Effects of the acceptors in triphenylamine-based D–A′–π–A dyes on photophysical, electrochemical, and photovoltaic properties

Journal of Power Sources 246 (2014) 831e839

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Effects of the acceptors in triphenylamine-based DeA0 epeA dyes on photophysical, electrochemical, and photovoltaic properties Ling Wang, Ping Shen*, Zhencai Cao, Xinping Liu, Yuanshuai Huang, Chunyan Liu, Pan Chen, Bin Zhao, Songting Tan College of Chemistry and Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, PR China

h i g h l i g h t s  We synthesized three DeA0 epeA structural dyes (Dye 1e3) to apply in DSSCs.  Results indicate both A0 and A have influence on optoelectronic properties.  Dye 1 with fluorenone as an additional acceptor gave a high PCE up to 5.33%.

g r a p h i c a l a b s t r a c t

CN CN O

CH N

Dye 2 PCE = 1.81%

CH

S

S

π

COOH CN

Dye1

PCE = 5.33%

A

D

NC

CN

Dye 3 PCE = 1.25% A'

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 May 2013 Received in revised form 5 August 2013 Accepted 8 August 2013 Available online 22 August 2013

Three DeA0 epeA structural triphenylamine dyes (Dye 1, Dye 2, and Dye 3) with cyanoacetic acid or malononitrile as an electron acceptor (A) and fluorenone or 9-(dicyanomethylene)fluorene as an additional acceptor (A0 ) have been designed and synthesized. Results revealed that both A and A0 have great influence on the optical, electrochemical and photovoltaic properties. In comparison with Dye 3, the photovoltaic performance of Dye 1 is remarkably improved by replacing 9-(dicyanomethylene)fluorene with fluorenone as the additional acceptor. The power conversion efficiency of dye-sensitized solar cell based on the Dye 1 (5.33%) is four times higher than that of Dye 3 (1.25%). The low efficiency of cell based on Dye 3 is mainly because the delocalization of the excited state is broken between the strong electronwithdrawing additional acceptor (9-(dicyanomethylene)fluorene) unit and cyanoacetic acid, which affects the efficient electron injection from Dye 3 to the conduction band of TiO2. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Acceptor Cyanoacetic acid DeA0 epeA dyes Dye-sensitized solar cells Fluorenone Photovoltaic performance

1. Introduction Due to the advantages of low cost, easy production, flexibility, and transparency relative to conventional crystalline silicon solar cells, dye-sensitized solar cells (DSSCs) have emerged as an important class of photovoltaic devices, since the breakthrough was made by Grätzel and co-workers [1]. As a key part of DSSCs, dyes play a crucial role in * Corresponding author. Tel.: þ86 15801195346; fax: þ86 0731 58292251. E-mail address: [email protected] (P. Shen). 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.08.034

high power conversion efficiencies (PCEs) and have been studied in depth by many researchers. There are two kinds of dyes, namely, metal-complex and metal-free types. Important metal-complex dyes are ruthenium complexes, such as N3/N719 [2,3] and others [4], including C101 [5,6] and CYC-B1 [7], which display PCEs over 11% under AM 1.5 irradiation. In view of the limited availability and environmental issues associated with ruthenium dyes, metal-free organic dyes are considered to be an alternative for use in DSSCs because they have high molar absorption coefficients and can be prepared more easily and economically. Many of metal-free organic

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L. Wang et al. / Journal of Power Sources 246 (2014) 831e839

Here, we present the design and synthesis of three DeA0 epeA dyes, Dye 1e3 (Scheme 1) with different A (2-cyanoacetic acid vs. malononitrile) and A0 (fluorenone vs. 9-(dicyanomethylene)fluorine) aiming of studying the effects of acceptors (A and A0 ) on the optoelectronic properties. For Dye 1, fluorenone is used as an additional acceptor, substituted triphenylamine is used as a donor, hexylthiophene is a p-bridge, and 2-cyanoacetic acid is an acceptor/anchor. To change of the electron-deficient ability of A and A0 , malononitrile was used instead of cyanoacetic acid to obtain Dye 2, and ketone group in fluorenone was replaced with dicyanovinylene to create Dye 3. Once again, our results showed that DeA0 epeA configuration is an effective motif and not only the electron-deficient ability of A but also A0 has a great influence on the photophysical, electrochemical and photovoltaic properties of this kind of dyes.

