Rigidifying the π-Linker to Enhance Light ... - ACS Publications

Jan 24, 2014 - Ning Cai,. †,§,∥. Xiandui Dong,. ‡ and Peng Wang*. ,†. †. State Key Laboratory of Polymer Physics and Chemistry, Changchun I...
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Rigidifying the π‑Linker to Enhance Light Absorption of Organic DyeSensitized Solar Cells and Influences on Charge Transfer Dynamics Zhaoyang Yao,†,§ Lin Yang,†,§ Yanchun Cai,† Cancan Yan,†,§ Min Zhang,† Ning Cai,†,§,∥ Xiandui Dong,‡ and Peng Wang*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China § University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: The usage of coplanar π-conjugated segments represents a feasible strategy on reducing the energy gap of organic push−pull dyes for mesoscopic titania solar cells. In this paper, we report two new dyes coded as C254 and C255 with the respective 1,4di(thiophen-2-yl)benzene and indacenodithiophene π-linkers, in combination with the electron-releasing triphenylamine and electron-withdrawing cyanoacrylic acid units. The energy-gap reduction stemming from the rigidity of the π-linker is accompanied by a negative shift of the ground-state redox potential, which however does not affect the yield of hole injection from the oxidized state of dye molecules to a cobalt redox electrolyte. On the other side, we have identified from femtosecond transient absorption measurements a diminished rate of electron injection from the relaxed, low-energy excited state of C255 to titania, albeit a comparable rate of electron injection from the high-energy excited states of these two dyes. The bulkier C255 dye with four hexyl side chains tethered on the two sp3 carbons of the fused indacenodithiophene unit can form a more compact self-assembling monolayer on titania, considerably attenuating the charge recombination of photoinjected electrons in titania with the cobalt electrolyte and thus enhancing the cell photovoltage and efficiency.

1. INTRODUCTION The dye-sensitized solar cell (DSC) technology is with great promise for the conversion of solar light to electricity,1 if its power conversion efficiency (PCE) can be further improved to the level of 15% at the air mass 1.5 global (AM1.5G) conditions. Even with the state-of-the-art materials, it has already been commercialized to power some wireless keyboards, owing to its peculiar performance operated under diffusive visible light. To further boost its economical efficiency for a larger scale of applications, enormous efforts have been devoted to insightful physical analysis and new material development in the past years. Ruthenium bipyridyl and zinc porphyrin complexes have been demonstrated thus far to be the most efficient light harvesters in DSCs.2−5 In addition, a lot of research interest has been caught on metal-free organic dyes in the past decade, due mainly to the abundance of raw materials, the flexibility of molecular design, and the bright color.6−10 Apart from the electron-releasing and electron-withdrawing blocks in a D-π-A dye, the conjugated π-linker is also of paramount importance in modulating the energy gap and the microstructure of a self-assembling dye layer on titania. In this context, a mass of thiophene-based organic dyes full of structural variety have been prepared since the initial work by Arakawa and his co-workers11 on integrating vinylthiophene or © 2014 American Chemical Society

2,2′-dithiophene (DT) with coumarin and cyanoacrylic acid units.12−22 In conjunction with the hydrophilic cyanoacrylic acid electron acceptor11 and the hydrophobic dihexyloxysubstituted triphenylamine electron donor,23 we have evaluated the merit of improving light absorption by π-linker rigidification, via comparing cyclopentadithiophene (CPDT) with DT as well as cyclopenta[1,2-b:5,4-b′]dithiophene[2′,1′:4,5]thieno[2,3-d]thiophene (CPDTTT) with 2,5-di(thiophen-2yl)thieno[3,2-b]thiophene (DTTT).24,25 Meanwhile, Wong et al. have exploited the coplanar indacenodithiophene (IDT)26−28 as the central π-spacer to construct a series of Dπ-A dyes with remarkable power conversion efficiencies (PCEs) up to 6.7% under AM1.5G radiation.29 Herein we will report two new dyes shown in Figure 1a by use of either the twisting 1,4-di(thiophen-2-yl)benzene (DTB) unit30 for C254 or IDT as the conjugated π-linker for C255. Furthermore, we will systematically analyze the impacts of structural alteration on light absorption and femtoseconds (fs) to milliseconds (ms) charge transfer dynamics, which underpin the photovoltaic performance of DSCs made with these dyes. Received: December 10, 2013 Revised: January 17, 2014 Published: January 24, 2014 2977

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mesoporous titania films shown in Figure 1c. It should be pointed out that there is a decreased dye loading amount (cm) of 1.3 × 10−8 mol cm−2 μm−1 for C255, in contrast to that of 2.0 × 10−8 mol cm−2 μm−1 for C254. This scenario could be intuitively comprehended in terms of a three-dimensional bulkier character of C255 compared to C254 (Figure S1, Supporting Information). Presented in Figure 1d are the photocurrent action spectra of DSCs made from the bilayer (4.2 + 5.0 μm thick) titania films grafted with these two dyes, in combination with a tris(1,10phenanthroline)cobalt(II/III) (Co-phen) electrolyte. The composition of Co-phen as well as the details for cell fabrication can be found in the Experimental Section. The maximum incident photon-to-collected electron conversion efficiencies (IPCEs) are ∼80% for C254 and ∼75% for C255. The red-shifted onset wavelength of photocurrent response for C255 is in good accordance with optical measurements (Figure 1c). Even though we consider the optical losses stemming from the light absorption and scattering by the FTO glass, it is still valuable to realize that the IPCEs observed for C254 and C255 are 10−15% lower than that of 90% for other organic dyes.22 The nonunity IPCEs perceived for these two dyes will be further analyzed by resorting to several time-resolved photophysical and electrical measurements. Nanosecond (ns) transient absorptions (TA) at a probe wavelength of 782 nm upon the excitation of 5 ns laser pulses were further recorded to examine the impact of rigidifying the π-linker on the average time constant (⟨τr1⟩) of charge recombination between the oxidized state of the dye molecules (D+) and the photoinjected electrons in titania. This recombination reaction can be analyzed by looking at the transient absorption traces i and ii (Figure 2) of the control Figure 1. (a) Molecular structures of the C254 and C255 photosensitizing dyes. Herein C6H13 represents the n-hexyl substituent. (b) UV−vis absorption spectra of the THF solutions of C254 and C255. (c) Light-harvesting efficiencies of the 4.2 μm thick, mesoporous titania films grafted with C254 and C255 and immersed in the Co-phen electrolyte. (d) Photocurrent action spectra of DSCs employing the bilayer titania films (4.2 + 5.0 μm) grafted with C254 and C255 in conjunction with the Co-phen electrolyte.

