Dye-Sensitized Solar Cells Based on Quinoxaline Dyes - American

Article pubs.acs.org/JPCC

Dye-Sensitized Solar Cells Based on Quinoxaline Dyes: Effect of π‑Linker on Absorption, Energy Levels, and Photovoltaic Performances Kai Pei,† Yongzhen Wu,† Ashraful Islam,‡ Shiqin Zhu,† Liyuan Han,‡ Zhiyuan Geng,§ and Weihong Zhu*,† †

Shanghai Key Laboratory of Functional Materials Chemistry, Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, P. R. China ‡ Photovoltaic Materials Unit, National Institute for Materials Science, Sengen 1-2-1, Tsukuba, Ibaraki, Japan § Gansu Key Laboratory of Polymer Materials, College of Chemistry and Chemical Engineering, Key Laboratory of Eco-environment-related Polymer Materials, Ministry of Education, Northwest Normal University, Lanzhou, 730070 Gansu, P. R. China S Supporting Information *

ABSTRACT: Quinoxaline derivatives show great potential in recent organic photovoltaics, not only as polymer acceptors for bulk heterojuction (BHJ) solar cells but also as molecular sensitizers for dye-sensitized solar cells (DSSCs). This work focuses on the effect of π-linkers on photovoltatic performances of D−A−π−A quinoxaline-based sensitizers used for DSSCs. The extension of πlinkers is one of the viable tactics to improve the molar absorption coefficient and red-shift the absorption peak, which is beneficial to light harvesting. With respect to IQ4, a series of quinoxaline sensitizers IQ6, IQ7, and IQ8 were synthesized on the basis of a promising building block of 2,3-diphenylquinoxaline with π-linker modification. Dye IQ8, with an additional thienyl unit near the anchor group, shows little change in absorption spectra and energy levels, while in IQ6 and IQ7, the additional thienyl group close to the donor group obviously red-shifts the absorption band and positively shifts the HOMO levels. In the series of sensitizers, their adsorption amounts on the TiO2 surface are slightly decreased by introduction of a thienyl unit near the donor part and/or the introduction of alkyl chains. Their photovoltaic performances are well evaluated by the electron collection length values (Lcol), first-principles calculations, the conduction band edge (ECB), and the fluctuation of electron density or charge recombination rate in DSSCs. Instead of the electron injection efficiency (Φinj), the low charge collection efficiency (Φcol) of IQ6, IQ7, and IQ8 results in their unsatisfactory incident photonto-current conversion efficiency (IPCE) plateaus. Also the difference of Voc among these dyes mainly arises from the fluctuation of TiO2 electron density, which is closely related to the recombination resistance. Upon increasing the thiophene number, the electron collection lengths of IQ6, IQ7, and IQ8 based DSSCs become shorter, which dramatically decreases their photocurrent with an unbeneficial preferable photovoltaic performance. As demonstrated, it is essential to have a judicious design on π-linker modification for high-performance D−A−π−A quinoxaline-based sensitizers.

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INTRODUCTION Owing to the high power conversion efficiency and low cost, dye-sensitized solar cells (DSSCs) have been regarded as one of the most prospective and potential photovoltaic devices.1 In a typical DSSC, the dye sensitizer plays a key role in absorbing the sunlight and separating charges.2−4 As such an important component, the molecular structure of sensitizers should be elaborately tailored with the aim of broad absorption, suitable energy levels, controllable adsorption pattern, and compact layer morphology as well as excellent stability. After decades of effort, a benchmark efficiency of 12.3% has been achieved from porphyrin/organic dye cosensitization.5 In recent years, enormous efforts have been devoted to the development of metal-free sensitizers, which show greater superiority in cost© XXXX American Chemical Society

effectivity and environmental compatibility. Encouragingly, the performance of pure organic dye-based DSSCs also arrives to the range of 8−11%.6−12 Recently, to broaden spectral response and optimize energy levels, novel D−A−π−A-featured sensitizers have been focused,13−19 in which an additional electron-withdrawing unit is specifically incorporated into the π bridge as the additional electron-deficient acceptor. Special Issue: Michael Grätzel Festschrift Received: December 15, 2013 Revised: January 24, 2014

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dx.doi.org/10.1021/jp412259t | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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the LHE of organic sensitizers could be enhanced by extending their π-spacer. In this regard, a great amount of organic dyes involving more thienyl units and its derivatives have been developed, and the corresponding influences have been deeply researched.28−31 However, the π-spacer extension is still little studied in quinoxaline-based sensitizers. Bearing all this in mind, we introduce the thiophene/n-hexylthiophene units to the IQ4 skeleton into a different manner, obtaining dyes IQ6, IQ7, and IQ8 (Figure 1) to prolong the conjugated linker and further get insight into the effect of π-extension on photovoltaic properties. Utilizing IQ4 as the control sensitizer, we can make a clear motivation to investigate the influence of the π-spacer in quinoxaline-based organic dyes upon the photovoltaic behaviors via a joint photophysical and electrochemical characterization.

