Efficient Solar Cells Based on Porphyrin Dyes with Flexible Chains

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Efficient Solar Cells Based on Porphyrin Dyes with Flexible Chains Attached to the Auxiliary Benzothiadiazole Acceptor: Suppression of Dye Aggregation and the Effect of Distortion Guosheng Yang, Yunyu Tang, Xin Li, Hans Ågren, and Yongshu Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12066 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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ACS Applied Materials & Interfaces

Efficient Solar Cells Based on Porphyrin Dyes with Flexible Chains Attached to the Auxiliary Benzothiadiazole Acceptor: Suppression of Dye Aggregation and the Effect of Distortion Guosheng Yang†, Yunyu Tang‡, Xin Li§, Hans Ågren§, 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

‡

East China Sea Fisheries Research Institute, Chinese Fisheries Academy of Fishery Science, 300 Jungong Road, Shanghai 200090, P. R. China

§

Division of Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal Institute

of Technology, SE-10691 Stockholm, Sweden KEYWORDS: dye sensitized solar cells, porphyrin, benzothiadiazole, dye aggregation, molecular distortion ABSTRACT: D−π−A type porphyrin dyes have been widely used for fabricating efficient dye-sensitized solar cells (DSSCs) owing to their strong absorption in the visible region and the ease of modifying their chemical structures and photovoltaic behavior. Based on our previously reported efficient porphyrin dye XW11 which contains a phenothiazine-based electron donor, a π-extending ethynylene unit, and an auxiliary benzothiadiazole acceptor, we herein report the syntheses of novel porphyrin dyes XW26-XW28 by introducing one or two alkyl/alkoxy chains into the auxiliary acceptor. The introduced chains can effectively suppress the dye aggregation. As a result, XW26-XW28 show excellent photovoltages of 700, 701 and 711 mV, respectively, obviously higher than 645 mV obtained for XW11. Nevertheless, the optimized structures of XW26 and XW27 exhibit severe distortion, showing large dihedral angles of 57.2°, and 44.0°, respectively, between the benzothiadiazole and the benzoic acid units, resulting from the steric hindrance between the benzoic acid unit and the neighboring alkyl/alkoxy chain on the benzothiadiazole unit, and thus blue-shifted absorption, decreased photocurrents and low efficiencies of 5.19% and 6.42% were observed for XW26 and XW27, respectively. Interestingly, XW26 exhibits more blue-shifted absorption spectrum relative to XW27, indicating that the steric hindrance of the alkyl/alkoxy chains has a more pronounced effect than the electronic effect. Different from XW26 and XW27, XW28 contains only one alkyl chain neighboring to the ethynylene unit, which does not induce obvious steric hindrance with the benzoic acid unit, and thus the distortion of the molecule is not seriously aggravated as compared with XW11. Hence, its absorption spectrum and photocurrent are similar to those of XW11. As a result, a higher efficiency of 9.12% was achieved for XW28 because of its suppressed dye aggregation and higher photovoltage. It is noteworthy that a high efficiency of 10.14% was successfully achieved for XW28 upon coadsorption with CDCA, which is also higher than the corresponding efficiency obtained for XW11. These results provide a novel approach for developing efficient porphyrin dyes by introducing chains into the suitable position of the auxiliary benzothiadiazolyl moiety to suppress the dye aggregation, without seriously aggravating the distortion of the dye molecules.

INTRODUCTION Dye-sensitized solar cells (DSSCs) have attracted intensive attention because of their ease of fabrication, relatively high photovoltaic efficiency and aesthetically vivid color and transparency.1,2 A typical DSSC consists of a photoanode, sensitizer, electrolytes and a counter electrode.3-5 Among these components, the sensitizer is used to absorb photons and generate electrons, and thus the development of novel and efficient sensitizers is one of the most

direct and effective ways to develop efficient DSSCs.6-8 On the basis of the initial research on ruthenium complex sensitizers,9-12 organic sensitizers with the donor−π−acceptor (D−π−A) configuration have been demonstrated to be promising DSSC dyes.13-18 In recent years, D−π−A type porphyrin derivatives have attracted great interests as DSSC sensitizers because of their strong absorption in the visible region and the ease of modifying their chemical structures and photovol-

