism as well as improving DSSC performances.2-12 The operation of DSSC is based on the photoexcitation of dyes bonded to a nanocrystalline TiO2 surface. Electron injection from the photoexcited dye molecules to the conduction band (CB) of the TiO2 electrode is the initiating process of the dye sensitization. The resulting oxidized dye molecules are regenerated by the I-/I3- redox electrolyte. The injected electrons travel through the network of TiO2 nanoparticles to reach the transparent * To whom correpondence should be addressed. E-mail: imahori@ scl.kyoto-u.ac.jp. † Department of Molecular Engineering, Graduate School of Engineering, Kyoto University. ‡ Kyoto University International Innovation Center. § Institute of Multidisciplinary Research for Advanced Materials, Tohoku University. | Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University. ⊥ Fukui Institute for Fundamental Chemistry, Kyoto University.
conducting glass substrate and then are transported through the external circuit to the counter electrode. At the counter electrode, the electrons are given to I3- in the electrolyte to regenerate I-, leading to the photocurrent generation in DSSC. To date, Ru(II) bipyridyl complexes have proven to be the most efficient TiO2 sensitizers (η ) 9-11%).6 Nevertheless, considering practical applications in the future, further improvement of DSSC performance is needed without using an expensive metal (i.e., Ru) in the dyes. In this context, a variety of organic dyes using inexpensive metals or without using metals has been prepared for DSSC.7-12 In spite of the extensive effort, the cell performance is still lower or comparable to that of Ru dye-based DSSC. Since porphyrins possess an intense Soret band at 400-450 nm and moderate Q bands at 500-650 nm, they have been regarded as potential photosensitizers in DSSC.11,12 However, the poor light-harvesting properties relative to Ru dyes at 450-500 and 650-900 nm have limited the cell performance of porphyrin-sensitized TiO2 cells. One promising way to surmount this problem is to modulate the electronic structures of porphyrins so that one can match the light-harvesting properties with the solar energy distribution on the earth. Soret
10.1021/jp805122z CCC: $40.75 2008 American Chemical Society Published on Web 09/10/2008
Naphthyl-Fused π-Elongated Porphyrins
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15577 unsymmetrical π-elongation on the photovoltaic properties (Figure 1). Finally, two porphyrins with complementary absorption properties in the visible region (fused-Zn-1 and Zn-3) are coadsorbed onto TiO2 electrodes to further improve the cell performance in DSSC (Figure 1). Experimental Section
Figure 1. Porphyrin derivatives used in this study.
and Q bands arise from π-π* transitions and can be explained in terms of a linear combination of transitions from slightly splitted HOMO and HOMO-1 to a degenerated pair of LUMO and LUMO-1. The configuration interaction leads to the intense Soret band at the short-wavelength and the moderate Q bands at the long-wavelength.13 Elongation of the π-conjugation and loss of symmetry in porphyrins cause splitting in the π and π* levels and decrease in the HOMO-LUMO gap, resulting in broadening and red-shift of the absorption bands, together with an increased intensity of the Q bands relative to that of the Soret band. In such a case, the cell performance of the porphyrinsensitized solar cells would be improved by the enhanced light absorption. Although there have been several reports on the aromatic ring-fused, π-extended porphyrins that were synthesized by oxidative coupling between the aromatic ring and the porphyrin core, they have yet to be applied to organic solar cells.14,15 In this paper, we report the synthesis of naphthalene ringfused, π-elongated porphyrins with low symmetry and their first applications to dye-sensitized solar cells.16 The porphyrin molecules used in this study are shown in Figure 1. Naphthyl group is introduced at the meso-position and then fused at the β-position to elongate the π system unsymmetrically. The unsymmetrically π-elongated porphyrin is highly expected to collect visible light efficiently, leading to the improvement of the photovoltaic properties. To evaluate the effects of the electronic coupling between the porphyrin and a TiO2 surface on the photovoltaic properties, carboxyl group is attached to the fused-naphthyl moiety in the porphyrin core (fused-Zn-1) or to the meso-phenyl group at the opposite side (fused-Zn-2). Bulky mesityl groups are also introduced at the other mesopositions of the porphyrin core to reduce the porphyrin aggregation on the TiO2 surface.12 Nonfused porphyrin references, Zn-1 and Zn-2, are also used to evaluate the effects of