The quinoxaline unit has attracted increasing attention in recent years due to its impressive performance in both quinoxaline-based polymers for bulk heterojuction (BHJ) solar cells 20−22 and quinoxaline-based sensitizers for DSSCs.23−25 Recently, Chou’s group reported a new alternating copolymer containing a fluorinated quinoxaline unit as the acceptor which leads to superior power conversion efficiencies (PCEs) up to 8%.20 Wang and Li’s group incorporated the quinoxaline unit into the conjugated bridge as an auxiliary acceptor to build pure organic sensitizers, obtaining high PCE in both the liquid electrolyte and the quasi-solid-state electrolyte.15,16 In our previous work, we also constructed highly efficient and stable D−A−π−A-featured sensitizers based on quinoxaline.26,27 The quinoxaline-based sensitizers have shown promising potential for achieving high performance in optoelectronic DSSCs. For instance, IQ4 (Figure 1) with 2,3-

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RESULTS AND DISCUSSION Design and Synthesis. In our previous work, a promising dye, IQ4, based on the quinoxaline unit was well studied. With the IPCE onset around 800 nm, the JSC value (17.55 mA cm−2) of IQ4 almost stretched to its theoretical limits.32 Given that the photoresponse range of sensitizers is the prerequisite and foundation of JSC values, the molecular engineering strategy has been focused on extending the absorption range based on the skeleton of IQ4 in this context. The modification in this work was conducted by appending thiophene/n-hexylthiophene units. As shown in Figure 1, a thienyl group was incorporated close to the indoline segment in the backbone of IQ4, and the flexible hexyl chains were tethered on the two thienyl groups to obtain IQ6, ensuring the solubility. On the other hand, another thienyl group was inserted into the π-spacer next to the anchor group based on IQ4 and IQ6, obtaining IQ8 and IQ7, respectively. We also planned to synthesize IQ10 (Figure S1 in the Supporting Information, SI), making a persuasive comparison of the structure−activity relationship, but the poor solubility of its intermediate dissuaded us from further synthesis. Nevertheless, in view of the fact that the alkyl chain has an ignorable influence on photochemical character which is well proved by our previous research,27,33 we are convinced that

Figure 1. Chemical structures of IQ4, IQ6, IQ7, and IQ8.

diphenylquinoxaline as the auxiliary acceptor in the π bridge showed a fairly high efficiency (9.2%).27 One of the crucial restrictions for IQ4 to acquire higher efficiency seems to be its relatively low photocurrent density, which stems from the relatively narrow IPCE (incident photon-to-current conversion efficiency, onset 99% of LHE at those wavelengths (Figure S3 in SI). Thus, we expect that the low Φcol of IQ6, IQ7, and IQ8 might result in their unsatisfactory IPCE plateaus. Charge collection is represented by the process that the injected electrons migrate to the fluorine-doped tin oxide (FTO) through the network of interconnected TiO2 nanoparticles, while holes on oxidized dyes transfer to a redox couple in the electrolyte and are further transported to the counter electrode.36 Once the injected electrons in TiO2 meet holes on dye cations or oxidized components in a redox couple, separated electron−hole recombination will occur and charge collection efficiency decline concomitantly. Generally, the ratio between electron diffusion length in TiO2 (LD) and thickness (L) of TiO2 films, LD/L, is used to evaluate Φcol. LD is normally 3−5 times larger than L in high performance DSSCs.38,39 O’Regan’s group has certified that the Φcol could also be evaluated by electron collection length values Lcol, which can be obtained by measuring the wavelength-dependent ratio (r(CE/PE)) of photocurrents derived from illuminating DSSCs from the counter-electrode side (CE) versus photoelectrode side (PE).40−42 If Lcol is less than the thickness of the photoelectrode TiO2 film, most electrons far from the electrode will be lost because of recombination during the process through the porous TiO2, while the majority of electrons adjacent to the electrode will be collected and contribute to the photocurrent. In other words, the ratio r(CE/PE) cannot reach unity. However, if Lcol exceeds the thickness of the film, almost all the electrons will be captured whether the illumination from back or front, making r(CE/PE) reach the maximum. The effective Lcol involves a critical photoelectrode thickness and exceeds it, and the r(CE/PE) drops rapidly if we measure the r(CE/PE) values of IQ4, IQ6, IQ7, and IQ8.43 IPCE plots of devices based on the four dyes with various thicknesses (2, 4, 6, and 9 μm) were recorded on condition of either PE-side or CE-side illumination, and the plots of r(CE/PE) as a function of thickness are depicted in Figure 5A. Considering the fixed light losses

