Photoelectrochemical Properties of Doubly β-Functionalized Porphyrin

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J. Phys. Chem. C 2008, 112, 16691–16699

16691

Photoelectrochemical Properties of Doubly β-Functionalized Porphyrin Sensitizers for Dye-Sensitized Nanocrystalline-TiO2 Solar Cells Jong Kang Park,† Hye Ryun Lee,† Jinping Chen,‡ Hiroshi Shinokubo,*,‡ Atsuhiro Osuka,*,‡ and Dongho Kim*,† Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea, and Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8502, Japan ReceiVed: May 14, 2008; ReVised Manuscript ReceiVed: July 18, 2008

Functionalized porphyrins at meso- and β-positions with different carboxylic acid groups were prepared to investigate electronic and photovoltaic properties as dye-sensitized nanocrystalline-TiO2 solar cells. The electronic structures of the porphyrin macrocyclic core are strongly coupled with olefinic side chains so that the absorption spectrum exhibits largely broad and red-shifted Soret and Q-bands, especially up to 475 nm at the Soret band in a porphyrin doubly functionalized with malonic diacid groups. Among porphyrin derivatives prepared in this study, 2b-bdta-Zn exhibits the maximum overall conversion efficiency of 3.03% and the maximum incident photon to current efficiency of 60.1% in the Soret band region, superior to the others. From such photovoltaic performances, we can suggest that multiple pathways through olefinic side chains at two β-positions enhance the overall electron injection efficiency and the moderate distance between the porphyrin sensitizer and the TiO2 semiconductor layer is important, retarding the charge recombination processes. As a consequence, these combined effects give rise to higher photovoltaic efficiency in photovoltaic regenerative solar cells. 1. Introduction There has been an increasing demand for regenerative solar cells because of an easily anticipated exhaustion of conventional energy sources for the last several decades. The extensive investigations on dye-sensitized solar cells (DSSC), which have attractive advantages such as low-cost production over other type of solar cells, were ignited by Gra¨tzel group using Ru(II) bipyridyl complexes and TiO2.1 They reported the power conversion efficiency over 10% for photoelectrochemical cells based on nanoporous TiO2 films sensitized by Ru(II) bipyridyl complexes and their ananlogues.2 In this device, commonly called Gra¨tzel type cell, electrons excited by photons are injected into the conduction band of semiconductors such as TiO2 and ZnO, in which the oxidized sensitizers are regenerated by the redox reaction with an electrolyte.3 This remarkable accomplishment has been drawing a huge number of studies in this field with the same basic concept.4 Despite numerous investigations on new dyes in DSSC, however, no further enhancement in photocurrent generation better than Ru(II) complexes has been achieved. As a new candidate for dye molecules in DSSC, many researchers have investigated chlorophyll derivatives to mimic the photosynthetic system of nature.5 Among them, porphyrins have been recognized as the most promising dyes mainly because of their photochemical and electrochemical stabilities, easy synthetic process, strong absorbing ability in the visible region, and handy control of redox potential by metalation.6-10 In DSSC, the sensitizer dyes including porphyrins are adsorbed on the TiO2 surface through anchoring groups such as carboxylic acid, sulfonic acid, phosphonic acid, etc.11 The effective coupling * Corresponding author. E-mail: [email protected], osuka@kuchem. kyoto-u.ac.jp, [email protected]. † Yonsei University. ‡ Kyoto University.

between anchoring group and nanocrystalline TiO2 has typically been realized from the complexation of carboxylate and phosphonate groups via a bridging bidentate or ester-like mode.12,13 To connect these binding groups, various forms of linkers have been used between the porphyrin and the end-group. However, despite the considerable efforts for utilizing porphyrins in DSSC and an augmentation of overall conversion efficiency, there have been dissimilar reports for the same porphyrin or on the exact influence of linker groups attached at the outer position of the porphyrin.14 Some reported negligible effect of the linker regardless of an effective conjugation through the bridge, the other discovered great change in efficiency of photovoltaic cell depending on the change of the linker character.11,15,16 However, according to the most recent reports, the degree of electronic coupling of the linker with the porphyrin core and the distance controlled by the linker length largely affect the photocurrent generation of nanocrystalline-TiO2 photovoltaic cells based on various functionalized porphyrins.13,16-18 On the other hand, the role of the functionalized position of porphyrins for the electron injection efficiency has been another issue and generally recognized that β-functionalized porphyrins are more efficient than meso-functionalized ones.11,15b,16,19 Therefore, a selective introduction of the carboxylic group at the desired position with different bridge lengths is important to characterize the influence by linkers. In this context, we have introduced unsaturated carboxylic acid groups at double β-positions of porphyrins through Rh catalysis.20 Seven β-substituted and one meso-substituted porphyrins with different linker distances were prepared to compare the effect of the position of functionalization, the distance, and the number of bridges. The carboxylate group, introduced in this series, at the side chain of porphyrins serves as an anchoring moiety to ensure efficient adsorption on the surface of TiO2, which is expected to enhance the efficiency of DSSCs. The carboxylic acid porphyrin derivatives investigated in this work

10.1021/jp804258q CCC: $40.75  2008 American Chemical Society Published on Web 09/27/2008