General Procedure. All commercial reagents and solvents were used without further purification. 1H NMR spectra were recorded on either a JEOL EX270 KS or a JEOL AL 300 NMR spectrometer. MALDI-TOF mass spectra were measured with COMPACT MALDI II (SHIMADZU) mass spectrometer with CHCA as a matrix. High-resolution mass spectra (HRMS) were recorded on a JEOL JMS-HX 110A spectrometer using 3-nitrobenzylic acid as a matrix. UV-visible absorption spectra of the porphyrins in dichloromethane and of the porphyrin monolayer on TiO2 electrodes were recorded using a Perkin-Elmer Lambda 900 UV/vis/NIR spectrometer. Steady-state fluorescence spectra were acquired by a SPEX Fluoromax-3 spectrofluorometer. Dichloromethane (spectroscopy grade) was used for the measurements of UV-visible absorption and fluorescence spectra. Fluorescence lifetimes were measured with a streak scope (Hamamatsu Photonics, model C4334-01) using a second-harmonic generation (SHG, 400 nm) of a Ti:sapphire laser (Spectra-Physics, model Tsunami 3950-L2S, with a full width at half-maximum (fwhm) of 1.5 ps) as an excitation source.17 Electrochemical measurements were made using an ALS 630a electrochemical analyzer. Redox potentials were determined in dichloromethane containing 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) at a scan rate of 0.1 V s-1. A glassy carbon working electrode, Ag/AgNO3 reference electrode, and Pt wire counter electrode were employed. Ferrocene (0.64 V vs NHE) for Zn-1 and Zn-2 and decamethylferrocene (0.15 V vs NHE) for fused-Zn-1 and fused-Zn-2 were used as internal standards. Dichloromethane was purified just before use and Bu4NPF6 was recrystallized from methanol. Synthesis of Porphyrins. The porphyrins (Zn-1, Zn-2, fusedZn-1, fused-Zn-2) were synthesized following the same procedures as described previously (see Supporting Information, S1).15f,18-20 The synthetic procedure and characterization of Zn-3 have been reported.12a Density Functional Theory (DFT) Calculations. Geometry optimization and electronic structure calculations of the porphyrins were performed using B3LYP functional and 3-21G* basis set implemented in the Gaussian 03 program package.21 Molecular orbitals were visualized by Molstudio 3.0 software. Preparation of Porphyrin-Modified TiO2 Electrode. Nanoporous TiO2 films were prepared from colloidal suspension of TiO2 nanoparticles (P25, Nippon Aerogel) dispersed in deionized water and Triton X-100.12,16 The suspension was deposited on a transparent conducting glass (Asahi Glass, SnO2: F, 9.4 ohm/sq) by using the doctor blade technique. The films were annealed at 673 K for 10 min, followed by similar deposition and annealing (723 K, 2 h) for the 10-µm-thick TiO2 films. The thickness of the films was determined using surface roughness/profile measuring instrument (SURFCOM 130A, ACCRETECH). The TiO2 electrodes were immersed into each of the 0.2 mM porphyrin methanol solutions at room temperature. After dye adsorption, the dye-coated electrodes were rinsed with methanol. The amounts of the porphyrins adsorbed onto the TiO2 films were determined by measuring absorbance at the Soret band of the dye molecules that were dissolved from the dye-adsorbed TiO2 films into DMF containing 0.1 M NaOH(aq).12,16
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SCHEME 1a
a Reagents and conditions: (a) BF3(OEt2), CHCl3, r.t., 2 h; (b) DDQ, 1 h, 9% (2 steps); (c) Ni(acac)2, toluene, reflux, 2 days, 82%; (d) FeCl3, CH2Cl2, CH3NO2, reflux, 12 h; (e) TFA, H2SO4, 16% (2 steps); (f) KOH, H2O, THF, EtOH, reflux, 4 h; and (g) Zn(OAc)2, CHCl3, r.t., 4 h, 77% for fused-Zn-1 (2 steps).
SCHEME 2a
a Reagents and conditions: (a) BF3(OEt2), CHCl3, r.t., 2 h; (b) DDQ, 1 h, 11% (2 steps); (c) Ni(acac)2, toluene, reflux, 2 days, 91%; (d) FeCl3, CH2Cl2, CH3NO2, r.t., 1.5 h; (e) TFA, H2SO4, 67% (2 steps); (f) KOH, H2O, THF, EtOH, reflux, 2 h; and (g) Zn(OAc)2, CHCl3, r.t., 2 h, 77% for fused-Zn-2 (2 steps).
Photovoltaic Measurements. The photovoltaic measurements were performed in a sandwich cell consisting of the porphyrin-sensitized TiO2 electrode as the working electrode and a platinum-coated conducting glass as the counter electrode.12,16 The two electrodes were placed on top of each other using a thin transparent film of Surly polymer (Dupont) as a spacer to form the electrolyte space. A thin layer of electrolyte (0.1 M LiI, 0.05 M I2, 0.6 M 2,3-dimethyl-1propylimidazolium iodide, and 0.5 M 4-t-butylpyridine in acetonitrile) was introduced into the interelectrode space. The IPCE values and photocurrent-voltage characteristics were determined by using a potentiostat (Bunko-Keiki Co., Ltd., Model HCSSP-25) irradiated with simulated AM 1.5 solar light (100 mW cm-2, Bunko-Keiki Co., Ltd., Model CEP-2000). All of the experimental values are given as an average from five independent measurements. Results and Discussion Synthesis and Characterization. The synthesis procedures of fused-Zn-1 and fused-Zn-2 are shown in Schemes 1 and 2, respectively. Porphyrin H2-1 was synthesized by the cross-
condensation of mesityldipyrromethane with 4-carbomethoxynaphthyl-aldehyde and mesitylaldehyde. Porphyrin H2-1 was treated with Ni(acac)2 at 110 °C for 2 days to yield Ni-1. Oxidative ring-close reaction of Ni-1 with FeCl315f and subsequent demetalation with TFA and H2SO4 afforded fused-H2-1. Oxidative ring-close reactions of Ni-1 with 2,3,-dichloro-5,6dicyano-1,4-benzoquinone (DDQ), Sc(OTf)3, or PhI(OTf)3 (Tf ) trifluoromethanesulfonyl) did not give the desirable product (fused-H2-1).15c Hydrolysis of fused-H2-1 and subsequent Zn(II)-metalation gave fused-Zn-1. Porphyrin fused-Zn-2 was synthesized by following the same procedure as that for fused-Zn-1 (Scheme 2). Methoxy group was introduced into the naphthyl group at the meso-position to promote oxidative ring-close reaction of Ni-2, because the reaction of Ni-2 without the methoxy group yielded a complex mixture including trace amounts of the desirable naphthyl-fused porphyrin. The reference nonfused porphyrins (Zn-1 and Zn2) were also prepared from H2-1 and H2-2, respectively. Structures of the new compounds were verified by spectroscopic analyses including 1H NMR and mass spectra (see Experimental Section).