Figure 5. (A) Ratios of photocurrent density generated by PE-side and CE-side illumination as a function of TiO2 film thicknesses in photoanodes coated with IQ4, IQ6, IQ7, and IQ8. (B) Light intensity dependence of photocurrent density for IQ4, IQ6, IQ7, and IQ8 based devices under short-circuit conditions.

assignable to less than complete transparency for the counterelectrode, the values of r(CE/PE) cannot attach the unity. The r(CE/PE) values of IQ4 are basically constant and over 0.87, indicating that Lcol of IQ4-based cells is no less than 9 μm. As for IQ8, r(CE/PE) is large enough for electrodes with a thickness of 2, 4, or 6 μm but decreases substantially as the film becomes thicker. Notably, in the cases of IQ6 and IQ7, the measured r(CE/PE) is around 0.8 only with the 2 μm electrodes, decreasing drastically along with the increase of the film thickness. Lcol values of the reported dyes decrease in the sequence of IQ4 (≥9 μm) > IQ8 (∼6 μm) > IQ6 ≈ IQ7 (∼4 μm). These results indicate that the amounts of electrons injected by IQ6, IQ7, and IQ8 cannot be efficiently collected while using the TiO2 photoelectrode with a thickness of 17 μm. Reasonably, the low Φcol of IQ6, IQ7, and IQ8 based devices should be responsible for their lower IPCE. However, the reason for low Φcol caused by additional thienyl units needs further investigation. As is well-known, charge collection efficiency is the result of competition between charge recombination and collection.44 Here we focuse on the charge recombination occurring on these IQ series sensitizers. It has been reported that expansion of the π-conjugated system often facilities the approach of I3− ions to the TiO 2 surface, resulting in faster charge recombination.45,46 Given that the order of the π-conjugated E



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length for these four dyes is IQ4 < IQ6 ≈ IQ8 < IQ7, we conjecture that the longest π-conjugated system of IQ7 might be apt to suffer the fastest charge recombination, thus leading to the lowest charge collection efficiency. To verify this suggestion, light intensity dependence of the photocurrent density of IQ4, IQ6, IQ7, and IQ8 based devices was measured under short-circuit conditions (Figure 5B).47 Within the light intensity range of 0.5−75 W m−2, the short-circuit photocurrent of IQ4, IQ6, and IQ8 based cells scales linearly with light intensity, whereas the JSC of IQ7 rises nonlinearly with the increasing light intensity. The curved slope of IQ7 decreased gradually even under such a weak light intensity range. It is conceivable that the JSC loss under stronger light intensity (such as AM 1.5, 1000 W/m2) will become much more serious. This phenomenon arises from the serious Columbic attraction between the cationic dyes and I3−.48 The density functional theory calculation method was used to investigate the different π-conjugated length effects on the Columbic attraction of the cells.49 We choose the typical longest and shortest π-conjugated length dyes (IQ4 and IQ7) to investigate the dye···I2 complex interaction by modeling the binding of I2 to the −CN groups in the dye molecules. The optimized molecular structures of dyes IQ4 and IQ7 with I2 were listed in Figure 6 and Table S1 in the

determined by the interfacial recombination rate of titania electrons with electron-accepting species in electrolytes and/or dye cations. To obtain a complete analysis of the injection and the difference of VOC among these four dyes, EIS measurements were performed on the DSSCs at different bias. Figure 7A shows the relationship between VOC and capacitance. Cμ, as the capacitive response of the cell, depends on the density of electronic states in TiO2 and gives specific information about the position of the conduction band (CB). Cμ can be represented by eq 1.52,53 Cμ = L(1 − p)qg (E Fn)

(1)

where L is the thickness of TiO2 film; p is the porosity; and g(EFn) is expressed by eq 2. g (E Fn) =

αqNL exp[(E Fn − ECB)/kBT ] kBT

(2)

