DOI: 10.1002/chem.201503514

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& Sensitizing Dyes |Hot Paper |

Molecular Engineering of Pyrido[3,4-b]pyrazine-Based Donor– Acceptor–p-Acceptor Organic Sensitizers: Effect of Auxiliary Acceptor in Cobalt- and Iodine-Based Electrolytes Bo Liu,[a, b] Fabrizio Giordano,[c] Kai Pei,[a] Jean-David Decoppet,[c] Wei-Hong Zhu,*[a] Shaik M. Zakeeruddin,*[c] and Michael Gr•tzel*[c] Abstract: Due to the ease of tuning its redox potential, the cobalt-based redox couple has been extensively applied for highly efficient dye-sensitized solar cells (DSSCs) with extraordinarily high photovoltages. However, a cobalt electrolyte needs particular structural changes in the organic dye components to obtain such high photovoltages. To achieve high device performance, specific requirements in the molecular tailoring of organic sensitizers still need to be met. Besides the need for large electron donors, studies of the auxiliary acceptor segment of donor–acceptor–p-acceptor (D-A-p-A) organic sensitizers are still rare in molecular optimization in

Introduction As attractive and alternative solar energy conversion devices, dye-sensitized solar cells (DSSCs) are under extensive investigation.[1, 2] As one of the key components of DSSCs, organic sensitizers based on the donor–p-acceptor (D-p-A) motif have been widely explored due to their high molar extinction coefficients, rare-metal-free structures, and, most importantly, their facile molecular tailoring that allows tunable photovoltaic performances.[3–12] Recently, an improved D-p-A construction, the so-

[a] Dr. B. Liu,+ Dr. K. Pei, Prof. W.-H. Zhu Shanghai Key Laboratory of Functional Materials Chemistry, Key Laboratory for Advanced Materials and Institute of Fine Chemicals East China University of Science & Technology 130 Meilong Road, Shanghai 200237 (P. R. China) E-mail: [email protected]

[b] Dr. B. Liu+ College of Chemistry and Material Science Hebei Normal University No. 20, East Road of Nan ErHuan, Shijiazhuang 050024 (P. R. China) [c] Dr. F. Giordano,+ Dr. J.-D. Decoppet, Dr. S. M. Zakeeruddin, Prof. M. Gr•tzel Laboratory for Photonics and Interfaces Institute of Chemical Sciences and Engineering Swiss Federal Institute of Technology 1015 Lausanne (Switzerland) E-mail: [email protected] [email protected] [+] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201503514. Chem. Eur. J. 2015, 21, 18654 – 18661

the context of cobalt electrolytes. In this work, two novel organic D-A-p-A-type sensitizers (IQ13 and IQ17) have been developed and exploited in cobalt- and iodine-based redox electrolyte DSSCs, specifically to provide insight into the effect of p-bridge modification in different electrolytes. The investigation has been focused on the additional electronwithdrawing acceptor capability with grafted long alkoxy chains. Optoelectronic transient measurements have indicated that IQ17 containing a pyrido[3,4-b]pyrazine moiety bearing long alkoxyphenyl chains is more suitable for application in cobalt-based DSSCs.

called D-A-p-A, in which an auxiliary heteroannulated acceptor[13] is introduced into the molecular framework, has been systematically developed, which has led to excellent photovoltaic performance and photostability.[14–30] To date, several D-A-p-Abased organic sensitizers have exhibited high power conversion efficiencies (PCEs) of over 10 % under full sun illumination (AM 1.5G, 100 mW cm¢2), such as the indoline sensitizer YA422 (10.65 %),[31] the porphyrin sensitizer GY50 (12.75 %),[32] and the record one SM315 (13.0 %).[33] Compared to classical ruthenium sensitizers, such pure organic sensitizers are very promising candidates for future commercial and practical application, showing comparably high PCEs and long-term stability. High PCEs of over 10 % have hitherto predominantly been obtained by using cobalt(II/III) tris-bipyridyl ([Co(bpy)3]2 + /3 + ) redox electrolytes instead of I¢/I3¢ electrolytes. Compared with traditional I¢/I3¢ redox electrolytes, cobalt electrolytes offer relatively high redox potentials and negligible light absorption in the visible region.[34] However, the cobalt shuttle still suffers from a low mass-transport capability and serious charge recombination, which critically limit the thicknesses of TiO2 electrodes.[35, 36] Consequently, organic sensitizers with high molar extinction coefficients are preferable to ruthenium sensitizers for thinner TiO2 films in cobalt electrolyte-based DSSCs. In this regard, expanding the donor part can enhance the open-circuit photovoltage (Voc) by around 70 mV, as exemplified by the values for YA422 and YA421.[31] Even in molecular systems as large as porphyrins, such simple modification of the donor part still has a discernible improving effect on Voc. Using alkoxyphenyl-substituted diphenylamine as the donor part, the

