Role of Adsorption Structures of Zn-Porphyrin on TiO2 in Dye

Article pubs.acs.org/JPCC

Role of Adsorption Structures of Zn-Porphyrin on TiO2 in DyeSensitized Solar Cells Studied by Sum Frequency Generation Vibrational Spectroscopy and Ultrafast Spectroscopy Shen Ye,*,† Arunkumar Kathiravan,‡ Hironobu Hayashi,§ Yujin Tong,† Yingyot Infahsaeng,‡ Pavel Chabera,‡ Torbjörn Pascher,‡ Arkady P. Yartsev,‡ Seiji Isoda,∥ Hiroshi Imahori,*,§,∥ and Villy Sundström*,‡ †

Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan Department of Chemical Physics, Lund University, Box 124, SE-22100 Lund, Sweden § Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ∥ Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ‡

S Supporting Information *

ABSTRACT: Several Zn-porphyrin (ZnP) derivatives were designed to build highly efficient dye-sensitized solar cells (DSC). It was found that solar cell efficiencies normalized for surface coverage (ηrel) are affected by the molecular spacer connecting the porphyrin core to the TiO2 surface, the sensitization conditions (solvent and time), and, to a lesser extent, the nature of the terminal group of the ZnP. Ultrafast transient absorption spectroscopy shows that electron transfer rates are strongly dependent on spacer and sensitization conditions. To understand this behavior at a molecular level, surface-sensitive vibrational spectroscopy, sum frequency generation (SFG), has been employed to investigate the adsorption geometries of these ZnP derivatives on the TiO2 surface for the first time. The average tilt angles and adsorption ordering of the ZnP molecules on the TiO2 surface were measured. A simple linear correlation between adsorption geometry of the adsorbed ZnP molecules, ηrel, and the concentration of long-lived electrons in the conduction band of TiO2 was shown to exist. The more perpendicular the orientation of the adsorbed ZnP (relative to the TiO2 surface), the higher the concentration of long-lived electrons in the conduction band, which contributes to the increase of photocurrent and solar cell efficiency. This result indicates that the electron transfer between ZnP and TiO2 occurs “through-space” rather than “through the molecular spacer”. It is also revealed that the sensitization solvent (methanol) may affect adsorption geometry and adsorption ordering through coadsorption and modify the electron transfer dynamics and consequently solar cell efficiency. Aggregation effects, which were observed for the longer sensitization times, are also discussed in relation to adsorption geometry and radiationless quenching processes. With the work reported here we demonstrate a novel strategy for DSC material characterization that can lead to design and manufacturing of photoactive materials with predictable and controlled properties.

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transport.13−55 In addition to the ruthenium polypyridyl complexes originally proposed by Grätzel et al.,1−12 many organic dye molecules have also been investigated as alternative sensitizers for DSCs from the viewpoint of cost and environmental demand.56−101 In this respect, porphyrin derivatives are frequently employed as potential candidates (so-called porphyrin-sensitized solar cells, PSSCs) due to their strong light absorption with Soret (400−450 nm) and Q bands (550−600 nm) in the visible spectral region, and in fact, solar

INTRODUCTION

Dye-sensitized solar cells (DSCs) introduced by Grätzel and co-workers have received extensive development efforts over the past two decades and now stand out as one of the most promising alternatives for photovoltaic solar energy conversion.1−12 A dye-coated nanocrystalline titanium dioxide (TiO2) thin film, in contact with an electrolyte solution containing iodine ions, absorbs photons over a wide range of the solar spectrum, and electrons are injected into the conduction band of the TiO2 to achieve efficient solar energy conversion. High power conversion efficiency of a DSC can be attributed to efficient solar energy harvesting by the dye sensitizer, efficient electron injection to the TiO2, and subsequent charge © 2013 American Chemical Society

Received: January 11, 2013 Revised: February 25, 2013 Published: February 25, 2013 6066

dx.doi.org/10.1021/jp400336r | J. Phys. Chem. C 2013, 117, 6066−6080

The Journal of Physical Chemistry C

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cell efficiencies (η) for PSSCs up to 12.3% have been reported,102−115 exceeding the highest obtained for other sensitizers.1−12,56−101 To further improve the performance of PSSCs, it is becoming increasingly important to get detailed knowledge of the factors controlling η. For example, a wide range of incident photon to current conversion efficiency (IPCE) and η values have been reported for the PSSCs using TiO2 electrodes sensitized by zinc(II)-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin.116−121 These results imply that the performance of PSSCs is quite sensitive to the sensitization conditions, such as solvent and time. To achieve high performance and reproducibility of the solar cell performance it is essential to elucidate the relationship between η and electron transfer dynamics at a molecular level, in which porphyrin−TiO2 adsorption geometry is expected to be a key factor. Recently, we took the first steps in this direction by investigating the photoinduced electron injection and recombination in Zn-porphyrin (ZnP)-sensitized TiO2 electrodes,122 for several different ZnPs at various sensitization conditions. Contrary to expectation, it was observed that the electron transfer rates (both injection and recombination) varied in a way inconsistent with electron transfer occurring through the molecular connecting spacer. With support from transmission electron microscopy (TEM) reporting on the adsorption geometries of the ZnPs to the nanostructured TiO2 surface, it was suggested that the electron transfer occurs “through space” rather than “through the molecular spacer”. For the family of ZnP molecules studied, a direct correlation between the amount of long-lived electrons in the TiO2 conduction band and the surface coverage-normalized solar cell efficiency (ηrel) was observed.122 With the work reported here we now take the next step toward a material characterization that can lead to design and manufacturing of DSC materials with predictable and controlled properties. To achieve this, not only ηrel and electron transfer dynamics but also the adsorption geometry of the sensitizers on the TiO2 surface have been characterized in detail. In the present work, sum frequency generation (SFG) vibration spectroscopy was employed to probe the structure and orientation of ZnP derivatives on the TiO2 surface at a molecular level. It is known that SFG spectroscopy is a secondorder nonlinear optical technique that allows us to obtain vibrational spectra at surfaces and interfaces.123−130 SFG has attracted much attention in recent years in multidisciplinary research fields, including surface science, materials chemistry, and biophysics, because of its high surface selectivity and sensitivity.123−130 Nevertheless, SFG has yet to be applied to DSCs to elucidate dye−TiO2 structure at the interface.131−136 By combining the dye-adsorption geometry information from SFG spectroscopy and the electron transfer dynamics information from ultrafast transient absorption spectroscopy, we are able to get a detailed picture about the correlation between the sensitizer adsorption geometry, electron injection/ recombination dynamics, and solar cell performance. To make a reliable orientation estimate from SFG measurements, a cyanophenyl group (denoted as CN-labeled, Figure 1) was specially designed and synthesized instead of the trimethylphenyl group (denoted as nonlabeled, Figure 1) in the ZnP derivatives used in our previous work.122 The adsorption geometry of the sensitizer can be determined from the CN stretching mode of the CN-labeled ZnP molecules adsorbed on the TiO2 surface without influence from other vibration modes,

