Donor-Substituted β-Functionalized Porphyrin Dyes on

(7) In fact, a representative example of a meso-aryl-substituted zinc porphyrin ..... E0–0 values were estimated from the intersection of the absorp...
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Donor-Substituted β-Functionalized Porphyrin Dyes on Hierarchically Structured Mesoporous TiO2 Spheres. Highly Efficient Dye-Sensitized Solar Cells Masatoshi Ishida,† Sun Woo Park,† Daesub Hwang,†,‡ Young Bean Koo,† Jonathan L. Sessler,*,†,§ Dong Young Kim,*,‡ and Dongho Kim*,† †

Department of Chemistry, Yonsei University, Seoul 120-749, Korea Opto-electronic Materials Lab, Korea Institute of Science and Technology, Seoul, 136-791 Korea § Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station Texas 78712-0165, United States ‡

A5300, Austin,

bS Supporting Information ABSTRACT: Novel zinc porphyrin dyes for use in dyesensitized solar cells (DSSCs) have been synthesized. These dyes are based on a molecular design that relies on donor π acceptor interactions, a concept implemented by introducing a bis(4-tert-butylphenyl)amino group at the meso position of the porphyrin opposite to what are 2-propenoic or 2,4pentadienoic acid anchoring groups at the β-pyrrolic positions. Incorporating an electron-donating group (i.e., the diarylamine) on the porphyrin core serves as the considerable electronic coupling between the donor site and porphyrin core, and hence, the HOMO LUMO energy gap is decreased. This change is reflected in the remarkable red shift and broadening of the absorption spectra relative to an unfunctionalized parent system. This substitution, in conjunction with functionalization with carboxylic acid moieties on the β-pyrrolic positions, also provides what is an effectively aligned donor π acceptor dipolar architecture. This, in turn, gives rise to advantageous chargetransfer properties, including what are significant improvements of the electron injection efficiency on titanium oxide (TiO2) compared to our previous models without a donor substituent. The DSSCs of this study were composed of zero-dimensional hierarchical structured TiO2 spheres with a diameter of 600 800 nm prepared from P-25 and anatase TiO2 nanoparticles, which functioned as the photoelectrodes. With the anatase TiO2-based DSSCs, the power conversion efficiencies (η) as well as the photocurrent action spectra were relatively enhanced, an effect ascribed to the characteristic mesoporous effect and associated electrophysical properties of the anatase TiO2 spheres. Among the dyes prepared in the context of the present study, the doubly functionalized carboxylic acid derivative, tda-2b-bd-Zn, gave rise to the highest power conversion. The η value was 7.47%, and the maximum incident photon-to-current efficiency was 77.3% at the Soret band. The overall η value of tda-2b-bd-Zn is comparable to the performance of typical ruthenium-based dyes, such as N3 (η = 7.68%), under the same conditions.

’ INTRODUCTION Dye-sensitized solar cells (DSSCs) have been regarded as promising candidates for the next generation of solar cells because they provide an economic alternative to silicon-based solar cells and may provide advantages in terms of cost and processing.1 The most efficient sensitizers employed in DSSCs are ruthenium (Ru) polypyridyl complexes. The best of these have produced photovoltaic power conversion efficiencies up to 11%. However, the limited number of available Ru(II) polypyridyl complexes and both cost and environmental concerns associated with the use of ruthenium-derived materials have inspired strenuous efforts to find novel dyes that do not rely on this or other expensive metals.2 To date, numerous organic dyes composed of π-conjugated sensitizing units, such as perylenes, phthalocyanines, squaraines, cyanines, and coumarins, have been r 2011 American Chemical Society

reported.3 Porphyrins have also attracted considerable attention as the light-harvesting antenna unit. These latter chromophores are attractive because they offer advantages, such as photochemical and electrochemical stability, appropriate molecular orbital (MO) levels, efficient capture of solar energy in the visible region with distinct high molar extinction coefficients, and an ability to control the redox potential by installing substituents.4 However, the overall performance of porphyrin dyes in DSSCs remains below that of Ru(II)-based sensitizers.5 It is, therefore, important to design and synthesize novel porphyrin derivatives so as to modulate their fundamental MO and light absorption properties; Received: March 11, 2011 Revised: July 14, 2011 Published: August 29, 2011 19343

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Scheme 1. Molecular Structures of the Porphyrin Sensitizers Used in This Work

in due course, this could lead to new sensitizers with improved conversion efficiencies. We have previously reported the photovoltaic activities of various β-functionalized porphyrin and porphyrin dimer dyes.6 This work was motivated by the consideration that functionalization of porphyrins at the pyrrolic β-positions with π-conjugative linkers could represent a possible strategy for the preparation of more efficient DSSCs. The resulting extension of the π-conjugation framework and the use of substituents (e.g., carboxylic acid) that permit anchoring to a TiO2 anchoring moiety might allow more effective injection of electrons into the conduction bands (CB) of the TiO2, which would function as a photoelectrode. This direct electronic interaction could enhance the splitting of the key filled or empty orbitals, thereby causing a red shifting in the Q and B bands typical of porphyrins and an increase in the oscillator strength.7 In fact, a representative example of a mesoaryl-substituted zinc porphyrin bearing a butenylidene malonic acid moiety has been prepared; it was characterized by a remarkably high conversion efficiency of 7.1% under standard conditions.8 Another design feature that appears attractive involves the use of two separate conjugated acid anchoring groups; if these are located on the same side of the porphyrin at the β-positions, no problems arising from steric hindrance would be expected.9 As reported earlier, such doubly functionalized sensitizers did, in fact, display higher conversion efficiencies in comparison to analogous porphyrins bearing a single substituent.6a This improvement was ascribed to the enhancement in chemical stability resulting from a more effective attachment to the TiO2 surface, as well as improvements in the electron injection efficiency arising from the presence of more than one electron injection pathway from the porphyrin to the CB of TiO2. We propose that the sensitizing efficiencies of β-pyrrolefunctionalized, carboxyl-bearing porphyrins could be further enhanced by installing a strong electron donor moiety on the other of the macrocyclic ring (e.g., at the meso position essentially opposite from the site of β-pyrrole substitution). Several groups have recently reported meso-electron donor substituted porphyrin dyes that displayed enhanced conversion efficiencies relative to controls.10 In particular, dyes incorporating mesodiarylamino groups as a strong electron donor group were found to be characterized by a decreased rate of charge recombination between the injected electrons and the oxidized dyes on the TiO2 surface.10a This modification also led to a broadening as well as a red shift of the absorption bands, thus improving the light