where α is the parameter that describes the exponential trap distribution of electrons below the conduction band; q represents the elementary charge (1.6 × 10−19 C); NL represents the total density of bandgap states; and kB is the Boltzmann constant (1.38 × 10−23 J K−1). Since the capacitance stands for the density of states in the bandgap of TiO2, the displacement of capacitance plots exhibits the displacement of the TiO2 conduction band in DSSCs (Figure 7A).52 Because the film thickness and porosity are identical for all the devices, the plots suggest that the conduction bands in IQ4 and IQ8 based devices are prompted with perceivable descent about 30 mV with respect to its position in IQ6 and IQ7. Considering that the alkyl chains show little influence on the position of the conduction band,19 we can conclude that introducing the thienyl group adjacent to the donor part might lift upward the conduction band edge of TiO2, while the additional thienyl unit next to the anchor induces negligible variation of the conduction band edge. As known, the downward shift of ECB decreases the energy difference between ECB and the redox potential of the electrolyte, leading to a lower theoretical photovoltage. However, it is worth mentioning that with a lower conduction band the VOC value of IQ8 is similar to IQ6 and IQ7. VOC of IQ4 is even 30 mV higher than that of IQ6 and IQ7 (Figure 4A, Table 2). Accordingly, we speculate that the difference in VOC among these dyes should be dominantly attributed to the fluctuation of TiO2 electron density, which is closely related to the recombination resistance. Figure 7C shows the recombination resistance as a function of corrected potential at open circuit. It is visually observed that the Rrec values of IQ8 are lower than that of the other three dyes, indicating that the charge recombination is more significant for the former than the latter. Meanwhile, IQ4, IQ6, and IQ7 show the similar recombination resistance without any exception. Additionally, to analyze the recombination resistance, Rrec, on the basis of a similar situation (i.e., the same distance between the electron Fermi level, EFn, and conduction band of TiO2, ECB), the shift of the CB has been corrected in the potentials of Figure 7B, where the voltage is at the same electron concentration in the semiconductor.54 As illustrated, chemical capacitance is directly related to the difference ECB − EFn, thus the criterion for the modified scale is that the chemical capacitances of the analyzed samples overlap (Figure 7C). Taking IQ7 as a reference, the relative shift of the ECB in the TiO2 could be easily estimated by displacing the

Figure 6. Geometries of the dye···I2 complexes. Left: IQ4. Right: IQ7.

SI. On the basis of our calculations, the distance of the CN···I2 in dye IQ4 (2.713 Å) is larger than IQ7 (2.703 Å), and the bond length of I···I in dye IQ4 is also larger than IQ7. This result suggests that the Columbic attraction of the IQ7···I2 complex, which possesses the longest π-conjugated length, is more stable. Thus the electron loss for IQ7 becomes more serious, causing fast electron recombination in the electrolyte to occur. Combined with the measurement of Lcol, undoubtedly, the increasing numbers of thienyl units do not benefit for the IPCE and JSC. Incorporating the thienyl group to the donor part even seriously shortens the collection length which is disadvantageous to the photoelectric conversion efficiency of DSSCs. Influence on Open-Circuit Photovoltage (VOC). With the same redox electrolyte for these four dye-based DSSCs, the alternation of VOC originates from an upward or downward displacement of electron quasi-Fermi-level (Efn) in titania, which intrinsically stems from a charge of titania conduction band edge (ECB) and/or a fluctuation of electron density (charge recombination rate in DSSCs).6,40,51 Generally, at a given photocarrier generation flux, the electron density is F



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Figure 7. Results from electronic impedance data for IQ4, IQ6, IQ7, and IQ8 in the dark at the different applied potentials.50 (A) Evolution of capacitance in TiO2. (C) Recombination resistance. In (B) and (D) the capacitance and recombination resistance are displaced, to make all capacitances overlap and ensure the same electron concentration in TiO2.

Inviting IQ4 as the reference sensitizer, we demonstrate a clear motivation to investigate the general influence of different πspacers in these D−A−π−A organic sensitizers on their absorption, energy levels, and photovoltaic performances. It is found that, on one hand, the additional thienyl unit near the anchor group has no distinct obvious effect on the absorption spectra and energy levels. On the other hand, the additional thienyl group close to the donor group obviously red-shifts the absorption band and positively shifts the LUMO levels. Moreover, the photovoltaic performances of these three novel dyes are discussed systematically, evaluated by the electron collection length values (Lcol), first-principles calculations, the conduction band edge (ECB), and the fluctuation of electron density or charge recombination rate in DSSCs. In distinct contrast with IQ4, the low charge collection efficiency (Φcol) of IQ6, IQ7, and IQ8 might result in their unsatisfactory IPCE plateaus. Moreover, the difference of VOC among these dyes should be dominantly attributed to the fluctuation of TiO2 electron density, which is closely related to the recombination resistance. Although IQ6 and IQ7 possess a higher conduction band (ECB) than IQ4, the relatively faster charge recombination rate limits its VOC performance. After extending the π-linker on the basis of IQ4, the diffusion lengths of IQ6−IQ8 become shorter without exception, which dramatically decreases their photocurrent and impairs their photovoltaic performances. The large difference between IQ4 and the other three dyes indicates

chemical capacitances until they all match, as shown in Figure 7B. Following this way, ECB − EFn is found to be 32 mV for IQ4, 30 mV for IQ8, and 2 mV for IQ6. The same shift applied to the Cμ has also been applied to the Rrec (Figure 7D). In the case of IQ6, IQ7, and IQ8, when the data are corrected for the observed band shift of the conduction band, the match in Rrec is perfect, and the same recombination rate at the same electron concentration in the semiconductor verifies their same VOC values. By fitting the EIS curves, electron lifetime (τ), which is closely related to recombination resistance, could be represented by the equation τ = CμRCT.54 To correctly evaluate the electron lifetime of different devices at the same equivalent value of the position of the conduction band (Vecb), we drew the electron lifetime as a function of Vecb for the DSSCs based on IQ4, IQ6, IQ7, and IQ8, respectively (Figure S4 in SI).54 At the same Vecb, the electron lifetime of the IQ4 is much longer than those of other dyes, while IQ6, IQ7, and IQ8 show the similar electron lifetime. As a result, although IQ6 and IQ7 possess the higher ECB than IQ4, the relatively faster charge recombination rate and shorter electron lifetime limit their VOC performance.