Figure 1. Molecular structure of CN-labeled and nonlabeled Znporphyrins. MP was defined as 2,4,6-Me in our previous report.121,122

as the direction of the molecular axis of the CN is the same as that of the symmetry axis of the ZnP molecule. As will be shown below, the adsorption geometries of CN-labeled and nonlabeled ZnP molecules are very similar, making the labeled molecules excellent probes of the adsorption of the nonlabeled molecules. Although SFG spectra of the carboxylic moieties of ZnP molecules can also be employed to determine the molecular orientation, the vibrational modes of the porphyrin ring make the interpretation of the experimental results complicated. Therefore, we preferred the CN-labeled ZnP molecules to determine the molecular adsorption geometry in the present work. Since electron transfer is a sensitive measure of distance as well as molecular structure, the rates of electron injection and recombination were also compared in detail at various sensitization conditions for labeled and nonlabeled molecules. Finally, the systematic approach to correlate sensitizer surface adsorption geometry, electron transfer dynamics, and solar cell performance is graphically illustrated in Scheme 1.

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EXPERIMENTAL SECTION Synthesis of ZnP Derivatives. As shown in Figure 1, four kinds of ZnP derivatives were synthesized in the work. They are labeled and nonlabeled ZnP molecules with monophenylene spacers (denoted as MP and CNMP) and biphenylene spacers (denoted as BP and CNBP). Details about the synthesis procedures of the novel CN-labeled molecules are given in the Supporting Information. Synthesis of the nonlabeled molecules has been reported in our previous paper.122

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on 1 kHz amplified Ti:sapphire femtosecond lasers, one with a central output wavelength of 795 nm and 6 mJ per pulse (Spectra-Physics) and the other centered at 775 nm with 0.9 mJ pulse energy (Clark MXR). The output beam of the 6 mJ laser system was split into two parts, each for pumping one of two collinear optical parametric amplifiers (TOPAS, Light Conversion) to generate pump and probe beams, and transient absorption kinetics were measured as previously described.122 The other laser system was used to pump two broad-band noncollinear parametric amplifiers and used to measure transient spectra.122 In both transient absorption experiments, excitation at 558 nm, in the linear range of fluency (∼1013 photons/cm2/pulse), was used. The ZnP-modified TiO2 films for the transient absorption spectroscopy measurement were prepared by a similar procedure as described above for the photovoltaic measurements. A spacer frame of 0.13 mm was placed on top of the ZnP/TiO2 film on a thin glass slide, and the volume between the slides was filled with ACN. Paper clips were used to press the glass assembly to prevent solvent evaporation during measurements. Multiexponential fitting of kinetics was performed in Matlab software.137,138 Details are given in the Supporting Information. SFG Spectroscopy. SFG measurements were carried out with a broad-band SFG system developed at Hokkaido Univeristy.139−145 A Ti:sapphire femtosecond laser oscillator (MaiTai, Spectra-Phycis) and a regenerative amplifier (SpitFire PRO, Spectra-Phycis) pumped by a Nd:YLF laser (EMPower, Spectra-Phycis) generate a 2.2 mJ pulse at 800 nm of 120 fs duration at a repetition rate of 1 kHz. Half of the output was used to pump a TOPAS (Light Conversion) to generate infrared pulses tunable from 2.5 to 10 μm with a spectral width of ∼250 cm−1. The remaining output from the amplifier was sent to a home-built spectral shaper to generate a narrow-band pulse (∼10 cm−1) at 800 nm for improving the spectral resolution. The narrow band 800 nm pulse and the tunable broad-band infrared pulse were overlapped on the sample surface, and SFG signals were recorded by a CCD camera attached to a spectrograph. The SFG spectra were normalized by a SFG spectrum recorded for a gold substrate. Two polarization combinations of the SFG, 800 nm, and infrared beams were used, i.e., ssp and sps. Generally, a SFG measurement in ssp polarization can probe a vibrational mode that has an infrared dipole component perpendicular to the surface, while sps polarization is sensitive to a vibrational mode that has an infrared dipole component parallel to the surface; this fact can be employed to determine the molecular orientation from different polarized SFG spectra. The SFG spectra as a function of infrared frequency (ωIR) were quantitatively analyzed and fitted by the following equation.146,147

Scheme 1. Illustration of the Experimental Approach Used in This Worka

a

Sum frequency generation (SFG) measures the adsorption geometry on the TiO2 surface of CN-labeled Zn-porphyrin molecules (CNZnPs), and comparison of electron transfer dynamics of CN-labeled and nonlabeled molecules reveals the suitability of the CN-ZnPs as structural probes. Finally, adsorption geometry and electron transfer dynamics are correlated to solar cell performance.