absorption capability. Importantly, linearly aligned monosubstituted zinc porphyrin dyes were found to be characterized by higher conversion efficiencies than analogous sensitizers containing multiple diarylamino substituents.11 On this basis, the use of a porphyrin containing both two conjugated anchoring carboxyl groups and a donor group attached at a meso position was expected to give rise to highly effective DSSCs. In fact, a push pull-type porphyrin dye with a diarylamine and ethynylbenzoic acid substitutents at meso positions, respectively, has recently achieved a power conversion efficiency of 11%, which is both the highest value yet recorded for a porphyrin-based solar cell device10h and one that is competitive with what is achieved with a standard of Ru(II)-based dye system (N719 dye; ∼11%). 12 Further, donor substituted organic dyes have achieved conversions as high as ∼9.5%.13 We thus sought to prepare and study new unsymmetric porphyrin dyes that are characterized by both an overall linear donor π acceptor dipolar structure and the presence of groups suitable for attaching to a TiO2 surface. In this study, we have focused on zinc porphyrin dyes bearing a bis(4-tert-butylphenyl)amino group appended to the meso position “opposite” to a β-pyrrolic position functionalized with a 2-propenoic acid or 2,4-pentadienoic acid moiety. The first of these substitutents was expected to function as an electron donor group, whereas the latter would serve as anchoring groups for interacting with the TiO2 surface. The specific porphyrin targets in question, denoted as tda-1b-Zn, tda-1b-d-Zn, and tda-2b-bdZn, would be expected to enhance the efficiency of DSSCs due to the distinct anisotropic charge-transfer properties created by installing synthetically the strong electron donor bis(4tert-butylphenyl)amino group at one of the meso positions (Scheme 1).14 As detailed below, we have evaluated the success of this design by analyzing the photovoltaic properties of the porphyrin sensitizers in question through use of optical and electrochemical measurements, density functional theory (DFT) calculations, incident photon-to-photocurrent efficiency (IPCE), and power conversion efficiency measurements. In particular, we have utilized either mesoporous P-25 or anatase TiO2 spheres with a diameter of 600 800 nm composed of granular nanoparticles by an electrostatic spray technique to improve the photovoltaic efficiency.15 The critical activities of the TiO2 photoelectrodes in DSSCs are believed to depend on the nanoparticle nature of the composited particle, particularly its size, morphology, and surface area, among other attributes.16 Although various kinds of one-dimensional (1D) TiO2 nanostructures, including nanofibers, nanowires, nanotubes, and nanorods, 19344

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The Journal of Physical Chemistry C have been examined, we favor the use of zero-dimensionally (0D) hierarchical spherelike structured TiO2 as photoelectrodes in DSSCs because of their relatively large surface for dye adsorption, fast electron transport properties, and good lightscattering features.17 They also possess large pores, which permits effective electrolyte penetration throughout the spheres. We also favor the use of the coadsorbent chenodeoxycholic acid (CDCA) because it reduces the propensity of porphyrin sensitizers to aggregate on the TiO2 surface, an intermolecular phenomenon that usually reduces the overall cell performances when it occurs.18 Finally, in this study, we have modified the TiO2 surface to increase the surface roughness through a posttreatment with TiCl4, an adjustment that increases the amount of porphyrin sensitizers that may be adsorbed effectively onto the TiO2 surface.19 As a consequence of this combination of synthetic design and substrate optimization, we have reached a power conversion efficiency of 7.47%. This efficiency, achieved when tda-2b-bd-Zn was used as the sensitizer in conjunction with pretreated mesoporous anatase TiO2 spheres in the presence of 2 equiv of CDCA, compares well with the value of 7.68% measured for a standard ruthenium complex dye complex (N3) under identical experimental conditions.

’ EXPERIMENTAL SECTION Synthesis of Porphyrins. The synthesis and characterization of the functionalized porphyrins used in this study are described in the Supporting Information. Preparation of Hierarchically Structured Mesoporous TiO2 Nanospheres. TiO2 dispersions for use in an electrostatic spray process were prepared in accord with the method we have previously reported.17f To prepare modified mesoporous P-25 TiO2 spheres with diameters between 600 and 800 nm, 10 wt % P-25 TiO2 (Degussa) was dispersed in ethanol using an ultra apex mill (model UAM-015, Kotobuki). For the preparation of mesoporous anatase TiO2 spheres with a diameter of 600 800 nm, 0.3 mol of acetic acid was added dropwise to 0.3 mol of titanium isopropoxide with stirring at room temperature. The modified precursor that resulted was stirred for about 30 min and poured into 400 mL of water as fast as possible with vigorous stirring. A white precipitate was formed instantaneously. Stirring was continued for an additional hour to allow the hydrolysis reaction to go to completion. After adding 6.2 mL of 65% aqueous nitric acid, the mixture was heated from room temperature to 80 °C over the course of 60 min and peptized for 1 2 h. The resultant mixture was kept in a titanium autoclave and heated at 250 °C for 12 h. The two types of dispersed TiO2 solutions generated as per the above protocols were loaded into a plastic syringe that was connected to a high-voltage power supply (Bertan Series 205B). The dispersed TiO2 solutions were then electrosprayed directly onto a conducting FTO substrate (10 cm  10 cm). An electric field of 15 kV was applied between the metal orifice and the conducting substrate. The feed rate was controlled by a syringe pump at 35 30 μL/min. To form a uniform thickness over a large area, the nozzle and the substrate were placed on a motion control system regulated by a microprocessor. Device Fabrication. To prepare the DSSC working electrode, a fluorine-doped SnO2 (FTO) conducting glass substrate (TEC 8, Pilkington) was first washed in a detergent solution using a sonic bath for 10 min and then rinsed with water and ethanol. The resulting washed FTO glass substrates were then coated with