2. RESULTS AND DISCUSSION The electronic absorption spectra (Figure 1b) of C254 and C255 dissolved in tetrahydrofuran (THF) were first recorded for us to take a preliminary look at the impact of rigidifying the π-linker on the power of light harvesting. The C255 dye featuring the fused IDT segment displays a maximum absorption wavelength (λabs max) of 509 nm, which is red-shifted by 43 nm with respect to that of 456 nm for the C254 reference dye with the nonplanar DTB unit. Moreover, the rigidification of the π-linker also gives rise to an improved molar absorption coefficient at the maximum absorption wavelength, εabs max, from 43.8 × 103 to 66.9 × 103 M−1 cm−1. The bathochromic and hyperchromic effects, which are implemented here by fusing the phenyl and thiophene blocks with the cyclopentadienyl group for organic D−A dyes, are in general accord with our previous observations on the other three pairs of dyes.24,25,31 Note that the electron-releasing effect endowed by the alkyl substitution also should contribute to the light absorption improvements. The energy-gap reduction concomitant with the π-linker alteration presented here is further proved by measuring the light-harvesting efficiencies (ϕlh) of dye-grafted

Figure 2. Absorption decays at a probe wavelength of 782 nm upon 5 ns laser excitation for the 4.2 μm thick, mesoporous titania films grafted with C254 (panel a) and C255 (panel b), which are also immersed in the inert electrolyte (curves i and ii) and the Co-phen electrolyte (curves iii and iv). The wavelengths of pump pulses are 595 nm for the C254 samples and 622 nm for the C255 samples. The pump wavelengths are selected in terms of a 0.2 optical density of the testing sample, and the pulse fluence is kept at 17 μJ cm−2 to afford an alike distribution profile of excitons. The gray lines are stretched exponential fittings.

cells, which are made from dye-grafted mesoporous titania films in conjunction with an inert electrolyte composed of 0.5 M 4tert-butylpyridine (TBP) and 0.1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in acetonitrile. The solid gray lines in Figure 2 are fittings of the temporal absorption changes (ΔA) with a stretched exponential decay function ΔA ∝ A0 exp[−(t/τr1)α], where A0 is the pre2978

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exponential factor, τr1 the characteristic time, and α the stretching parameter. ⟨τr1⟩ can be further derived via the equation ⟨τr1⟩ = (τr1/α)Γ(1/α), where Γ is the gamma function.32 The use of a stretched function is based upon the consideration of several factors, predominantly including the heterogeneity of electron-occupying states in the nanocrystalline titania film, the inconstant orientation of dye molecules on the surface of titania, and the exponential density distribution of photoinjected electrons and D+ in the studied samples. It is found that C255 displays a ⟨τr1⟩ of 2.3 ms, which is similar to that of 1.6 ms for C254. These time constants lie in the common range from 0.5 to 30 ms observed for other arylaminecyanoacrylic dyes.16,19,25,31,33−38 However, taking the place of the inert electrolyte with the Co-phen electrolyte initiates an acceleration of absorption decays (curves iii and iv of Figure 2) in significant measure, owing to the occurrence of hole injection from D+ to the divalent cobalt(II) cation. Using the same protocol for ⟨τr1⟩, the calculated mean time constants (⟨τhi⟩) of hole injection are 22 μs for C254 and 26 μs for C255. It is noted that there is a 75 mV negative shift of ground-state redox potential for C255 with respect to C254 (Figure S2, Supporting Information). However, the two time constants of D+ involving charge transfer reactions are both insensitive to the changes of free energy. Also, the total reorganization energy of transformation between D and D+ is mainly contributed from the solventdependent outer-sphere reorganization energy, the latter of which is only weakly dependent on the dye structure.39 Thereby, it is very likely that in terms of the Marcus electron transfer theory40−42 the parameters of electronic couplings of D+ with cobalt(II) ions and photoinjected electrons in titania play some decisive roles in these charge transfer processes. Overall, due to the weak influences of π-linker rigidification on ⟨τr1⟩ and ⟨τhi⟩, the kinetic branch ratios of these two charge transfer channels both exceed 70 for C254 and C255, suggesting the yields of hole injection from D+ to Co(II) are close to unity. The yield of electron injection from excited-state dye molecules to titania (ϕei) was further evaluated by means of the fs fluorescence upconversion technique. A control cell made from a dye-grafted mesoporous alumina film was first constructed to simulate the nonelectron-injecting, excited state evolution of a titania counterpart. Note that the Cophen electrolyte was also infiltrated to make the measurements close to the realistic conditions of DSC operation. Our supplementary measurements on the alumina cells using the inert electrolyte have excluded any charge transfer and energy transfer from the excited dye molecules to the cobalt(II/III) species. The PL decays (Figure 3b) featuring amplitudeaveraged lifetimes ⟨τex⟩ of 30.3 ps for C254 and 43.6 ps for C255 (Table 1) could be mainly ascribed to the radiative and radiationless deactivation of excited-state dye molecules. Note that the ⟨τex⟩ value of C255 grafted on alumina is considerably shorter than that of 231.1 ps in THF (Table 1). However, the ⟨τex⟩ value of C254 on alumina is significantly longer than that of 3.7 ps in THF (Figure 3a and Table 1). It can be imaged that the existence of twisting motions among the conjugated segments to a large extent triggers the nonradiative deactivation of C254 in THF; however, there is an evident aggregationinduced elongation of exciton lifetime in the self-assembling dye layer. Furthermore, the substitution of alumina by titania (Figure 3c) induces a significant shortening of ⟨τex⟩ (1.3 ps for C254 and 5.2 ps for C255, Table 1), owing to carrier