Naphthyl-Fused π-Elongated Porphyrins
Figure 2. UV-visible absorption spectra of (a) fused-Zn-1 (solid line) and Zn-1 (dashed line) and (b) fused-Zn-2 (solid line) and Zn-2 (dashed line) in CH2Cl2.
Optical and Electrochemical Properties. The UV-visible absorption spectra of fused-Zn-1, fused-Zn-2, Zn-1, and Zn-2 were measured in CH2Cl2 (Figure 2). The peak positions and molar absorption coefficients () of Soret and Q bands are listed in Table 1. The UV-visible absorption spectra of Zn-1 and Zn-2 exhibit typical a strong Soret band and moderate Q bands. However, as a result of the expansion of the π-system and lowering of the symmetry, the Soret and Q bands of fused-Zn-1 are broadened and red-shifted by ∼60 and 130 nm compared with those of Zn-1, respectively (Table 1). Similar broadening and red-shift of the absorption bands are noted for fused-Zn-2 relative to Zn-2. The absorption edge of the fused porphyrins is extended close to 750 nm. The molar absorption coefficients at the Soret bands of fused-Zn-1 and fused-Zn-2 are ∼30% of those for Zn-1 and Zn-2, but the integrated values of molar absorption coefficients with respect to wavenumber at the Soret band region (400-550 nm) are rather comparable for Zn-1 (4.2 × 108), Zn-2 (4.3 × 108), fused-Zn-1 (3.3 × 108), and fusedZn-2 (3.1 × 108). More importantly, the integrated values of molar absorption coefficients at the Q-band region (550-750 nm) of fused-Zn-1 (5.0 × 107) and fused-Zn-2 (4.7 × 107) are 2 times larger than those of Zn-1 (2.5 × 107) and Zn-2 (2.6 × 107). These results reveal that fused-Zn-1 and fusedZn-2 can absorb the incident light in the visible and nearinfrared regions intensively. The steady-state fluorescence spectra of fused-Zn-1, fusedZn-2, Zn-1, and Zn-2 were also measured in CH2Cl2 by exciting at the peak position of the Soret band (Figure 3). The emission maxito ma are also summarized in Table 1. The emission maxima of fused-Zn-1 (716 nm, 766 nm) and fused-Zn-2 (702 nm, 759 nm) are red-shifted by ∼100-110 nm relative to those
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15579 of Zn-1 (601 nm, 652 nm) and Zn-2 (604 nm, 656 nm), which is consistent with the results of the UV-visible absorption spectra. From the intercept of the normalized UV-visible absorption spectra and steady-state fluorescence spectra, the zeroth-zeroth energies (E0-0) are determined to be 1.77 eV for fused-Zn-1, 1.80 eV for fused-Zn-2, and 2.15 eV for Zn-1 and Zn-2. The E0-0 energies of fused-Zn-1 and fused-Zn-2 are smaller than the value of Zn-1 and Zn-2. These results imply that the HOMO-LUMO gaps of the fused porphyrins are small in comparison with the nonfused porphyrins owing to the extension of π system. To evaluate the lifetime of the porphyrin excited singlet state, the fluorescence lifetimes (τ) were measured in MeOH excited at 400 nm and monitored at 650 nm for Zn-1, and Zn-2 and 700 nm for fused-Zn-1 and fused-Zn-2. The fluorescence decays of fused-Zn-1, fused-Zn-2, Zn-1, and Zn-2 were analyzed by a single component and the results are summarized in Table 1. The fluorescence lifetimes of fused-Zn-1 (1.8 ns) and fused-Zn-2 (1.6 ns) are slightly shorter than those of Zn-1 (2.6 ns) and Zn-2 (2.8 ns), but the fluorescence lifetimes are still much longer than the electron injection rate (1011-1013 s-1) from the porphyrin excited singlet state to the conduction band of TiO2 electrodes.11 The first oxidation and reduction potentials of porphyrins were determined by differential pulse voltammetry. The redox potentials were measured in dichloromethane containing 0.1 M tetra-n-butylammonium hexafluorophosphate (nBu4NPF6) at