Sample Preparation of ZnP-Sensitized TiO2 Films. Nanoporous films were prepared from a colloidal suspension of TiO2 nanoparticles (P25, Nippon Aerogel) dispersed in distilled water.122 This TiO2 colloidal suspension was deposited on a transparent conducting glass substrate (Asahi Glass, SnO2:F, 8 ohm/sq) by using the doctor blade technique. The films were annealed for 10 min at 673 K for the 4 μm thick TiO2 films, followed by similar deposition and annealing (723 K, 2 h) for the 10 μm thick TiO2 films. The thickness of the films was determined using a surface roughness tester (SURFCOM 130A, ACCRETECH). The TiO2 electrodes were sensitized in a 0.2 mM porphyrin solution of t-butyl alcohol:acetonitrile (v/v = 1:1) (denoted as t-BuOH:ACN) or methanol (MeOH) at room temperature. After removal from the dye bath, the nonadsorbed molecules were washed off by rinsing the film with the same solvent, and the film was dried at room temperature for ∼20 s to yield the ZnP-sensitized TiO2 films (denoted as ZnP/TiO2). The amount of porphyrin adsorbed on the TiO2 films was determined by measuring the absorbance at the peak of the Soret band for each dye molecule dissolved from the dye-sensitized TiO2 films into a known amount of dimethylformamide (DMF) containing 0.1 M NaOH aqueous solution. Photovoltaic Measurements. The photovoltaic measurements were performed in a sandwich cell consisting of the dyesensitized electrode as the working electrode and a platinumcoated glass electrode as the counter electrode.122 The two electrodes were placed on top of each other using a thin transparent Surlyn polymer film (Dupont) as a spacer for the electrolyte solution (0.5 M LiI, 0.01 M I2, 0.6 M dimethylpropyl imidazolium iodide, and 0.5 M 4-t-butylpyridine in ACN). The IPCE and current−voltage characteristics were determined by a PEC-L10 solar simulator (Peccell, Japan) irradiated with simulated air mass (AM) 1.5 solar light (100 mW/cm2). All experimental values were given as an average of five independent measurements. Transient Absorption Spectroscopy. Two femtosecond laser setups were used for the transient absorption spectroscopy at Lund University.137,138 Both measurement setups are based

(2) 2 I(ωIR )SFG ∝ |χR(2) + χNR |

χR(2) = N ∑ n

ωIR

An − ωυ , n + i Γn

(1a)

(1b)

(2) χ(2) R and χNR are the second-order resonant and nonresonant susceptibilities, respectively. N is molecular density on the surface. χ(2) R directly relates with the resonant contribution for the species on the surface. An, ωυ,n, and Γn are the amplitude, resonant frequency, and damping coefficient of the nth vibrational mode, respectively.

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TiO2 cell, which reflects the difference in the light harvesting efficiency. Electron Transfer Dynamics of ZnP-Sensitized TiO2 Films. Figure 2 shows the normalized steady state absorption

The TiO2 layers for SFG characterization were spin-coated on a CaF2 substrate and sintered at 673 K for 10 min and then at 723 K for 2 h. The thickness of the TiO2 layer was ∼120 nm as estimated from an AFM measurement. Other procedures were the same as those mentioned above. No difference in the surface morphologies of the electrode surface was found.

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RESULTS AND DISCUSSION Photovoltaic Properties of CN-Labeled ZnP-Sensitized TiO2 Electrodes. First, the surface coverage (Γ) of each ZnP molecule adsorbed on the TiO2 films was determined as described in our previous paper.121 Both CN-labeled and nonlabeled ZnP molecules122 reached a saturated value after 1 h adsorption in MeOH or t-BuOH:ACN (Table S1, Supporting Information). Almost no difference in the experimental Γ was observed for these ZnP molecules adsorbed on the TiO2 surface at different sensitization time (1 and 12 h). Assuming that the ZnPs are densely packed to form a perfect monolayer on the TiO2 surface that can be regarded to be flat at a scale of 10 nm from TEM measurements,122 calculated Γ values for CNMP and CNBP were estimated to be Γ = 1.2 × 10−10 mol cm−2 based on the molecular dimensions. By taking into account the good agreement between the calculated and experimental Γ values together with the saturated adsorption behavior of CNMP and CNBP, we conclude that densely packed monolayers of the CN-labeled ZnPs are formed on the TiO2 surface, similar to those of nonlabeled ZnPs.122 Then, the performances of the CN-labeled ZnP-sensitized TiO2 solar cells were evaluated. Note that η = Jsc × Voc × f f, in which Jsc is the short-circuit current, Voc the open-circuit potential, and f f the fill factor. For CNMP (Table S2, Supporting Information) sensitized in MeOH (or t-BuOH:ACN), it was found that η reached 3.4% (2.1%) after 1 h adsorption but decreased to 2.9% (1.3%) for 12 h sensitization. A similar sensitization time dependence of η was obtained for CNBP in both solvents, and η is lower in t-BuOH:ACN than in MeOH. The maximum η value (3.4%) and the maximum IPCE (59% at 420 nm) of the CNMP-sensitized TiO2 cell, with MeOH as a sensitization solvent, are higher than the corresponding values (2.0% and 37% at 420 nm) of the CNBP-sensitized TiO2 cell. The performances of the CNlabeled and nonlabeled ZnP/TiO2 solar cells are summarized in Table S2 (Supporting Information). On the other hand, the solar cell efficiency depends strongly on the device fabrication conditions, and it is hard to directly compare the efficiencies reported by different research groups. Typically we prepared the solar cell system using N719 ruthenium dye on the TiO2 nanoparticle under the same fabrication and optimized conditions (P25, sensitization solvent of t-BuOH:ACN for 12 h) as a reference system and obtained an efficiency of 6.5%.121 The maximum solar cell efficiency (4.6%) of the MP-sensitized TiO2 cell is ∼70% of that (6.5%) of the N719-sensitized TiO2 cell under our optimized conditions with P-25 TiO 2 particle and methanol as sensitization solvent with sensitization time of 1 h.121 Moreover, the maximum IPCE (76%) of the MP-sensitized TiO2 cell is comparable to that (73%) of the N719-sensitized TiO2 cell. These results show that (yield of electron injection) × (yield of long-lived conduction band electrons that can be extracted) is similar for the two cells and (efficiency of light harvesting) limits the cell performances. In fact, integration of the IPCE values for the MP-sensitized TiO2 cell with respect to wavelength (380−800 nm) is ∼70% that of the N719-sensitized

