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Multiply Wrapped Porphyrin Dyes with a Phenothiazine Donor: A High Efficiency of 11.7% Achieved through a Synergetic Coadsorption and Cosensitization Approach Yunyue Lu, Heli Song, Xin Li, Hans Ågren, Qingyun Liu, Jiwei Zhang, Xuan Zhang, and Yongshu Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19077 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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ACS Applied Materials & Interfaces

Multiply Wrapped Porphyrin Dyes with a Phenothiazine Donor: A High Efficiency of 11.7% Achieved through a Synergetic Coadsorption and Cosensitization Approach Yunyue Lu†, Heli Song†, Xin Li‡, Hans Ågren‡, Qingyun Liu§, Jiwei Zhang¶, Xuan Zhang¶*, Yongshu Xie†* Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science & Technology, Shanghai 200237, P. R. China. ‡ Division of Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal Institute of Technology, SE-10691 Stockholm, Sweden. § College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao, P. R. China. †

¶

College of Chemistry, Chemical Engineering & Biotechnology, Donghua University, Shanghai 201620, P. R. China.

KEYWORDS: phenothiazine, porphyrin, wrapped structure, coadsorption, cosensitization ABSTRACT: Photocurrent (Jsc) and photovoltage (Voc) are two important parameters for dye-sensitized solar cells (DSSCs) to achieve high power conversion efficiencies (PCE). Herein, we synthesize four novel porphyrin dyes, XW36-XW39, using an Nphenyl substituted phenothiazine donor to pursue higher PCE. For XW36 and XW37, the N-phenyl group is wrapped with two orthoalkoxy chains. In contrast, it is substituted with a para-alkoxy group in XW38 and XW39. The phenothiazine-wrapping in XW36 and XW37 induce more serious distortion, which is beneficial for anti-aggregation, but unfavorable for the electron transfer from donor to porphyrin framework. Thus, individual porphyrin dyes XW36 and XW37 exhibit efficiencies of 9.05% and 9.58%, respectively, lower than those of 9.51% and 10.00% achieved for XW38 and XW39, respectively. Besides, the introduction of a methyl group into the benzoic acid acceptor unit is conducive to anti-aggregation, and thus improves the Voc and efficiencies. Therefore, higher efficiencies were achieved for XW37 and XW39, compared with XW36 and XW38, respectively. Interestingly, although the individual XW36 dye shows a lowest efficiency among the four dyes, a highest efficiency of 11.70% was obtained for XW36 on the basis of synergetic adsorption with CDCA and PT-C6 because of simultaneously improved Jsc and Voc, which may be ascribed to the lowest dye loading amount of XW36 among all of these porphyrin dyes, with the largest vacancy area left on the TiO2 surface available for co-sensitizer PT-C6, resulting in a highest Jsc. The high efficiency of 11.70% is one of the highest efficiencies using I-/I3- electrolyte in DSSCs. These results provide an effective strategy for developing efficient DSSCs by the targeted coadsorption and cosensitization of porphyrin sensitizers optimized through introducing a bis(ortho-alkoxy) wrapped phenyl group into the phenothiazine donor and/or methyl groups into the benzoic acid acceptor unit.

INTRODUCTION In the past decades, DSSCs have been developed rapidly owing to their simple syntheses and modest to high PCE. Since the first report in 1991, many donor-π-acceptor type (D-π-A) dyes exhibit excellent photovoltaic efficiencies.1-34 Among the various synthetic dyes, porphyrin dyes have received widespread attention in DSSCs because of their prominent spectral bandwidth in 400-700 nm and easily modulated structures.35-48 In spite of the advantageous characters, porphyrin dyes also have their specific absorption drawbacks around 500550 nm and in the near infrared (NIR) region. Hence, the cosensitization approach has been applied to further enhance photovoltaic efficiencies by applying a co-sensitizer with complementary absorption character.39-46 Up to now, record efficiencies have been reported for SM31522 and