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SCHEME 1: Structures of the Carboxylic Acid Porphyrins Substituted at Meso- and β-Positions

are named as follows: the prefix nb represents that n is the number of funtionalized positions and b is β-position. The middle indices d, bd, and ta mean diene, bis-diene, and tetracarboxylic acid and the suffix FB and Zn indicate freebase porphyrin and Zn porphyrin, respectively (Scheme 1). Here we have investigated the electronic properties and cell performances of unsaturated carboxylate group substituted porphyrins at β-positions adsorbed on TiO2 nanocrystalline surface through UV-vis absorption, adsorption behavior on TiO2, density functional theory calculations, incident photonto-current efficiency (IPCE) measurements, and power conversion efficiency. All compounds maintain the same aryl group (Ar ) 3,5-di-tert-butylphenyl) to remove the effect of different size of the aryl group and reduce aggregation through π-stacking between porphyrin units. With these objectives in our mind, the linker length was controlled by changing 2-propenoic acid group to (E,E)-2,4-pentadienoic acid group in two β-substituted porphyrins. To enhance the electron injection efficiency, the malonic diacid group, which exhibits more efficient charge injection efficiency than a simple carboxylic acid group,19 was introduced in 2b-bdta-Zn. To compare the influence of the bridge number, 1b-d-Zn which has (E,E)-2,4-pentadienoic acid group at one β-position was prepared. The porphyrin sensitizer functionalized at two β-pyrrolic positions with (E,E)-2,4-pentadienoic acid groups, 2b-bd-Zn, achieves η ) 2.37% while DSSC based on N3 dye used as the reference system and 1b-d-Zn attained 5.85 and 2.08%, respectively, under the same conditions. Moreover, 2b-bdtaZn exhibits the highest η ) 3.03% in these series. Importantly, the doubly functionalized porphyrin is better than the single bridged one, because of more rigid binding onto TiO2 and multiple pathways for photoinduced electron injection processes. This work provides further insight into the design of porphyrin sensitizers for more efficient performance in DSSC. 2. Experimental Section Synthesis of Porphyrins. The details of the synthesis of samples are described elsewhere20 and in the Supporting Information. Steady-State Absorption and Emission. Steady-state absorption spectra in solution and on transparent TiO2 were acquired using a UV–vis–NIR spectrometer (Varian, Cary5000). Steady-state fluorescence spectra were recorded on a fluorescence spectrometer (Hitachi, FL2500).

Preparation of TiO2 Film. A commercial paste of pure anatase nanoporous TiO2 (Ti-Nanoxide D, Solaronix) was coated by the doctor blade technique onto a fluorine doped SnO2 conducting glass (TEC 8, Pilkington) for electrochemical studies. After drying at ∼80 °C, the films were annealed at 450 °C for 15 min. The TiO2 electrodes were immersed into each of the 0.2 mM ethanol solution of the porphyrins at room temperature for 12 h. The sensitized film were rinsed by ethanol carefully to remove physisorbed dye and dried under air conditions. The TiO2 film thickness was measured as 7-8 µm using an Alphastep 200 surface profiler (Tencor P-10). The transparent TiO2 films were prepared by the same method with commercial paste of highly transparent TiO2 (Ti-Nanoxide HT, Solaronix). For absorption measurement on TiO2, highly transparent TiO2 paste was used for semiconductor layer and the thickness was measured as ∼4 µm. Since TiO2 nanoporous material starts to absorb light strongly from 350 nm, all the absorption spectra were corrected with the absorption spectrum of TiO2 to retrieve the absorption spectra of porphyrin molecules adsorbed on TiO2 layer. The adsorption quantity in 2 × 10-4 M porphyrin solution dissolved in ethanol increases with immersing time. When the immersing time is over 1 h, the Soret band absorption rise too intense to show spectral changes by excitonic interactions between porphyrin units on TiO2. The moderate Soret band absorption intensity of porphyrins adsorbed on TiO2 was seen by nearly 5-10 min immersing time under our experimental conditions. The Photoelectrochemical Measurement. The incident photon-to-current efficiency (IPCE) measurements were performed in sandwich type cell by using porphyrin adsorbed TiO2 film and Pt sputtered conducting glass as counter electrode. The counter electrode was prepared by coating sputtered Pt onto ITO conducting glass for IPCE measurement. Electrolyte (0.05 M I2, 0.1 M LiI, 0.6 M butylmethylimidazolium iodide, 0.5 M 4-tert-butylpyridine in 3-methoxypropionitrile) was introduced into the interelectrode space. The IPCE of the cells were measured by using a potentiostat (Keithley 2400 source meter), a 300 W xenon lamp (Oriel Co.) in combination with a spectrapro-150 monochromator (Acton Research Co.), in the range of 400-800 nm. The cutoff filter of 400 nm was attached at the output slit of the monochromator to remove UV light. The light intensities were measured with calibrated power meter (LM-2 VIS, Coherent) at each wavelength.