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CONCLUSIONS A series of sensitizers based on the auxiliary acceptor unit of quinoxaline, containing indoline as electron donor and cyanoacetic acid as acceptor/anchor, have been specifically developed as IQ6, IQ7, and IQ8 for high efficiency DSSCs. G



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that the modification of the π-segment should be screened with caution.

thienyl−H), 7.69 (s, 1H, thienyl−H), 7.49−7.66 (m, 5H, Ph− H), 7.36−7.48 (m, 4H, Ph−H), 7.25−7.36 (m, 2H, Ph−H), 6.99 (d, J = 8.0 Hz, 1H, Ph−H), 6.83 (d, J = 8.4 Hz, 1H), 4.75−4.88 (m, 1H, NCHCH−), 3.71−3.84 (m, 1H, NCHCH−), 2.59−2.73 (m, 2H, hexyl−CH2C5H11), 2.50− 2.59 (m, 2H, hexyl−CH2C5H11), 2.29 (s, 3H, Ph−CH3), 1.94− 2.11 (m, 1H, indoline−H), 1.68−1.88 (m, 3H, indoline−H), 1.47−1.68 (m, 5H, indoline−H and hexyl− CH2CH2C3H6CH3), 1.13−1.46 (m, 13H, indoline−H and hexyl−C2H4C3H6CH3), 0.75−0.94 (m, 6H, hexyl−CH3), 0.85−0.86 (m, 6H). 13C NMR (100 MHz, THF-d8, ppm): δ 151.58, 145.44, 143.21, 141.36, 140.43, 138.87, 138.74, 138.60, 137.07, 136.82, 135.16, 134.81, 133.07, 130.98, 130.49, 129.84, 129.49, 129.20, 129.01, 128.79, 128.18, 127.90, 126.39, 125.72, 125.23, 125.16, 124.93, 119.83, 107.02, 68.98, 45.41, 35.07, 33.58, 31.75, 31.01, 30.36, 29.87, 29.34, 28.82, 22.60, 22.57, 19.87, 13.50. HRMS (ESI, m/z): [M + H]+ calcd for C66H63N4O2S3, 1039.4113; found, 1039.4117. Synthesis of IQ8. A mixture of aldehyde 2c (300 mg, 0.42 mmol) and cyanoacetic acid (170 mg, 2.0 mmol) in acetonitrile (20 mL) was refluxed in the presence of piperidine (2 mL) for 8 h under argon. After cooling, the mixture was evaporated. Then, the solid was dissolved with CH2Cl2 (40 mL), washed with water (30 mL × 3) and brine, dried over Na2SO4, and evaporated under reduced pressure. The crude product was purified by column chromatography with 1% AcOH in CH2Cl2 on silica gel to yield the product as a purple powder (200 mg, 0.25 mmol, yield 61%). 1H NMR (400 MHz, CDCl3, ppm): δ 8.46 (s, 1H, alkene−H), 8.41 (d, J = 4.0 Hz, 1H, thienyl−H), 8.00 (d, J = 4.0 Hz, 1H, thienyl−H), 7.94 (d, J = 8.0 Hz, 1H, Ph−H), 7.82 (d, J = 8.0 Hz, 1H, Ph−H), 7.69−7.79 (m, 3H, Ph−H), 7.64 (d, J = 4.0 Hz, 1H, thienyl−H), 7.54−7.63 (m, 3H, Ph−H), 7.42−7.54 (m, 4H, Ph−H), 7.30−7.42 (m, 3H, Ph−H), 7.23 (d, J = 8.4 Hz, 2H, Ph−H), 7.19 (d, J = 8.4 Hz, 2H, Ph−H), 6.85 (d, J = 8.4 Hz, 1H), 4.83−4.93 (m, 1H, NCHCH−), 3.81−3.93 (m, 1H, NCHCH−), 2.29 (s, 3H, Ph− CH3), 2.04−2.19 (m, 1H, indoline−H), 1.88−1.96 (m, 1H, indoline−H), 1.72−1.88 (m, 2H, indoline−H), 1.59−1.71 (m, 1H, indoline−H), 1.38−1.52 (m, 1H, indoline−H). 13C NMR (100 MHz, THF-d8, ppm): δ 162.45, 161.77, 158.99, 158.65, 149.04, 148.84, 146.12, 145.74, 144.40, 143.72, 138.71, 137.76, 136.49, 135.89, 134.95, 134.87, 132.38, 129.81, 129.48, 129.42, 128.97, 128.75, 128.27, 128.01, 127.62, 126.53, 126.13, 125.69, 124.63, 117.91, 117.59, 114.59, 111.71, 111.40, 104.87, 92.85, 67.17, 52.69, 52.60, 43.67, 43.11, 33.20, 31.79, 27.80, 18.02. HRMS (ESI, m/z): [M + H]+ calcd for C50H37N4O2S2, 789.2358; found, 789.2357. DSSC Fabrication and Photovoltaic Performance Measurements. A double-layer TiO2 photoelectrode 17 μm in thickness, composed of a 12 μm thick nanoporous layer and a 5 mm thick scattering layer (area: 0.25 cm2), was prepared by screen printing on a conducting glass substrate. A dye solution of IQ4 and IQ5 with 3 × 10−4 M concentration in CHCl3/ EtOH (v/v, 3/7) was used to uptake the dye onto the TiO2 film. The TiO2 films were immersed in the dye solution and then kept at 25 °C for 40 h. Photovoltaic measurements were performed in a sandwich-type solar cell in conjunction with an electrolyte consisting of a solution of 0.6 M dimethylpropylimidazolium iodide (DMPII), 0.05 M I2, 0.1 M LiI, and 0.5−1.0 M tert-butylpyridine (TBP) in acetonitrile (AN). The dyedeposited TiO2 film and a platinum-coated conducting glass were separated by a Surlyn spacer (40 μm thick) and sealed by heating the polymer frame. Photocurrent density−voltage (I−