a scan rate of 0.1 V s-1 and the values are listed in Table 1. As the results of the π-elongation, the first oxidation potentials of fused-Zn-1 (0.99 V vs NHE) and fused-Zn-2 (1.02 V vs NHE) are negatively shifted compared to Zn-1 (1.04 V vs NHE) and Zn-2 (1.04 V vs NHE), whereas the first reduction potentials of fused-Zn-1 (-0.69 V vs NHE) and fused-Zn-2 (-0.67 V vs NHE) are positively shifted relative to Zn-1 (-0.80 V vs NHE) and Zn-2 (-0.74 V vs NHE), respectively. Totally, the HOMO-LUMO gaps of fused-Zn-1 (1.68 eV) and fused-Zn-2 (1.69 eV) are smaller than those of Zn-1 (1.84 eV) and Zn-2 (1.78 eV), respectively, which are in good agreement with the results of the UV-visible absorption spectra and the steadystate fluorescence spectra. From the first oxidation potentials and the E0-0 energies, the excited-state oxidation potentials are determined to be -0.78 V for fused-Zn-1 and fused-Zn-2 and -1.11 V for Zn-1 and Zn-2 (vs NHE). In the porphyrinsensitized solar cells, the driving forces for electron injection from the porphyrin singlet excited-state to the CB of TiO2 (-0.5 V vs NHE)12 electrodes (∆Ginj) and regeneration of the porphyrin radical cation by I-/I3- redox couple (0.5 V vs NHE) (∆Greg) are summarized in Table 1. Both processes are thermodynamically feasible. DFT Calculations. Geometry optimization and vibrational frequency analysis were performed by DFT methods for fusedZn-1, fused-Zn-2, Zn-1, and Zn-2.21 All of the optimized structures show no negative frequencies. The energies of frontier molecular orbitals (FMOs) are summarized in Table 2. The results of DFT calculations are consistent with those of the UV-visible absorption spectra, fluorescence spectra, and the electrochemical measurements. In the cases of Zn-1 and Zn-2, the energies of LUMO and LUMO+1 as well as those of HOMO and HOMO-1 are nearly degenerated. In contrast, for fusedZn-1 and fused-Zn-2, the degeneracy of LUMO and LUMO+1 together with that of HOMO and HOMO-1 is relieved by the π-elongation. It is noteworthy that the energy levels of LUMOs are more influenced than those of HOMOs by the π-elongation. Overall, the HOMO-LUMO gaps of fused-Zn-1 (2.31 eV) and
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TABLE 1: Optical and Electrochemical Data for Porphyrins porphyrin
λaabs/nm (, 104 M-1 cm-1)
λbem (nm)
fused-Zn-1
482 (12.4)
716
fused-Zn-2
682 (2.46) 480 (14.4)
766 702
Zn-1
679 (2.57) 422 (43.3)
759 601
Zn-2
551 (2.07) 424 (45.1)
652 604
552 (2.12)
656
τc (ns)
E0-0d(eV)
Eoxe (V)
Eredf (V)
Eoxg* (V)
∆Ginjh (eV)
∆Gregi (eV)
1.8
1.77
0.99
-0.69
-0.78
-0.28
-0.49
1.6
1.80
1.02
-0.67
-0.78
-0.28
-0.52
2.6
2.15
1.04
-0.80
-1.11
-0.61
-0.54
2.8
2.15
1.04
-0.74
-1.11
-0.61
-0.54
a Wavelengths for the maxima of Soret and Q bands in CH2Cl2 solution. b Wavelengths for emission maxima in CH2Cl2 by exciting at the peak position of Soret band. c In MeOH by exciting at the 400 nm. d Determined from the intercept of the normalized absorption and emission spectra. e First oxidation potentials (vs NHE). f First reduction potentials (vs NHE). g Excited-state oxidation potentials approximated from Eox and E0-0 (vs NHE). h Driving forces for electron injection from the porphyrin excited singlet state (Eoxg*) to the conduction band of TiO2 (-0.5 V vs NHE). i Driving forces for regeneration of the porphyrin radical cation by I-/I3- redox couple (+0.5 V vs NHE).
Figure 3. Steady-state fluorescence spectra of (a) fused-Zn-1 (solid line) and Zn-1 (dashed line) and (b) fused-Zn-2 (solid line) and Zn-2 (dashed line) in CH2Cl2. The spectra were obtained by exciting at the peak position of the Soret band.