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either the mesoporous P-25 TiO2 spheres or the mesoporous anatase TiO2 spheres via electrospray. Electrodes coated with TiO2 were then annealed at 500 °C for 30 min. The film thickness of the annealed TiO2 layer was measured and found to be 11 12 μm using an Alpha-step 200 surface profiler (Tencor P-10). A transparent TiO2 film was prepared by using a commercial paste of highly transparent TiO2 (Ti-Nanoxide HT, Solaronix). The film thickness of the transparent TiO2 layer prepared in this way was about 4 μm. The working electrode was immersed into 0.1 M aqueous TiCl4, a solution made up from an aqueous stock solution of 2 M TiCl4 by appropriate dilutions. After treatment with TiCl4, the TiO2 films were washed with deionized water and ethanol and then sintered at 500 °C for 30 min. After cooling at 80 °C, the working electrodes were immersed into solutions of the porphyrin dyes of this study (0.2 mM in ethanol) and allowed to stand overnight (15 20 h). To prepare the counter electrode, the FTO plates were drilled and then washed first with deionized water and then 0.1 M HCl in ethanol with sonication for 10 min. After the FTO glass was washed in this way, the counter electrodes themselves were prepared by drop-coating a solution of H2PtCl6 (0.5 mM in isopropyl alcohol) onto the FTO glass and then heating at 400 °C for 20 min. The dye-adsorbed TiO2 films and the platinum counter electrodes were assembled into sealed sandwich-type cells by heating with a hot-melt film (Surlyn 1702, 25 μm thickness, Dupont) used as a spacer between the electrodes. An aliquot of the electrolyte solution (0.05 M I2, 0.3 M LiI, 0.6 M butylmethylimidazolium iodide, 0.5 M 4-tert-butylpyridine in 4:1 acetonitrile/ valeronitrile) was then placed into a hole in the counter electrode of the assembled cells. The hole was sealed with a microscope cover glass and Surlyn (25 μm thickness) to prevent leakage of the electrolyte solution. Steady-State Absorption and Fluorescence Measurement. 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). The absorption spectra were corrected for contributions from the TiO2 itself by subtracting the absorption of the bare TiO2 layer. Photovotaic Properties Measurement. The power conversion efficiencies (PCEs) of the DSSCs of this study were measured using a solar simulator (450 W, Oriel Co.) with an AM 1.5 global filter. The incident light intensity was adjusted with an NREL-calibrated Si solar cell (PV Measurements Inc.) to an intensity corresponding to 1 sun (100 mW cm 2). The tested solar cells were masked to an aperture area of 0.25 cm2. The photovoltaic characteristics of the DSSCs were obtained by applying an external potential bias to the cells and measuring the generated photocurrent with a Keithley model 2400 digital source meter. The voltage step and delay time of the photocurrent were 10 mV and 0.1 s, respectively. Each cell performance value was taken as the average of three independent samples. The incident photon-to-current efficiencies (IPCEs) were measured by using a potentiostat (Keithley 2400 source meter) and a 300 W xenon lamp (Oriel Co.) in combination with a spectrapro150 monochromator (Acton Research Co.), in the range of 400 800 nm. A cutoff filter of 400 nm was attached at the output slit of the monochromator to remove UV light. The light intensities were measured with a calibrated power meter (LM-2 VIS, Coherent) at each wavelength. 19345

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Scheme 2. Synthesis of Donor-Substituted β-Functionalized Porphyrinsa

a Reagents and conditions: (a) (tBuPh)2NH, Pd(OAc)2, DPEphos, NaH, THF; (b) [Ir(OMe)(cod)]2, dtbpy, (BPin)2, 1,4-dioxane; (c) Rh(cod)(OH)2, methyl acrylate, 1,4-dioxane/H2O; (d) Rh(cod)(OH)2, methyl 2,4-pentadienoate, 1,4-dioxane/H2O; (e) NaOH/EtOH/THF.

Density Functional Theory (DFT) Calculations. Theoretical calculations were performed with the Gaussian 03 program suite using a supercomputer.20 All calculations were carried out using the density functional theory (DFT) method with Becke’s threeparameter hybrid exchange functionals and the Lee Yang Parr correlation functional (B3LYP) employing the 6-31G(d) basis set for all atoms.21

’ RESULTS AND DISCUSSION Synthesis. The molecular structures of porphyrin dyes, tda1b-Zn, tda-1b-d-Zn, and tda-2b-bd-Zn, used in this study are shown in Scheme 1. One of our two main design objectives consisted of introducing a meso-donor amine moiety onto the central porphyrin framework. This, it was thought, would produce push pull porphyrin dyes and would allow an enhancement of the absorption in the visible region as the result of strong electronic interactions.22 The other design goal involved introducing β-olefinic bridges between the porphyrin core and the carboxylic acid groups that would anchor the dye to the TiO2 surface. This latter feature was expected to provide for good electronic conjugation between the porphyrin and the TiO2 surface and thus facilitate electron injection from the excited porphyrin into the TiO2 conduction band following photoexcitation. The use of more than one carboxylic acid group was expected to ensure efficient adsorption onto the TiO2 surface.

Figure 1. UV visible absorption (black line) and fluorescence (red line) spectra of (a) tda-1b-Zn, (b) tda-1b-d-Zn, and (c) tda-2b-bd-Zn measured in EtOH.

Finally, several bulky meso-aryl groups bearing tert-butyl substitutents were incorporated into the design so as to decrease the propensity of the porphyrins to undergo aggregation in solution or on the semiconductor surface. With such considerations in mind, several new porphyrins were synthesized using various metal-catalyzed reactions as the key bond construction step (Scheme 2). Specifically, the mesodiarylamino-substituted zinc porphyrin (5) was prepared by the 19346

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Table 1. Optical Properties and Electrochemical Data of Porphyrins and Driving Forces for Electron-Transfer Processes on TiO2 λabs (nm)a

λem (nm)c

tda-1b-Zn

421 (1.74)b, 569, 618

665

1.93

0.84

1.30

1.09

0.59

0.44

tda-1b-d-Zn

422 (1.87)b, 573, 628

671

1.92

0.84

1.22

1.08

0.57

0.44

tda-2b-bd-Zn

441 (1.82)b, 582, 650

688

1.85

0.86

1.17

0.99

0.49

0.46

compound

E0

0

(eV)d

Eox (V)e

Ered (V)e

Eox* (V)f

ΔGinj (eV)g

ΔGreg (eV)h

Wavelengths for the absorption maxima of the B and Q bands in EtOH. b Extinction coefficients of samples, ε (10 5 3 M 1 3 cm 1), obtained in ethanol. c Wavelengths for emission maxima in EtOH obtained by excitating at the B-band wavelength. d E0 0 values were estimated from the intersection of the absorption and emission spectra. e First oxidation and reduction potentials determined by using cyclic voltammetry in CH2Cl2 containing 0.1 M n-Bu4NPF6 as a supporting electrolyte (vs NHE). f Excited-state oxidation potentials approximated from Eox and E0 0 (vs NHE). g Driving forces for electron injection from the porphyrin excited singlet state (Eox*) to the CB of TiO2 ( 0.5 V vs NHE). h Driving force for regeneration of the porphyrin radical cation (Eox) by reaction with the I /I3 redox couple (+0.5 V vs NHE). a