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EXPERIMENTAL SECTION Characterization. 1H and 13C NMR spectra were recorded on Bruker AVANCE III-400 MHz (100 MHz for 13C NMR) instruments with tetramethylsilane (TMS) as internal standard. HRMS were performed using a Waters LCT Premier XE spectrometer. The absorption spectra of dyes in solution and adsorbed on TiO2 films were measured with a Varian Cary 500 spectrophotometer. The cyclic voltammograms (CVs) were determined with a Versastat II electrochemical workstation (Princeton Applied Research). Dye-adsorbed TiO2 on conductive glass was used as the working electrode, a Pt wire as the counter electrode, and a saturated calomel (SCE) reference electrode in saturated KCl solution as the reference electrode. 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) was used as the supporting electrolyte in CH2Cl2. The ferrocenium/ferrocene (Fc/Fc+) redox couple was used as an external potential reference. Synthesis of IQ6. A mixture of aldehyde 3a (240 mg, 0.27 mmol) and cyanoacetic acid (85 mg, 1.0 mmol) in acetonitrile (50 mL) was refluxed in the presence of piperidine (1 mL) for 8 h under argon. After cooling, the mixture was evaporated. Then, the solid was dissolved with CH2Cl2 (40 mL), washed with water (30 mL × 3) and brine, dried over Na2SO4, and evaporated under reduced pressure. The crude product was purified by column chromatography with 1% AcOH in CH2Cl2 on silica gel to yield the product as a deep purple powder (150 mg, 0.16 mmol, yield 58%). 1H NMR (400 MHz, DMSO, ppm): δ 8.42 (s, 1H, alkene−H), 7.87 (d, J = 7.6 Hz, 2H, Ph− H), 7.71−7.80 (m, 3H, Ph−H, thienyl−H), 7.67 (s, 1H, thienyl−H), 7.60 (s, 1H, Ph−H), 7.48−7.55 (m, 2H, Ph−H), 7.32−7.48 (m, 5H, Ph−H), 7.13−7.25 (m, 6H, Ph−H), 7.10 (d, J = 8.0 Hz, 1H, Ph−H), 6.91 (d, J = 6.8 Hz, 1H, Ph−H), 4.72−4.89 (m, 1H, NCHCH−), 3.71−3.91 (m, 1H, NCHCH−), 2.70−2.88 (m, 2H, hexyl−CH2C5H11), 2.50− 2.69 (m, 2H, hexyl−CH2C5H11), 2.34 (s, 3H, Ph−CH3), 1.99− 2.13 (m, 1H, indoline−H), 1.86−1.99 (m, 2H, indoline−H), 1.73−1.86 (m, 1H, indoline−H), 1.49−1.73 (m, 6H, indoline− H and hexyl−CH2CH2C3H6CH3), 1.15−1.47 (m, 12H, hexyl− C2H4C3H6CH3), 0.78−0.94 (m, 6H, hexyl−CH3). 13C NMR (100 MHz, THF-d8, ppm): δ 161.87, 151.26, 149.94, 145.66, 144.78, 142.08, 141.51, 138.53, 137.10, 136.40, 135.07, 134.98, 133.35, 132.72, 131.69, 131.52, 129.19, 128.87, 128.65, 128.51, 127.65, 126.96, 126.82, 126.62, 126.56, 126.45, 126.19, 126.10, 126.06, 123.50, 123.30, 122.97, 118.03, 114.25, 105.15, 94.80, 67.14, 43.54, 33.22, 31.71, 29.89, 29.76, 29.52, 29.15, 27.46, 27.14, 26.95, 26.74, 20.74, 20.64, 18.02, 11.64, 11.58. HRMS (ESI, m/z): [M + H]+ calcd for C62H61N4O2S2, 957.4236; found, 957.4236. Synthesis of IQ7. A mixture of aldehyde 3b (110 mg, 0.11 mmol) and cyanoacetic acid (45 mg, 0.53 mmol) in acetonitrile (40 mL) was refluxed in the presence of piperidine (1 mL) for 10 h under argon. After cooling, the mixture was evaporated. Then, the solid was dissolved with CH2Cl2 (50 mL), washed with water (40 mL × 3) and brine, dried over Na2SO4, and evaporated under reduced pressure. The crude product was purified by column chromatography with 1% AcOH in CH2Cl2 on silica gel to yield the product as a deep purple powder (64 mg, 0.06 mmol, yield 58%). 1H NMR (400 MHz, CDCl3, ppm): δ 8.35 (s, 1H, alkene−H), 7.99 (d, J = 8.0 Hz, 1H, Ph− H), 7.92 (d, J = 8.0 Hz, 1H, Ph−H), 7.83 (d, J = 3.2 Hz, 1H, H