ADEKA-1 / LEG426 for individual dyes and cosensitization systems, respectively, based on the cobalt electrolyte. However, extra structural characters and more challenging device fabrication techniques are usually required for DSSCs devices using the cobalt electrolyte. Hence, researches on developing preeminent sensitizers using I-/I3- electrolyte are still highly desirable.41 Thus, a number of iodine-based DSSCs have been reported showing efficiencies beyond 10%.2-4,44,46 In this respect, high efficiencies of 10.45% and 11.50% have been achieved by us for the DSSCs based on XW4 and XW11, respectively, employing delicately designed cosensitization approaches by carefully selecting co-sensitizers with absorption spectra complementary to those of the porphyrin dyes. As we know, typical efficient porphyrin dyes have the structural feature of two bis(ortho-alkoxy)-wrapped meso-phenyl groups,20,35-51 and

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the wrapping character has been demonstrated to be effective for suppressing dye aggregation.42,44 For the donor moieties, long alkoxy / alkyl chains are usually employed to suppress dye aggregation.39-51 Based on the reported results, it can be envisioned that the wrapping strategy may be applicable to the donor moiety as well. Meanwhile, an alkyl substituent may be introduced into the acceptor moiety5 for further suppressing the dye aggregation. Thus, we herein design porphyrin dyes XW36XW39 using a phenothiazine donor with its N-phenyl group wrapped with two ortho-alkoxy chains (XW36 and XW37) or substituted with a para-alkoxy unit (XW38 and XW39). Besides, a methyl-substituted benzoic acid acceptor was introduced into dyes XW37 and XW39 with the purpose to suppress the dye aggregation. Meanwhile, the introduced methyl group will not cause obvious unfavorable steric hindrance. As we expected, higher efficiencies were successfully achieved for all the new porphyrin dyes, compared with XW10. Among the four porphyrin dyes, XW39 exhibited the highest PCE of 10.0%. OH C12H25O

OC12H25 H

R1

N

N N

S

N

C12H25O

HO

H

H

H

OC12H25

HO

XW10

Spectral and Electrochemical Properties Figure 2 showed absorption spectra of XW36-XW39, XW10 and PT-C6 in 300-750 nm, and the emission spectra of XW36-XW39 are exhibited in Figure S2. All the four dyes show a Soret band and Q bands around 450 nm and 570-700 nm, respectively, covering a large wavelength range, which is favorable for harvesting the sunlight.46 The corresponding data are shown in Table 1. Compared with the alkyl substituent in XW10, the hexyloxy-substituted phenyl groups attached to the nitrogen atom of the phenothiazine moiety in XW36-XW39 were observed to cause slight bathochromic-shift of Q bands from 665 to 666~669 nm, indicative of the electron-donating character of the hexyloxyphenyl moieties. By comparison of XW37 and XW39 with XW36 and XW38, it is obvious that the introduction of a methyl group into the benzoic acid unit does not induce obvious electronic effect or steric hindrance effect. PT-C6 presents broad spectral response from the UV region to about 560 nm52 (Figure 2), which may be used to fill up the weak absorption regions at both sides of the Soret bands of XW36-XW39.34

R2

C6H13O

R1

RESULTS AND DISCUSSION

H COOH

Zn

N

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R2

O

CDCA

H OC6H13

OC6H13

XW36

H OC6H13 OC6H13

CH3

XW37

C6H13 N

S

OC6H13

XW38

C6H13O

H

XW39

C6H13O

CH3

NC COOH

PT-C6

Figure 1. Molecular structures of XW36-XW39, coadsorbent CDCA, cosensitizer PT-C6,52 and previously reported molecule XW10.46

On the other hand, chenodeoxycholic acid (CDCA) (Figure 1) was an effective coadsorbent for suppressing dye aggregation.28,42,46 In spite of many successful examples of separately used coadsorbents and co-sensitizers, reports on simultaneous use of these two components are rather rare.37 Herein, PT-C6 was selected as a co-sensitizer owing to its good photovoltaic performance both as an individual dye52 and as a co-sensitizer,34 and its size is similar to that of CDCA, which may adsorb into the voids between the porphyrin dyes like CDCA. Consistent with our expectation, XW36-XW39 exhibit excellent performance upon synergetic coadsorption and cosensitization. It is noteworthy that a highest efficiency of 11.70% was achieved for XW36 with simultaneously enhanced photovoltage and photocurrent.