palladium(Pd)-catalyzed amination of the meso-bromo zinc porphyrin (4) with N,N-diarylamine.23 A subsequent iridium(Ir)catalyzed borylation reaction afforded the β-pyrrole-functionalized mono- and diborylated porphyrins (6) and (7).24 The borylated derivatives obtained in this way were reacted with different vinyl esters under rhodium(Rh)-catalyzed reactions to give the β-conjugated porphyrin ester derivatives (1 3).9 Subsequently, the ester derivatives 1 3 were hydrolyzed under basic conditions to afford the donor-substituted β-functionalized porphyrins, tda-1b-Zn, tda-1b-d-Zn, and tda-2b-bd-Zn, respectively. Optical and Electrochemical Properties. The absorption spectra of the porphyrin dyes considered in this study are shown in Figure 1, and the corresponding optical data are summarized in Table 1. In general, ordinary porphyrins show a characteristic sharp and intense Soret absorption band at about 400 450 nm along with moderately intense Q absorption bands at about 500 650 nm.25 All porphyrin dyes studied in this work exhibited a broadened Soret band and red shifted Q-bands in the 400 500 and 550 700 nm spectral ranges, respectively. In comparison with our previously reported diarylamine-unsubstituted derivatives, the differences in the optical features are ascribed to an electronic interaction between the porphyrin core and the diarylamino group.6a In addition, it was found that the electronic interaction between the porphyrin macrocyclic core part and the unsaturated bridging group also affects the electronic structure, presumably as the result of increasing the extent of π-conjugation. The absorption spectrum of tda-1b-Zn, a porphyrin derivative bearing an acrylic acid chain, displays a broadened Soret band, along with a new shoulder at 450 500 nm as compared to normal porphyrins. When the side chains were replaced by 2,4-pentadienoic acid to give tda-1b-d-Zn, a similar effect was seen in the absorption spectra. The porphyrin derivative, tda-2b-bd-Zn, exhibits the most pronounced broadening and the greatest bathochromic shift in the Soret band, a result that is consistent with the doubly olefinic side chains at the β-positions, effectively perturbing the electronics of the porphyrin core. Functionalization at the β-positions leads to a broadened Soret band due to an electronic coupling between the olefinic chains and the porphyrin core.9 The wide shoulder bands near the Soret region are considered to be a characteristic feature of donor-substituted porphyrins.11 At least they are consistently seen in the compounds prepared for this study. On this basis, we conclude that the combination of a strong electron-donating group and longer β-side conjugated chains has a significant influence on the optical properties of these porphyrin-type sensitizers. In particular, they produce features that are considered desirable for light-harvesting ability in the green spectral region.

The steady-state fluorescence spectral data of the porphyrins are summarized in Table 1 and Figure 1. In accordance with the trend noted in the case of the absorption spectra, the peak maxima of the emission spectra were found to be red shifted in the order of tda-1b-Zn < tda-1b-d-Zn < tda-2b-bd-Zn; thus, the shift was found to be correlated with the degree of the β-olefinic side chain elongation as well as the number of side chains present on the porphyrin core. As estimated from an analysis of the optical data, the zero zero excitation energies (E0 0) were found to be 1.93 eV for tda-1b-Zn, 1.91 eV for tda-1b-d-Zn, and 1.85 eV for tda-2b-bd-Zn (Table 1). To determine redox properties of the porphyrins, cyclic voltammetry (CV) measurements were carried out in CH2Cl2 containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte (Table 1). All porphyrin dyes exhibited reversible oxidation waves and quasi-reversible reduction waves. The first oxidation potentials (Eox) corresponding to the HOMO level of dyes are virtually the same, viz. tda-1bZn (0.84 V vs NHE), tda-1b-d-Zn (0.84 V), and tda-2b-bd-Zn (0.86 V). In contrast, positive shifts in the first reduction potentials (Ered) of the porphyrins were seen, with the degree of shift again correlating with the extent of π-extension at the β-pyrrolic positions. On this basis, it is concluded that increasing π-conjugation leads to a reduction in the electrochemical HOMO LUMO energy gap, an inference that is in agreement with the structure-correlated red shifts seen in the absorption spectra. The values of Eox of all the porphyrins are more positive than the potential of the I /I3 redox mediator (0.5 V vs NHE) present in the electrolyte; this means that, once oxidized, the porphyrins of the present study are thermodynamically capable of accepting an electron from this key transfer material (Figure 2).26 The LUMO levels of the porphyrins were calculated from the difference Eox E0 0. The resulting values were found to be more negative than the conduction band (CB) of TiO2 ( 0.5 V vs NHE); therefore, electron injection into the TiO2 spheres is thermodynamically favored (ΔGinj < 0) following porphyrin photoexcitation.27 This dual ability, namely, to transfer an electron from the porphyrin excited singlet state into the CB of the TiO2 to produce a porphyrin radical cation and then reduce this latter species via electron capture from the I /I3 redox mediator (ΔGreg < 0) makes the dyes of the present study of interest for use in DSSCs. DFT Calculations. To gain insight into the electronic structure of the frontier molecular orbitals of the modified porphyrins of this study, DFT calculations were performed at the B3LYP/ 6-31G(d) level (Figure 3 and Table 2). The optimized geometries of all porphyrins revealed planar porphyrin cores and diarylamino groups that lie perpendicular to these planar scaffolds, presumably to avoid steric congestion around the meso positions. 19347

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Figure 2. Schematic energy levels of the porphyrins, (a) tda-1b-Zn, (b) tda-1b-d-Zn, and (c) tda-2b-bd-Zn, the TiO2 conduction band, and energy levels of the redox mediator. HOMO = Eox and LUMO = E0 0* (= Eox E0 0) .

Figure 3. Selected molecular orbital diagrams for the porphyrins of this study, (a) tda-1b-Zn, (b) tda-1b-d-Zn, and (c) tda-2b-bd-Zn. These representations were obtained from DFT calculations carried out at the B3LYP/6-31G(d) level.

Table 2. Molecular Orbital Energy Levels for the Porphyrins of the Present Studya compound

HOMO 1

HOMO

LUMO

LUMO+1

ΔE

(eV)

(eV)

(eV)

(eV)

(eV)b

tda-1b-Zn

5.28

4.84

2.50

2.25

2.34

tda-1b-d-Zn

5.24

4.85

2.55

2.26

2.30

tda-2b-bd-Zn

5.33

4.93

2.76

2.52

2.17

a

Estimated by DFT calculations using the B3LYP/6-31G(d) level basis set. b Energy gap between the HOMO and LUMO levels.