TABLE 2: Calculated Energies of Frontier Molecular Orbitals for fused-Zn-1, fused-Zn-2, Zn-1, and Zn-2 E E E E
(LUMO+1)/eV (LUMO)/eV (HOMO)/eV (HOMO-1)/eV
fused-Zn-1
fused-Zn-2
Zn-1
Zn-2
-2.22 -2.73 -5.04 -5.28
-2.10 -2.56 -4.82 -5.13
-2.17 -2.19 -5.23 -5.25
-2.13 -2.17 -5.17 -5.23
fused-Zn-2 (2.56 eV) become smaller than those of Zn-1 (3.04 eV) and Zn-2 (3.00 eV). These results agree well with those of the electrochemical measurements (vide supra). The molecular orbitals of fused-Zn-1, fused-Zn-2, Zn-1, and Zn-2 are shown in Figure 4. The cores of Zn-1 and Zn-2 display
analogous electronic structures, but the change of the spacer moiety (naphthyl spacer for Zn-1 vs phenyl spacer for Zn-2) causes the difference in electron densities on the anchor group (i.e., carboxyl group) in the LUMOs. The decrease of the dihedral angles between the porphyrin plane and the spacer moiety (90° for Zn-1 > 69° for Zn-2) leads to an increase of the electron densities on the carboxyl group of Zn-2 over Zn1. However, the electron densities of HOMO and LUMO in fused-Zn-1 and fused-Zn-2 are delocalized over the whole π-systems including the fused naphthyl groups. It should be noted here that the electron density on the carboxyl group in the LUMO of fused-Zn-1 is larger than that in the LUMO of Zn-1, whereas there exists small electron density on the carboxyl group in the LUMO of fused-Zn-2 in comparison with that of Zn-2. The larger electron density may lead to efficient electron injection from the porphyrin singlet excited-state to the CB of TiO2 electrodes owing to the strong electronic coupling between the porphyrin and the TiO2 surface through the carboxyl group, resulting in the difference in the cell performances between the two systems (vide infra). The optimized molecular structures of fused-Zn-1 and fusedZn-2 are also shown in Figure 5. The π-elongated porphyrin planes of fused-Zn-1 and fused-Zn-2 are twisted due to the steric congestion between the β-proton and the hydrogen atom of the fused-naphthalene ring. The strain of porphyrin ring is likely to cause fast nonradiative relaxation in the porphyrin excited singlet state,22 which is in good agreement with the results of the fluorescence lifetime measurements. The nonradiative relaxation would compete with electron injection to the CB of TiO2, resulting in a decrease of the cell performances (vide infra). Preparation of Porphyrin-Modified TiO2 Electrodes. The mesoporous TiO2 electrodes were prepared as described in the Experimental Section. The TiO2 electrodes were immersed into MeOH containing 0.2 mM porphyrin for a certain period of time (0.25-12 h) at room temperature to give the porphyrinmodified TiO2 electrode (denoted as TiO2/fused-Zn-1, fusedZn-2, Zn-1 or Zn-2). The total amounts of the porphyrins adsorbed onto the TiO2 electrodes were determined by measuring absorbance of the porphyrins, which were dissolved from the porphyrin-modified TiO2 electrodes into DMF containing 0.1 M NaOH(aq).12 Taking into account the surface area of P25 (54 m2 g-1),23 the porphyrin densities (Γ) on the actual surface area are estimated (Table 3). The Γ value is increased rapidly with increasing the immersing time, but levels off at the
Naphthyl-Fused π-Elongated Porphyrins
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15581
Figure 5. Optimized geometries of fused-Zn-1 and fused-Zn-2 calculated at B3LYP/3-21G* level.
Figure 4. HOMOs and LUMOs of fused-Zn-1, fused-Zn-2, Zn-1, and Zn-2.
immersing time of 0.5-1 h to yield the saturated Γ value; TiO2/ fused-Zn-1: 9.4 × 10-11 mol cm-2; TiO2/fused-Zn-2: 1.2 × 10-10 mol cm-2; TiO2/Zn-1: 1.0 × 10-10 mol cm-2; TiO2/Zn2: 1.0 × 10-10 mol cm-2 (see Supporting Information, S2). Assuming that the porphyrin molecules are densely packed onto TiO2 surface to make a monolayer where the single bond between the carboxyl group and the aromatic moiety is perpendicular to the TiO2 surface, the calculated Γ values are estimated to be 9.1 × 10-11 mol cm-2 for fused-Zn-1, 1.2 × 10-10 mol cm-2 for fused-Zn-2, and 1.0 × 10-10 mol cm-2 for Zn-1 and Zn-2. The experimental Γ values are in good accordance with the ideal Γ values, implying that a well-packed