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taic behavior.19-22 Hence, numerous porphyrin dyes have been reported for fabricating efficient DSSCs.23-34 Nevertheless, typical porphyrin dyes suffer from the disadvantages of poor light harvesting capability beyond 700 nm and unfavorable severe dye aggregation associated with the extended π-conjugation framework, which may be unfavorable for the photocurrent and photovoltage, respectively.4 To address these problems, additional ethynylene bridges may be introduced to extend the π framework and thus improve the absorption in the NIR region.35-38 On the other hand, flexible substituents have been introduced to suppress the dye aggregation.39,40 In this respect, we have reported a series of porphyrin dyes. 41-49 For example, XW11 was designed by introducing a phenothiazine moiety as the electron donor, an ethynylene unit for extending the π-framework, and a benzothiadiazole moiety as the auxiliary acceptor, which exhibits absorption up to 730 nm and a high efficiency of 11.5% by utilizing a cosensitization approach.48 In spite of the successful results obtained for the cosensitized systems, the individual XW11 dye exhibits a moderate efficiency of 7.8%. In this work, we are aimed to optimize the porphyrin dye structure based on XW11 to further improve the efficiency of the individual porphyrin dye by introducing alkyl or alkoxy chains into the benzothiadiazolyl moiety. Thus, we have successfully designed and synthesized three novel porphyrin dyes XW26-XW28 (Chart 1). The introduced chains are favorable for suppressing the dye aggregation, resulting in enhanced photovoltages of 700 − 711 mV relative to that of 645 mV obtained for XW11.48 However, the introduced chains unfavorably induce more severe distortion between the benzothiadiazole and the benzoic acid units, resulting in blueshifted absorption and decreased photocurrents. As a result of the contradictory effects on the photovoltage and photocurrent, XW26-XW28 exhibit different cell efficiencies of 5.19%, 6.42%, and 9.12%, respectively. It is noteworthy that XW28 contains an alkyl group neighboring to the ethynylene unit, which does not obviously affect the planarity at the acceptor part. Hence, XW28 exhibits the highest efficiency among all these dyes. After coadsorption with CDCA, a high efficiency of 10.14% was achieved for XW28. This work provides a novel approach for optimizing the porphyrin dyes by introducing chains into the suitable position of the auxiliary benzothiadiazolyl acceptor moiety to suppress the dye aggregation, without seriously aggravating the distortion of the molecules.

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Chart 1. Molecular structures of the porphyrin dyes. RESULTS AND DISCUSSION Syntheses and characterization Scheme 1. Synthetic routes for XW26 − XW28. C5H11 Br

i

Br

O2N

Br

O2N

Br

O2N

ii

iii

O2N

H2N

C7H15

H2N

C7H15

iv

N

C7H15

N

C7H15

S

C5H11

1-1 N

S

v R1

1-3

1-2 N

N

S

N

N X

X R1

R2

1a-1c

vi

2a-2c

N

R1

3a-3c

N Br

OC12H25

S

viii

R2

OC12H25 N

N

x

COOMe

D

N

A

Zn N

N

R2 C12H25O

OC12H25

OC12H25

XW26-XW28

6a-6c

N

S

C7H15 N

C6H13 N

1a, 2a, 3a, 4a, 5a, 6a: R1 = R2 = C7H15; 2a, 3a: X = I 1b, 2b, 3b, 4b, 5b, 6b: R1 = R2 = OC7H15; 2b, 3b: X = Br 1c, 2c, 3c, 4c, 5c, 6c: R1 = C6H13 , R2 = H; 2c, 3c : X = I

R2

5a-5c

N

C12H25O

D-Zn-Br

COOMe R1

N

Zn N

N

C12H25O N

N

D

N

S

N

OC12H25

R1 C12H25O

N

4a-4c

C12H25O

ix

Zn N

S

COOMe

R2

OC12H25

D

N COOMe vii TMS

R1

1a

1-4

N

X

R2

C12H25O

S

N

COOH

XW26

COOH

XW27

COOH

XW28

C7H15 N

OC7H15

C7H15O S

C6H13O

S

N

S

N

C6H13

D

A

Reaction conditions: (i) 1-1, H2SO4, H2SO4•SO3, HNO3; (ii) 1-2, 1-Heptyne, Pd(PPh3)2Cl2, CuI, Et3N; (iii) 1-3, 10% Pd/C, H2; (iv) 1-4, SOCl2, Et3N, DCM. (v) KIO3, I2, H2SO4, AcOH, H2O for 1a and 1c; Br2, AcOH, DCM for 1b; (vi) 4(methoxycarbonyl)phenylboronic acid, Pd(PPh3)4, CsF, DME for 2a and 2c; 4-(methoxycarbonyl)phenylboronic acid, Pd(PPh3)4, K2CO3, THF for 2b; (vii) TMSA, Pd(PPh3)2Cl2, CuI, Et3N, THF; (viii) KOH, MeOH, THF; (ix) 5a, 5b, or 5c, Pd2(dba)3, AsPh3, THF, Et3N; (x) LiOH·H2O, THF, H2O. The syntheses of the porphyrin dyes are illustrated in Scheme 1. The key steps for the synthetic routes are the syntheses of the properly substituted acceptor units. We firstly synthesized the alkyl and alkoxy substituted benzothiadiazoles, which were subsequently subjected to bromination (for 2b) or iodation(for 2a, 2c) and Sonogashira