Incorporation of an electron-donating group to the porphyrin ring serves to reduce the HOMO LUMO gap in comparison with the corresponding porphyrin without a meso-diarylamine group. This effect is attributed to the considerable electronic coupling between the electron donor group and the porphyrin core.6a Notably, the HOMO energy levels are slightly increased in the order of

tda-1b-Zn < tda-1b-d-Zn < tda-2b-bd-Zn; in contrast, the LUMO levels are significantly decreased in the same order. The calculated energetic features of these three porphyrins are thus fully consistent with what was inferred from the experiments described above, particularly the electrochemical potentials. It was also found that the greatest electronic density present in the ground-state forms of tda1b-Zn, tda-1b-d-Zn, and tda-2b-bd-Zn is present within the diarylamine substituent, as well as in the π-system of the porphyrin ring at the HOMOs. In contrast, the densities of the LUMOs are distributed on the porphyrin core and the β-olefinic terminal anchoring groups. During photoexcitation, electrons are transferred from the donor diarylamine through the β-olefinic bridge to the surface-bound carboxylate. This produces a strong coupling between the excited-state wave function of the dye with the Ti(3d, t2g) orbitals that make up the CB of the TiO2. On the basis of these calculations, the dipolar structure present in the porphyrins of this study was expected to facilitate an efficient and rapid electron injection from the porphyrin excited state into the CB of TiO2. Moreover, within the present series of dyes, the chargetransfer effects were expected to be enhanced for tda-2b-bd-Zn (a compound bearing longer pentadiene-bridged conjugation groups) relative to the other species; the high jinj value for tda2b-bd-Zn could reflect the doubly conjugated electron injection pathway resulting from the symmetric electron distribution seen for the LUMO of tda-2b-bd-Zn (vide infra). Adsorption on Hierarchically Structured P-25 TiO2 Spheres. One of the key elements in DSSCs is the nanomorphology of the TiO2 used for the photoelectrodes. Because of a competition between the generation and the recombination of the photogenerated electrons on the TiO2 layer, these electrodes often provide a bottleneck for the development of high photocurrents. Typically, films that consist of 10 20 nm TiO2 nanoparticles are used for the fabrication of working electrodes on a transparent conductive oxide. This gives a high surface area. However, the power conversion efficiency of DSSCs based on small-sized TiO2 nanoparticles is not always high due to grain boundaries and defect sites on the TiO2 surface, which can delay the initial electron transport and increase the charge recombination rate. Because of these limitations, we favor one-dimensional nanostructured mesoporous TiO2 spheres.17f Such TiO2 spheres provide large pore volumes, allow for fast electron transport, and have adequately high surface areas and light-scattering effects when used as photoelectrode films. The large pore volumes promote the infiltration of electrolytes, which, in turn, improves the charge collection efficiency and permits fast electron transport within the photoelectrode. The high surface area and light-scattering features increase the light-harvesting capability and permit adequate dye loading. As detailed earlier, we produce mesoporous TiO2 spheres suitable for use as working electrodes via an electrostatic spray technique.17 A schematic illustration of the DSSCs prepared in this way is given in Figure 4a. In the present study, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images revealed that the morphology and integrity of the sprayed mesoporous TiO2 spheres were well-preserved after construction of the photoelectrode (Figure 4b h). The electrostatic sprayed photoelectrodes were treated further by hot-pressing at 120 °C (120 ton for 12 min) to enhance the external connection among the mesoporous TiO2 spheres (Figure 4g). Although the pore volume was slightly reduced as the result of this pressing treatment, it still remains larger than that of P-25 nanoparticles, as inferred from the fact that better penetration of electrolytes and longer electron diffusion lengths 19348

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Figure 4. (a) Schematic illustration of the stepwise fabrication of DSSCs using an electrostatic spray process. SEM images of (b, f) as-sprayed mesoporous TiO2 spheres, (c, g) the cross-sectional images after carrying out the pressing amd thermal annealing steps at 120 °C, 12 ton for 10 min, and (d, h) the mesoporous TiO2 spheres with a diameter of 600 700 nm after TiCl4 post-treatment. TEM image of (e) the mesoporous spheres.

are observed. Figure 4h shows TiO2 spheres that have been subjected to a final post-treatment with an aqueous TiCl4 solution (vide infra). To prepare the final porphyrin-modified photoelectrodes, the TiO2 electrodes generated as above were immersed into a solution of the porphyrin under study (0.2 mM) for 12 h. The total amount of porphyrins adsorbed onto the TiO2 layers was estimated by measuring the absorbance of the dissolved porphyrins in the mixed aqueous organic solvent and the weight of the TiO2 layer. The 12 h immersion time was considered adequate to saturate the surface of the TiO2 films in the case of all three porphyrins and is consistent with what has been used previously to prepare TiO2 electrodes from porphyrins bearing carboxylic acid anchors.10g,11b,28 The packing density (Γ, mol cm 2) of the adsorbed porphyrin sensitizers is one of the critical factors that can be correlated directly with the light-harvesting ability of TiO2-supported DSSCs.29 The Γ values, measured as detailed above, were taken as an average of three independently prepared porphyrin/TiO2 electrodes. As a reference dye, the loading amount of N3 on TiO2 was estimated to be 1.37  10 10 mol cm 2, which is comparable to the values reported in a previous report.30 The singly functionalized porphyrin, tda-1b-Zn, was found to be adsorbed at the 8.32  10 11 mol cm 2 TiO2 level. The corresponding value for tda-1b-d-Zn was 9.58  10 11 mol cm 2, which represents a 13% relative increase. This finding lead us to propose that longer linkages between the porphyrin core and then anchoring carboxyl moieties reduce the steric hindrances between the neighboring porphyrin moieties as porphyrins once attached to the TiO2 layer. The Γ value of the electrodes built up from tda-2b-bd-Zn exhibits the highest loading level among the three porphyrin sensitizers, namely, 1.15  10 10 mol cm 2. Presumably, this reflects the fact that tda-2b-bd-Zn provides for a doubly conjugated bridge between the porphyrin β-pyrrolic positions and the TiO2 surface. Although detailed insights into the binding

interactions and the geometries of the porphyrins on the TiO2 surface were not obtained in this study, on the basis of previous work with similar donor substituted porphyrins, which showed similar Γ values (i.e., 7  10 11 1  10 10 mol cm 2), we propose that all three electrodes created in the context of the present study contain the constituent porphyrins densely packed as monolayers on the TiO2 surface.10g,11b Accordingly, the higher loading density obtained with the doubly bridged, flexible derivative tda-2b-bd-Zn led us to predict that it would give rise to relatively improved DSSCs. This prediction was tested as described below. Photovoltaic Properties of Porphyrin-Sensitized DSSCs. The photovoltaic performances of the porphyrin-sensitized TiO2 cells built up from the three porphyrins used in the present study were evaluated under various conditions with AM 1.5 irradiation (100 mW cm 2). The power conversion efficiency (η) was obtained according to the equation, η = Jsc  Voc  ff, where Jsc is the short-circuit photocurrent density (mA cm 2), Voc is the open-circuit voltage (V), and ff is the fill factor.31 The overall power conversion efficiencies exhibit a systematic trend across this small series of porphyrins that can be related to the nature of the β-substituents (Table S1 in the Supporting Information). Specifically, the maximal η values were found to increase as the size and complexity of the β-substituents increased, that is, tda-1b-Zn, 3.20%; tda-1b-d-Zn, 4.22%; and tda-2b-bd-Zn, 4.36%. The overall cell efficiencies (η values) for the present set of porphyrins bearing a diarylamino donor group proved to be remarkably better than those for cells created without such a substituent.6a The addition of coadsorbents, such as CDCA, during the sensitizer adsorption step to reduce the molecular aggregation on the surface of the TiO2 spheres resulted in a considerable improvement in the cell performance. For instance, a notable increase in current density and energy conversion efficiency was seen upon coadsorption of the dyes with 2 equiv of CDCA (Table S1 in the Supporting Information). This result leads us to 19349