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The power conversion efficiency (PCE) measurements were performed in a sandwich cell consisting of the porphyrinsensitized TiO2 electrode as the working electrode and a platinum-coated counter electrode as well. The Pt counter electrodes were prepared by coating H2PtCl6 solution (5 mM in isopropyl alcohol) on to the FTO and then sintering at 400 °C for 20 min. The electrodes were separated by a 25 µm-thick Thermoplast hot-melt sealing film (SX1170-25, Solaronix SA) as a spacer and sealed at 120 °C for 20 s. The liquid electrolyte (0.05 M I2, 0.1 M LiI, 0.6 M butylmethylimidazolium iodide, 0.5 M 4-tert-butylpyridine in 4:1 acetonitrile/valeronitrile) was introduced into the interelectrode space from the Pt counter electrode side through a predrilled hole. The hole was sealed with a microscope cover slide and Surlyn polymer to avoid leakage of the electrolyte solution. Cell performance was evaluated using a collimated beam from a 300 W Xe lamp (Oriel Co.) with AM filter and the incident light intensity was adjusted with a NREL-calibrated Si solar cell (PV Measurements Inc.) for 1 SUN (100 mW cm-2) intensity. The J-V curves were measured at AM1.5 illumination using a Keithley 2400 source meter. Each value for cell performance was taken as the average of three independent samples. Density Functional Theory Calculations. Theoretical calculations were performed with the Gaussian03 program suite.21 All calculation were carried out by the density functional theory (DFT) method with Becke’s three-parameter hybrid exchange functionals and the Lee-Yang-Parr correlation functional (B3LYP),22,23 employing 3-21G and 6-31G* basis set for all atoms. 3. Results and Discussion 3.1. Steady-State Absorption. Figure 1 shows the absorption spectra of meso- and β-functionalized porphyrins, which indicate the effect of olefinic side chains at β-positions on the electronic properties of porphyrins. The electronic interaction between the macrocyclic core part of the porphyrin and the unsaturated group considerably affects the electronic states and the absorption character by expansion of π-electron conjugation pathway, a decrease in the symmetry of the porphyrin, and an increase of flexibility because of the anchoring groups. Especially as the olefinic side chains with strong electron-withdrawing groups are connected, the feature of absorption bands of the porphyrin is strongly perturbed. The absorption spectrum of 2m-s-Zn prepared as a reference molecule shows nearly the same electronic properties as Zn(II)TPP because of negligible electronic coupling between the porphyrin core and the propanoic acid group at the mesoposition. The perpendicular dihedral angle of side chain at the meso-position as shown by the X-ray crystallographic structure is ascribed to the steric hindrance between β-hydrogens of the porphyrin and the side chain at the meso-position.20 The strong and sharp Soret band was observed at 426 nm along with weak Q(1,0) band at 562 nm and Q(0,0) band at 603 nm in ethanol, being distinctly characteristic of a Zn porphyrin anchoring with a perpendicular orientation through aryl groups at mesopositions.24 (Table 1) On the other hand, the carboxylic acid group connected to the β-position through olefinic chains allows the electron density to be delocalized through the bridge with the porphyrin core. 2b-FB, which is a free base porphyrin with 2-propenoic acid groups at two β-positions, shows the red-shifted Soret band at 438 nm with a reduced extinction coefficient and discrete four Q-bands, Qx(1,0), Qx(0,0), Qy(1,0) and Qy(0,0). The zero-zero excitation energy of 1.87 eV and the calculated HOMO-LUMO

Figure 1. UV-vis absorption spectra of 2m-s-Zn, 2b-FB, 2b-Zn, 4bFB, 4b-Zn, 1b-d-Zn, 2b-bd-Zn, and 2b-bdta-Zn in ethanol. The spectra are normalized for comparison.

gap of 2.47 eV originated from a more negative shift of LUMO and LUMO+1 levels compared with 2m-s-Zn by effective electronic coupling through olefinic side chains (Table 2). The Zn metalated form of 2b-FB, 2b-Zn, shows the more red-shifted Soret band at 452 nm in contrast to that of 2b-FB at 438 nm and a couple of Q-bands, Q(1,0), and Q(0,0), by reduced C2V symmetry. The red-shifted Soret band up to 450 and 466 nm and the broadened absorption spectra for 4b-FB and 4b-Zn are indicative of effective delocalization of electron density through the bridge and the planar geometry of side chains as confirmed by optimized structures. The expansion of electron density along one axis causes dual Q-bands instead of four Q-bands for 4b-Zn. 1b-d-Zn, which has a diene chain at the β-position, was prepared for comparison with the previous works by Gra¨tzel and co-workers.16,19 Similar porphyrin sensitizers bearing malonic diacid group instead of a simple carboxylic acid group were observed to be the most efficient porphyrin derivatives for DSSC up to now, exhibiting broad absorption which is particularly good for light harvesting. The Soret band observed at 441 nm and its fwhm are nearly three times broader than 2m-s-Zn, but the vibronic feature of the Q-band remains at 565

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TABLE 1: UV–Vis Absorption and Fluorescence Data Measured in Ethanol compound 2m-s-Zn 2b-FB 2b-Zn 4b-FB 4b-Zn 1b-d-Zn 2b-bd-Zn 2b-bdta-Zn

Soret

 (105 M-1 cm-1)

426 438 452 450 466 441 466 475

4.56a 1.57b 1.76b 1.91a 2.09a 1.95b 1.93b 1.14a

524, 527, 572, 537, 542, 565, 576, 584,

Q-band

Emi (nm)

∆E (cm-1)

E0-0c

562, 561, 618 572, 580, 599 607 621

611, 664 667, 736 633, 684 677, 750 647, 709 617, 672 642, 698 649, 700

217.1 113.2 383.4 87.8 193.5 487.0 898.1 694.7

2.04 1.87 1.98 1.84 1.93 2.04 1.98 1.95

603 603, 662 612, 673 616, 639

a The extinction coefficients of samples were obtained in THF. b The extinction coefficients of samples were obtained in MC. c The zero-zero excitation energy (also called as Q-state energy) was obtained from the intercept of the normalized absorption and emission spectra measured in ethanol for all samples.25,26