Figure 2. Normalized absorption spectra of (a) MP (black solid line) vs CNMP (red dotted line) and (b) BP (black solid line) vs CNBP (red dotted line) in MeOH and normalized steady state fluorescence spectra of (a) MP (magenta solid line) vs CNMP (green dotted line) and (b) BP (magenta solid line) vs CNBP (green dotted line) in MeOH. The normalized absorption and emission intensities are given on the left and right axis, respectively.

(left axis) and fluorescence spectra (right axis) for ZnPs with (a) the monophenylene spacer (MP, CNMP) and (b) biphenylene spacer (BP, CNBP) dissolved in MeOH. The absorption and fluorescence maximum wavelengths of CNlabeled molecules are quite similar to those of nonlabeled ones. The CN-labeled molecules show an ∼3 nm red-shift of the Q(0,0) absorption band as compared to the nonlabeled ones. This can be attributed to a larger steric hindrance of the 2,4,6trimethylphenyl than the 4-cyanophenyl group, so that the phenyl group is twisted out of the porphyrin plane, resulting in less conjugation in the nonlabeled ZnP molecules. The effect of this difference in electronic conjugation is also visible as a broadening of the absorption and fluorescence bands of the CN-labeled molecules relative to the nonlabeled ones. On the other hand, the monophenylene (CNMP) and biphenylene (CNBP) molecules show very similar absorption and fluorescence spectra, implying that the electronic structure of the porphyrin core is not perturbed by the different spacers. As reported in our previous paper,122 the electron injection and recombination of the nonlabeled ZnP-sensitized TiO2 films can be examined by time-resolved transient absorption spectroscopy. In the present paper, the same technique has been employed to investigate the CN-labeled ZnP-sensitized TiO2 films. As described previously,122 the electron transfer 6069



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Figure 3. (a) Transient absorption kinetics of MP (black circles), BP (cyan triangles), CNMP (green squares), and CNBP (wine diamonds) sensitized TiO2 in MeOH for 1 h. (b) Transient absorption kinetics of MP (red circles), BP (magenta triangles), CNMP (blue squares), and CNBP (olive diamonds) sensitized TiO2 in t-BuOH:ACN for 1 h. (c) Transient absorption kinetics of MP (full, black circles) and CNMP (full, green squares) sensitized TiO2 in MeOH for 1 h and of MP (open, black circles) and CNMP (open, green squares) sensitized TiO2 in MeOH for 12 h. (d) Transient absorption kinetics of BP (open, cyan triangles) and CNBP (open, wine diamonds) sensitized TiO2 in MeOH for 12 h.

Table 1. Power Conversion Efficiency Normalized for Surface Coverage (ηrel) Correlated to Amplitude of Long Lived Recombination (A>50 ns) and Tilt Angle (θ) cell MP/TiO2

BP/TiO2

CNMP/TiO2

CNBP/TiO2

sensitization solvent

sensitization time (h)

MeOH MeOH t-BuOH:ACN t-BuOH:ACN MeOH MeOH t-BuOH:ACN t-BuOH:ACN MeOH MeOH t-BuOH:ACN t-BuOH:ACN MeOH MeOH t-BuOH:ACN t-BuOH:ACN

1 12 1 12 1 12 1 12 1 12 1 12 1 12 1 12

ηrel (%) 5.4a 4.3a 4.8a 2.3a 3.3a 2.7a 2.8a 2.4a 4.3 ± 3.4 ± 2.5 ± 1.5 ± 2.5 ± 1.5 ± 1.4 ± 0.7 ±

0.2 0.2 0.1 0.1 0.2 0.1 0.1 0.1

A>50 nsb (%)

tilt anglec θ (degrees)