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Full Paper Voc of SM315 increased to 910 mV compared to 885 mV for GY50, thereby giving the record PCE of 13.0 %.[32, 33] Besides the donor part, the auxiliary acceptor moiety in D-Ap-A motif dyes is also expected to influence the photovoltaic performance, especially when using different electrolytes. In our previous work, diphenyl-substituted benzo[3,4-b]pyrazine (quinoxaline) was successfully introduced as an auxiliary acceptor in the p-bridge to afford the highly promising organic sensitizer IQ2, which exhibited less aggregation due to the twisted steric conformation of its two phenyl groups.[19] However, the electron-withdrawing capability of benzo[3,4-b]pyrazine is insufficient, leading to a narrow incident photon-to-current efficiency (IPCE) action spectrum of IQ2-based DSSCs, with an onset wavelength of only around 750 nm. To enhance the electron-withdrawing capability and to broaden the IPCE spectrum, diphenyl-substituted pyrido[3,4-b]pyrazine, incorporating an additional unsaturated nitrogen atom, was alternatively exploited as an auxiliary acceptor moiety instead of diphenyl-benzo[3,4-b]pyrazine, leading to the novel D-A-p-A motif sensitizers IQ13 and IQ17 (Scheme 1). Focusing on the additional electron-withdrawing acceptor capability and the appended alkoxy chains, we have systematically evaluated the effect of p-bridge modification of the auxiliary acceptor on the photovoltaic performances of DSSCs with cobalt- and iodinebased redox electrolytes.

Scheme 1. Chemical structures of dyes IQ13, IQ17, and reference IQ2 showing the dihedral angles between the pyrazine and phenyl planes. Note: the dihedral angles were calculated on the basis of hybrid density functional theory (B3LYP) with the 6-31G* basis set as implemented in the Gaussian 09 program. The optimized ground-state geometries of IQ13, IQ17, and IQ2 are shown in Figure S1 in the Supporting Information.

Figure 1. Absorption spectra of sensitizers IQ17 and IQ13: a) in CH2Cl2 and b) on 4 mm TiO2 thin films.

sorption bands (lmax) for both dyes are located at 563 nm, corresponding to strong intramolecular charge transfer (ICT) from the donor unit to the anchoring group. Evidently, the two grafted long alkoxy chains do not affect the position of the absorption bands. IQ17 showed a slightly higher molar extinction coefficient compared to IQ13 (33 300 vs 30 200 mA cm¢2, Table 1). Upon adsorption onto a TiO2 film, the dyes also showed similar spectral properties with small differences in the region 650–750 nm. As expected, upon insertion of the additional unsaturated nitrogen center, the pyrido[3,4-b]pyrazine acceptor exhibited stronger electron-withdrawing capability, resulting in a large redshift by 40 nm from 523 nm (IQ2)[19] to 563 nm (IQ13 and IQ17). Although similar absorption properties were found for IQ13 and IQ17, their fluorescence proper-

Table 1. Photophysical and electrochemical data for sensitizers IQ13 and IQ17. Dye

Results and Discussion [19]

As in IQ2, the two phenyl groups show large dihedral angles with respect to the pyrido[3,4-b]pyrazine plane in both IQ13 and IQ17 (Scheme 1), implying that they could again act as aggregation-preventing substituents. Thus, in the photovoltaic performance characterization, co-adsorbents such as chenodeoxycholic acid (CDCA) were not required. Both IQ13 and IQ17 in CH2Cl2 exhibit three absorption bands (Figure 1 a). AbChem. Eur. J. 2015, 21, 18654 – 18661

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lmax[a] [nm]

IQ13 563 IQ17 563

e[a] [m¢1 cm¢1]

lem (f)[b] [nm]

HOMO[c] [V]

E0-0[d] [eV]

LUMO[e] [V]

30200 33300

707 (3.1 %) 736 (1.0 %)