TABLE 2: Energy Level from DFT Calculation and Measured Redox Potentialsa compound HOMO-1 (eV) HOMO (eV) LUMO (eV) LUMO+1 (eV) ∆Ecalb (eV) E1/2,oxc (V) E1/2,redc (V) ∆EHOMO-LUMOd (V) E/1/2,ox (eV) ZnTPP 2m-s-Zn 2b-FB 2b-Zn 4b-FB 4b-Zn 1b-d-Zn 2b-bd-Zn 2b-bdta-Zn

-5.20e -5.28 -5.50 -5.42 -5.74 -5.69 -5.30 -5.33 -5.61

-5.06e -5.09 -5.22 -5.36 -5.52 -5.61 -5.20 -5.31 -5.58

-2.10e -2.29 -2.75 -2.67 -3.10 -3.05 -2.53 -2.72 -3.13

-2.10e -2.23 -2.59 -2.50 -2.94 -2.59 -2.26 -2.53 -2.83

2.96e 2.80 2.47 2.69 2.42 2.56 2.67 2.59 2.45

0.29 0.26 -f 0.33 -f 0.44 0.30 0.32 0.34

-1.91 -1.91 -1.52 -1.66 -1.34 -1.51 -1.75 -1.65 -1.58

2.20 2.17 2.10 1.99 2.06 1.95 2.05 1.97 1.92

-1.78 -1.65 -1.49 -1.74 -1.66 -1.61

a Calculated at B3LYP/6-31 (d)//B3LYP/3-21 G level. b Difference in the HOMO-LUMO energy. c These values were measured by cyclic voltametry in ester forms of all porphyrins using a platinum working electrode, a platinum wire counter electrode and Ag/0.01 M AgClO4 reference electrode. The measurements were carried out in dichloromethane solutions containing 0.1 M Bu4NBF6 as a supporting electrolyte. (in V vs ferrocene/ferrocenium ion pair). d ∆EHOMO-LUMO ) E1/2,ox - E1/2,red. e Reference 16. f Not resolved.

and 599 nm as Q(1,0) and Q(0,0), respectively. The previous efficient photovoltaic performance in mono β-functionalized porphyrins have led us to prepare 2b-bd-Zn and 2b-bdta-Zn, which possess additional bis-diene side chains at the same side β-position. The absorption spectra of 2b-bd-Zn and 2b-bdtaZn exhibit remarkably broad and red-shifted Soret bands appearing at 466 and 475 nm, respectively. Especially, the Soret band ranging from 400 to 520 nm and a continuous density of states over S1 and S2 states in 2b-bdta-Zn implies strongly perturbed electronic state of the porphyrin core by malonic diacid groups. Interestingly, the change in the Q-band position in all samples is not significant in contrast to considerable changes in the Soret band. Even in 2b-bdta-Zn, the Q-band appeared as Q(1,0) at 584 nm and Q(0,0) at 621 nm similar to 2b-Zn. All the fluorescence spectra exhibit the typical features of porphyrins with two vibronic structures as mirror image to the absorption bands. From the largest E0-0 value of 2m-s-Zn (2.04 eV), we can notice that as the π-conjugation length increases, the zero-zero excitation energy value decreases. This trend was observed in going from 2b-FB (1.87 eV), 2b-Zn (1.98 eV), and 1b-d-Zn (2.04 eV) to 4b-FB (1.84 eV), 4b-Zn (1.93 eV), and 2b-bd-Zn (1.98 eV). The E0-0 value of 1.95 eV in 2bbdta-Zn is smaller than that of 2b-bd-Zn because of the extension of electron density through the bridge. Unsaturated carboxylic acid functions at β-positions of porphyrins commonly showed further red-shifted and broader Soret bands as compared with 2m-s-Zn. This feature can be explained by the effective electronic coupling through olefinic side chains as can be seen in the density functional theory calculations. The broadened UV-vis absorption spectra of β-substituted porphyrins with strong electronic interactions through diene or ene bridges show a prospective possibility for broad light harvesting into green region. Additionally, it is worthy to note that the characteristic absorption features of

porphyrins, a strong Soret band and vibronic structures of weak Q-bands, are maintained in the absorption spectra of all samples in spite of the reduced symmetry and the delocalization of electron density to olefinic side chains. In the meantime, the perturbation of the porphyrin core by carboxylic acid groups is not so significant as compared with other stronger electronwithdrawing groups such as 1,3-diethyl-2-thiobarbituric acid (TBA), indicating that the energy levels for the excited states of the porphyrins still remain at the proper level with respect to the conduction band edge of TiO2 semiconductor for electron injection process.27 3.2. DFT Calculation and Electrochemical Properties. We have carried out DFT calculations to gain further insight into the electronic structure depending on the side chains of β-substituted porphyrins with different numbers, positions, and lengths (Figure 2). The initial geometry was introduced from the X-ray crystallographic structures.20 We have first compared the molecular orbitals of 2m-s-Zn and Zn(II)TPP to examine the influence of the propanoic acid group at meso-positions. As seen in the steady-state absorption spectrum, the molecular orbitals of 2m-s-Zn are similar to those of Zn(II)TPP possessing a1u(HOMO-1), a2u(HOMO), and eg(LUMO, LUMO+1) according to Gouterman’s four orbital model.28,29 The calculated HOMO-LUMO gap of 2.80 eV is slightly smaller than that of Zn(II)TPP, 2.91 or 2.96 eV, depending on the level of calculation (Table 2).16,24 The negligible electronic coupling between the porphyrin core and the side chain at two meso-positions of 2m-s-Zn makes it retain the MO structures of Zn(II)TPP except a small change in HOMO and LUMO levels by replacing the perpendicular phenyl ring by the propanoic acid group. On the other hand, while the π-orbitals of macrocyclic core of the other porphyrins interact with olefinic side chain at β-positions, the molecular orbital shapes of HOMO, HOMO-

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Figure 2. Selected four molecular orbitals for carboxylic acid porphyrins calculated at B3LYP/6-31G(d)//B3LYP/3-21G level.