Figure 3. PL traces upon 130 fs laser excitation of the THF dye solutions (panel a), dye-grafted alumina films (panel b), and dyegrafted titania films (panel c). The dyed oxide films are immersed in the Co-phen electrolyte. The pulse fluence of excitation light at 530 nm is 38 μJ cm−2, and the probe wavelengths are 700 nm for C254 and 720 nm for C255. The solid gray lines are fittings of normalized PL intensity (IPL) via an equation comprising an exponential rise and three-exponential decays convoluted with a Gaussian instrument response function I(t), IPL = A0 − Ar exp(−t/τr) ⊕ I(t) + ∑3i=1 Ai exp(−t/τi) ⊕ I(t), where A0 is the baseline signal; Ar and Ai denote the amplitudes; and τr and τi are the time constants.

photogeneration at the energy-offset titania/dye interface. The estimated ϕei of C254 is 96% by referring to the relationship ϕei = 1 − ⟨τex,titania⟩/⟨τex,alumina⟩, which is slightly higher than that of 88% for C255. The lower electron injection yield stemming from the π-linker rigidification has given a clue on the aforementioned IPCE height reduction (Figure 1d). We further recorded fs-TA spectra (Figures 4a and 4b) to directly probe the electron injection dynamics of the preceding dye-grafted titania films immersed in the Co-phen electrolyte.43−45 Owing to a serious spectral superposition of the ground state, excited state, and oxidized state for these two dyes, it is infeasible to survey the electron injection dynamics at a single wavelength in the visible and near-IR (not shown) range. On the basis of the kinetic model illustrated in Scheme S1 (Supporting Information), we thereby carried out the target analysis46−49 of our fs-TA data and identified three species in the kinetic evolution, including the high-energy excited singlet state (D*H), the relaxed, low-energy excited singlet state (D*L), and the oxidized state (D+) as represented in the speciesassociated difference spectra (SADS) of Figures 4c and 4d. The extracted time constants are compiled in Table 2 and can be used to nicely fit the kinetic traces at various wavelengths (Figures S3 and S4, Supporting Information). The kinetic traces of these components are presented in Figures 4e and 4f for C254 and C255, respectively. The time constants (τtr) for the evolution of D*H to D*L by energy relaxation were 2.8 ps for the self-assembling C254 dye layer and 1.9 ps for C255. The averaged time constants (τTA ei,av) for the two-exponential generation of D+ from D*H and D*L are 2.8 ps for C254 and 2979

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Table 1. Time Constants and Fractional Amplitudes of PL Decay Traces sample

τ1 [ps]

a1a

τ2 [ps]

a2a

τ3 [ps]

a3a

⟨τexb⟩ [ps]

C254 in THF C255 in THF C254@Al2O3 C255@Al2O3 C254@TiO2 C255@TiO2

1.6 101.7 0.2 4.5 0.7 1.1

0.43 0.37 0.14 0.52 0.65 0.45

1.6 107.9 4.2 20.0 1.7 3.4

0.43 0.31 0.51 0.01 0.26 0.42

16.5 500.0 80.3 87.4 5.2 25.2

0.14 0.32 0.35 0.47 0.09 0.13

3.7 231.1 30.3 43.6 1.3 5.2

a The fractional amplitudes a1, a2, and a3 are derived by the equation ai = Ai/∑3i=1Ai. bThe average time constants ⟨τex⟩ are derived by the equation ⟨τex⟩ = ∑3i=1aiτi.

Figure 4. 3D graphs of fs-TA spectra of 2.1 μm thick, mesoporous titania films grafted with C254 (panel a) and C255 (panel b), which are also immersed in the Co-phen electrolyte. The pulse fluence of excitation light at 530 nm is 18 μJ cm−2. Species-associated difference spectra of D*H, D*L, and D+ for the C254 (panel c) and C255 (panel d) samples, which are generated by the target analysis of the TA spectra in panels a and b with the model shown in Scheme S1 (Supporting Information). Note that there is a small contribution from the transition of photoinjected electrons in titania to the nominal spectra of D+. Kinetic traces generated by the target analysis, for D*H (red), D*L (blue), and D+ (green) of the C254 (panel e) and C255 (panel f) samples. The gray lines in panels e and f represent the instrument response functions.

ferrocene/ferrocenium (Fc/Fc+) redox couple. An only 10 meV variation of driving force is not likely to give a good answer for the observed difference in the time constants of electron injection from D*L. One may argue that this could be due to the physical absorption of the C255 dye molecules on titania, which has actually been excluded by our attenuated total reflection Fourier transform infrared (ATR-FTIR) spectrum measurements (Figure S6, Supporting Information). The exact reason for the slower electron injection of C255 is not very clear at the moment but could be related to a weaker electronic coupling of its relaxed excited state and titania.51 This rationalization is based upon the fact that the π-linker rigidification can lead to a reduced distribution of the lowest unoccupied molecular orbital on the cyanoacrylic acid moiety.52 The typical photocurrent density−voltage (j−V) characteristics at an irradiance of 100 mW cm−2, simulated AM1.5 sunlight (Figure 5a) of DSCs fabricated with the C254 and C255 dyes were also tested, and Table 3 lists their detailed parameters. The C254 dye has a short-circuit photocurrent density (jsc) of 11.02 mA cm−2, an open-circuit photovoltage (Voc) of 823 mV, and a fill factor (FF) of 0.737, affording a PCE of 6.7%. Furthermore, the C255 dye with a smaller energy gap displays a higher jsc of 12.94 mA cm−2 and an over 80 mV

Table 2. Time Constants Derived from the Target Analysis of fs-TA Spectra dye

τtr [ps]

τei,H [ps]

τei,L [ps]

τTA ei,av [ps]

a τPL ei,av [ps]

C254 C255

2.8 1.9

1.3 1.5

6.5 18.0

2.8 7.4

1.4 5.9

a

The time constants (τPL ei,av) for electron injection derived from the fs fluorescence upconversion experiments are also included for comparison.