2


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coupling reactions to afford the desired acceptor moieties 5a−5c. Then, the acceptors were introduced to the porphyrin frameworks through Sonogashira coupling reactions. Finally, the target dyes were successfully obtained through hydrolysis of the esters. The products were fully characterized with 1H NMR, 13C NMR and HRMS (See the Supporting Information).

no blue-shift as compared with XW11, indicating that the alkyl group neighboring the ethynylene unit does not induce obvious distortion of the molecule, which is consistent with the optimized molecular structure (vide infra).

Optical Properties The UV-vis absorption spectra of the dyes in THF are shown in Figure 1, with the corresponding absorption and emission data listed in Table 1. Table 1. Absorption and emission data for the dyes in THF Dye

a

Absorption λmax [nm] (ε [103 M-1 cm-1])

Emission λmax [nm]a

XW11

465 (201.6), 622 (14.6), 683 (103.4)

709

XW26

465 (269.6), 620 (16.9), 675 (116.2)

691

XW27

465 (275.8), 621 (19.3), 678 (127.4)

696

XW28

465 (202.9), 623 (16.4), 683 (111.3)

705

Figure 1. Absorption spectra of the porphyrin dyes in THF.

Excitation wavelength: 465 nm.

As expected, the absorption spectra of the porphyrin dyes exhibit a typical intense Soret band within 400−500 nm and less intense Q bands in a range of 550−740 nm (Figure 1, Table 1). Compared with XW11, XW26 and XW27 exhibit a slightly blue-shifted Q band (by 8 and 5 nm, respectively), which may be ascribed to the distortion between the benzothiadiazolyl moiety and the benzoic acid unit caused by the presence of the attached chains (vide infra). Compared with the two alkyl chains attached to the auxiliary benzothiadiazole acceptor in XW26, XW27 contains two strongly electron-donating alkoxy chains on the benzothiadiazole unit. It can be anticipated that the intramolecular charge transfer (ICT) effect for XW27 may be weaker than XW26, which will result in blue-shifted absorption spectra. This inference is contradictory to the observed data, which exhibit a slightly redshifted absorption spectrum for XW27 relative to XW26. This disagreement may be ascribed to the relatively weaker steric hindrance of the alkoxy chain as compared with the alkyl chain, which result in less seriously distorted structure of XW27 as compared with XW26 (vide infra). The spectral characters observed for XW26 and XW27 indicate that the steric hindrance induced by the alkyl/alkoxy chains has a more pronounced effect on the absorption spectrum than the electronic effect.50 In contrast to the blue-shifted absorption spectra for XW26 and XW27, the Q band for XW28 exhibits almost

Figure 2. Absorption spectra of XW11, XW26-XW28 adsorbed on transparent TiO2 films (3 μm). Figure 2 shows the absorption spectra of XW11, XW26XW28 on TiO2 films. All the spectra are broadened after anchoring on the transparent TiO2 films, which will be favorable for sunlight harvesting. It is noteworthy that the spectra of XW26-XW28 on the TiO2 films are blue-shifted by 15-18 nm relative to the corresponding solution spectra, which may be induced by the H-aggregation as well as deprotonation on the TiO2 films.51 In contrast, XW11 displays a more pronounced blue-shift of 25 nm.48 These results indicate that the dye aggregation effect for XW26-

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XW28 can be obviously suppressed by introducing the alkyl or alkoxy chains, as compared with XW11. Electrochemical Properties Cyclic voltammetry was used to evaluate the energy level distributions and the feasibility of the electron transfer processes. The ground state oxidation potentials (Eox) of XW26− −XW28 were estimated to be 0.84, 0.85, and 0.86 V (Figure 3, Table 2), respectively, versus the normal hydrogen electrode (NHE). Their Eox values are obviously more positive than the iodide/triiodide redox couple (∼0.4 V), indicating that the oxidized dyes can be efficiently regenerated by the electrolyte. The energy gaps of XW26-XW28 were estimated to be 1.85 V, 1.83 V and 1.81 V, respectively, and the LUMO levels of dyes XW26-XW28 were thus estimated to be -1.01, - 0.98, and -0.95, respectively, more negative than the conduction band edge of TiO2 (-0.5 V), indicative of enough driving forces for the electron injection processes from the excited dyes to the conduction band of TiO2. From these values, we can attain a schematic energy-level diagram for the HOMOs and LUMOs of XW26−XW28 (Figure 4).52 Consistent with the absorption spectra, the HOMO-LUMO gaps of XW26XW28 successively decrease, which can be ascribed to the successively improved planarity of the dyes (vide infra).