1, LUMO, and LUMO+1 of β-tethered porphyrins show no significant change from those of Zn(II)TPP or 2m-s-Zn. Thus, the characteristic Soret and Q-bands of porphyrins still appear despite the electronic delocalization through olefinic groups. For 2b-FB, HOMO and HOMO-1 still remain as a1u and a2u-like MO while LUMO and LUMO-1 as eg-like MO at the porphyrin core. However, LUMO shows a significant electron density on the olefinic bridge, which provides a good pathway for electron injection through the bridge. The HOMO and LUMO levels were shifted to more negative because of the expansion of π-electron pathway. In the case of 2b-Zn, while the overall molecular orbital structures were conserved despite the modification by Zn metalation from free-base form, the negative shift of the HOMO and HOMO–1 energy level and the positive shift of the LUMO and LUMO+1 level contribute to the zero-zero excitation energy of 2b-Zn than that of 2b-FB (Table 2). As the number of bridges increases, the energy levels of both HOMO and LUMO decrease, and thereby the tendency was observed in 4b-FB and 4b-Zn. In contrast to multiple substituted porphyrin systems, notable differences were seen in 1b-d-Zn, which has a different axis of rotation at the β-position. The energy gap of 0.27 eV between LUMO and LUMO+1 is the largest among seven samples except 2b-bdta-Zn, and LUMO+2 moves to -1.61 eV (-0.76 eV for 2m-s-Zn). Despite the increased bridge length of the diene side chain, electron density delocalization occurs effectively as shown in molecular orbital diagram, indicating considerable electronic coupling through the diene bridge. 2b-bd-Zn, which has one more diene-bridged carboxylic acid group at another β-position, shows similar molecular orbitals

to those of 2b-Zn, indicating still effective electronic coupling through the bis-diene bridge in LUMO (-2.72 eV) and LUMO+1 (-2.53 eV). Accordingly, we can expect effective multiple charge injection pathways to the semiconductor through two bridges at β-positions. In contrast with other molecules, however, this molecule has HOMO and HOMO-1 at -5.31 and -5.33 eV, respectively, and nearly degenerate HOMO-2 and HOMO-3 are shifted up to -6.00 eV, being close to HOMO level. Additional diacid groups in 2b-bdta-Zn, totally accompanying four COOH groups, caused further electron density shift toward bridges. The LUMO of 2b-bdta-Zn was calculated to be -3.13 eV, nearly 1 eV negatively shifted compared with 2m-s-Zn, and HOMO and HOMO-1 appeared at -5.58 and -5.61 eV, respectively, which were found at -5.09 and -5.28 eV for 2m-s-Zn. Overall, all porphyrin samples except 2m-s-Zn exhibited more negatively shifted LUMO+2 and LUMO+3 and more positively shifted HOMO–2 and HOMO–3, respectively, by the influence of olefinic groups at β-positions. These changes in molecular orbital levels mainly contribute to the broadening in the absorption spectra except 2m-s-Zn which shows a large gap between LUMO+1 (-2.23 eV) and LUMO+2 (-0.76 eV). The redox potentials of porphyrin derivatives have been measured by cyclic voltammetry to estimate the excited-state oxidation potentials in V vs Fc/Fc+ (0.64 V NHE).13 Electrochemical measurements were performed on the ester derivatives in dichloromethane because of low solubility of their acid forms. All samples exhibited reversible one- or two-electron redox waves, which are in accordance with DFT calculation and spectroscopic results (Table 2). Redox potentials of Zn(II)TPP

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were also measured as a reference and 2m-s-Zn showed virtually similar redox potential to Zn(II)TPP in 0.29 and -1.91 V as the first oxidation and reduction potentials. With an increase in the π-conjugation length and the number of bridges in going from 1b-d-Zn to 2b-bdta-Zn, the first oxidation potential was slightly shifted to more positive values from 0.30 (1b-d-Zn), 0.32 (2b-bd-Zn), to 0.34 V (2b-bdta-Zn) while the first reduction potential was altered less negatively from -1.75, -1.65, to -1.58 V, more steeply than oxidation potentials. These contribute to the reduced HOMO-LUMO gap over 2bbd-Zn and 2b-bdta-Zn than 1b-d-Zn and are consistent with the tendency on the change of HOMO-LUMO gap calculated from DFT calculation and UV-vis data in these porphyrin series. Accordingly, the reduced HOMO-LUMO gap in these series could be mainly attributed to the change in the first reduction potentials while the first oxidation levels retain around 0.3 V. The excited-state oxidation potentials were obtained from the first oxidation potential measured by cyclic voltammetry, and the zero-zero excitation energy was calculated by the absorption and emission spectra using eq 1.