7.4 ps for C255 obtained by fs-TA measurements. As listed in PL Table 2, τTA ei,av is in rough accord with τei,av estimated from the fluorescence upconversion data. While both dyes feature a similar time constant (τei,H) of 1.4 ± 0.1 ps for electron injection from D*H to titania, the C255 dye displays a nearly three times deceleration for electron injection from D*L to titania (Table 2). Taking the ground-state redox potentials (E0D/D ′ +) from cyclic voltammograms (Figure S1, Supporting Information) and the 0−0 emission transition energies (E0−0) derived from the steady-state photoluminescence spectra (Figure S5, Supporting Information),50 we can further estimate the excited-state redox potential of (E0D*/D ′ +), being −1.533 V for C254 and −1.523 V for C255 with respect to the 2980

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Figure 6. (a) Dependence of extracted charge on open-circuit photovoltage. (b) Plots of lifetime of photoinjected electrons in titania as a function of charge. Figure 5. (a) j−V characteristics of the C254 and C255 cells measured at an irradiance of 100 mW cm−2, simulated AM1.5 sunlight. (b) Plots of open-circuit photovoltage against short-circuit photocurrent density. An antireflection film was adhered to a testing cell during measurements. Aperture area of the employed metal mask: 0.160 cm−2.

titania conduction band edge, probably owing to other important factors such as a relatively lower dye loading amount of C255, a different tilt angel of dye molecules on titania, and different surface concentrations of some electrolyte components. Thereby, in this work we did not take into account the possible but very complicated impacts of the dipole moments of the ground state, excited state, and oxidized state of dye molecules on multichannel charge transfer dynamics. This issue could be detailed in future studies. In addition, a nonunity electron collection yield stemming from a shorter electron lifetime could also be a controlling factor for the only 80% IPCE maximum of the C254 cell. In view of the nearly quantitative yields of hole injection from the photooxidized dyes to cobalt (II) ions in the electrolyte for the C254 and C255 cells revealed by the ns-TA measurements, the prolonged τe could be mainly ascribed to a reduced overlap of the electronic wave functions of titania and cobalt (III) ions. To gain a microscopic insight, we also employed the X-ray reflectivity (XRR)55−58 technique to examine the microstructure features of the self-assembling layers of dye molecules on the surface of planar titania. The planar titania made via atomic layer deposition (ALD) on monocrystalline silicon has an atomically smooth surface and also possesses a much higher X-ray scattering length density (SLD) with respect to an organic ultrathin film.22 It can be derived by fitting the reflectivity data in Figure S7 (Supporting Information) with a bilayer model that the dye alteration from C254 to C255 leads to a reduced thickness (d) of the dye layer from 1.8 to 1.3 nm but a larger SLD from 7.1 × 10−6 to 8.9 × 10 −5 nm −1 (Table S1, Supporting Information). The interaction strength of scatters in the dye layer with incident X-ray is directly associated with SLD, as described by the relation SLD = (reNAρZ)/MR, where re is the Bohr electron radius, NA the Avogadro’s number, ρ the mass density of the dye layer, Z the sum of the atomic number (i.e., total number of electrons) of a dye molecule, and MR the relative molecular

augmented Voc of 906 mV, generating a markedly improved PCE of 9.0%. For a DSC at the short-circuit condition, the density of electrons stored in the mesoporous titania film is pretty low so that the charge collection yield is at least higher than 75% for the cells studied here, and the measured jsc is in rough proportion to the flux of carrier photogeneration. Thereby, we further recorded j−V curves at a set of light irradiances and plotted Voc versus jsc. It can be derived from Figure 5b that at a fixed jsc there is an about 80 mV higher Voc for C255 in comparison with C254. It is well documented that for DSCs with an invariable redox electrolyte their Voc values under light are intimately related to the electron quasi-Fermi levels (EF,n) of titania, which are determined by the density of photoinjected electrons in titania and/or the conduction band edge (Ec) of titania.53,54 At a given flux of carrier photogeneration, the density of electrons in titania is controlled by the rate of recombining photoinjected electrons with holes of the electrolyte and/or the oxidized state of dye molecules. Therefore, charge extraction (CE) and transient photovoltage decay (TPD) measurements were performed to dissect the energetic and kinetic origins of the observed Voc difference. As presented in Figure 6a, the dye alternation from C254 to C255 does not bring forth a marked variation of the conduction band edge of titania but to a large extent elongates the lifetimes (τe, Figure 6b) of photoinjected electrons at a given charge (Q) stored in titania. We noted the difference for the ground state dipole moments of these two dyes (15.1 D for C254 and 17.6 D for C255) from DFT calculation but did not observe its influence on the shift of

Table 3. Averaged Parameters of Five Cells Measured at an Irradiance of the 100 mW cm−2, Simulated AM1.5 Sunlighta dye

jIPCE [mA cm−2] sc

jsc [mA cm−2]

Voc [mV]

FF

PCE [%]

C254 C255

10.57 ± 0.17 12.33 ± 0.15

11.02 ± 0.15 12.94 ± 0.13

823 ± 3 906 ± 4

0.737 ± 0.005 0.764 ± 0.006

6.7 ± 0.2 9.0 ± 0.2

a IPCE jsc

is derived via wavelength integration of the product of the standard AM1.5 emission spectrum (ASTM G173-03) and the measured IPCE with the experimentally measured jsc, spectrum. The validness of measured photovoltaic parameters is examined by comparing the calculated jIPCE sc exhibiting a less than 5% error. 2981