Figure 2. Absorption spectra of XW10, PT-C6 and XW36-XW39 in THF. Table 1. Absorption and Emission Data for the Porphyrin Dyes in THF Dye

Absorption λmax [nm]

Emission

(ε [103 M-1 cm-1])

λmax [nm]a

XW10

459 (310.7), 590 (11.6), 665 (80.8)

678

XW36

453 (222.1), 590 (8.9), 668 (63.0)

685

XW37

454 (244.8), 590 (10.1), 669 (65.8)

686

XW38

458 (269.9), 590 (10.5), 666 (78.4)

684

XW39

459 (268.6), 590 (10.5), 668 (74.1)

685

Excitation wavelength: 453 nm (XW36), 454 nm (XW37), 458 nm (XW38), 459 nm (XW39). a

Furthermore, broadened absorption bands were observed on TiO2 films with respect to the corresponding solution spectra (Figure S1), which may result from the H-aggregation on TiO2 films caused by the dyes themselves,34,54 and this character is also beneficial for light harvesting.46,53

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Electrochemical Properties The energy level and whether the electron transfer processes was feasible of the DSSCs can be evaluated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements (Figure S3). The estimated ground state oxidation potentials (Eox) of XW36–XW39 are 0.78 V, 0.78 V, 0.81 V and 0.80 V, respectively, vs. normal hydrogen electrode (NHE) (Figure 3, Table 2). The Eox of all these porphyrin dyes are more positive than the energy level of I-/I3- electrolyte (0.4 V), indicative of enough driving force for regenerating the oxidized dyes by I-/I3- electrolyte. And the E*0-0 of dyes XW36XW39 were obtained to be -1.05 V, -1.05 V, -1.03 V and -1.03 V, respectively. And there were also enough driving forces for injecting electron from XW36-XW39 into the conduction band edge (ECB) of TiO2 (-0.5 V) (Figure 3).55-57

anti-aggregation. For XW36 and XW37, they have wrapped structures in the presence of two hexyloxy chains, and thus they exhibit even larger  values around 85, and because of the extra steric hindrance associated with the wrapped structures, the angles between the phenothiazine moieties and the porphyrin macrocycles in XW36 and XW37 are larger than those in XW38 and XW39 (Table S1, Figure S5), which also may be favourable for suppressing dye aggregation but may unfavourably impede the electron transfer process (vide infra). Photovoltaic Performance of the DSSCs The novel porphyrin dyes XW36-XW39 were adsorbed onto the titania film as the photoanode for fabricating DSSCs. The photovoltaic parameters were shown in Table 3. Figure 4a was the photocurrent-voltage curves and Figure 4b showed the incident photon-to-current conversion efficiency (IPCE) action spectra. The achieved efficiencies lied in the range of 9.05%10.00%, higher than XW10 (8.60%), which was in accord with the synergetically enhanced Jsc and Voc.

Figure 3. Schematic energy level of XW36-XW39. Table 2. Electrochemical Data of XW36-XW39 Adsorbed on TiO2 Films Dye XW36

E

a *

Eox / V (vs. NHE) 0.78

E0-0 / V 1.83

E*0-0 / V (vs. NHE)a -1.05

XW37

0.78

1.83

-1.05

XW38

0.81

1.84

-1.03

XW39

0.80

1.83

-1.03

0-0

= Eox - E0–0.

Theoretical Calculations Density functional theory (DFT) calculations and timedependent density functional theory (TDDFT) calculations can help us gain further understanding of the electron distribution on the molecular structures in the frontier molecular orbitals of these porphyrin dyes, thus, we use the Gaussian 09 program package to perform theoretical calculations.58-61 According to Figure S4, the donor units and the porphyrin macrocycles are predominantly distributed by the HOMO orbitals, while the acceptor units and the porphyrin macrocycles are mainly distributed by LUMO orbitals, which is beneficial for redistributing the electron from HOMO orbitals to LUMO orbitals, which means the electron can effectively transfer from phenothiazine donor and to benzoic acid acceptor followed by injecting into ECB of TiO2. From the optimized molecular structures of XW38 and XW39 (Figure S5, Table S1), the dihedral angles  between the N-phenyl groups and the phenothiazine units exhibit large values of 82, favourable for

Figure 4. (a) J-V curves and (b) IPCE action spectra of DSSCs based on XW36-XW39.