E1⁄2*(P+/P) ) E1⁄2(P+/P) - E0-0

(1)

where E1/2(P+/P) is the first oxidation potential of porphyrin and E0-0 is the zero-zero excitation energy obtained by the intersection of the normalized lowest energy absorption peak and highest energy fluorescence peak. All E0-0 values in our experiment were obtained by the absorption and fluorescence spectra measured in ethanol which was also used in the preparation of sandwich cell. The electron injection process from adsorbed dyes to the conduction band of the semiconductor occurs efficiently when the driving force is appropriate. The conduction band edge of TiO2 has been well-known as -0.5 V (vs NHE).3,30 The excitedstate oxidation level of typical Zn(II) porphyrin has been reported to be virtually -1.0 V (vs NHE) so that the driving force for electron injection in porphyrin-TiO2-based cell is ∼0.5 V.31 Therefore, the estimated excited-state oxidation potentials, -1.91 (2m-s-Zn), -1.66 (2b-Zn), -1.75 (1b-d-Zn), -1.65 (2bbd-Zn), and -1.58 (2b-bdta-Zn) V (vs Fc/Fc+) are still sufficient for electron injection. 3.3. UV-vis Absorption on the TiO2 Layer. The geometry of porphyrins adsorbed on the TiO2 layer largely affects the photoelectrical performance depending on a perpendicular or a horizontal orientation with respect to TiO2 surface. UV-vis absorption spectra on TiO2 were measured for all porphyrin samples to find a clue to the molecular geometry (Figure 3). The most notable feature in the absorption spectra on TiO2 films is that there is no peak shift for 2m-s-Zn compared with its solution spectra while there are blue-shift and broadening of the Soret bands for singly and doubly β-substituted porphyrins. The Soret band remaining at 426 nm for 2m-s-Zn in solution as well as on TiO2 is attributable to the negligible interaction between porphyrin units because of the flexible single bond bridge. On the other hand, the Soret bands at 438 and 452 nm for 2b-FB and 2b-Zn in solution were shifted to 427 and 448 nm in TiO2 films, respectively. Such a peak shift and asymmetrical broadening are indicative of the geometry of porphyrins bound onto TiO2 film. The blue-shift and broadening of the Soret band represent excitonic coupling between porphyrin units with major H-type interactions, indicating highly packed condition on TiO2.18,32 In the cases of 4b-FB and 4bZn, which are expected to be adsorbed on TiO2 layers with a horizontal orientation, a low adsorption quantity with indistin-

Figure 3. UV-vis absorption spectra of carboxylic acid porphyrins on HT-TiO2/glass after dipping for (a) 5 min and (b) 18 h.

guishable Q-band in absorption spectrum on TiO2 was observed despite long immersing time. The previous studies reported the higher efficiency of the planar binding mode than the perpendicular geometry of porphyrins on TiO2 layer.18 Nevertheless, in our case the origin of scarce adsorption quantity and very low IPCE is not clear at this stage. Presumably the bridge length is not long enough to bind onto the TiO2 surface with the planar geometry. Moreover, the perpendicular binding mode of 4bFB and 4b-Zn causes a decrease in the electron injection efficiency because of the electron-withdrawing group at unbound carboxylic acid groups. The formation of a J-type aggregate in solution, which is large in size, also may prevent the porphyrin aggregates from penetrating into the pores of TiO2 films (Supporting Information). Besides 4b-FB and 4b-Zn, similar behaviors were also observed in the Soret band region for 1b-d-Zn, 2b-bd-Zn, and 2b-bdta-Zn. The Soret bands of 1b-d-Zn, 2b-bd-Zn, and 2bbdta-Zn on TiO2 were shifted from 441, 466, and 475 nm in solution to 427, 450, and 469 nm in TiO2 films, respectively, and exhibited asymmetric shapes, indicating H-type interactions due to the cofacial and perpendicular geometries of β-functionalized porphyrins on TiO2 layer. Therefore, from these absorption characteristics and previous works, we can expect highly packed monolayers of porphyrins on TiO2 surface at the final stage. The quantitative measurements of adsorbed sensitizers were not successful because of the low solubility and formation of aggregates in NaOH solution and even in DMF mixed NaOH solution. 3.4. Photoelectrochemical Properties. In a regenerative photovoltaic cell, sensitizers are excited by incident photons. The hot electrons can decay through several relaxation processes; electron injection process to the conduction band of semiconductor, intersystem crossing to the triplet state and

Doubly β-Functionalized Porphyrin Sensitizers

J. Phys. Chem. C, Vol. 112, No. 42, 2008 16697

Figure 4. Photocurrent action spectra of functionalized porphyrins adsorbed on TiO2 layer. Efficiency of N3 dye also was measured as reference molecule at the same condition.

radiative or nonradiative decay to the ground state.33 Among these, charge injection process has been recognized as ultrafast process occurring in ∼100 fs to a few tens picosecond time scale while radiative decay process occurs in a few nanoseconds.34 Therefore, after relaxation to the S1 state in porphyrins the charge injection process is highly dominant than other channels, 103-104 times faster than radiative decay rates. The hot electrons are injected to the conduction band edge of TiO2 at -0.5 V (vs NHE) and move to the counter electrode by driving force induced by the gap between redox potential of I-/I3- liquid-electrolyte typically.31 The oxidized porphyrins can be retrieved to its singlet ground-state by electron donation through redox reaction of I-/I3- (+0.5 V vs NHE) in electrolytes. To understand photovoltaic behavior depending on bridge nature and length, the incident photon-to-current efficiency (IPCE) action spectra given by the following equation were measured for all samples as shown in Figure 5.35

IPCE )

1240(eV nm) × Jph(mA cm-2) λ(nm) × φ(mW cm-2)