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The Journal of Physical Chemistry C suggest that this quantity of CDCA does indeed serve to prevent molecular aggregation. To optimize the cell performance further, we employed an empirical surface modification technique that involves a posttreatment of the TiO2 films to a dilute aqueous TiCl4 solution.32 This TiCl4 post-treatment has been used previously to improve the performances of P-25 TiO2 nanoparticles and is thought to reflect the specific deposition of an ultrapure TiO2 shell on the mesoporous TiO2.18 This procedure can affect the downward shift in the TiO2 conduction band-edge potential and a significant decrease in the electron/electrolyte recombination rate constant.33 In the event, it is important to appreciate that, from an operational perspective, the effective electron diffusion length at the interface can be increased through TiCl4 post-treatment. The overall η values of the optimized DSSCs prepared from the present donor-substituted porphyrins proved considerably more efficient than those created using analogous unsubstituted porphyrins (η; ∼2.4%).34 A maximum η value of 6.35% with a Jsc of 16.8 mA cm 2, a Voc of 0.63 V, and an ff of 0.60 were achieved in the case of the tda-2b-bd-Zn-sensitized cell (Figure 5a and Table 3). Judging from the similar values of Voc, the difference in solar cell performance originates primary from the nonparallel values of Jsc. Each of the photocurrent action spectra follows the absorption features of the corresponding porphyrin absorbed on the electrode, a finding consistent with the reasonable conclusion that the porphyrin is the main source for the photocurrent generation (Figure 5b,c). The photocurrent action spectra of the tda-2b-bd-Zn-sensitized cell exhibited superior IPCE values (77.6% and 50.1% for the Soret band and Q band, respectively) compared to those built up from tda-1b-Zn and tda-1b-d-Zn. This trend, seen over all wavelength regions, matches well with the power conversion efficiency values. To rationalize the difference in η, as well as the IPCE values, between the porphyrin-sensitized cells of the present studies, the IPCE was analyzed in terms of three components in accord with the following equation:35 IPCE = LHE  jinj  ηcol, where LHE (light-harvesting efficiency) is the number of absorbed photons per the number of incident photons, jinj is the quantum yield for electron injection from the porphyrin excited state to the CB of the TiO2 electrode, and ηcol is the efficiency of charge collection. Although the absorption coefficients for each constituent porphyrin are likely to be comparable, the larger surface coverage of tda-2b-bd-Zn on the TiO2 surface was expected to give rise to an increase in the LHE value compared to the other two sensitizers, tda-1b-Zn and tda-1b-d-Zn. However, the driving force for electron injection is greater in the case of the latter two porphyrins. Independent of the sensitizer, improvements in the LHE values can also result from an increase in the overall light scattering engendered, for example, by the use of larger TiO2 particles in the film. Essentially, a greater degree of scatter, caused by multiple reflections within the particles, would serve to increase the net optical path length substantially beyond what would be predicted based on the film thickness. In the present case, particles with a diameter of 600 800 nm were used. As a result, the absorption of solar light was enhanced, particularly in the red and near-IR spectral regions, relative to what is seen in the case of smaller particles. This enhancement is reflected in terms of an increase in the Jsc values of the DSSCs.36 Considering that electronic coupling between the LUMO of the porphyrin and the CB of the TiO2 is stronger in the case of tda-2b-bd-Zn than it is for tda-1b-Zn and tda-1b-d-Zn, the jinj term for DSSCs based

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Figure 5. (a) Characteristic photocurrent voltage curves of the porphyrins of the present study analyzed under the conditions of simulated global AM 1.5 solar radiation at 100 mW cm 2. The samples were standard sandwich-type cells fabricated using mesoporous P-25 TiO2 spheres. (b) Photocurrent action spectra of the devices made using the three porphyrins of this study. (c) UV visible absorption spectra of the TiO2/porphyrin constructs.

on tda-2b-bd-Zn was expected to be larger than for the other two dyes (vide supra). The term ηcol primarily depends on the relative rates of charge transport versus charge recombination.37 Considering the similar values of Voc, the degree of the charge recombination is not expected to be substantially different in the case of the three dyes utilized in this study. However, the shorter bridges provided by tda-1b-Zn and tda-1b-d-Zn and the resulting closer distribution of electron density between the porphyrin HOMO and the TiO2 surface could lead to a slight increase in the rate of charge recombination between the electron injected into the CB of the TiO2 electrode and the porphyrin cation radical produced as the result of this transfer (Figure 3). The relatively loose packed porphyrin monolayer provided by the use of tda-1b-Zn and tda1b-d-Zn rather than tda-2b-bd-Zn could also allow for a greater degree of direct charge recombination between the electrons on 19350

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Table 3. Photoelectrochemical Performances of P-25 TiO2 Spheres Films Sensitized with Porphyrinsa IPCE Jsc (mA cm 2)

Voc (V)

ff

η (%)

Soret (nm)

Q band (nm)

tda-1b-Zn

10.7

0.62

0.70

4.60

52.1 (450)

23.3 (580), 19.9 (620)

tda-1b-d-Zn

14.1

0.63

0.63

5.46

67.7 (470)

40.2 (580), 35.7 (620)

tda-2b-bd-Zn

16.8

0.63

0.60

6.35

76.6 (490)

50.1 (590), 33.3 (660)

compound

a

DSSCs were fabricated with mesoporous P-25 TiO2 spheres treated with TiCl4 and added 2 equiv of CDCA. The conversion efficiency parameters were measured under conditions of 100 mW cm 2 illumination. N3-sensitized TiO2 electrode was annealed at 773 K for 30 min and immersed into a 0.3 mM ethanol solution of the dye at room temperature for 12 h (η = 7.44%, Jsc = 17.44 mA cm 2, Voc = 0.75 V, ff = 0.61, IPCEmax = 68.2% (530 nm)).