0.91 0.95

1.83 1.86

¢0.92 ¢0.91

[a] Absorption peaks (lmax) and molar extinction coefficients (e) in CH2Cl2. [b] The fluorescence quantum efficiency was measured with reference to Rhodamine B. [c] The formal oxidation potentials (vs. NHE) in CH2Cl2 were calibrated using ferrocene as a reference and taken as the HOMOs. [d] Estimated from the band gap derived from the wavelength at 10 % maximum absorption intensity for the dye in CH2Cl2. [e] The LUMO was calculated from the expression LUMO = HOMO¢E0-0. Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper ties were slightly different. The lem of IQ17 was observed at 736 nm, redshifted by almost 30 nm with respect to that of IQ13, suggesting a larger Stokes shift in the former. That is to say, upon light irradiation, the excited electrons of IQ17 can release energy more readily through the decay process than those of IQ13, as corroborated by the fluorescence quantum efficiencies shown in Table 1. Furthermore, the absorption spectral changes were also measured under light irradiation (> 400 nm) for different durations to evaluate the photostabilities of IQ13 and IQ17 (Figures S2 and S3 in the Supporting Information). After 30 min of irradiation, blueshifts of 6 and 5 nm were observed for IQ13 and IQ17 with only 7 % and 5 % decreases in absorption intensity, respectively, indicating good photostability of these dyes. Besides their optoelectronic properties, the two sensitizers also exhibited similar electrochemical properties. As shown in Table 1, the oxidation potentials of IQ13 and IQ17 were measured as 0.91 and 0.95 V versus NHE, respectively, higher than those of I¢/I3¢ and [Co(bpy)3]2 + /3 + , meaning that the two sensitizers can be regenerated from their oxidized forms by electron donation from both redox couples following interfacial electron injection. The LUMO levels were evaluated from the HOMO levels and band gaps (E0-0) as ¢0.92 and ¢0.91 V versus NHE, respectively. These energy levels are very close, indicative of the similar electron-injection driving forces to the TiO2 conduction band for sensitizers IQ13 and IQ17. Quantum calculations were also performed with the Gaussian 09 package[37] to further highlight the electron density distributions in the HOMO and LUMO. TD-DFT calculations were performed based on the ground-state geometries optimized by DFT using the hybrid B3LYP[38] functional and the standard 6-31G(d) basis set. The hybrid MPW1K[39] functional including 42 % Hartree–Fock (HF) exchange was used with the 6-31 + G(d) basis set. As shown in Figure 2, in contrast to traditional D-p-A organic sensitizers, the HOMOs of IQ13 and IQ17 are mainly distributed on the D-A junction, whereas the LUMOs are mainly localized on the A-p-A moiety, with distinct overlap with the HOMOs on the pyrido[3,4-b]pyrazine cores. This suggests that under illumination the excited electrons may be smoothly transferred to the acceptor part and then injected into the conduction band of TiO2.

Figure 2. Frontier molecular orbitals (HOMOs and LUMOs) of IQ2, IQ13, and IQ17 calculated with the Gaussian 09 program. Chem. Eur. J. 2015, 21, 18654 – 18661

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Even though IQ13 and IQ17 show similar absorption spectra and energy levels both in solution and when adsorbed on a TiO2 surface, their IPCE spectral responses are quite different. The IPCE spectrum of IQ13 is redshifted by 25 nm compared to that of IQ17, and by 50 nm compared to that of IQ2 (Figure 3). However, the maximum IPCE value of IQ13 is lower

Figure 3. Current–voltage characteristics (I–V, a) and IPCE action spectra (b) for IQ13, IQ17, and reference IQ2 with the cobalt-based electrolyte. The I–V characteristics and IPCE spectra with the iodine-based redox electrolyte are shown in Figure S4 in the Supporting Information.

than those of the other two dyes. In general, a lower IPCE could be due to lower light-harvesting efficiency, a smaller quantum yield for electron injection into the conduction band of TiO2, or a lower charge-collection efficiency (EC) due to a higher recombination rate. We firstly evaluated the electron lifetime, tlifetime, in the TiO2 by small signal photovoltage and photocurrent transient measurements, as reported previously,[40–42] details of the set-up for which are given in the Experimental Section. Values of tlifetime versus charge density (noc) in the TiO2 film under open-circuit conditions are plotted in Figure 4. IQ2 was also tested for comparison. For the cobalt-based redox electrolyte, the difference in tlifetime is related to the ability of the dye molecule to protect the TiO2 surface from approach by the oxidized redox component ([Co(bipy)3]3 + ). Due to the two long alkoxy chains grafted onto the benzo[3,4-b]pyrazine and pyrido[3,4-b]pyrazine moieties, IQ2 and IQ17 showed similar electron lifetimes (Figure 4 a), suggesting that the insertion of nitrogen in the auxiliary acceptor of dye IQ17 has no influence. However, compared with IQ17, IQ13 presented a shorter tlifetime, which might be due to the smaller molecular volume resulting from the absence of long alkoxy chains on the pyrido[3,4-b]pyrazine moiety, leading to a higher recombination rate. Thus, IQ13 gave lower IPCE

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Full Paper decrease in thickness, although the loaded amount of dye IQ17 was sharply decreased by more than 60 % from 0.94 to 0.35 Õ 10¢7 mol cm¢2 (Table S1 in the Supporting Information), the decrease in Jsc was only around 9.5 %, suggesting that IQ17 is quite appropriate for use in cobalt-based DSSCs. Although IQ2 and IQ17 have similar tlifetime values in cobaltbased DSSCs, the Voc values showed a difference of 51 mV, much larger than that of iodine-based DSSCs (16 mV, listed in Table 2). On the other hand, the difference in Voc values between IQ13 and IQ17 with cobalt-based DSSCs was measured as 47 mV, smaller than that with iodine-based DSSCs (63 mV). Thus, the influence of dye structural changes on the photovoltaic performances, optoelectronic transient analysis was further applied.[40, 41] The corresponding densities of states (DOS) within the devices under open-circuit conditions were calculated from differential capacitance measurements,[41] which were made by applying a red-light pulse superimposed on the bias white light. From such differential capacitance measurements, we could estimate the DOS in the TiO2 at a given potential bias:

DOS ¼ 6:24x1018

Figure 4. Recombination time constants for IQ2, IQ13, and IQ17 for a) cobalt- and b) iodine-based electrolytes.