× 100

(2)

where Jph is the photocurrent density for monochromatic irradiation, λ is the wavelength of monochromatic light, and φ is the incident photon intensity. As a reference device, the efficiency of the N3-TiO2 sandwich type cell was measured under the same conditions. TiO2 electrode was immersed in N3 dye solution in ethanol (0.3 mM) for 12 h, and the power conversion efficiency, η, was obtained to be 5.85% with 12.57 mA cm-2 of Jsc, 0.73 V of Voc, and 0.64 of fill factor which was reported to exhibit over 10% in the previous work by Gra¨tzel group1 (Figure 4). The overall photocurrent action spectra were observed to correspond to the absorption spectra of sensitizer on TiO2 layer. Commonly, the ratio of Soret to Q-band region notably decreased compared to solution phase as more efficient porphyrin sensitizer. The IPCE values of 2b-bd-Zn and 1b-d-Zn were measured as 53.6 and 51.6% at the Soret band maximum and 27.3 and 23.0% at the Q-band maximum, respectively. In the cases of 2b-FB, 2b-Zn, lower IPCE values were measured because of a bridge length shorter than that of 2b-bd-Zn. Such low and similar shape of IPCE action spectra was measured for 2m-s-Zn with maximum 8.8% in the Soret region for 2ms-Zn due to ineffective electronic coupling through the propanoic acid bridge at the meso-position. The most efficient system was discovered in the DSSC based on 2b-bdta-Zn with a maximum IPCE value of 60.1 and 21.8% at 450 and 590 nm in the Soret and Q-band regions, respectively. The main aspect

Figure 5. Photocurrent-voltage characteristic curves of porphyrins and N3 in the condition of simulated global AM1.5 solar radiation at 100 mW cm-2 as a sandwich type cell.

TABLE 3: Photoelectrochemical Results of the Porphyrin-Sensitized Solar Cella IPCE (%)/(nm) compound 2m-s-Zn 2b-FB 2b-Zn 4b-FB 4b-Zn 1b-d-Zn 2b-bd-Zn 2b-bdta-Zn

Soret

Q-band

8.8 (430) 1.0 (570) 18.3 (420) 4.9 (540) 41.7 (450) 9.5 (580) 1.0 (430) 1.0 (680) 1.4 (460) 1.2 (680) 51.6 (450) 23.0 (580) 53.6 (470) 27.3 (590) 60.1 (450) 21.8 (590)

Jsc (mA cm-2) Voc (V) 0.51 1.84 4.45 -b -b 6.20 6.74 8.38

0.45 0.51 0.54 -b -b 0.54 0.56 0.59

ff

η (%)

0.60 0.67 0.61 -b -b 0.62 0.62 0.62

0.14 0.63 1.48 -b -b 2.08 2.37 3.03

a Overall conversion efficiency parameters were measured in the light condition of 100 mW cm-2 illumination. b Not measured.

of the highest efficiency in 2b-bdta-Zn is the stronger electronwithdrawing group (malonic diacid group) compared with 2bbd-Zn (carboxylic diacid group) and the longer distance from TiO2 compared with 2b-Zn and 2b-FB. The IPCE value of virtually 1% efficiency for 4b-Fb and 4b-Zn obtained in the Soret region showed that these porphyrins are not efficient sensitizers in the DSSC system (Supporting Information). The main reason for low efficiency of 4b-Fb and 4b-Zn is not clear at present, but the low adsorption quantity, the aggregation property in dense solution, the relatively low first oxidation potential, and high reduction potential seems to contribute to the unpromising results. The traits in these series have also been confirmed in power conversion efficiency measurements. The photovoltaic results of porphyrin-TiO2 cells under AM1.5 irradiation (100 mW cm-2) were summarized in Table 3. The power conversion efficiencies, η, were obtained from the following equation,36

η ) Jsc × Voc × ff

(3)

where Jsc is the short-circuit photocurrent density, Voc is the open-circuit voltage, and ff represents the fill factor. Under the standard global AM 1.5 solar conditions, the 2bbdta-Zn-based TiO2 cell gives a maximum efficiency of η ) 3.03% with a Jsc of 8.38 mA cm-2, a Voc of 0.59 V, and a fill factor of 0.62. The overall efficiencies under the same irradiation condition were observed in the order of 2b-bdta-Zn (3.03%), 2b-bd-ta (2.37%), 1b-d-Zn (2.08%) 2b-Zn (1.48%), 2b-FB (0.63%), and 2m-s-Zn (0.14%), which is in good agreement with the tendency of IPCE values although the overall efficiencies slightly lessened after heat treatment (Figure 5).