porphyrin monolayer is formed on the TiO2 surface. The UV-visible absorption spectra of TiO2/porphyrin electrodes are illustrated in Figure 6. The thickness of the TiO2 electrode was adjusted to be 700-1000 nm to accurately obtain the shape and peak position of the spectra. The splitting of the Soret bands result from the exciton coupling of the porphyrins due to the close proximity of the porphyrins on the TiO2 electrode,24 in spite of introducing methyl groups at the meso-phenyl groups of the porphyrins.12 Photovoltaic Properties of Porphyrin-Modified TiO2 Electrodes. Current-voltage characteristics of porphyrin-modified TiO2 electrodes were measured under AM 1.5 conditions (100 mW cm-2). The η value is derived from the equation η ) JSC × VOC × ff, where JSC is short circuit current density (mA cm-2), VOC is open circuit potential (V), and ff is fill factor. The η value of porphyrin-sensitized cells increases rapidly with increasing the immersing time to reach maximum η values (ηmax) of fused-Zn-1 cell for 0.5 h, fused-Zn-2 cell for 0.25 h, Zn-1 cell for 1.0 h, and Zn-2 cell for 0.5 h, respectively (Figure 7). Further increase of the immersing time leads to a gradual decrease in the η value. The decrease in the η values with increasing the immersing time does not match the saturation behavior of the porphyrin densities (Γ) on the TiO2 electrodes (see Supporting Information S1). The decrease may associate with the accelerated nonradiative relaxation in the porphyrin excited singlet state due to the porphyrin aggregation on TiO2. Table 3 summarizes the photovoltaic properties of the porphyrin-sensitized TiO2 electrodes under the optimized conditions. The current-voltage characteristics are shown in Figure 8. The η value (4.1%) of fused-Zn-1 cell is larger by 50% than the value (2.8%) of Zn-1 cell. In contrast, the η value (1.1%) of fused-Zn-2 cell is smaller by 70% than the value (3.4%) of the Zn-2 cell. Moreover, the η value of Zn-1 cell (2.8%) is smaller than that of Zn-2 cell (3.4%). Figure 9 depicts the photocurrent action spectra of all the porphyrin-sensitized TiO2 cells under the optimized conditions in which the highest η value is obtained. The IPCE values are calculated by normalizing the photocurrent densities for incident light energy and intensity and by use of the eq 16d (Table 3):
IPCE(%) ) 100 × 1240 × i ⁄ (Win × λ) cm-2),
(1)
where i is the photocurrent density (A Win is the incident light intensity (W cm-2), and λ is the excitation wavelength
15582 J. Phys. Chem. C, Vol. 112, No. 39, 2008
Hayashi et al.
TABLE 3: Cell Performance of Porphyrin-Sensitized Cells under the Optimized Conditions cell
saturated Γ (10-10 mol cm-2)
JSC (mA cm-2)
VOC (V)
ff
η (%)
IPCE (APCE)/%
TiO2/fused-Zn-1c TiO2/fused-Zn-2d TiO2/Zn-1e TiO2/Zn-2c
0.94 1.1 1.1 1.0
10.6 3.6 6.7 8.2
0.62 0.53 0.61 0.64
0.62 0.58 0.68 0.65
4.1 1.1 2.8 3.4
55 (55)a, 42 (43)b 29 (29)a, 18 (18)b 59 (59)a, 32 (33)b 64 (64)a, 40 (41) b
a At Soret band (470 nm for fused-Zn-1 and fused-Zn-2 and 420 nm for Zn-1 and Zn-2). b At Q-band (690 nm for fused-Zn-1 and fused-Zn-2 and 560 nm for Zn-1 and Zn-2). c Immersing time of 0.5 h. d Immersing time of 0.25 h. e Immersing time of 1 h.
Figure 7. Immersing time profiles of η values for fused-Zn-1 (triangle), fused-Zn-2 (close circle), Zn-1 (cross), and Zn-2 (open circle) adsorbed onto TiO2 electrodes in MeOH.
Figure 6. UV-visible absorption spectra of (a) TiO2/fused-Zn-1 (solid line) and TiO2/Zn-1 (dashed line) and (b) TiO2/fused-Zn-2 (solid line) and TiO2/Zn-2 (dashed line). Thickness of the TiO2 electrode was adjusted to be 700-1000 nm to obtain the shape and peak position of the spectra accurately. The spectra of TiO2/Zn-1 and TiO2/Zn-2 are normalized for comparison. The porphyrin-modified TiO2 electrodes were obtained from the solution of MeOH containing the porphyrin (0.2 mM) for optimized immersing time.
(nm). Absorbed photon-to-current efficiency (APCE) values of the porphyrin-sensitized cells were also calculated from the IPCE value and absorbance at the Soret and Q bands by the following equations (eqs 2 and 3)6d (Table 3):
LHE ) 1 - 10-Abs APCE ) IPCE ⁄ LHE
(2) (3)
where LHE is light harvesting efficiency and Abs is absorbance of porphyrin-modified TiO2 electrode. The overall photocurrent generation response of the porphyrin-modified TiO2 electrodes parallels the absorption spectral features of the corresponding porphyrin/TiO2 electrodes (Figure 6), implying the involvement of porphyrins in the photocurrent generation. The η value (2.8%)
Figure 8. Current-voltage characteristics under the optimized conditions exhibiting maximum η values for porphyrin-sensitized TiO2 cells. (a) fused-Zn-1 (η ) 4.1%), (b) fused-Zn-2 (η ) 1.1%), (c) Zn-1 (η ) 2.8%), and (d) Zn-2 (η ) 3.4%). Conditions: electrolyte 0.1 M LiI, 0.05 M I2, 0.6 M 2,3-dimethyl-1-propylimidazolium iodide, and 0.5 M 4-t-butylpyridine in CH3CN; input power: AM 1.5 under simulated solar light (100 mW cm-2).