XW36 exhibits a lower Jsc (18.04 mA·cm-2) and a higher Voc (0.742 V), compared with XW38 (18.40 mA·cm-2 and 0.731 V, respectively). These observations may be rationalized by the difference in the dihedral angles β between the phenothiazine units and the porphyrin framework (Figure S5, Table S1), observed in the optimized structures obtained from DFT calculations. Compared with the β angle of 0.9 in XW38, a larger β angle of 7.4 is observed in XW36, which may be induced by the steric effect related to the wrapped donor in XW36, and the more distorted structure may hinder the electron transfer from the phenothiazine group to the porphyrin



framework, resulting in a lower Jsc, but the more distorted structure of XW36 is beneficial for anti-aggregation, and thus a higher Voc was obtained, relative to that of XW38. Because of the paradoxical effects of the wrapped structural character on Jsc and Voc, an efficiency of 9.05% was obtained for XW36, which is lower than that of 9.51% obtained for XW38. A similar trend was observed for XW37 and XW39, with the efficiencies of 9.58% and 10.0%, respectively. Besides, the methyl-substituted benzoic acid acceptor is favourable for anti-aggregation, and thus enhances the Voc values, as can be evidenced by the higher photovoltages achieved for XW37 and XW39, compared with those of XW36 and XW38, respectively. We also investigated the dye-loading amounts on the photoanode (Table S2), which indicated that XW37 and XW39 have higher dye loading amounts, compared with XW36 and XW38, respectively, which may partly rationalize the observation that XW37 and XW39 exhibit higher Jsc values than the respective values of XW36 and XW38. Interestingly, the dye loading amounts on TiO2 for XW37 and XW39 are slightly higher than the corresponding values for XW36 and XW38, although they contain a bulkier methyl group on the acceptor moiety. This observation may be rationalized as following: 1) the size of the methyl group is much smaller than the alkoxyl chains in these porphyrin dyes, and thus the loading amounts are not decreased by the methy group; 2) the methyl substitution may enhance the steric hindrance in the acceptor part, and thus dyes XW37 and XW39 may tend to anchor on the TiO2 film in a more vertical manner, and thus more dyes may be accommodated on the TiO2 surface. On the other hand, the methyl group also can suppress dye aggregation, and thus simultaneously improve the Jsc and Voc values, resulting in obviously enhanced efficiencies in XW37 and XW39, compared with XW36 and XW38, respectively.

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lifetime (τ) as well.62 Thus, we carried out EIS measurements. The chemical capacitance and electron lifetime were achieved by means of the EIS spectra. As shown in Figure 5a, Cμ values of XW36-XW39 based DSSCs were smaller than Cμ of XW10, indicative of negative shifts of TiO2 ECB, which was favorable for enhancing the photovoltages. And the Cμ values of XW36XW39 lie roughly in the sequence of XW37 < XW36 ≈ XW39 < XW38, according with the order of the decreasing Voc values. In addition, the electron lifetime of XW36-XW39 were longer than the τ of XW10 (Figure 5b), which may be owing to the better ability of anti-aggregation of the dyes by the alkoxy-substituted phenyl and methyl groups in XW36XW39. And in the voltage bias range of 0.65-0.85 V, the order of τ is consistent with that of Voc, ranking as XW38 < XW39 ≈ XW36 < XW37.63 Consistent with the sequences of Cμ and τ, the highest Voc was achieved for XW37, and the lowest Voc was obtained for XW38 among these porphyrin dyes. These results indicate that the electron quasi-Fermi-level in TiO2 and the charge recombination process determined the Voc values of DSSCs.

Table 3. Photovoltaic Parameters of the Porphyrin DSSCs under AM1.5 Illumination (Power, 100 mW·cm-2).a Dye

JSC

VOC

[mA cm-2]

[mV]

FF [%]

PCE [%]

XW36

18.04±0.05

742±2

68.16±0.43

9.05±0.03

XW37

18.77±0.23

749±4

68.14±0.16

9.58±0.01

XW38

18.40±0.33

731±2

70.22±0.23

9.51±0.12

XW39

19.36±0.15

741±3

69.83±0.18

10.0±0.03

XW1046

17.90±0.04

711±5

68.40±0.50

8.60±0.06

The photovoltaic data are the averaged values obtained from three cells.

a

Electrochemical Impedance Spectroscopy Electrochemical impedance spectroscopy (EIS) data can help us understand these DSSCs’ photovoltaic behavior, because the Voc of the DSSC device is related to the electron quasi-Fermilevel (Ef) of TiO2, which is in connection with chemical capacitance (Cμ). In addition, Voc is in connection with electron

Figure 5. Plots of (a) Cμ and (b) τ vs. potential bias for DSSCs based on XW10 and XW36-XW39. The insets shows the partially enlarged figures at the bias voltages of 0.70 and 0.75 V, respectively, close to the Voc values.