100 97 88 82 85 86 37 46 100 90 82 70 90 64 47 61

n/a n/a n/a n/a n/a n/a n/a n/a 42 38 63 66 61 55 68 64

a Taken from ref 122 (accuracy is given value ±0.1%). bAmplitudes fitted with ±5% error. cValues were extracted from SFG experiments with 50 ns recombination kinetic component (denoted as A>50 ns, see Table 1) is most important to determine the number of electrons collected in the external circuit per absorbed photon. Thus, this parameter along with the parameters describing sensitizer adsorption and solar cell efficiency are collected in Table 1. As shown in Figures 3a and 3b, the electron transfer (both injection and recombination) kinetics at 660 nm are very similar for the CN-labeled and nonlabeled dyes (i.e., MP vs CNMP and BP vs CNBP). This is particularly true for the ZnP molecules with monophenylene spacers (MP and CNMP) where the kinetics profiles are more or less identical. The close similarities of injection and recombination kinetics of the labeled and nonlabeled molecules imply that both types of ZnP molecules must have similar interaction with the TiO2 surface. Taking into account that the spacer lengths between the porphyrin core and carboxylic group (i.e., MP and CNMP vs BP and CNBP) are different, we can find the spacer dependence of the electron transfer dynamics in Figures 3a and 3b. Both labeled and nonlabeled molecules with the longer biphenylene spacer (BP and CNBP) generally have higher amplitudes of fast electron injection and recombination, i.e., overall faster electron transfer. This result is in good agreement with findings in our previous paper.122 The electron injection process for the biphenylene spacer molecules (BP and CNBP) is essentially complete before ∼10 ps, while the recombination process shows a large component (10−50%) on the time scale of several hundred picoseconds (Table S3, Supporting Information). As a comparison, electron injection for the monophenylene spacer molecules (MP, CNMP) shows a large fraction of slow kinetics (∼100 ps), and most of the recombination occurs with a >50 ns time constant (Table 1 and Table S3, Supporting Information). This behavior of slower electron transfer for a shorter spacer is counterintuitive if electron transfer is imagined to occur through the molecular spacer connecting the molecule to the TiO2 surface. We will discuss this issue further in relation with the adsorption geometry results obtained from the SFG measurements. Figure 3 also demonstrates that the ZnP molecules with monophenylene and biphenylene spacers respond quite differently to change of sensitization solvent and sensitization time. MP/TiO2 and CNMP/TiO2 exhibit only small changes in electron transfer dynamics upon changing the sensitization solvent from MeOH to t-BuOH:ACN (Figures 3a and 3b) or extending sensitization time from 1 to 12 h (Figure 3c and Figure S7, Supporting Information). For the ZnP molecules with biphenylene spacers (BP and CNBP), changing the sensitization solvent from MeOH to t-BuOH:ACN leads to a dramatic change of the injection and recombination dynamicsthe recombination kinetics in MeOH has only a ∼10% amplitude on the time scale of several hundred picoseconds, which increases to ∼50% in t-BuOH:ACN (Figures 3a and 3b 6071



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where Ns is the number of molecules at the interface and R is (2) the hyperpolarizability ratio between β(2) aac and βccc for the CN 152 group. The expression ⟨ ⟩ carries out an ensemble average of (2) the orientation angle. Since χ(2) yyz and χyzy for the CN group can be obtained from the ssp- and sps-polarized spectra, its tilt angle (θ) can be estimated based on the equations above (details for the procedures are given in the Supporting Information). θ estimated from the SFG measurements are summarized in Table 1 assuming a δ-function distribution. As shown in Table 1, both CNMP and CNBP are attached to the TiO2 surface at a considerable tilt angle to the surface normal. The estimated tilt angles basically agree with those from the average monolayer thickness determined by our previous TEM measurements (Figure S10, Supporting Information).122 Generally, CNBP has on average a somewhat larger tilt angle than that of CNMP under the same sensitization conditions. On the other hand, the sensitization in MeOH gives a lower tilt angle than in mixed solvents for the same ZnP dye (Table 1). Here, it should be noted that simple geometrical considerations show that the hydrogen atoms on the phenylene spacer closest to the TiO2 surface exert steric interactions with the surface, such that the tilt angle of the ZnP molecules is limited to ∼65°. This implies that they cannot adopt a more tilted geometry on the TiO2 surface, which is in agreement with the maximum tilt angles observed from SFG measurements (Table 1). We also notice that an X-ray reflectometry study on ZnP molecules adsorbed on an atomically smooth TiO2 surface prepared by atomic layer deposition (ALD) arrived at a similar picture of the sensitizer adsorption geometry as presented here.153,154 As shown in Figure 4, the spectral intensities of the SFG spectra with t-BuOH:ACN as sensitization solvent are always higher than that with MeOH. The SFG intensity at the interface depends on molecular density (N), tilt angle θ (and its angular distribution width σ), as well as conformational ordering.146,147 As mentioned in the photovoltaic characterization above, the total coverage for CNMP and CNBP adsorbed on the TiO2 surface is comparable (Table S1, Supporting Information). Although the calculation based on eq 2 shows that a ZnP adlayer with a larger tilt angle in the present case (Table 1) will give a slightly higher SFG intensity of the CN mode, this difference is too small to explain the intensity difference observed from the SFG spectra of ZnP adsorbed in MeOH and t-BuOH:ACN solvents. Therefore, higher SFG intensity with t-BuOH:ACN as a sensitizing solvent also suggests that the porphyrins are adsorbed on the TiO2 surface in a more ordered geometry in t-BuOH:ACN as compared to MeOH. As will be shown later, MeOH molecules are coadsorbed at the TiO2 surface with ZnP molecules. Additionally, the SFG intensities of the ssp- and sps-spectra with sensitization time of 1 h are higher than those with sensitization time of 12 h in both solvents (Figure S11, Supporting Information). Given that a long sensitization time leads to an increase in the porphyrin aggregation on the TiO2 surface,121,122 this result suggests enhanced disordering among ZnP molecules with extending sensitization time. It is known that protic solvents such as alcohols adsorb on a TiO2 surface,131,132 which would also affect solar cell performance as well as electron transfer dynamics on the ZnP-sensitized TiO2 surface. To understand this issue, SFG measurements were also carried out to clarify the solvent adsorption on the TiO2 surface. Figure 5a shows SFG spectra of a fresh TiO2