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(6) Cai, N.; Zhang, J.; Xu, M. F.; Zhang, M.; Wang, P. Improving the Photovoltage of Dithienopyrrole Dye-Sensitized Solar Cells via Attaching the Bulky Bis(octyloxy)biphenyl Moiety to the Conjugated π-Linker. Adv. Funct. Mater. 2013, 23, 3539−3547. (7) Lin, R. Y.; Lin, H. W.; Yen, Y. S.; Chang, C. H.; Chou, H. H.; Chen, P. W.; Hsu, C. Y.; Chen, Y. C.; Lin, J. T.; Ho, K. C. 2,6Conjugated Anthracene Sensitizers for High-Performance DyeSensitized Solar Cells. Energy Environ. Sci. 2013, 6, 2477−2486. (8) Cai, N.; Li, R. Z.; Wang, Y. L; Zhang, M.; Wang, P. Organic DyeSensitized Solar Cells with a Cobalt Redox Couple: Influences of πLinker Rigidification and Dye−Bath Solvent Selection. Energy Environ. Sci. 2013, 6, 139−147. (9) Zhang, J.; Yao, Z. Y.; Cai, Y. C.; Yang, L.; Xu, M. F.; Li, R. Z.; Zhang, M.; Dong, X. D.; Wang, P. Conjugated Linker Correlated Energetics and Kinetics in Dithienopyrrole Dye-Sensitized Solar Cells. Energy Environ. Sci. 2013, 6, 1604−1614. (10) Zhang, M.; Wang, Y. L.; Xu, M. F.; Ma, W. T.; Li, R. Z.; Wang, P. Design of High-Efficiency Organic Dyes for Titania Solar Cells Based on the Chromophoric Core of CyclopentadithiopheneBenzothiadiazole. Energy Environ. Sci. 2013, 6, 2944−2949. (11) Liu, B.; Li, W. Q.; Wang, B.; Li, X. Y.; Liu, Q. B.; Naruta, Y.; Zhu, W. H. Influence of Different Anchoring Groups in Indoline Dyes for Dye-Sensitized Solar Cells: Electron Injection, Impedance and Charge Recombination. J. Power Sources 2013, 234, 139−146. (12) Liu, J.; Yang, X. D.; Islam, A.; Numata, Y. H.; Zhang, S. F.; Salim, N. T.; Chen, H.; Han, L. Y. Efficient Metal-Free Sensitizers Bearing Circle Chain Embracing π-Spacers for Dye-Sensitized Solar Cells. J. Mater. Chem. A 2013, 1, 10889−10897. (13) Wu, Y. Z.; Marszalek, M.; Zakeeruddin, S. M.; Zhang, Q.; Tian, H.; Grätzel, M.; Zhu, W. H. High-Conversion-Efficiency Organic DyeSensitized Solar Cells: Molecular Engineering on D−A−π−A Featured Organic Indoline Dyes. Energy Environ. Sci. 2012, 5, 8261−8272. (14) Li, S. R.; Lee, C. P.; Kuo, H. T.; Ho, K. C.; Sun, S. S. HighPerformance Dipolar Organic Dyes with an Electron-Deficient Diphenylquinoxaline Moiety in the π-Conjugation Framework for Dye-Sensitized Solar Cells. Chem.Eur. J. 2012, 18, 12085−12095. (15) Shi, J.; Chen, J. N.; Chai, Z. F.; Wang, H.; Tang, R. L.; Fan, K.; Wu, M.; Han, H. W.; Qin, J. G.; Peng, T. Y.; et al. High Performance Organic Sensitizers Based on 11,12-Bis(Hexyloxy) Dibenzo[a,c]phenazine for Dye-Sensitized Solar Cells. J. Mater. Chem. 2012, 22, 18830−18838. (16) Lu, X. F.; Feng, Q. Y.; Lan, T.; Zhou, G.; Wang, Z. S. Molecular Engineering of Quinoxaline-Based Organic Sensitizers for Highly Efficient and Stable Dye-Sensitized Solar Cells. Chem. Mater. 2012, 24, 3179−3187. (17) Ying, W. J.; Yang, J. B.; Wielopolskic, M.; Moehlb, T.; Moserc, J. E.; Comteb, P.; Hua, J. L.; Zakeeruddin, S. M.; Tian, H.; Grätzel, M. New Pyrido[3,4-b]pyrazine-Based Sensitizers for Efficient and Stable Dye-Sensitized Solar Cells. Chem. Sci. 2014, 5, 206−214. (18) Li, H. R.; Koh, T. M.; Hagfeldt, A.; Grätzel, M.; Mhaisalkar, S. G.; Grimsdale, A. C. New Donor−π−Acceptor Sensitizers Containing 5H-[1,2,5]thiadiazolo[3,4-f ]isoindole-5,7(6H)-dione and 6H-Pyrrolo[3,4-g]quinoxaline-6,8(7H)-dione Units. Chem. Commun. 2013, 49, 2409−2411. (19) Feng, Q. Y.; Zhang, W. Y.; Zhou, G.; Wang, Z. S. Enhanced Performance of Quasi-Solid-State Dye-Sensitized Solar Cells by Branching the Linear Substituent in Sensitizers Based on Thieno[3,4-c]pyrrole-4,6-dione. Chem. Asian J. 2013, 8, 168−177. (20) Duan, R. M.; Ye, L.; Guo, X.; Huang, Y.; Wang, P.; Zhang, S. Q.; Zhang, J. P.; Huo, L. J.; Hou, J. H. Application of Two-Dimensional Conjugated Benzo[1,2-b:4,5-b′]dithiophene in Quinoxaline-Based Photovoltaic Polymers. Macromolecules 2012, 45, 3032−3038. (21) Kim, Y.; Yeom, H. R.; Kim, J. Y.; Yang, C. High-Efficiency Polymer Solar Cells with a Cost-Effective Quinoxaline Polymer Through Nanoscale Morphology Control Induced by Practical Pocessing Additives. Energy Environ. Sci. 2013, 6, 1909−1916. (22) Chen, H. C.; Chen, Y. H.; Liu, C. C.; Chien, Y. C.; Chou, S. W.; Chou, P. T. Prominent Short-Circuit Currents of Fluorinated