Coadsorption and Cosensitization CDCA is commonly used to suppress dye aggregation by minimizing the TiO2 surface area in direct contact with the electrolyte, which is beneficial for improving Voc values.64,65 Although the phenothiazine donor is nonplanar with a butterfly


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conformation33 and some alkoxy groups are introduced into the porphyrin dyes43, which can suppress aggregation to some extent, dye aggregation still occurs judging from the differences between the absorption spectra on TiO2 films and in THF. Thus, we dipped the TiO2 electrode in solutions containing the individual porphyrin dyes and CDCA, and then fabricated the DSSCs. The corresponding photovoltaic parameters are shown in Table S4. As expected, the coadsorption and cosensitization system performed better in Voc than the individual porphyrin dyes, and the Jsc values were slightly enhanced at low CDCA concentrations, but decreased with the increasing concentrations of CDCA, which may be related to the competitive adsorption of CDCA. Finally, the efficiencies were enhanced to 9.68%~10.3% with 2 mM CDCA.

cosensitization after the photoanodes were immersed in 0.2 mM porphyrin dye and 2 mM CDCA. Consistent with our expectation, the Jsc and Voc for the porphyrin dyes were effectively enhanced upon simultaneous coadsorption with 2 mM CDCA and cosensitization with PT-C6, but the Jsc and PCE values of all the porphyrin dyes were decreased when additional PT-C6 was added, which may be rationalized by the competitive adsorption between PT-C6 and the porphyrin dyes as well as the aggregation of PT-C6 itself. Finally, the optimal concentration for PT-C6 was found out to be 0.3 mM. The photovoltaic behavior and the corresponding photovoltaic parameters were exhibited in Figure 6, Figure S6, Table 4 and Table S5. Table 4. Photovoltaic Parameters of the Porphyrin Based DSSCs upon Synergetic Coadsorption and Cosensitization. JSC [mA cm-2]

VOC [mV]

FF [%]

PCE [%]

XW36+CDCAa

18.12±0.35

750±2

71.23±0.43

9.68±0.10

XW37+CDCA

a

18.66±0.25

759±1

72.23±0.87

10.2±0.03

XW38+CDCAa

18.87±0.16

745±2

72.98±0.23

10.3±0.14

XW39+CDCAa

18.86±0.32

753±2

71.93±0.44

10.2±0.08

PT-C6+CDCAb

14.87±0.12

781±1

74.29±0.17

8.63±0.04

XW36+CDCA+ PTC6c

20.89±0.17

765±3

73.22±0.05

11.7±0.10

XW37+CDCA+ PTC6c

20.12±0.35

768±2

72.41±0.25

11.2±0.21

XW38+CDCA+ PTC6c

20.23±0.05

759±1

71.92±0.10

11.0±0.04

XW39+CDCA+ PTC6c

19.83±0.10

763±2

72.47±0.14

11.0±0.05

XW4+C144

20.15

736

71.0

10.5

XW11+WS-546

20.33

760

74.4

11.5

Dye

The TiO2 photoanode was immersed in 2 mM CDCA together with porphyrin dye solution in toluene / ethanol (v/v, 1/4) for 10 h, rinsed with ethanol; bthe TiO2 photoanode was immersed in 2 mM CDCA together with 0.3 mM PT-C6 dye solution in CH3CN / t-BuOH (v/v, 1/1) for 2.5 h, rinsed with ethanol; cthe TiO2 photoanode was immersed in 2 mM of CDCA together with 0.2 mM porphyrin dye solution in toluene / ethanol (v/v, 1/4) for 10 h, rinsed with ethanol, and then dipped in a solution of PT-C6 (0.3 mM) in CH3CN / t-BuOH (v/v, 1/1) for 2.5 h, rinsed with ethanol. a

Figure 6. (a) J-V curves and (b) IPCE action spectra of DSSCs based on XW36-XW39+CDCA+PT-C6.