Figure 4. (a) ssp- and (b) sps-polarized SFG spectra of CNMPsensitized (black symbols) and CNBP-sensitized (red symbols) TiO2 films with sensitization solvent of MeOH. (c) ssp- and (d) spspolarized SFG spectra of CNMP-sensitized (black circles) and CNBPsensitized (red circles) TiO2 films with sensitization solvent of a tBuOH:ACN mixture. Sensitization time is 1 h. Solid traces are fitted SFG curves based on eq 1.

group does not interact with the TiO2 substrate. Furthermore, SFG spectra of the ZnP-sensitized TiO2 surface in the lower frequency region (1800−1400 cm−1) show the presence of asymmetric and symmetric stretching modes of the carboxylate group and the disappearance of the CO stretching mode of the carboxylic acid (results not shown). ATR-IR characterization of the ZnP-modified TiO2 surface also confirmed the existence of a carboxylate group on the TiO2 surface (Figure S9, Supporting Information). These results imply that the CNlabeled ZnP molecules are bound to the TiO2 surface through the carboxylic acid anchor moiety, which is deprotonated to the carboxylic anion and adsorbed via a bridging bidentate mode. This finding is consistent with previous observations for analogous porphyrins on TiO2.102−111 Thus, the CN moiety is free from interaction with the TiO2 surface, and the presence of the CN moiety in the labeled ZnP derivatives does not change their adsorption interaction with the TiO2 surface. The quantitative analysis on the polarization-dependent SFG measurement for the CN stretching mode (Figure 4) enabled us to determine the orientation of the CN-labeled ZnP molecules adsorbed on the TiO2 surface. By fitting the sspand sps-polarized SFG spectra and correcting for the local field factors (L factor, see Figure S1, Supporting Information) according to the present optical geometry and the TiO2 film thickness (∼120 nm), the second-order susceptibilities of χ(2) yyz and χ(2) yzy can be determined. As the CN vibrational mode can be regarded as belonging to the C∞ν symmetry group, χ(2) yyz and 150,151 χ(2) yzy can be expressed as 1 (2) (2) χyyz = Nsβccc [⟨cos θ ⟩(1 + R ) − ⟨cos3 θ ⟩(1 − R )] 2 (2a) (2) χyzy =

1 Nsβ (2)(1 − R )[⟨cos θ ⟩−⟨cos3 θ ⟩] 2 ccc

(2b) 6072



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Figure 5. (a) SFG spectra of TiO2 films sensitized with MeOH (red) and with t-BuOH:ACN (black). The spectra in the C−H region of 2800−3000 cm−1 are shown without normalization. The background signal was subtracted. (b) SFG spectra in the C−H region (2800−3000 cm−1) of 4-CF3/ TiO2 film sensitized in MeOH for 1 h (red) and 4-CF3/TiO2 (black) film sensitized in t-BuOH:ACN for 1 h. The background signal was subtracted. (c) Molecular structure of 4-CF3.

It should be noted here that we could find the SFG peaks attributed to the adsorbed MeOH species even when the TiO2 surface was sensitized by the ZnP derivatives dissolved in MeOH. To avoid possible spectral contribution from the methyl groups of the ZnP derivatives, a new MP derivative 4CF3 in which the 2,4,6-trimethylphenyl group was replaced by 4-trifluoromethylphenyl121,122 (i.e., the molecule does not have the C−H stretching mode in the frequency region) was synthesized and then sensitized in MeOH or t-BuOH:ACN (Figure 5b). The C−H stretching peaks are still clearly observed for sensitization in MeOH, while the peaks are absent in t-BuOH:ACN. These results strongly support that MeOH can coadsorb on the TiO2 surface together with the ZnP derivative. In summary of the adsorption geometry of ZnP adsorption on the TiO2 surface determined by SFG vibrational spectroscopy, the tilt angle of CNMP was found to be smaller than CNBP under the same sensitization conditions. The sensitization of ZnP in MeOH solvent gives a lower SFG spectral intensity in comparison with that in the mixed solvent, which is attributed to a smaller tilt angle of adsorbed ZnP as well as the more disordered adsorption geometry induced by the coadsorption of MeOH. As will be discussed below, these features will directly affect the electron transfer behaviors and

surface after immersion into (a) MeOH (red trace) and (b) tBuOH:ACN (black trace), in the frequency region of the C−H stretching (2800−3000 cm−1). No signal was found for a fresh TiO2 surface before the immersion. After adsorption in MeOH, the SFG spectrum shows three broad peaks around 2825, 2880, and 2945 cm−1 and a shoulder around 2970 cm−1. After ozone (O3) treatment for 5 min, these peaks disappeared completely and thus could be attributed to adsorbed species from MeOH. Shultz and co-workers reported that methanol adsorbs to the TiO2 surface in two adsorption modes: physisorbed as molecular methanol and chemisorbed as a methoxy species.131,132 Two pairs of C−H stretch peaks were observed and ascribed to the methoxy species (2828 and 2935 cm−1) and molecular MeOH (2855 and 2968 cm−1), which is consistent with the present SFG observations. One can expect that the coadsorption of MeOH definitely occurs on the TiO2 surface. On the other hand, after immersion in t-BuOH:ACN, no SFG peak on the TiO2 surface is observed in both the C−H region (Figure 5a, black trace) and CN stretching region (data not shown), indicating that neither t-BuOH nor ACN form ordered adsorption structures on the TiO2 surface. The tbutyl group of t-BuOH is sterically bulkier than the methyl group of MeOH, resulting in no adsorption of t-BuOH on the TiO2 surface. 6073