of the Zn-1 cell is slightly smaller than the value (3.4%) of the Zn-2 cell, which correlates with the difference in the IPCE values of the Zn-1 cell and the Zn-2 cell (Table 3). Considering the similar absorption features of TiO2/Zn-1 and TiO2/Zn-2, the difference in the η values is associated with the electron injection from the porphyrin excited-state to the CB of the TiO2 electrode and/or the charge collection to respective electrode after collecting the light. From the results of DFT calculations, the electron density on the carboxyl group in LUMO of Zn-1 is smaller than that in LUMO of Zn-2 (Figure 4). Therefore, the small electronic coupling between the porphyrin and the TiO2 surface through the carboxyl group may rationalize the inferior cell performance of the Zn-1 cell in comparison with that of the Zn-2 cell. The photocurrent generation of the fusedZn-1 cell is improved relative to that of Zn-1-sensitized cell, but that of the fused-Zn-2 cell is lower than that of Zn-2 cell,
Naphthyl-Fused π-Elongated Porphyrins
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15583
Figure 9. Action spectra of (a) fused-Zn-1 (bold line), (b) fusedZn-2 (dashed line), (c) Zn-1 (dotted line), and (d) Zn-2 (solid line) sensitized TiO2 cells. Each of the porphyrin-modified TiO2 electrodes was prepared under the optimized conditions. Conditions: electrolyte 0.1 M LiI, 0.05 M I2, 0.6 M 2,3-dimethyl-1-propyl imidazolium iodide, and 0.5 M 4-t-butylpyridine in CH3CN; input power: AM 1.5 under simulated solar light (100 mW cm-2).
which is consistent with the trend in the η values. The enhancement of cell performances in the fused-Zn-1 cell relative to that of the Zn-1 cell is explained by the improvement of light-harvesting abilities (Figure 6) together with the strong electronic coupling of the fused-Zn-1 cell relative to the Zn-1 cell (Figure 4). However, irrespective of the stronger electronic coupling, the maximum IPCE value of the fused-Zn-1 cell (55%) is rather lower than that of the Zn-1 cell (59%). In the fused-Zn-1 cell, the accelerated nonradiative relaxation of the porphyrin excited singlet state, which is caused by the strain of the porphyrin ring22 (Figure 5), may prevent the efficient electron injection from the porphyrin excited singlet state to the CB of TiO2 electrodes. This accounts for the slight decrease in the maximum IPCE value of the fused-Zn-1 cell. The reduced cell performances in the fused-Zn-2 cell relative to the Zn-2 cell may be rationalized by the smaller electronic coupling in the spacer, the enhanced nonradiative relaxation in the porphyrin excited singlet state, and the larger porphyrin aggregation due to the less steric hindrance of fused-Zn-2 relative to Zn-2, although the light-harvesting abilities are improved. It should be emphasized here that the photocurrent response of the fusedZn-1 and fused-Zn-2 cells is extended up to 800 nm, which is the longest wavelength as an edge of photocurrent generation among porphyrin-sensitized solar cells ever reported.11,12 In short, further improvement of the cell performances would be possible by designing novel unsymmetrically, π-elongated porphyrins with a planar porphyrin ring. Co-adsorption of Porphyrins with Different Absorption Features. Cosensitization of dyes with different absorption features is one of the promising approaches to widen the photocurrent response area.25 In the fused-Zn-1-sensitized cell,
the η value reached to 4.1%, but the photocurrent response at around 400 and 550 nm rather declines compared with the Zn-1 cell because of the red-shifted photocurrent response exhibiting the peaks at around 470 and 690 nm. One way to surmount the problem is to mix fused-Zn-1 with a dye with compensative absorption features. We have already revealed the high cell performance (η ) 4.6%) of Zn-3-sensitized TiO2 cell (Figure 1).12d The Zn-3-sensitized TiO2 cell exhibited photocurrent response at around 430 and 560 nm, which is complementary to the photocurrent response of the fused-Zn-1-sensitized TiO2 cell. Thus, a combination of fused-Zn-1 and Zn-3 in DSSC is expected to improve the cell performance relative to the respective single component cells. The amounts of fused-Zn-1 and Zn-3 on the TiO2 electrode were systematically varied by the competitive coadsorption onto the TiO2 electrode from MeOH solutions containing various molar ratios of fused-Zn-1 and Zn-3 with a total concentration of 0.2 mM (molar ratio of fused-Zn-1: Zn-3 ) 1:0, 1:1, 1:2, 1:3, 1:4, 1:8, 1:10, 0:1). The ratios of porphyrin densities (Γ) of fused-Zn-1 and Zn-3 on the TiO2 electrodes are comparable to the initial molar ratios of fused-Zn-1 and Zn-3 in the immersing MeOH solution (Table 4). This is