Although CDCA can be used to enhance the Voc values, it does not exhibit light response and photocurrent generation. Hence, the efficiencies of the porphyrin dyes were only slightly enhanced through coadsorption with 2 mM CDCA (Table S4). Therefore, it’s necessary and feasible to fill up the weak absorption of porphyrin dyes by a co-sensitizer with a synergetic improvement in Voc and Jsc. In many cosensitization systems, a co-sensitizer is used to fill up the absorption valley in the range of 500600 nm for porphyrin dyes.42-46 In fact, the absorptions in 350400 nm of porphyrin dyes are also weak. Herein, we employ PT-C6 as a co-sensitizer, considering its broad absorption in 370560 nm,34 which can effectively fill up the weak absorption around 450 nm of these porphyrin dyes. Thus, various concentrations of PT-C6 were tried for

It is obvious that all of the porphyrin dyes exhibit obviously enhanced Jsc values of 19.83-20.89 mA·cm-2 upon simultaneous coadsorption and cosensitization. The achieved Jsc values are 9% ~ 15% higher than those integrated from the IPCE spectra (Figure S7). This is a common phenomenon in porphyrin dyes based DSSCs,34,40 which can be rationalized by the following three factors: 1) FTO glasses may reflect and absorb the sunlight, which may induce about 10% to 15% loss in IPCE spectrum;66,67 2) we use full irradiation in J-V test, which provides more electrons than in IPCE measurement, and the charge transport / collection is more efficient in J-V test;34 3) an increase in Jsc associated with the elevated temperature



due to the heat generated by the near-infrared light included in the full irradiation.34,68 On the other hand, the Voc value of a cosensitization system is usually located between the values of the individual cells.44,45,69 Thus, the co-sensitized DSSCs exhibit Voc values around 760 mV, dramatically higher than the individual XW36-XW39, but lower than 775 mV achieved for PT-C6.52 As a consequence of the simultaneously improved Voc values and Jsc values, obviously enhanced efficiencies of 11.0%11.7% were achieved, with the highest efficiency of 11.7% based on XW36+CDCA+PT-C6. Compared with our previously reported cosensitization systems of XW4+C1 and XW11+ WS-5,44,46 the simultaneously coadsorption and cosensitization system of XW36+ CDCA+ PT-C6 exhibits both higher Voc and higher Jsc, which may be rationalized by the fact that CDCA can effectively suppress dye aggregation and PTC6 can effectively enhance the IPCE spectra. It is interesting that the individual XW36 dye exhibits the lowest efficiency among the porphyrin dyes, but a highest efficiency was obtained for it upon synergetic coadsorption and cosensitization. This observation may be rationalized by the fact that the lowest dye loading amount was observed for XW36 among all of these porphyrin dyes, with the largest vacancy area left on the TiO2 surface available for the adsorption of co-sensitizer PT-C6 (Table S3), and thus a highest Jsc was achieved upon cosensitization, which can be clearly evidenced by the observation that the cosensitized cells based on XW36 exhibit the highest IPCE plateau in 420530 nm (Figure 6), mainly contributed by the absorption peak of PT-C6. Photostability