Table 4. Photoelectrochemical Performance of Anatase TiO2 Spheres Films Sensitized with Porphyrinsa IPCE Jsc (mA cm 2)

Voc (V)

ff

η (%)

Soret (nm)

Q band (nm)

tda-1b-Zn

12.1

0.66

0.62

4.95

48.0 (460)

34.0 (580), 29.2 (630)

tda-1b-d-Zn tda-2b-bd-Zn

13.9 18.4

0.67 0.71

0.64 0.57

5.91 7.47

54.1 (488) 77.3 (489)

45.4 (580), 37.8 (630) 67.5 (590), 51.2 (650)

compound

a DSSCs were fabricated with mesoporous anatase TiO2 spheres treated with TiCl4 and 2 equiv of CDCA. The conversion efficiency parameters measured under conditions of 100 mW cm 2 illumination. The N3-sensitized TiO2 electrode was annealed at 773 K for 30 min before being immersed into a 0.3 mM ethanol solution of the dye at room temperature for 12 h (η = 7.68%, Jsc = 15.28 mA cm 2, Voc = 0.78 V, ff = 0.66, IPCEmax = 71.6% (530 nm)).

the TiO2 and any I3 anions that could penetrate to the surface. To the extent that this occurs, it would lead to a lower Jsc value. In the event, the best efficiency was seen for the DSSCs generated using the porphyrin with the most extensively conjugated β-olefinic linker, namely, tda-2b-bd-Zn. As noted above, this relative benefit is ascribed primarily to a better light-harvesting ability.38 Effect of Nanocrystal Forms of TiO2 Spheres on Photovoltaic Performances. The morphology of the TiO2 films has a strong influence on the photovoltage, the fill factor, and the IPCE features of the solar cells. Several crystal forms of TiO2, such as rutile, anatase, and brookite phases, occur naturally. Of these, the anatase phase has been perceived as the most attractive phase for use in DSSC construction because of its phase stability, ease of fabrication, and its potentially higher conduction band edge energy.15 In addition, anatase TiO2 has a wider optical band gap (3.2 eV) than rutile (3.0 eV), a difference that originates in the higher conduction band energy. This, in turn, leads to a higher Fermi level and better Voc in DSSCs for the same conduction band electron concentration.3b Typically, the mesoporous P-25 TiO2 spheres employed as working electrodes in DSSCs exist as a mixture of anatase and rutile phases in a ratio of 8:2. For the present study, we have used TiO2 spheres with a diameter of 600 800 nm composed of anatase nanoparticles prepared via an e-spray method, a technique that is known to improve the overall performances of DSSCs.39 The actual electrodes were further treated by hot-pressing at 120 °C to increase the interconnectivity among the TiO2 spheres. The composition of the anatase phase of the resulting annealed TiO2 film was tested by powder X-ray diffraction (XRD) methods, and the absence of an appreciable rutile phase contamination was confirmed (Figure S1 in the Supporting Information). The morphology and size of the anatase TiO2 spheres were also determined by inspection of TEM images, XRD diffraction ring patterns, and SEM micrographs (Figure S2 in the Supporting Information).

The photovoltaic properties of the mesoporous anatase TiO2 sphere-based solar cells were compared with those of the mesoporous P-25 TiO2 sphere-based solar cells (Table 4). On this basis, it was concluded that all porphyrins exhibited an enhancement in the total conversion efficiency due to a significant increase in the underlying Voc and Jsc values (Figure 6a). Complementary IPCE measurements provided support for the conclusion that the overall photocurrent action spectra were enhanced, especially in the Q-band region (550 700 nm) (Figure 6b). Among the dyes considered in the present study, tda-2b-bd-Zn produced cells with the highest conversion efficiency (7.47% with a Jsc = 18.36 mA cm 2, Voc = 0.71 V, and ff = 0.57) under these conditions. This efficiency rivals that seen for DSSCs based on a typical ruthenium-based dye, N3 (η = 7.68%). Probably, the higher value of η reflects improvements in the light-harvesting efficiency in the longer wavelength region, as well as favorable electron injection properties arising from the use of anatase TiO2 spheres. The values of Jsc for these latter DSSCs are larger than those created using the P-25 TiO2. An increase in the Voc value is also seen, which may be attributed by the improved LHE or light-scattering effect seen in the case of cells made up from the anatase TiO2 spheres, as reflected in the IPCE spectra. This enhancement of Voc is particularly noticeable in the case of the DSSCs produced from the doubly β-pyrrolefunctionalized sensitizer tda-2b-bd-Zn (almost a 0.1 V increase as compared to the analogous P-25-based systems). This enhancement is less apparent in the case of the singly modified derivatives, tda-1b-Zn and tda-1b-d-Zn, a difference that may be attributed to variations in the extent of dye loading. In the event, the highest IPCE values were observed in the case of DSSCs produced from tda-2b-bd-Zn with the recorded value being 77.3% at the Soret band maximum and 67.5% at the Q-band maximum; these values are nearly the same as those seen for the control DSSC made up from the control dye N3. More optimal surface states and phase boundary characteristics, which could lead to improved forward electron transport or reduced charge 19351

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produced. The maximum IPCE value proved to be 77.3% at the Soret band of tda-2b-bd-Zn. These values were considered to reflect both a favorable light-harvesting capability and an efficient electron injection ability. The results reported here lead us to suggest that an optimization of both the donor π acceptor structure of the porphyrin dye and the morphology of nanostructured TiO2 on which it is supported could lead to DSSCs with yetimproved efficiencies.40 Work along these lines is in progress.

’ ASSOCIATED CONTENT

bS

Supporting Information. Synthetic procedures, characterization data for new compounds, summaries of the photoelectrochemical performance of porphyrin-sensitized solar cells, and characterization data for the anatase mesoporous TiO2 spheres. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

Figure 6. (a) Characteristic photocurrent voltage curves for the porphyrins of this study as analyzed under conditions of simulated global AM 1.5 solar radiation at 100 mW cm 2. The dyes were studied as sandwich-type cells fabricated using mesoporous anatase TiO2 spheres. (b) Photocurrent action spectra of the devices made up using the porphyrins of this study.

recombination, could also underlie the better efficiency seen with the mesoporous anatase TiO2 spheres. However, these putative benefits do not extend to the control N3-based DSSCs prepared in the context of this study since little enhancement is seen with this ruthenium complex upon moving from P-25 to anatase TiO2 spheres. Presumably, this reflects the different ways in which these two obviously disparate classes of dye orient on the TiO2 surface. However, it is appreciated that further analysis will be required before the determinants responsible for these differences are fully known.