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of our measurements. This distribution of slow lifetimes would then correspond to the part of the tilt angle distribution representing the most upright molecules. All this means that a small tilt angle should be seen as a larger fraction of sensitizers exhibiting slow recombination. This view nicely agrees with the correlation between tilt angle and amplitude of very slow recombination, A>50 ns. Electron injection and recombination are both controlled by the dye-TiO2−distance and should therefore both respond to a change in dye−semiconductor distance, through a change of tilt angle. If this is correct there should exist a correlation between the rate of electron injection and rate of recombination. Figure S12 (Supporting Information) shows that there in fact exists such a correlationthe higher the amplitude of instantaneous rise of the transient absorption signal (≪ 1 ps electron injection) the lower the amplitude of slow recombination. The very fast injection corresponds to a fraction of molecules, making the shortest distance between the edge of the porphyrin core and the TiO2 surface and thus also very fast recombination (∼100 ps time scale recombination). From this we can see that both injection and recombination give the same picture of the adsorption geometry, which is corroborated by the tilt angle from the SFG measurements. The discussion above has shown how adsorption geometry of the ZnPs controls the amount of the long-lived electrons (>50 ns) in the TiO2 conduction band. The fraction of molecules that recombines with electrons on a faster time scale does so much faster (50 ns, is correlated to the solar cell efficiency and ultimately the tilt angle to solar cell efficiency, η. Before we can perform this correlation correctly we need to examine the parameters controlling η, which can be defined as

thus efficiency of the solar cells made by the corresponding ZnP dyes. Correlation between Adsorption Geometry, Electron−Cation Recombination Dynamics, and Solar Cell Efficiency. The rate of electron transfer with its exponential distance dependence is expected to depend critically on the sensitizer−TiO2 distance and therefore on the adsorption geometry of sensitizer on the TiO2 surface. Now, we are in a position to explore how the electron transfer dynamics correlate to the adsorption geometry of the sensitizers on the TiO2 surface as obtained from the SFG measurement. The inset in Figure 6 shows the relation between the concentration of

Figure 6. Solar cell efficiency normalized for surface coverage (ηrel) as a function of tilt angle. Closed and open symbols correspond to the sensitization time of 1 and 12 h, respectively. The inset shows the correlation between tilt angle and amplitude of >50 ns charge recombination, A>50 ns (i.e., concentration of long-lived electrons in the TiO2 conduction band (%)). The color and symbol coding is the same as in main figure.

η ∝ (efficiency of light harvesting) × (yield of electron injection) × (yield of long lived conduction band electrons that can be extracted) × (1 − other losses)

long-lived conduction band electrons (represented by the amplitude of the long-lived kinetic component, A>50 ns) and tilt angles for each labeled ZnP-sensitized TiO2 film prepared under different conditions. Although there is some scattering in the experimental points, a clear correlation exists between the two parametersa small tilt angle results in more long-lived electrons. This behavior should be associated with the distance dependence of electron transfer of ZnP adsorbed on the TiO2 surface. If the electron transfer takes place through the molecular spacer, such a tilt angle dependence should not exist for one type of sensitizer. As reported for the nonlabeled ZnP molecules in our previous paper,122 if the electron transfer between the edge of the porphyrin core and the TiO2 surface takes place mainly through space, this correlation can be understooda small tilt angle corresponds to a larger fraction of molecules having a longer electron transfer distance and therefore more of a slower electron transfer. It should be noted that the heterogeneity of the molecules as well as the TiO2 surface lead to a distribution of tilt angles. Thus, estimated tilt angles rather represent the average of a distribution of molecular orientations. The very slow, >50 ns, recombination most likely corresponds to a distribution of nonresolved very long recombination times because of the limited time window

Light harvesting is related to the surface coverage of the dye and how well it absorbs the wavelengths of the solar spectrum, and it varies substantially between the different dyes and various experimental conditions. Thus, variations in η are to a certain extent a result of variation in light harvesting. The absorption spectra are quite similar for all porphyrin dyes studied, while the surface coverage may vary somewhat. When discussing solar cell efficiency correlations between electron transfer dynamics and adsorption geometry, we therefore use an efficiency normalized to surface coverage (ηrel) to remove any uncertainty that stems from variations in light-harvesting efficiency (Table 1). Electron injection is more or less the same for all ZnP derivatives under short sensitization time (i.e., 1 h). Furthermore, electron injection is also fast as compared to the excited singlet state lifetime of the porphyrin (2 ns), so electron injection is close to 100% efficient (except for some of the 12 h sensitizations where porphyrin excited state quenching induced by strong dye−dye interaction decreases the efficiency of electron injection).121,122 Thus, in most cases electron injection does not influence the efficiency. In contrast, recombination, which we now know is controlled by the tilt angle of adsorbed ZnP on TiO2, directly controls the number of long-lived 6074



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PSSCs and proposed that suitable long alkoxyl chains are capable of wrapping the porphyrin core, thus resulting in decreased dye aggregation and improved photovoltaic performance.113 In the present study, the nonlabeled ZnP molecules (MP and/or BP) show more bulky structures than that of CNlabeled ones (CNMP and/or CNBP). It is expected that the bulky hydrophobic environment of the nonlabeled ZnP molecules can also reduce the aggregation of the dye molecules on the TiO2 surface and block the charge recombination between electrons in the TiO2 and I3− in comparison with that of CN-labeled ones, thus giving higher conversion efficiency.