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stirred for another 2 h. Then the mixture was extracted into dichloromethane, and the organic layer was washed with water and dried over anhydrous sodium sulfate. After removing solvent under reduced pressure, the residue was purified by column chromatography (dichloromethane/petroleum ether 60−90 °C, 1/1, v/v) on silica gel to yield a yellow solid as the desired product 2 (230 mg, 76% yield). 1H NMR (600 MHz, DMSO-d6) δ: 9.92 (s, 1H), 8.05 (d, J = 4.2 Hz, 1H), 7.86 (d, J = 8.4 Hz, 2H), 7.80 (d, J = 4.2 Hz, 1H), 7.77 (d, J = 8.4 Hz, 2H), 7.64 (d, J = 4.2 Hz, 1H), 7.18 (m, 2H). 13C NMR (150 MHz, CDCl3) δ: 183.97, 151.97, 142.29, 141.88, 139.22, 134.69, 131.34, 128.70, 126.87, 126.51, 126.04, 125.27, 124.54. MS (ESI) m/z calcd for (C15H10OS2): 270.0. Found: 271.0 ([M + H]+). Anal. Calcd for C15H10OS2: C, 66.63; H, 3.37. Found: C, 66.59; H, 3.40. 5-(4-(5-Bromothiophen-2-yl)phenyl)thiophene-2-carbaldehyde (3). In a three-neck round-bottom flask was dissoved 2 (0.218 g, 0.81 mmol) in THF (28 mL) and cooled to 0 °C using an ice salt bath. NBS (158 mg, 0.89 mmol) in THF (5 mL) was added to the reaction mixture dropwise. Then the resulting solution was stirred at the same temperature for 5 h before a yellow solid formed. Water was added to terminate the reaction, and the mixture was extracted three times with chloroform before the organic phase was washed with water and dried over anhydrous sodium sulfate. After removing partial solvent under reduced pressure, the residue was filtered and washed with water and dichloromethane to yield a yellow powder 3 (0.234 g, 83% yield). 1H NMR (600 MHz, CDCl3) δ: 9.90 (s, 1H), 7.75 (d, J = 4.2 Hz, 1H), 7.68 (d, J = 8.4 Hz, 2H), 7.57 (d, J = 8.4 Hz, 2H), 7.42 (d, J = 3.6 Hz, 1H), 7.12 (d, J = 3.6 Hz, 1H), 7.06 (d, J = 3.6 Hz, 1H). 13C NMR (150 MHz, CDCl3) δ: 182.69, 153.37, 144.69, 142.59, 137.35, 134.58, 132.39, 131.11, 126.99, 124.13, 123.90, 112.41. MS (ESI) m/z calcd for (C15H9BrOS2): 347.9. Found: 348.9 ([M + H]+). Anal. Calcd for C15H9BrOS2: C, 51.58; H, 2.60. Found: C, 51.56; H, 2.63. 5-(4-(5-(4-(Bis(4-(hexyloxy)phenyl)amino)phenyl)thiophen-2-yl)phenyl)thiophene-2-carbaldehyde (4). In a dried Schlenk tube were dissolved 3 (0.234 g, 0.67 mmol) and 4,4,5,5-tetramethyl-2-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}-1,3,2-dioxaborolane (0.450 g, 0.80 mmol) in dioxane/H2O (v/v, 5/1). Then Pd(OAc)2 (3 mg, 0.02 mmol), SPhos (6 mg, 0.02 mmol), and K3PO4 (0.710 g, 3.35 mmol) were added to the reaction mixture, which was stirred at 45 °C overnight under argon. After cooling to room temperature, water was added, and the mixture was extracted three times with chloroform before the organic phase was washed with water and dried over anhydrous sodium sulfate. After solvent removal under reduced pressure, the residue was purified by column chromatography (dichloromethane/petroleum ether 60−90 °C, 1/2, v/v) on silica gel to yield an orange solid as the desired product 4 (0.411 g, 86% yield). 1H NMR (600 MHz, DMSO-d6) δ: 9.92 (s, 1H), 8.06 (d, J = 3.6 Hz, 1H), 7.85 (d, J = 8.4 Hz, 2H), 7.79 (d, J = 3.6 Hz, 1H), 7.76 (d, J = 8.4 Hz, 2H), 7.61 (d, J = 4.2 Hz, 1H), 7.51 (d, J = 9.0 Hz, 2H), 7.38 (d, J = 4.2 Hz, 1H), 7.04 (d, J = 9.0 Hz, 4H), 6.92 (d, J = 9.0 Hz, 4H), 6.78 (d, J = 8.4 Hz, 2H), 3.94 (t, J = 6.6 Hz, 4H), 1.70 (m, 4H), 1.42 (m, 4H), 1.32 (m, 8H), 0.89 (t, J = 6.0 Hz, 6H). 13C NMR (150 MHz, THF-d8) δ: 183.28, 157.46, 154.00, 150.18, 146.21, 144.34, 142.05, 141.77, 138.62, 136.84, 133.23, 128.08, 128.02, 127.40, 127.38, 126.98, 126.26, 125.48, 124.11, 121.43, 116.52, 69.25, 33.07, 30.79, 27.22, 26.06, 24.01, 14.86. MS (ESI) m/z calcd for (C45H47NO3S2): 713.3. Found: 714.2 ([M

mass of a dye. Thereby, the averaged surface concentration (cp) of C254 on the ALD titania can be calculated by the equation cp = (SLDd)/reNAZ, being 1.8 × 10−10 mol cm−2, which is larger than that of 1.1 × 10−10 mol cm−2 for C255. The similar ratios for the cp values on the planar titania and the cm on the mesoporous titania have suggested an alike self-assembling of dye molecules on these two substrates. It can be derived that with respect to C254 the C255 dye molecules self-assemble on the surface of titania with a reduced number of molecules to form a thinner but higher mass density organic layer. The latter feature is closely related to its four n-hexyl side chains on the IDT unit. These chains along with the conjugated backbone function as components of an organic blocking shell on titania, which prevents the cobalt (III) ions from being in close proximity to the surface of titania and thus elongates the τe value of charge recombination.

3. CONCLUSIONS In summary, we have synthesized a new push−pull dye C255 characteristic of the hexyl-substituted indacenodithiophene πlinker, which features a red-shifted absorption peak, an improved molar absorption coefficient, and an elongated excited-state lifetime with respect to its reference dye C254 with 2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene. Our fs fluorescence upconversion and transient absorption experiments have shown that the overall electron injection rate and yield are both slightly reduced upon rigidifying the π-linker, even though the rate of electron injection from the high-energy excited states is hardly affected. Our work has also pointed to the urgency of designing a narrow energy-gap dye featuring a longlived excited state for quantitative electron injection. In addition, the quest for viable strategies to construct a favorite self-assembling layer of dye molecules on titania is very important to reduce the charge recombination at the titania/ electrolyte interface and thus enlarge the number and free energy of electrons converted from solar photons. 4. EXPERIMENTAL SECTION 4.1. Materials. N,N-Dimethylformide (DMF), 1,2-dichloroethane (DCE), dimethyl sulfoxide (DMSO), THF, chloroform, ethanol, and phosphorus oxychloride (POCl3) were distilled before use. LiTFSI, acetonitrile, TBP, N-bromosuccinimide (NBS), palladium(II) acetate (Pd(OAc)2), 2-(2,6-dimethoxybiphenyl)-dicyclohexylphosphine (SPhos), 2-cyanoacetic acid, and piperidine were purchased from Aldrich. The scattering paste was purchased from Dyesol, and the paste for the transparent layer was prepared according to the published procedure.59 DTB30 and 4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene28 were synthesized according to the corresponding literature methods. The synthesis of 4,4,5,5-tetramethyl-2-{4[N,N-bis(4-hexyloxyphenyl)amino]phenyl}-1,3,2-dioxaborolane was described in our previous paper.24 4.2. Dye Synthesis. The synthetic routes for C254 and C255 are illustrated in Scheme S2 of the Supporting Information. 5-(4-(Thiophen-2-yl)phenyl)thiophene-2-carbaldehyde (2). In a three-neck round-bottom flask was dissoved 1 (0.270 g, 1.11 mmol) in DCE (5 mL) and cooled to 0 °C using an ice salt bath. DMF (0.52 mL, 6.68 mmol) and POCl3 (0.15 mL, 10.67 mmol) were added to the reaction mixture. The resulting solution was stirred at 70 °C for 12 h. Saturated sodium acetate aqueous solution (10 mL) was added, and the mixture was 2982