V) of sealed solar cells was measured with a metal mask of 0.25 cm2, under illuminating the cell through the conducting glass from the anode side with a solar simulator (WXS-155S-10) at AM 1.5 illuminations (light intensities: 100 mW cm−2). IPCE measurements were made on a CEP-2000 system (BunkohKeiki Co. Ltd.). Electronic Impedance Spectra (EIS) Measurements. The EIS were measured with an impedance analyzer (Solartron Analytical, 1255B) connected with a potentiostat (Solartron Analytical, 1287) under illumination using a solar simulator (WXS-155S-10: Wacom Denso Co. Japan), recorded over a frequency range of 10−2 to 106 Hz at 25 °C. The applied bias voltage and AC amplitude were set at the VOC of the DSSCs, characterized using Z-View software (Solartron Analytical).

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ASSOCIATED CONTENT

S Supporting Information *

Chemical Structure of IQ10, absorption spectra between IQ4 and IQ8 in CH2Cl2, LHE spectra of IQ4, IQ6, IQ7, and IQ8 calculated from the absorption spectra of dye-loaded 8 μm TiO2 electrodes, spectrum of electron lifetime vs Vecb based on IQ4, IQ6, IQ7 and IQ8, and calculated iodide atomic charge on the iodine of various adducts and second order perturbation energy (E(2), kcal mol−1). 1H and 13C NMR and HRMS of IQ6, IQ7, and IQ8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (+86) 21-6425-2758. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National 973 Program (2013CB733 700), NSFC/China, NSFC for Distinguished Young Scholars (Grant No. 21325625), the Oriental Scholarship, National Major Scientific Technological Special Project (2012YQ15008709), SRFDP 20120074110002, the Fundamental Research Funds for the Central Universities (WK1013002), Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Agency, and the Open Funding Project of State Key Laboratory of Luminescent Materials and Devices (SCUT).

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REFERENCES

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