electrons in the conduction band of TiO2 and thus the photocurrent in the operating solar cell, suggesting that there should be a direct relationship between ηrel and the tilt angle. This expectation is verified in Figure 6 where we can see a linear correlation (with some scatter) between ηrel and tilt anglethe larger the angle the lower the solar cell efficiency (the corresponding plot for ηrel vs A>50 ns is shown in Figure S13, Supporting Information). The ηrel vs tilt angle plots depict that 12 h sensitization leads to lower efficiency than 1 h, for the same tilt angle. This suggests that efficiency is lost in the injection step, and it is reasonable to suggest that long sensitization leads to aggregation that decreases the injection efficiency by radiationless quenching processes competing with the injection, a phenomenon already observed in our earlier study of the nonlabeled ZnP molecules.122 We discussed above and more extensively in ref 122 how aggregation may change the electron transfer and therefore solar cell efficiency. These results taken together thus suggest that aggregation may influence solar cell efficiency through two effectschanged adsorption geometry and increased excited state quenching. We have seen that the DSC active electrodes based on CNlabeled and nonlabeled porphyrin molecules exhibit very similar electron transfer dynamics (Figure 4) and that trends of solar cell efficiency variations with degree of slow (>50 ns) charge recombination also are very similar (Figure S13 (Supporting Information) vs Figure 8 in ref 122). Nevertheless, the CNlabeled ZnP exhibits a somewhat lower absolute value of ηrel than the nonlabeled ZnP (Table S2, Supporting Information) under the same experimental conditions. It is recognized that the dipole moment of the sensitizer may influence the solar cell performance155−158 through a shift of the position of the TiO2 conduction band, which ultimately causes a variation of the Voc value. The interfacial dipole moment could also change the electron injection rate, which would result in a variation of the Jsc value. However, our measurements reveal that there is neither a significant difference in the Voc values for the labeled and nonlabeled sensitizers nor any significant difference in electron injection rates. Therefore, the effects of dipole moment on the solar cell performance are likely to be negligible for the studied porphyrin molecules. A difference in rereduction (regeneration) kinetics between the oxidized porphyrin and the I−/I3− redox couple in the electrolyte could be another possible explanation for the lower solar cell performance of the labeled porphyrins. A difference in driving force of the rereduction could give rise to a difference in rate. However, the first oxidation potentials of CNMP (0.98 V vs NHE) and CNBP (0.97 V vs NHE) in dichloromethane containing 0.1 M Bu4NPF6 as a supporting electrolyte are virtually identical to those of the nonlabeled porphyrins (1.00 V vs NHE for MP, 0.98 V vs NHE for BP),122 implying that a difference in driving force cannot rationalize a difference in the rereduction rate. On the other hand, it is known that the regeneration reaction proceeds via a transient intermediate complex [dye+•I−] formed through the reaction between a photogenerated dye cation and an iodide ion.159 If the interaction between the cyano group of the porphyrin cation and iodide ion in the electrolyte solution inhibits the [dye+•I−] formation,160 the regeneration kinetics between the oxidized porphyrin and I−/ I3− redox couple would slow down, resulting in a decrease in the photocurrent and solar cell efficiency. Recently, Diau and his co-workers investigated the roles of alkoxyl/alkyl chains in the photovoltaic performance of the

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CONCLUSIONS We have successfully established CN-labeled ZnP molecules attached to a nanostructured TiO2 surface as probes of the adsorption geometry of nonlabeled molecules by combining vibrational SFG and transient absorption spectroscopy for the first time. The transient absorption kinetic results of labeled and nonlabeled molecules show very similar dynamics. The SFG measurements reveal that all ZnP molecules studied here are attached to the TiO2 surface by a carboxylic group under an appreciable tilt angle, and this angle depends on the specifics of the porphyrin as well as properties of the sensitization solvent and duration of the sensitization. There is a linear correlation between average tilt angle and ηrelthe smaller the angle the higher the solar cell efficiency. Since electron transfer exhibits a strong distance dependence, the rate of electron injection and recombination constitutes a complementary measure (to SFGmeasured tilt angle) of sensitizer adsorption geometry. It is also revealed that the sensitization solvent (methanol) may affect adsorption geometry and adsorption ordering through coadsorption and modify the electron transfer dynamics and consequently solar cell efficiency. Long sensitization (i.e., 12 h vs 1 h) of the TiO2 electrode causes aggregation-induced decrease of solar cell efficiency through two effectschanges of adsorption geometry and increased radiationless quenching decreasing injection efficiency. The comprehensive results suggest a method to characterize the structure of the sensitizer/semiconductor interface and thus pave the way toward providing DSC materials with predictable electron transfer properties. Despite the very simple model, a strong correlation between tilt angle and electron transfer rate has been confirmed in the present work. Thus, this must be a strong guiding parameter in designing new materials. On top of that of course the additional complicating factors, detailed shape/structure of the molecules, coadsorbing solvent, etc. have to be considered in a next step refinement of the picture.

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ASSOCIATED CONTENT

S Supporting Information *

Details of synthesis of the CN-labeled Zn-porphyrins and determination of their orientation on the TiO2 surface are described. Detailed analysis results of cell performance, ultrafast kinetics, and adsorption geometry are also given. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]; villy. [email protected]. Notes

The authors declare no competing financial interest. 6075



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ACKNOWLEDGMENTS For funding of the work performed at Lund University, the Swedish Energy Agency (STEM), the Swedish Research Council, the Knut&Alice Wallenberg foundation, and the European Research Council (Advanced Investigator Grant to V. S., VISCHEM 226136) are acknowledged. This work was supported by Grant-in-Aids (No. 21350100 to H. I.), Advanced Low Carbon Technology Research and Development Program (ALCA, JST), and WPI Initiative, MEXT, Japan. H. H. is grateful for a JSPS fellowship for Young Scientists. S.Y. acknowledges a Grant-in-Aid for Scientific Research (B) 23350058 and a Grant-in-Aid for Scientific Research on Innovative Areas “Coordination program” (24108701) from MEXT, Japan.

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Role of Adsorption Structures of Zn-Porphyrin on TiO2 in Dye

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