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parameters for the efficient cells based on XW36-XW39 + 2 mM CDCA+ 0.3 mM PT-C6 over 500 h. After soaking in visible-light for 500 h, the PCEs of the DSSC devices remained at 83%-85% of the initial values, which indicated good photostability of the DSSC devices.13,43 CONCLUSIONS In this work, four novel porphyrin dyes, XW36XW39, were designed and synthesized for fabricating efficient DSSCs, using an N-phenyl substituted phenothiazine donor. For XW36 and XW37, an N-phenyl group is wrapped with two ortho-alkoxy chains. In contrast, it is substituted with a para-alkoxy group in XW38 and XW39. The phenothiazine-wrapping in XW36 and XW37 induce more serious distortion, which is favourable for suppressing the dye aggregation, but unfavourable for the electron transfer from the phenothiazine group to porphyrin framework. Thus, individual porphyrin dyes XW36 and XW37 exhibit efficiencies of 9.05% and 9.58%, respectively, lower than those of 9.51% and 10.0% achieved for XW38 and XW39, respectively. On the other hand, the introduction of methyl groups into the benzoic acid acceptor unit is favourable for antiaggregation, and finally enhances the Voc and efficiencies. Thus, higher efficiencies were achieved for XW37 and XW39, compared with XW36 and XW38, respectively. Interestingly, although the individual XW36 dye exhibits the lowest efficiency among all of these porphyrin dyes, a highest efficiency of 11.7% was achieved for XW36 upon synergetic coadsorption with CDCA and cosensitization with PT-C6 because of simultaneously improved Jsc and Voc. These results may be rationalized by the fact that XW36 exhibits the lowest dye loading amount among all of these porphyrin dyes, with the largest vacancy area left on the TiO2 surface available for cosensitizer PT-C6, resulting in a highest Jsc. The high efficiency of 11.7% is higher than that achieved for the cosensitization of XW10 with WS5, and it is among the highest efficiencies for DSSCs based on the traditional iodine electrolyte.5,6,46 These results provide an effective strategy for developing efficient DSSCs by the synergetic coadsorption and cosensitization of optimized porphyrin sensitizers containing a bis(ortho-alkoxy) wrapped phenyl group on the phenothiazine donor and/or methyl groups on the benzoic acid acceptor unit.

EXPERIMENTAL SECTION

Figure 7. Plots of photovoltaic parameters (Jsc, Voc, FF, PCE) as a function of illumination time for DSSC devices based on XW36-XW39 + 2 mM CDCA+ 0.3 mM PT-C6.

Photostability is also an important parameter for practical applications of DSSCs.70 Herein, we recorded the photovoltaic

1H NMR and 13C NMR spectra were obtained using a Bruker AM 400 or 500 MHz spectrometer at 298 K using tetramethylsilane (TMS) as the internal standard. Matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDITOF-MS) was measured on a Shimadzu-Kratos model Axima CFR+ mass spectrometer using dithranol as the matrix. UV-vis absorption spectra were recorded on a Varian Cary 100 spectrophotometer and fluorescence spectra were recorded on a Varian Cray Eclipse fluorescence spectrophotometer. The cyclic voltammograms of the dyes were obtained in acetonitrile with a Versastat II electrochemical workstation (Princeton Applied Research). The differential pulse voltammetry of the dyes were


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obtained in acetonitrile with a CHI600E electrochemical workstation. Eox were measured in acetonitrile using 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the electrolyte (working electrode: FTO / TiO2 / dye; reference electrode: SCE; calibrated with ferrocene / ferrocenium (Fc / Fc+) as an external reference. Counter electrode: Pt). E0-0 was calculated from the wavelength at the intersection (λinter) of normalized absorption and emission spectra using the equation E0-0 = 1240 / λinter. The E*0-0 was calculated from the equation of E*0-0 = Eox - E0–0. Photovoltaic measurements were performed by employing an AM 1.5 solar simulator equipped with a 300 W xenon lamp (model no. 91160, Oriel). The power of the simulated light was calibrated to 100 mW cm-2 using a Newport Oriel PV reference cell system (model 91150 V). J-V curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with a model 2400 source meter (Keithley Instruments, Inc.). The voltage step and delay time of the photocurrent were 10 mV and 40 ms, respectively. Action spectra of the incident monochromatic photonto-electron conversion efficiencies (IPCE) for the solar cells were obtained with a Newport-74125 system (Newport Instruments). The intensity of monochromatic light was measured with a Si detector (Newport-71640). The electrochemical impedance spectroscopy (EIS) measurements of all the DSSCs were performed using a Zahner IM6e Impedance Analyzer (ZAHNER-elektrik GmbH & Co. KG, Kronach, Germany), with the frequency range of 0.1 Hz-100 kHz and the alternative signal of 10 mV.

ASSOCIATED CONTENT Supporting Information Available: Detail of synthesis, characterization data for the compounds and additional figures were included in the Supporting Information. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially supported by the NSFC (21472047, 21772041, 201702062, 21811530005), the Program for Professor of Special Appointment (Eastern Scholar; GZ2016006) at Shanghai Institutions of Higher Learning, Shanghai Pujiang Program (17PJ1401700), and the Fundamental Research Funds for the Central Universities (WK1616004, 222201717003, 222201714013).

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