values than those of IQ17 in the region 400–650 nm and a lower Voc. In the case of the iodine-based electrolyte (Figure 4 b), the tlifetime trend was IQ2 > IQ17 > IQ13, following the order of Voc values (739, 723, and 660 mV, respectively), as shown in Table 2. The introduction of the nitrogen atom in the auxiliary acceptor of dye IQ17 seems to be responsible for accelerating the recombination rate in the iodine-based electrolyte. As reported earlier, the interaction between electrolyte and dye can enhance the recombination rate.[43] Evidently, compared with the cobalt-based redox electrolyte, there is a stronger interaction of dye IQ17 with iodine/triiodide. Moreover, to reduce charge recombination between the injected electrons present in the conduction band of TiO2 and the Co3 + species, 4 mm TiO2 electrodes were used in the cobalt-based DSSCs, half of the thickness (8 mm) used in the iodine-based DSSCs. With the

c dð1 ¢ pÞ

ð1Þ

where c is the differential capacitance, d is the thickness, and p is the porosity of the TiO2 film. Measurements were performed at different levels of light intensity, recording the differential capacitance and open-circuit voltage of the cells. The differential capacitance was then integrated with respect to the potential bias (V*), which can be approximated as follows: Z QQC ðV * Þ ¼

0

V

*

CTiO2 dV

ð2Þ

A plot of the total charge density (noc) at different Voc or of the DOS versus photovoltage is generally used to calculate EC shifts caused by additives in the electrolyte. This is acceptable under the assumption of similar DOS distributions between the cells under comparison.[40, 41] Plots of Voc versus calculated charge density for these dye-based DSSC devices with iodine and cobalt redox electrolytes are shown in Figure 5. IQ13 showed downward shifts of EC of DV … 59 and 28 mV for the cobalt- and iodine-based electrolytes, respectively. In contrast, IQ17 showed downward shifts of DV … 25 and 32 mV for the respective electrolytes. Generally, the density of adsorbed molecules on the TiO2 surface is believed to be responsible for the shift in the conduction band [Eq. (3)].[31]

Table 2. Photovoltaic parameters for IQ2, IQ13, and IQ17 with cobaltand iodine-based redox electrolytes. Dye

Electrolyte

Jsc [mA cm¢2]

Voc [mV]

FF [%]

PCE [%]

IQ13 IQ17 IQ2 IQ13 IQ17 IQ2

[Co(bpy)3] + 2/ + 3 [Co(bpy)3] + 2/ + 3 [Co(bpy)3] + 2/ + 3 I¢/I3¢ I¢/I3¢ I¢/I3¢

14.2 14.2 12.8 17.6 15.7 14.0

745 792 843 660 723 739

0.74 0.77 0.74 0.73 0.74 0.73

8.0 9.0 8.0 8.75 8.6 7.6

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DVOC ¼

kB T rlifetime;dye1 lnð Þ q rlifetime;dye2

ð3Þ

where T is the absolute temperature, KB is the Boltzmann constant, and q is the elementary charge. Because IQ13 lacks the long alkoxy chains of IQ17, it would clearly be expected to be adsorbed at a higher molecular density due to its smaller size, 18657

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Full Paper performances were quite different. Optoelectronic transient analysis revealed that, compared with IQ13, the grafted long alkoxy chains in IQ17 can effectively protect the TiO2 surface from the approach of the oxidized species (Co3 + ) in a cobaltbased electrolyte, thus resulting in a lower recombination rate and hence higher IPCE values and photovoltages. As demonstrated, the use of pyrido[3,4-b]pyrazine as an auxiliary acceptor can induce a shift in EC of 30 mV for IQ17 DSSCs in conjunction with a cobalt-based electrolyte. IQ17 incorporating pyrido[3,4-b]pyrazine bearing two long alkoxy chains is more suitable for application in cobalt-based DSSCs than in systems based on the I¢/I3¢ redox couple.

Experimental Section General NMR spectra were recorded on a Bruker AM 400 spectrometer operating at 400 MHz (for 1H NMR) and 100 MHz (for 13C NMR); chemical shifts are referenced to TMS. HRMS were obtained with a Waters ESI mass spectrometer. UV/Vis spectra were measured with a Cary 100 spectrophotometer. Cyclic voltammograms were recorded with a Versastat II electrochemical workstation (Princeton Applied Research) using a three-electrode cell with a Pt working electrode, a Pt wire auxiliary electrode, and a regular calomel reference electrode (SCE) in saturated KCl solution. A 0.1 m solution of tetrabutylammonium hexafluorophosphate (TBAPF6) in THF was used as the supporting electrolyte. The scan rate was 100 mV s¢1. The ferrocenium/ferrocene (Fc/Fc + ) redox couple was added after measurement as an internal potential reference.