reasonable considering the similar molecular structures. The η values of the cosensitized cells also show similar dependency as a function of the immersing time (see Supporting Information, S3), as seen for the fused-Zn-1, fused-Zn-2, Zn-1, and Zn-2-sensitized TiO2 cells (Figure 7). The photovoltaic properties of the cosensitized cells with an immersing time of 0.5 h, which is the best conditions for the cell performance, are listed in Table 4. With an increase in the ratio of Zn-3 vs fused-Zn-1, the η value is increased gradually together with an increase in the JSC value to reach the maximal η ) 5.0% where the molar ratio of fusedZn-1 and Zn-3 is 1: 8 (Figure 10). Further increase in the ratio of Zn-3 vs fused-Zn-1 leads to a decrease in the η value. Under the optimized conditions exhibiting the maximal η ) 5.0%, the photocurrent action spectrum discloses the photocurrent response with the peaks at around 420, 560, and 600 nm arising from Zn-3 and those at around 490 and 700 nm arising from fusedZn-1 (Figure 11). Although the improvement of cell performance is moderate, these results unambiguously corroborate that cosensitization of porphyrins with complementary absorption properties is a potential methodology to improve the cell performance in DSSCs. Conclusions We have successfully synthesized novel unsymmetrically π-elongated porphyrins for dye-sensitized TiO2 cells. The TiO2 cell with naphthyl-fused-porphyrin exhibited the power conversion efficiency of 4.1%, which was improved by 50% relative to the nonfused porphyrin reference cell. These results clearly show that elongation of the porphyrin π-system with low
TABLE 4: Cell Performances of Co-sensitized Cells under the Optimized Conditions
a
fused-Zn-1: Zn-3 (solution)a
Γ (fused-Zn-1) (10-11 mol cm-2)
Γ (Zn-3) (10-11 mol cm-2)
JSC (mA cm-2)
VOC
ff (V)
η (%)
1:0 1:1 1:2 1:3 1:4 1:8 1:10 0:1
7.8 6.1 3.7 2.3 2.4 1.1 0.8 0
0 6.4 6.8 7.5 9.1 9.5 10.7 12.0
10.6 11.0 11.5 11.6 11.6 11.7 10.8 9.4
0.62 0.63 0.64 0.65 0.66 0.67 0.67 0.76
0.62 0.61 0.60 0.60 0.64 0.64 0.64 0.64
4.1 4.2 4.4 4.5 4.9 5.0 4.6 4.6
Molar ratio of fused-Zn-1 and Zn-3 in MeOH where the total concentration of the porphyrins was fixed at 0.2 mM.
15584 J. Phys. Chem. C, Vol. 112, No. 39, 2008
Hayashi et al. Supporting Information Available: Synthetic detail of porphyrins (S1), immersing time profiles of Γ values for fusedZn-1 and fused-Zn-2 (S2), and immersing time profiles of Γ values and η values for cosensitized solar TiO2 cell (S3) are available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 10. Comparison of current-voltage characteristics under the optimized conditions. (a) fused-Zn-1 and Zn-3 mixed cell (solid line, η ) 5.0%), (b) fused-Zn-1 cell (dashed line, η ) 4.1%), (c) Zn-3 cell (dotted line, η ) 4.6%). Conditions: electrolyte 0.1 M LiI, 0.05 M I2, 0.6 M 2,3-dimethyl-1-propyl imidazolium iodide, and 0.5 M 4-tbutylpyridine in CH3CN; input power: AM 1.5 under simulated solar light (100 mW cm-2).
Figure 11. Action spectrum of cosensitized TiO2 cell under the optimized conditions (solid line). The initial molar ratio of fused-Zn-1 and Zn-3 in MeOH is 1: 8. Conditions: electrolyte 0.1 M LiI, 0.05 M I2, 0.6 M 2,3-dimethyl-1-propylimidazolium iodide, and 0.5 M 4-tbutylpyridine in CH3CN; input power: AM 1.5 under simulated solar light (100 mW cm-2). The UV-visible absorption spectrum of the cosensitized TiO2 electrode is also shown for comparison (dashed line). The thickness of the TiO2 electrode for UV-visible absorption measurement was adjusted to be 700-1000 nm to obtain the shape and peak position of the spectrum accurately.
symmetry is a useful tactic for collecting solar light in the visible and near-infrared regions, leading to improved cell performance of porphyrin-sensitized solar cells. The TiO2 cell with the naphthyl-fused-porphyrin exhibited the improved power conversion efficiency of 5.0% when the naphthyl-fused-porphyrin is cosensitized with the porphyrin with different absorption features. Thus, cosensitization of porphyrins with complementary absorption properties is a potential methodology to improve cell performance in DSSCs. Acknowledgment. This work was supported by Strategic University/Industry Alliance of the International Innovation Center, Kyoto University and Grant-in-Aid (No. 19350068 to H.I.) from MEXT, Japan. We gratefully acknowledge Prof. Susumu Yoshikawa (Kyoto University) and Prof. Shozo Yanagida and Dr. Naruhiko Masaki (Osaka University) for the use of equipment for photovoltaic measurements. Computation time was provided by the Supercomputer Laboratory and Academic Center for Computing and Media Studies, Kyoto University.
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