16698 J. Phys. Chem. C, Vol. 112, No. 42, 2008 From the detailed efficiency parameters, the major difference in power conversion efficiency is the photocurrent density which indicates the most efficient charge injection process in 2b-bdtaZn. In previous work, porphyrin sensitizers functionalized at one β-position prepared by the Gra¨tzel group show nearly 5-7% of η depending on aryl and anchoring groups, suggesting that the most efficient porphyrin system possesses a malonic diacid group (Ar ) 4-methylphenyl).19 The overall structure is similar to 1b-d-Zn despite a different ending group and aryl group at the meso-position. For a more systematic interpretation of these results, IPCE can also be divided into three terms: the light harvesting efficiency (LHE), the quantum yield of charge injection (φinj), and the charge collection efficiency at the back electrode (ηc).13,14c,17 First of all, the LHE term can be eliminated because β-functionalized porphyrins have a similar extinction coefficient of ∼2 × 105 M-1 cm-1 and 2m-s-Zn showed very low efficiency although it has three times higher light absorbing ability in the Soret region than the others. Then the photocurrent generation efficiency can be considered as the competition process between charge injection and charge recombination. 2bZn adsorbed would have the closer distance between the porphyrin and the semiconductor surface than would 2b-bdZn onto the TiO2 layer. This indicates a higher efficiency in the charge injection process of the 2b-Zn-adsorbed TiO2 cell. However, 2b-bd-Zn shows a higher IPCE value and overall efficiency both in the Soret and Q-band regions. This can be explained by the reduced charge recombination process that has a greater effect than the reduced charge injection process in the 2b-bd-Zn-sensitized TiO2 cell. The higher efficiency of 2bbd-Zn than that of 1b-d-Zn indicates that multiple pathways and increased carboxylic acid groups exhibit better performance compared with monofunctionalized porphyrins. Notably this feature leads us to suggest that doubly β-functionalized porphyrin systems would be a good candidate for more efficient porphyrin-based TiO2 cells. As can be seen in the molecular orbital calculations, the malonic diacid group in 2b-bdta-Zn induces a larger electron density shift from the porphyrin macrocycle to the olefinic side chain. Namely, the stronger electron-withdrawing group is able to induce more efficient electron injection from the porphyrin to TiO2, resulting in the largest IPCE values of 2b-bdta-Zn. Collectively, we can envision several patent possibilities from this work and previous studies. First of all, the malonic diacid group, which is a stronger electron-withdrawing group than a simple carboxylic acid group, can improve the efficiency. The malonic diacid group not only draws electron density toward polyene bridges, which finally facilitate more charge injection to the semiconductor, but also offers stronger binding strength to the surface of TiO2. At the moment it does not disturb first excited-state oxidation potential, allowing the porphyrin to maintain sufficient driving force for charge injection of hot electrons to the conduction band of TiO2. Second, the multiple electron injection pathway is more beneficial than just one path through the bridge. Despite almost similar distance from the TiO2 layer to the porphyrin core and first excited-state oxidation potential of 2b-bd-Zn and 1b-d-Zn, 2b-bd-Zn revealed better efficiency than the mono-β-functionalized one. Although the difference is not large, it demonstrates that the doubly β-functionalized porphyrin is superior. Lastly, the electron injection process can sometimes be considered through the degree of frontier orbital overlap between porphyrin and TiO2. The high efficiency in the horizontal geometry can be explained by this model. However, it seems that the major electron injection

Park et al. process occurs through the covalent bond bridge when porphyrins are connected in perpendicular geometry onto TiO2. The increased efficiency of 2b-bd-Zn than 2b-Zn demonstrates that the moderate distance between the porphyrin and the semiconductor properly compensates for the competition between the electron injection and the charge recombination process.11 Therefore, from the present study on the systematic tailored porphyrin functionalized at the β-position with broader light harvesting into the green region, we can suggest that exploiting multiple β-positions of porphyrins lead to enhanced photovoltaic cells for a future framework. 4. Conclusions We have investigated the electronic properties and photoconversion efficiency of unsaturated carboxylic acid substituted porphyrins at meso- and β-positions adsorbed onto a TiO2 nanocrystalline surface. To clarify the effect of the bridge length and the position of side chains, steady-state absorption and photoelectrochemical measurements have been performed. All porphyrins were adsorbed onto TiO2 layers by carboxylic acid groups effectively. The absorption spectrum of porphyrins adsorbed on TiO2 surface suggests H-type interactions in mono and dual β-substituted porphyrin systems indicating highly packed monolayers of porphyrins on the TiO2 surface. The photoelectrochemical study shows that the effective electronic coupling through the bridge plays an important role in the charge injection process. Additionally, the moderate distance between the adsorbed porphyrin and the TiO2 nanocrystalline surface shows better performance in photoelectrochemical conversion because of the reduced rate of charge recombination processes. We have found that the doubly diene substituted porphyrin at two β-positions exhibits better photovoltaic performance than that of the mono-β-dienylporphyrin. On the basis of these results on doubly β-substituted porphyrins, we can suggest that the effective π-conjugation through the bridge plays a momentous role in the electron injection process in DSSC, and multiple electron transport pathways enhance the photovoltaic efficiency in porphyrin sensitizer based solar cells. As an extension of our current work, a future study will test the photovoltaic performance of orthogonal meso-meso linked and completely fused meso-meso, β-β, β′-β′ triply linked porphyrin dimers functionalized also at β-positions which should have much broader absorption spectra. Acknowledgment. The work at Yonsei University was financially supported by the Star Faculty Program of the Ministry of Education and Human Resources Development. The work at Kyoto University was supported by Grant-in Aids for Scientific Research (Nos. 19205006 and 18655013) from MEXT. H.R.L. acknowledges the fellowship of the BK 21 program from the Ministry of Education, Science and Technology and Human Resources Development. S.H. acknowledges the Research Fellowships of the JSPS for Young Scientists. Dr. Dong Young Kim (KIST) and Byong Hong Lee are gratefully acknowledged for the use of equipments for photovoltaic measurement and help in preparation of sandwich type cell. Supporting Information Available: Synthetic details, UV-vis absorption and emission, cyclic voltammogram, UV-vis absorption of 4b-FB and 4b-Zn in the dense solution, and IPCE action spectra of 4b-FB and 4b-Zn. This material is available free of charge via the Internet at http://pubs.acs.org.

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