Figure 5. Charge densities for IQ2, IQ13, and IQ17 versus Voc for a) iodineand b) cobalt-based electrolytes.

thus resulting in a higher conduction band edge. However, the conduction band shifts of IQ13 and IQ17 were comparable in the iodine-based electrolyte. Even though IQ2 and IQ17 have similar molecular structures, EC shifts of DV … 30 and DV … ¢9 mV were observed in the cobalt- and iodine-based redox electrolytes, respectively. Considering that 9 mV is within the experimental variance, we can conclude that the insertion of an additional nitrogen atom into benzo[3,4-b]pyrazine to form pyrido[3,4-b]pyrazine only influences the conduction band edge in the cobalt-based redox electrolyte. Moreover, values of DVoc (IQ2¢IQ17) + EC = 38 and 9 mV obtained for the cobalt- and iodine-based redox electrolytes, respectively, are close to the VOC differences measured from the I–V data (51 and 16 mV). Similarly, values of DVoc (IQ2¢IQ13) + EC = 74 and 56 mV are close to those from I–V data (98 and 79 mV) obtained in the cobalt- and iodine-based redox electrolyte devices, respectively.

Conclusion Through inserting an additional unsaturated nitrogen atom into benzo[3,4-b]pyrazine we have prepared the more strongly electron-withdrawing pyrido[3,4-b]pyrazine and used it as an auxiliary acceptor in two D-A-p-A dyes IQ13 and IQ17. As a result of this modification, the IPCE spectra were successfully broadened. Although similar optical and electrochemical properties were observed for IQ13 and IQ17, their photovoltaic Chem. Eur. J. 2015, 21, 18654 – 18661

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Synthesis and characterization The synthetic routes to IQ13 and IQ17 are shown in Scheme S1 (see the Supporting Information). Through cyclization, Suzuki, and Knoevenagel reactions, IQ13 and IQ17 were obtained as purple powders. The new sensitizers and important intermediates were characterized by 1H and 13C NMR and HRMS. Relevant spectra are shown in Figures S5–S13 (in the Supporting Information).

Synthesis of IQ13 Synthesis of 2 a: 2,5-Dibromopyridine-3,4-diamine (1.0 g, 3.75 mmol) and benzil (1,2-diphenylethane-1,2-dione (0.79 g, 3.75 mmol) were dissolved in nBuOH (10 mL). The reaction mixture was heated under reflux for 5 h and then cooled to RT. The product 2 a was obtained as yellow needles, which were collected by filtration (0.5 g, 33 %). 1H NMR (400 MHz, CDCl3): d = 8.76 (s, 1 H), 7.61–7.70 (m, 4 H), 7.42–7.49 (m, 2 H), 7.34–7.42 ppm (m, 4 H). Synthesis of 3 a: Compound 2 a (2.0 g, 3.28 mmol), 2 m aqueous K2CO3 (20 mL), and Pd(PPh3)4 (0.02 g) were added to THF (40 mL). After heating to reflux, a solution of indoline-boronic acid (0.632 g, 1.7 mmol) in THF (40 mL) was added and the resulting mixture was maintained under reflux for a further 8 h. The product was extracted with dichloromethane (3 Õ 40 mL). The combined extracts were dried, concentrated, and subjected to column chromatography on silica gel eluting with dichloromethane/petroleum ether (1:2, v/v) to afford 3 a (1.08 g, 61 %). 1H NMR (400 MHz, CDCl3): d = 8.97 (s, 1 H), 8.16 (s, 1 H), 8.11 (dd, J1 = 8.5 Hz, J2 = 1.7 Hz, 1 H), 7.70–7.76 (m, 2 H), 7.60–7.65 (m, 2 H), 7.30–7.47 (m, 6 H), 7.23–7.28 (m, 2 H), 7.19 (d, J = 8.4 Hz, 2 H), 7.00 (d, J = 8.5 Hz, 1 H), 4.86–4.93 (m, 1 H), 3.89– 3.98 (m, 1 H), 2.36 (s, 3 H), 2.09–2.18 (m, 1 H), 2.00–2.09 (m, 1 H),