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+ H]+). Anal. Calcd for C45H47NO3S2: C, 75.70; H, 6.63; N, 1.96. Found: C, 75.63; H, 6.72; N, 1.92. 4,9-Dihydro-4,4,9,9-tetrahexyl-s-indaceno[1,2-b:5,6-b′]-dithiophene (6). To a suspension of 5 (0.288 g, 1.08 mmol) in anhydrous DMSO (15 mL) was added sodium tert-butoxide (1.99 g, 6.49 mmol) in parts. The reaction mixture was stirred for 10 min, followed by the addition of 1-bromohexane (1.033 g, 6.49 mmol) dropwise. After complete addition, the resultant mixture was stirred overnight at room temperature, then poured into ice water. The crude product was extracted into diethyl ether, and the organic layer was washed with brine and water and dried over anhydrous sodium sulfate. After removing solvent under reduced pressure, the residue was purified by column chromatography (petroleum ether 60−90 °C) on silica gel to yield a viscous colorless oil (0.530 g, 81%). 1H NMR (600 MHz, DMSO-d6) δ: 7.50 (d, J = 4.8 Hz, 2H), 7.46 (s, 2H), 7.08 (d, J = 4.8 Hz, 2H), 1.99 (m, 4H), 1.90 (m, 4H), 1.02 (m, 24H), 0.70 (t, J = 6.9 Hz, 12H), 0.69 (m, 8H). 13C NMR (150 MHz, CDCl3) δ: 155.08, 153.21, 141.65, 135.58, 126.09, 121.69, 113.11, 53.65, 39.18, 31.58, 29.70, 24.12, 22.56, 14.00. MS (ESI) m/z calcd. for (C40H58S2): 602.4. Found: 603.4 ([M + H]+). Anal. Calcd for C40H58S2: C, 79.67; H, 9.69. Found: C, 79.59; H, 9.73. 4,9-Dihydro-4,4,9,9-tetrahexyl-s-indaceno[1,2-b:5,6-b′]-dithiophene-2-carbaldehyde (7). In a three-neck round-bottom flask was dissoved 6 (0.230 g, 0.38 mmol) in DCE (5 mL), and the solution was cooled to 0 °C using an ice salt bath. DMF (0.078 g, 1.06 mmol) and POCl3 (0.069 g, 0.45 mmol) were added to the reaction mixture. The resulting solution was stirred at room temperature for 4 h. Saturated sodium acetate aqueous solution (10 mL) was added before the mixture was stirred for another 2 h. Then the mixture was extracted into dichloromethane, and the organic layer was washed with water and dried over anhydrous sodium sulfate. After removing solvent under reduced pressure, the residue was purified by column chromatography (dichloromethane/petroleum ether 60−90 °C, 1/1, v/v) on silica gel to yield a yellow solid as the desired product 7 (0.230 g, 96% yield). 1H NMR (600 MHz, DMSO-d6) δ: 9.88 (s, 1H), 8.02 (s, 1H), 7.73 (s, 1H), 7.60 (d, J = 4.8 Hz, 1H), 7.59 (s, 1H), 7.12 (d, J = 4.8 Hz, 1H), 1.95 (m, 8H), 1.03 (m, 24H), 0.70 (t, J = 6.4 Hz, 12H), 0.69 (m, 8H). 13 C NMR (150 MHz, CDCl3) δ: 182.83, 156.49, 154.98, 154.83, 153.91, 144.39, 138.52, 133.75, 127.77, 121.80, 114.88, 113.19, 54.04, 53.85, 39.07, 39.05, 31.54, 29.60, 24.21, 24.11, 22.52, 13.97. MS (ESI) m/z calcd for (C41H58OS2): 630.4. Found: 631.3 ([M + H]+). Anal. Calcd for C41H58OS2: C, 78.04; H, 9.26. Found: C, 77.97; H, 9.31. 7-Bromo-4,9-dihydro-4,4,9,9-tetrahexyl-s-indaceno[1,2b:5,6-b′]-dithiophene-2-carbaldehyde (8). To a cold solution of 7 (0.244 g, 0.39 mmol) in THF (15 mL) was added NBS (0.076 g, 0.43 mmol) at 0 °C under argon. The reaction mixture was stirred at the same temperature for 5 h, and then water (15 mL) was added. The crude product was extracted into dichloromethane, and the organic layer was dried over anhydrous sodium sulfate. After removing solvent under reduced pressure, the residue was purified by column chromatography (dichloromethane/petroleum ether 60−90 °C, 1/1, v/v) on silica gel to yield a yellow-green solid (0.260 g, 95% yield). 1H NMR (600 MHz, DMSO-d6) δ: 9.89 (s, 1H), 8.02 (s, 1H), 7.75 (s, 1H), 7.64 (s, 1H), 7.37 (s, 1H), 1.99 (m, 8H), 1.03 (m, 24H), 0.71 (t, J = 6.6 Hz, 12H), 0.69 (m, 8H). 13C NMR (150 MHz, CDCl3) δ: 182.85, 155.26, 155.14, 154.99, 152.58, 152.35, 144.64, 141.44, 137.98, 134.09,