Figure 3. Cyclic voltammetry curves of XW26-XW28 absorbed on the TiO2 films.

Table 2. Electrochemical data of XW26-XW28 adsorbed on the TiO2 films. Dye XW26

HOMOa /V (vs. NHE) 0.84

E0-0b/V 1.85

LUMOc /V (vs. NHE) -1.01

XW27

0.85

1.83

-0.98

XW28

0.86

1.81

-0.95

a

HOMO levels 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). b E0-0 was calculated from the wavelength at the intersection (λinter) of normalized absorption and emission spectra using the equation E0-0 = 1240/λinter. c The LUMO was calculated from the equation of LUMO = HOMO - E0–0.

Figure 4. Schematic energy-level diagrams of XW26XW28. DFT calculations Suitable spatial distributions of the frontier orbitals are essential for enabling the electron transfer processes for DSSCs. To gain further insight into the effect of varying the molecular structures on electron distribution in the frontier molecular orbitals, we employed density functional theory(DFT) calculations, and time-dependent density functional theory (TDDFT) calculations using the Gaussian09 program package.53−56 According to the corresponding molecular orbital profiles (Figure 5), the HOMO orbitals are predominantly distributed over the donor unit and the porphyrin framework, while the LUMO orbitals are mainly distributed over the porphyrin framework and the acceptor unit. These characters can result in redistribution of the electron from the HOMO to the LUMO and thus enable the electron transfer from the donor to the anchoring group, followed by electron injection from the LUMO to the conduction band of TiO2.

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In the optimized molecular structures of XW26-XW28, the dihedral angles between the benzothiadiazole units and the benzoic acid units are quite different. Compared with XW11, both XW26 and XW27 show obviously larger dihedral angles of 57.2° and 44.0°, respectively (Figure 6). In contrast, the corresponding angle for XW28 has a much smaller value of 36°, which is similar to that for XW11. These results indicate that the alkyl/alkoxy chain neighboring to the benzoic acid unit tends to induce more severe distortion, whereas, the chain neighboring to the ethynylene unit has little effect on the planarity. In addition, the alkyl chain has more pronounced steric hindrance than the alkoxy chain. Consistently, the most severely distorted dye XW26 exhibits the most blue-shifted absorption spectrum and the largest HOMO-LUMO gap (vide supra).

Table 3. Photovoltaic parameters of the porphyrin dye sensitized solar cells under AM1.5 illumination (power, 100 mW·cm-2). Dye

VOC / mV

JSC / mA cm-2

XW26

700±1

10.51±0.30

71.90±0.20 5.19±0.10

XW27

701±2

12.79±0.20

71.63±0.30 6.42±0.20

XW28

711±1

17.93±0.20

71.54±0.20 9.12±0.10

XW26 + 2 mM CDCA

708±1

11.37±0.20

69.13±0.20 5.57±0.10

XW27 + 2 mM CDCA

710±1

14.08±0.20

72.26±0.20 7.17±0.10

XW28 + 2 mM CDCA

715±2

19.38±0.20

72.96±0.20 10.14±0.10

XW11a

645±3

18.83±0.28

64.20±0.30 7.80±0.10

XW11 + 2 mM CDCAa

727±2

18.26±0.27

70.10±0.40 9.30±0.06

FF /%

PCE /%

The photovoltaic data are the averaged values obtained from three cells. a The data were reported in our previous work.48

Figure 5. Frontier molecular orbital profiles of XW26XW28 calculated by DFT.

Figure 6. The dihedral angles (in degrees) in the optimized porphyrin dyes. Photovoltaic performance of the DSSCs On the basis of the above-mentioned results, the dyes were used for fabrication of DSSCs based on the I2/I3- electrolyte, and the corresponding photovoltaic parameters are summarized in Table 3. The photocurrent−voltage curves and the incident photon-to-current conversion efficiency (IPCE) action spectra are shown in Figure 7.

Figure 7. (a) J-V characteristics and (b) IPCE action spectra of DSSCs based on XW26-XW28.

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It is noteworthy that XW28 exhibits the most redshifted IPCE up to 850 nm and a highest IPCE plateau up to 76% as compared with XW26 and XW27 (XW26

Efficient Solar Cells Based on Porphyrin Dyes with Flexible Chains

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