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Full Paper 1.90–1.99 (m, 1 H), 1.75–1.87 (m, 1 H), 1.66–1.75 (m, 1 H), 1.59– 1.66 ppm (m, 1 H); 13C NMR (100 MHz, CDCl3): d = 156.13, 153.36, 149.85, 147.92, 142.03, 139.80, 138.20, 137.86, 134.93, 134.79, 132.16, 130.28, 132.05, 130.28, 130.05, 129.99, 129.88, 129.43, 128.47, 128.36, 127.96, 126.95, 120.84, 115.96, 106.83, 69.42, 45.34, 35.21, 33.62, 24.47, 20.89 ppm. Synthesis of 4 a: Compound 3 a (0.64 g, 1 mmol), (5-formylthiophen-2-yl)boronic acid (0.45 g, 3 mmol), and Pd(PPh3)4 (0.02 g) were added to a mixture of 2 m aqueous K2CO3 (20 mL) and THF (30 mL). After heating under reflux for 12 h, the mixture was cooled to RT and water was added. The product was extracted with dichloromethane (3 Õ 30 mL). The combined extracts were dried, concentrated, and subjected to column chromatography on silica gel eluting with dichloromethane/petroleum ether (1:1, v/v) to afford 4 a (0.3 g, 47 %). 1H NMR (400 MHz, CDCl3): d = 9.99 (s, 1 H), 9.24 (s, 1 H), 8.27 (s, 1 H), 8.23 (dd, J1 = 8.5 Hz, J2 = 1.8 Hz, 1 H), 7.95 (d, J = 4.0 Hz, 1 H), 7.84 (d, J = 4.0 Hz, 1 H), 7.78 (dd, J1 = 7.4 Hz, J2 = 1.5 Hz, 2 H), 7.66 (dd, J1 = 7.7 Hz, J2 = 1.3 Hz, 2 H), 7.40–7.47 (m, 3 H), 7.32–7.40 (m, 3 H), 7.24–7.28 (m, 2 H), 7.18 (d, J = 8.4 Hz, 2 H), 7.01 (d, J = 8.5 Hz, 1 H), 4.88–4.95 (m, 1 H), 3.91–3.99 (m, 1 H), 2.37 (s, 3 H), 2.10–2.19 (m, 1 H), 2.02–2.10 (m, 1 H), 1.91–1.99 (m, 1 H), 1.76–1.88 (m, 1 H), 1.66–1.75 (m, 1 H), 1.60–1.66 ppm (m, 1 H); 13 C NMR (100 MHz, CDCl3): d = 183.29, 158.98, 155.34, 152.80, 150.10, 146.41, 144.73, 144.66, 140.34, 139.63, 138.40, 137.81, 136.10, 134.86, 133.42, 132.34, 130.40, 130.03, 130.01, 129.90, 129.40, 128.58, 128.39, 128.13, 127.24, 126.50, 121.76, 120.97, 106.82, 69.46, 45.32, 35.25, 33.57, 24.46, 20.91 ppm. Synthesis of IQ13: A 100 mL flask was charged with compound 4 a (0.32 g, 0.5 mmol), cyanoacetic acid (0.17 g, 2.0 mmol), piperidine (1.5 mL), and acetonitrile (50 mL). The mixture was heated under reflux for 8 h and then cooled to RT. After evaporation of the solvent, the residue was subjected to column chromatography on silica gel eluting with dichloromethane/AcOH (100:1) to afford IQ13 (0.16 g, 45 %). M.p. 308–310 8C; 1H NMR (400 MHz, [D8]THF): d = 10.74 (s, 1 H; -COOH), 9.25 (s, 1 H), 8.35 (s, 1 H), 8.33 (s, 1 H), 8.11 (dd, J1 = 8.5 Hz, J2 = 1.6 Hz, 1 H), 7.96 (d, J = 4.2 Hz, 1 H), 7.82–7.87 (m, 2 H), 7.78 (d, J = 4.2 Hz, 1 H), 7.60–7.68 (m, 2 H), 7.28–7.36 (m, 3 H), 7.20–7.28 (m, 3 H), 7.09 (s, 1 H), 7.07 (s, 1 H), 6.85 (d, J = 8.5 Hz, 1 H), 4.80–4.90 (m, 1 H), 3.80–3.87 (m, 1 H), 2.22 (s, 3 H), 2.01–2.10 (m, 1 H), 1.89–1.97 (m, 1 H), 1.79–1.85 (m, 1 H), 1.66–1.76 (m, 2 H), 1.44–1.54 ppm (m, 1 H); 13C NMR (100 MHz, [D8]THF): d = 163.30, 146.15, 144.40, 139.79, 137.79, 132.22, 131.86, 130.73, 129.99, 129.75, 129.61, 129.40, 128.97, 128.49, 128.34, 128.05, 125.71, 120.75, 106.23, 69.28, 45.29, 33.37, 29.66, 19.93 ppm; HRMS (ESI): m/z: [M+ +H] + calcd for C45H34N5O2S: 708.2433; found: 708.2436; FTIR (KBr): n˜ = 3419, 1620, 1517, 1369, 1122, 925, 565 cm¢1.

Synthesis of IQ17 Synthesis of 3 b: Compound 2 b (1.2 g, 1.7 mmol), 2 m aqueous K2CO3 (30 mL), and Pd(PPh3)4 (0.04 g) were added to THF (60 mL). After heating to reflux, indoline-boronic acid (1.26 g, 3.4 mmol) was added and the resulting mixture was maintained under reflux for a further 8 h. The product was extracted with dichloromethane (3 Õ 50 mL). The combined extracts were dried, concentrated, and subjected to column chromatography on silica gel eluting with dichloromethane/petroleum ether (1:3, v/v) to afford 3 b (0.5 g, 41 %). 1H NMR (400 MHz, CDCl3): d = 8.90 (s, 1 H), 8.12 (s, 1 H), 8.07 (dd, 1 H, J1 = 1.6 Hz, J2 = 8.4 Hz), 7.72 (d, 2 H, J = 8.8 Hz), 7.60 (d, 2 H, J = 8.8 Hz), 7.24 (d, 2 H, J = 8.4 Hz), 7.19 (d, 2 H, J = 8.4 Hz), 7.00 (d, 1 H, J = 8.4 Hz), 6.88 (d, 2 H, J = 8.8 Hz), 6.84 (d, 2 H, J = 8.8 Hz), 4.82–4.92 (m, 1 H), 3.87–4.04 (m, 1 H), 2.35 (s, 3 H), 2.08–2.18 (m, 1 H), 2.00–2.08 (m, 1 H), 1.90–1.98 (m, 1 H), 1.73–1.86 (m, 1 H), 1.63– Chem. Eur. J. 2015, 21, 18654 – 18661