130.45, 124.85, 114.86, 113.94, 113.10, 54.85, 54.09, 39.01, 38.95, 31.53, 29.58, 24.20, 24.08, 22.54, 22.51, 13.97. MS (ESI) m/z calcd for (C41H57BrOS2): 708.3. Found: 709.3 ([M + H]+). Anal. Calcd for C41H57BrOS2: C, 69.36; H, 8.09. Found: C, 69.32; H, 8.15. 7-(4-(Bis(4-(hexyloxy)phenyl)amino)phenyl)-4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2carbaldehyde (9). In a dried Schlenk tube were dissolved 8 (0.260 g, 0.37 mmol) and 4,4,5,5-tetramethyl-2-{4-[N,N-bis(4hexyloxyphenyl)amino]phenyl}-1,3,2-dioxaborolane (0.257 g, 0.45 mmol) in dioxane/H2O (6 mL, v/v, 5/1). Then Pd(OAc)2 (2 mg, 0.01 mmol), SPhos (4 mg, 0.01 mmol), and K3PO4 (0.392 g, 1.85 mmol) were added to the reaction mixture, which was stirred at 45 °C overnight under argon. After cooling to room temperature, water was added, and the mixture was extracted three times with chloroform before the organic phase was washed with water and dried over anhydrous sodium sulfate. After solvent removal under reduced pressure, the residue was purified by column chromatography (dichloromethane/petroleum ether 60−90 °C, 1/2, v/v) on silica gel to yield a red solid as the desired product 9 (0.375g, 90% yield). 1 H NMR (600 MHz, DMSO-d6) δ: 9.88 (s, 1H), 8.01 (s, 1H), 7.72 (s, 1H), 7.55 (s, 1H), 7.52 (d, J = 8.4 Hz, 2H), 7.39 (s, 1H), 7.03 (d, J = 8.4 Hz, 4H), 6.92 (d, J = 9.0 Hz, 4H), 6.80 (d, J = 9.0 Hz, 2H), 3.95 (t, J = 6.3 Hz, 4H), 1.99 (m, 8H), 1.71 (m, 4H), 1.42 (m, 4H), 1.32 (m, 8H), 1.04 (m, 24H), 0.89 (t, J = 6.0 Hz, 6H), 0.69 (m, 20H). 13C NMR (150 MHz, THF-d8) δ: 183.20, 158.77, 157.37, 156.31, 154.65, 152.76, 149.96, 149.41, 146.71, 141.91, 140.35, 140.22, 135.37, 131.71, 128.63, 127.88, 127.24, 121.86, 117.59, 116.51, 115.97, 114.19, 69.25, 55.59, 55.51, 40.41, 40.34, 33.07, 33.02, 32.99, 31.07, 30.80, 27.23, 23.92, 23.91, 14.82. MS (ESI) m/z calcd for (C71H95NO3S2) 1073.7. Found: 1074.3 ([M + H]+). Anal. Calcd for C71H95NO3S2: C, 79.35; H, 8.91; N, 1.30. Found: C, 79.29; H, 8.96; N, 1.27. 2-Cyano-3-{5-(4-(5-(4-(N,N-bis(4-hexyloxyphenyl)amino)phenyl)thiophen-2-yl)phenyl)thiophen-2-yl}acrylic Acid (C254). To a stirred solution of 4 (0.411 g, 0.58 mmol) and cyanoacetic acid (0.147 g, 1.73 mmol) in chloroform was added piperidine (0.345 g, 4.06 mmol). The reaction mixture was refluxed under argon for 12 h and then acidified with 2 M hydrochloric acid aqueous solution. The crude product was extracted into chloroform, washed with water, and dried over anhydrous sodium sulfate. After removing solvent under reduced pressure, the residue was purified by flash chromatography with toluene and then methanol/toluene (1/20, v/v) in turn as eluent to yield a red-violet powder (0.366 g, 81%). 1H NMR (600 MHz, DMSO-d6) δ: 13.74 (s, 1H), 8.51 (s, 1H), 8.04 (d, J = 3.6 Hz, 1H), 7.83 (d, J = 8.4 Hz, 2H), 7.82 (d, J = 3.6 Hz, 1H), 7.77 (d, J = 7.8 Hz, 2H), 7.62 (d, J = 3.6 Hz, 1H), 7.51 (d, J = 8.4 Hz, 2H), 7.38 (d, J = 3.6 Hz, 1H), 7.04 (d, J = 8.4 Hz, 4H), 6.92 (d, J = 8.4 Hz, 4H), 6.78 (d, J = 8.4 Hz, 2H), 3.94 (t, J = 6.6 Hz, 4H), 1.70 (m, 4H), 1.41 (m, 4H), 1.31 (m, 8H), 0.88 (t, J = 6.0 Hz, 6H). 13C NMR (150 MHz, DMSO-d6) δ: 163.57, 155.40, 152.16, 148.16, 146.45, 143.98, 141.32, 139.76, 139.45, 134.69, 134.48, 130.86, 126.83, 126.77, 126.10, 125.79, 125.56, 125.04, 124.83, 123.31, 119.00, 116.45, 115.41, 98.29, 67.58, 30.98, 28.69, 25.18, 22.04, 13.86. HR-MS (MALDI) m/z calcd for (C48H48N2O4S2): 780.30555. Found: 780.30974. Anal. Calcd for C48H48N2O4S2: C, 71.96; H, 7.60; N, 3.11. Found: C, 71.87; H, 7.68; N, 3.06. 2-Cyano-3-{7-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}-4,9-dihydro-4,4,9,9-tetrahexyl-s-indaceno[1,2-b:5,62983

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immersing it into a 150 μM dye solution (chloroform/ethanol, v/v, 1/9) for 10 h, was used as the negative electrode. The recipe of a Co-Phen electrolyte: 0.25 M tris(1,10phenanthroline)cobalt(II) di[bis(trifluoromethanesulfonyl)imide], 0.05 M tris(1,10-phenanthroline)cobalt(III) tris[bis(trifluoromethanesulfonyl)imide], 0.5 M TBP, and 0.1 M LiTFSI in acetonitrile. Details on cell fabrication and measurements of IPCE, j−V, transient photovoltage decay, and charge extraction can be found in our previous publications.31,33