’ CONCLUSIONS We have synthesized a set of three zinc porphyrin sensitizers possessing an overall donor porphyrin acceptor dipolar structure. The systems in question contain a diarylamino group as the electron-donating subunit and electron-withdrawing β-olefinic substituents as both electron-withdrawing groups and as anchors for attaching the porphyrin core to a TiO2 surface. The three porphyrins of this study display characteristic charge-transfer features. For instance, the electron density is predominantly localized on the diarylamine donor moiety in the HOMO, whereas the corresponding electrons are found on the acceptor moieties in the LUMO. The electronic features embodied in the functionalized porphyrins of this study are considered advantageous for the purpose of creating dyes for use in DSSCs. Under optimized conditions, such as the use of the best of the three porphyrins (i.e., tda-2b-bd-Zn) studied in the presence of 2 equiv of CDCA as a coadsorbent on pure mesoporous anatase TiO2 sphere electrodes, cells with performance conversion efficiencies of 7.47% could be

*E-mail: [email protected] (J.L.S.), [email protected] (D.Y.K.), [email protected] (D.K.). Phone: +1-512-471-5009 (J.L.S.), +82-2-958-5323 (D.Y.K.), +82-2-2123-2652 (D.K.). Fax: +1-512471-7550 (J.L.S.), +82-2-958-5309 (D.Y.K.), +82-2-364-7050 (D.K.).

’ ACKNOWLEDGMENT The work at Yonsei University was supported by World Class University (R32-2010-000-10217) Programs from the Ministry of Education, Science, and Technology (MEST) and the Fundamental R&D Program for Core Technology of Materials (2010A011-0033) from the Ministry of Knowledge Economy of Korea (DK) and KIST Institutional Program (Grant No. 2E21833 to D. Y.K.). The work in Austin was supported by the National Sciences Foundation and the Robert A. Welch Foundation (grants CHE1057904 and F-1018 to J.L.S.). The quantum calculations were performed using the supercomputing resource of the Korea Institute of Science and Technology Information (KISTI). The authors thank Prof. Woo Dong Jung (Yonsei University) for allowing accessing to his preparative GPC. ’ REFERENCES (1) (a) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; M€uller, E.; Liska, P.; Vlachopoulos, N.; Gr€atzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (b) O’Regan, B.; Gr€atzel, M. Nature 1991, 335, 737. (2) (a) Gr€atzel., M. Bull. Jpn. Soc. Coord. Chem. 2008, 51, 3. (b) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gr€atzel., M. J. Am. Chem. Soc. 2001, 123, 1613. (3) Recent reviews of organic dyes in DSSCs: (a) Mishra, A.; Fischer, M. K. R.; B€auerle, P. Angew. Chem., Int. Ed. 2009, 48, 2474. (b) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Chem. Rev. 2010, 110, 6595. (c) Ning, Z.; Fu, Y.; Tian, H. Energy Environ. Sci. 2010, 3, 1170. (d) Clifford, F. N.; Martínez-Ferrero, E.; Viterisi, A.; Palomares, E. Chem. Soc. Rev. 2011, 40, 1635. (e) Preat, J.; Jacquemin, D.; Perpete, E. A. Energy Environ. Sci. 2010, 3, 891. (4) (a) Imahori, H.; Umeyama, T.; Ito, S. Acc. Chem. Res. 2009, 42, 1809. (b) Campbell, W. M.; Burrell, A. K.; Officer, D. L.; Jolley, K. W. Coord. Chem. Rev. 2004, 248, 1363. (c) Wang, X.-F.; Tamiaki, H. Energy Environ. Sci. 2010, 3, 94. 19352

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Kamat, P. V. Nano Lett. 2002, 2, 29. (d) Kamat, P. V.; Barazzouk, S.; Hotchandani, S.; Thomas, K. G. Chem.—Eur. J. 2000, 6, 3914. (e) Hara, K.; Horiguchi, T.; Kinoshita, T.; Sayama, K.; Sugihara, H.; Arakawa, H. Chem. Lett. 2000, 29, 316–317. (32) O’Regan, B. C.; Durrant, J. R.; Sommeling, P. M.; Bakker, N. J. J. Phys. Chem. C 2007, 111, 14001. (33) (a) Sommeling, P. M.; O’Regan, B. C.; Haswell, R. R.; Smit, H. J. P.; Bakker, N. J.; Smits, J. J. T.; Kroon, J. M.; van Roosmalen, J. A. M. J. Phys. Chem. B 2006, 110, 19191. (b) O’Regan, B. C.; Durrant, J. R.; Sommeling, P. M.; Bakker, N. J. J. Phys. Chem. C 2007, 111, 14001. (c) Barnes, P. R. F.; Anderson, A. Y.; Koops, S. E.; Durrant, J. R.; O’Regan, B. C. J. Phys. Chem. C 2008, 113, 1126. (34) Although it is difficult to discuss the effect of donor substitution because of the considerably different measurement conditions associated with the use of different TiO2 preparations, the absolute values of η for the DSSCs generated with the porphyrins of this study are dramatically enhanced relative to earlier systems. See ref 6a. (35) Kusama, H.; Kurashige, M.; Sayama, K.; Yanagida, M.; Sugihara, H. J. Photochem. Photobiol., A 2007, 189, 100. (36) (a) Rothernberger, G.; Comte, P.; Gr€atzel, M. Sol. Energy Mater. Sol. Cells 1999, 58, 321. (b) Hore, S.; Nitz, P.; Vetter, C.; Prahl, C.; Niggemann, M.; Kern, R. Chem. Commun. 2005, 2011. (37) Rochford, J.; Chu, D.; Hagfeldt, A.; Galoppini, E. J. Am. Chem. Soc. 2007, 129, 4655. (38) An analogous spacer length dependence on the η values has been reported in the case of conjugated phyenylethylnyl and biphenyl spacers. See ref 29 and Lin, C.-Y.; Lo, C.-F.; Luo, L.; Lu, H.-P.; Hung, C.-S.; Diau, E. W.-G. J. Phys. Chem. C 2009, 113, 755. In the case of these systems, the total efficiency was found to decrease upon the extension of the conjugation, an effect ascribed to the presence of tilted geometric orientations on the TiO2 surface. This stands in contrast to our porphyrins (e.g., tda-2b-bd-Zn) where strong electronic coupling between the LUMO and the CB of TiO2 was inferred based on the DFT calculations. We thus propose that our linkers induce a predominantly perpendicular geometry for the sensitizers relative to the TiO2 surface. (39) Hwang, D.; Lee, H.; Kim, D.; Kim, D. Y. Multi-functional Photoelectrode of Hierarchically-structured Mesoporous TiO2 Beads for Highly Efficient Solid-state Dye-sensitized Solar Cells. Submitted. (40) Our TiO2 films, while highly effective, are not yet considered fully optimized because the cell performance of the representative ruthenium dye, N3, chosen as a reference standard, exhibited a smaller efficiency (η = 7.68%) than that noted in a previous report (η = 10%). See ref 1.

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