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1.72 (m, 2 H), 1.55–1.63 (m, 1 H), 1.40–1.52 (m, 4 H), 1.20–1.40 (m, 16 H), 0.83–0.96 ppm (m, 6 H); 13C NMR (100 MHz, [D6]DMSO): d = 160.86, 160.27, 158.46, 155.47, 152.82, 149.63, 147.40, 141.73, 139.91, 134.68, 131.99, 131.93, 131.44, 130.64, 130.19, 127.89, 127.17, 120.72, 115.93, 114.44, 114.34, 106.81, 69.38, 68.15, 68.12, 45.37, 35.20, 33.65, 31.84, 29.38, 29.27, 29.22, 26.06, 24.48, 22.69, 20.88, 14.14 ppm. Synthesis of 4 b: Compound 3 b (0.45 g, 0.52 mmol), (5-formylthiophen-2-yl)boronic acid (0.23 g, 1.0 mmol), and Pd(PPh3)4 (0.02 g) were added to a mixture of 2 m aqueous K2CO3 (10 mL) and THF (30 mL). After heating under reflux for 12 h, the mixture was cooled to RT and water was added. The product was extracted with dichloromethane (3 Õ 30 mL). The combined extracts were dried, concentrated, and subjected to column chromatography on silica gel eluting with dichloromethane/petroleum ether (1:1, v/v) to afford 4 b (0.3 g, 64 %). 1H NMR (400 MHz, [D6]DMSO): d = 9.97 (s, 1 H), 9.15 (s, 1 H), 8.23 (s, 1 H), 8.19 (dd, 1 H, J1 = 1.2 Hz, J2 = 8.4 Hz), 7.88 (d, 1 H, J = 4.0 Hz), 7.80 (d, 1 H, J = 4.0 Hz), 7.76 (d, 2 H, J = 8.8 Hz), 7.63 (d, 2 H, J = 8.8 Hz), 7.25 (d, 2 H, J = 8.4 Hz), 7.20 (d, 2 H, J = 8.4 Hz), 7.01 (d, 1 H, J = 8.4 Hz), 6.93 (d, 2 H, J = 8.8 Hz), 6.85 (d, 2 H, J = 8.8 Hz), 4.83–4.94 (m, 1 H), 3.99–4.08 (t, 2 H), 3.86–3.99 (m, 3 H), 2.36 (s, 3 H), 2.09–2.18 (m, 1 H), 2.01–2.09 (m, 1 H), 1.90–1.99 (m, 1 H), 1.74–1.87 (m, 5 H), 1.66–1.73 (m, 1 H), 1.56–1.66 (m, 1 H), 1.41–1.53 (m, 4 H), 1.21–1.41 (m, 16 H), 0.81–0.96 ppm (m, 6 H); 13 C NMR (100 MHz, [D6]DMSO): d = 183.33, 160.85, 160.24, 158.57, 154.78, 152.29, 149.90, 146.65, 144.64, 144.11, 140.63, 139.71, 136.13, 134.77, 133.13, 132.33, 132.18, 131.98, 131.42, 130.84, 130.10, 139.88, 128.06, 127.37, 126.16, 121.60, 120.85, 114.53, 114.34, 106.80, 69.41, 68.18, 68.12, 45.33, 35.24, 33.59, 31.85, 29.38, 29.40, 29.27, 26.08, 26.07, 24.47, 22.70, 20.90, 14.16 ppm; HRMS (ESI): m/z: [M+ +H] + calcd for C58H65N4O3S: 897.4777; found: 897.4777. Synthesis of IQ17: A 100 mL flask was charged with compound 4 b (0.30 g, 0.42 mmol), cyanoacetic acid (0.80 g, 1.0 mmol), piperidine (1.0 mL), and acetonitrile (50 mL). The mixture was heated under reflux for 8 h and then cooled to RT. After evaporation of the solvent, the residue was subjected to column chromatography on silica gel eluting with dichloromethane/AcOH (100:1) to afford IQ17 (0.15 g, 37 %). M.p.: 261–263 8C; 1H NMR (400 MHz, [D6]DMSO): d = 9.22 (s, 1 H), 8.43 (s, 1 H), 8.14 (s, 1 H), 7.98–8.10 (m, 2 H), 7.92 (d, 1 H, J = 3.6 Hz), 7.71 (d, J = 8.0 Hz, 1 H), 7.44 (d, 2 H, J = 8.0 Hz), 7.11–7.31 (m, 4 H), 6.84 (d, 3 H, J = 7.6 Hz), 6.78–6.85 (m, 5 H), 4.81–4.95 (m, 1 H), 3.85–4.01 (m, 2 H), 3.76–3.85 (m, 1 H), 2.29 (s, 3 H), 1.96–2.11 (m, 1 H), 1.89–1.76 (m, 2 H), 1.54–1.76 (m, 1 H), 1.33–1.49 (m, 4 H), 1.14–1.33 (m, 16 H), 0.77–0.91 ppm (m, 6 H); 13 C NMR (100 MHz, [D6]DMSO): d = 163.87, 160.04, 159.48, 156.57, 148.78, 145.50, 139.00, 138.59, 137.87, 134.09, 131.99, 131.30, 131.10, 130.46, 129.75, 129.18, 126.99, 120.63, 120.13, 114.29, 113.92, 97.49, 68.43, 67.44, 44.41, 31.23, 28.77, 28.65, 25.49, 23.96, 22.06, 20.40, 13.90 ppm; HRMS (ESI): m/z: [M+ +H] + calcd for C61H66N5O4S: 964.4836; found: 964.4836; FTIR (KBr): n˜ = 3421, 2924, 1605, 1511, 1378, 1127, 916, 562 cm¢1.