b′]-dithiophene-2-yl}acrylic Acid (C255). The same procedure as above was applied to give a purple-black powder. Yield: 84%. 1 H NMR (600 MHz, THF-d8) δ: 8.55 (s, 1H), 7.94 (s, 1H), 7.84 (s, 1H), 7.65 (d, J = 8.4 Hz, 2H), 7.61 (s, 1H), 7.44 (s, 1H), 7.20 (d, J = 8.4 Hz, 4H), 7.07 (d, J = 8.4 Hz, 2H), 7.02 (d, J = 8.4 Hz, 4H), 4.12 (t, J = 6.3 Hz, 4H), 2.25 (m, 8H), 1.93 (m, 4H), 1.67 (m, 4H), 1.54 (m, 8H), 1.30 (m, 24H), 1.09 (m, 14H), 0.94 (m, 12H). 13C NMR (150 MHz, THF-d8) δ: 165.04, 159.17, 157.39, 156.62, 156.48, 154.92, 150.03, 149.70, 148.06, 141.88, 140.72, 140.37, 139.31, 135.24, 133.40, 128.59, 127.91, 127.27, 121.84, 117.70, 116.51, 116.50, 114.19, 97.38, 69.26, 55.49, 55.45, 40.43, 40.32, 33.07, 33.03, 32.98, 31.12, 30.80, 27.23, 23.93, 23.90, 14.83. HR-MS (MALDI) m/z calcd for (C74H96N2O4S2): 1140.68115. Found: 1140.68166. Anal. Calcd for C74H96N2O4S2: C, 77.85; H, 8.48; N, 2.45. Found: C, 77.78; H, 8.53; N, 2.40. 4.3. UV−vis, ATR-FTIR, Voltammetric, and XRR Measurements. Static electronic absorption and vibration spectra were measured with an Agilent G1103A spectrometer and a BRUKER Vertex 70 FTIR spectrometer, respectively. Steady-state PL spectra were recorded by use of an Andor ICCD camera with laser excitation at 532 nm. Cyclic voltammograms of dye-grafted titania films were recorded on a CHI660C electrochemical workstation, and the potential was reported against the Fc/Fc+ reference. XRR measurements were performed on a Bruker D8 discover high-resolution diffractometer by using Cu Kα X-ray radiation (λ = 1.542 Å), and the experimental details have been described in our previous paper.22 4.4. fs Fluorescence Upconversion and TA Measurements. Both experiments used the same laser source. A modelocked Ti:sapphire laser (Tsunami, Spectra Physics) pumped by a Nd:YVO4 laser (Millennia Pro s-Series, Spectra Physics) was used as a source of a regenerative amplifier (RGA, Spitfire, Spectra Physics) to generate 3.7 mJ, 130 fs pulses at 800 nm, which were split into two parts at a ratio of 9/1 with a beam splitter. The large proportion was delivered to an optical parametric amplifier (TOPAS-C, Light Conversion) to produce pump pulses at 530 nm. For the fs fluorescence upconversion measurements, the pump light was focused on a rotating sample to bring forth the emitted photons at 720 nm. The minor part of the output of RGA employed as the optical gate was concentrated on a 0.3 mm thick BBO crystal with the 720 nm photons to get a sum frequency light, which was detected by a FOG 100-DA spectrometer (CDP Corp.). The instrument response function (fwhm) was about 180 fs. For the fs-TA experiments, the pump light went through a phase-locked chopper (1 kHz) and was focused on a rotating sample. A white light continuum generated by focusing the minor portion of the output of RGA on a sapphire was split into two equal beams as the probe and reference lights. The probe and reference lights were focused on two optical fibers connected with monochromators and detected by two multichannel optical sensors (1024 elements, MS 2022i, CDP Corp.). The polarization between pump and probe beams was set at the magic angle. The processed signal was displayed with the ExciPRO software (CDP Corp.). The raw data were analyzed by using the Glotaran software.49 The nanosecond transient absorption kinetic measurements have been described in our previous paper.33 4.5. Cell Fabrication and Characterization. A 4.2 + 5.0 μm thick, bilayer titania film screen-printed on fluorine-doped tin oxide (FTO) conducting glass, which was dyed by

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ASSOCIATED CONTENT

S Supporting Information *

Optimized geometries, synthetic route, and additional spectroscopic, voltammetric, and XRR data. 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]. Tel.: 0086-431-85262952. Present Address

∥ Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida Ushinomiya-cho, Sakyo-ku, Kyoto 6068501, Japan.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Acknowledgements are made to the National 973 Program (2011CBA00702) and the National Science Foundation of China (No. 51103146 and No. 91233206) for financial support.

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REFERENCES

(1) Graetzel, M.; Janssen, R. A. J.; Mitzi, D. B.; Sargent, E. H. Materials Interface Engineering for Solution-Processed Photovoltaics. Nature 2012, 488, 304−312. (2) Yu, Q.; Wang, Y.; Yi, Z.; Zu, N.; Zhang, J.; Zhang, M.; Wang, P. High-Efficiency Dye-Sensitized Solar Cells: The Influence of Lithium Ions on Exciton Dissociation, Charge Recombination, and Surface States. ACS Nano 2010, 4, 6032−6038. (3) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Grätzel, M. Porphyrin-Sensitized Solar Cells with Cobalt(II/III)-Based Redox Electrolyte Exceed 12% Efficiency. Science 2011, 334, 629−634. (4) Zhang, S.; Islam, A.; Yang, X.; Qin, C.; Zhang, K.; Numata, Y.; Chen, H.; Han, L. Improvement of Spectral Response by Cosensitizers for High Efficiency Dye-Sensitized Solar Cells. J. Mater. Chem. A 2013, 1, 4812−4819. (5) Ozawa, H.; Okuyama, Y.; Arakawa, H. Effects of Cation Composition in the Electrolyte on the Efficiency Improvement of Black Dye-Based Dye-Sensitized Solar Cells. RSC Adv. 2013, 3, 9175− 9177. (6) Mishra, A.; Fischer, M. K. R.; Bäuerle, P. Metal-Free Organic Dyes for Dye-Sensitized Solar Cells: From Structure: Property Relationships to Design Rules. Angew. Chem., Int. Ed. 2009, 48, 2474−2499. (7) Imahori, H.; Umeyama, T.; Ito, S. Large π-Aromatic Molecules as Potential Sensitizers for Highly Efficient Dye-Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1809−1818. (8) Li, C.; Wonneberger, H. Perylene Imides for Organic Photovoltaics: Yesterday, Today, and Tomorrow. Adv. Mater. 2012, 24, 613−636. (9) Yen, Y.-S.; Chou, H.-H.; Chen, Y.-C.; Hsu, C.-Y.; Lin, J. T. Recent Developments in Molecule-Based Organic Materials for DyeSensitized Solar Cells. J. Mater. Chem. 2012, 22, 8734−8747.

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