DSSC fabrication and photovoltaic performance measurements A screen-printed double-layer film of mesoporous TiO2 was used to sensitize the films as a photoanode. An 8 mm thick transparent film of 20 nm mesoporous TiO2 particles was first printed on fluorinedoped SnO2 (FTO) conducting glass and further coated by a 5 mm thick second layer of 400 nm light-scattering anatase particles. Photoanodes were prepared by immersing the double-layer TiO2 film into a dye solution (3 Õ 10¢4 m in CHCl3/EtOH, 3:7, v/v) for 8 h.

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Full Paper Counter electrodes were prepared by depositing Pt catalyst onto cleaned FTO glass by coating with a drop of H2PtCl6 solution (50 mm in ethanol) followed by heat treatment at 400 8C for 30 min. The two electrodes were sandwiched using a 25 mm thick Surlyn spacer and sealed by heating the polymer frame. In this work, 1.0 m 1,3-dimethylimidazolium iodide (DMII), 0.05 m LiI, 30 mm I2, 0.5 m tert-butylpyridine, and 0.1 m guanidinium thiocyanate (GNCS) in acetonitrile/valeronitrile (85:15, v/v) were used to prepare the redox electrolyte. For the cobalt devices, photoanodes (4 mm nanocrystalline TiO2 electrodes with a 4 mm scattering layer) were sensitized in dye solution for 4 h. The electrolyte composition was 0.1 m lithium trifluoromethanesulfonimide (LiTFSI), 0.055 m [Co(bpy)3](TFSI)3 (bpy = 2,2’-bipyridine), 0.2 m [Co(bpy)3](TFSI)2, and 0.8 m tert-butylpyridine (TBP) in acetonitrile. A 450 W xenon lamp (Oriel, USA) was used as a light source to study the current–voltage characteristics of the DSC devices. The spectral output of the lamp was filtered using a Schott K113 Tempax sunlight filter (Pr•zisions Glas & Optik GmbH, Germany) to reduce the mismatch between the simulated and actual solar spectrum to less than 2 %. A Keithley model 2400 digital source meter (Keithley, USA) was used for data acquisition. A photoactive area of 0.16 cm2 was defined by a black mask of 4 mm Õ 4 mm.

Optoelectronic transient measurements For transient photovoltage/photocurrent decay measurements, a white-light bias was generated from an array of diodes. Red-light pulse diodes (with a square pulse width, 100 ns rise and fall time) controlled by a fast solid-state switch were used as the perturbation source. Voltage dynamics were recorded on a PC interfaced Keithley 2602 source meter with a response time of 500 ms. The perturbation light source was set at a suitably low level such that the system could be assumed to be linear. By varying the white light bias intensity, the electron recombination lifetime could be estimated over a range of applied biases. Before the LEDs were switched to the next light intensity, a charge-extraction routine was executed to measure the electron density in the film. In the charge-extraction technique, the LED illumination source was turned off in < 1 ms, and simultaneously the cell was switched from open to short circuit. The resulting current, as the cell returned to V = 0 and J = 0, was integrated to give a direct measurement of the excess charge in the film at the applied VOC.

Acknowledgements This work was supported by the Natural Science Foundation of China for Creative Research Groups (21421004) and Distinguished Young Scholars (21325625), the Oriental Scholarship, Science and Technology Commission of Shanghai Municipality (15XD1501400), Oversea Scholars of Hebei DHRSS (C201400324) and Excellent Young Scholars of Hebei Education Department (Y2012017), the Swiss National Science Foundation and the CTI for project number 17622.1 PFNM-NM, glass2energy SA (g2e), Villaz-St-Pierre, Switzerland. Keywords: auxiliary acceptor · cobalt-based electrolyte · donor–acceptor systems · dyes/pigments · optoelectronic transient measurement · organic sensitizers

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Received: September 3, 2015 Published online on November 9, 2015

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