dyes based on coumarin [8], indoline [9,10], phenothiazine [11,12], porphyrin [13,14], phthalocyanine [15], and triphenylamine [16e18] derivatives have been developed as promising candidates with the impressive power conversion efficiencies. The donorep-bridgeeacceptor (DepeA) structural system is the basis of metal-free dyes owing to its effective photoinduced intramolecular charge transfer properties. The photoelectric properties of such dyes can be finely tuned by alternating independently or matching different groups within the DepeA structure [19,20]. In order to red-shift the spectral absorption to the visible light region, recent endeavors have involved the use of additional donor (D0 ) or acceptor (A0 ) units between the donor and the p-bridge of the dye molecule, creating a DeD0 epeA [21e 23] or DeA0 epeA [9,24e31] configuration, which facilitates intramolecular charge transfer and shifts the bandgap to the low energy region. In the previous researches, many electronwithdrawing blocks, such as benzothiadiazole [9,24,25], diketopyrrolopyrrole [26,27], quinoxaline [28,29], benzotriazole [30], and benzophenazine [31], were employed as A0 to design several DeA0 epeA dyes, and the resulting dyes exhibit good photovoltaic performance and stability. This shows that the DeA0 epeA is a very promising structural prototype for high performance dyes. Recently, fluorenone has been used as the additional acceptor to build DeA0 epeA dyes because it is intrinsically electron-deficient as well as commercially available and cheap [32]. However, it is lack of systematical investigation on the effects of acceptors (A and A0 ) on optoelectronic properties of this kind of DeA0 epeA dyes. Such purposeful results encourage us to continually develop new DeA0 epeA dyes using cheap electron-deficient groups as additional acceptors to explore the relationship between the structure and property. On the other hand, besides the dyes, the optimization of fabricated processes, such as the dye soaking time, a coadsorbate and the additive, are also play an important role to for high-performance DSSCs [33].

2. Experimental section 2.1. Materials and reagents All reactions were carried out under nitrogen atmosphere. Tributyl (4-hexylthiophen-2-yl) stannane [18] and pinacol(4-(N,Ndi-p-tolylamino)phenyl)boronate were prepared according to our reported methods [34]. N, N-Dimethylformamide (DMF) was dried by distillation over CaH2. Tetrahydrofuran (THF) was dried and distilled over sodium/benzophenone. All other reagents and solvents used in this work were commercially purchased and used without further purification. All chromatographic separations were carried out on silica gel (200e300 mesh). 2.2. Analytical instruments 1

H NMR and 13C NMR spectra were recorded on a Bruker Avance 400 instrument. UVevis spectra of the dyes were measured on a PerkineElmer Lamada 25 spectrometer. The PL spectra were

C6H13

O Bu3Sn Br

Br

O

C6H13

S

S

Pd(PPh3)4, DMF

S

NC

C6H13

C6H13

CNCH2CN pyridine

C6H13 S

S

1 R1

C6H13 ClCH2CH2Cl

2

S

NBS

CHO

S

DMF

OHC

N

O

C6H13 N

Pd(PPh3)4, Toluene

N

S

R1

R1

S

C6H13 S

C6H13 CN S

Br

4a R1 = O 4b R1 = C(CN)2

5a R1 = O 5b R1 = C(CN)2 C6H13

S

4

B

C6H13

S

3a R1 = O 3b R1 = C(CN)2

O

R1

C6H13

C6H13

DMF, POCl3

1, 2

CN

R2

Dye 1 R1 = O, R2 = COOH Dye 2 R1 = O, R2 = CN Dye 3 R1 = C(CN)2, R2 = COOH Scheme 1. Synthesis of the Dye 1, Dye 2, Dye 3.

CHO

Piperidine CNCH2R2 CH3CN

L. Wang et al. / Journal of Power Sources 246 (2014) 831e839

obtained using PerkineElmer LS-50 luminescence spectrometer. FT-IR spectra were obtained on Perkine Elmer Spectra One spectrophotometer. Molecular mass was determined by matrix assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS) using a Bruker Aupoflex-Z mass spectrometer. The elemental analysis of the compounds was carried out with an Elementar Vario EL Z element analyzer for C, H, and N determination. Electrochemical redox potentials were obtained by cyclic voltammetry (CV) using a three-electrode cell and an electrochemistry workstation (ZAHNER ZENNIUM). The working electrode was a Pt ring electrode; the auxiliary electrode was a Pt wire, and saturated calomel electrode (SCE) was used as reference electrode. Tetrabutylammonium hexafluorophosphate (Bu4NPF6) 0.1 M was used as supporting electrolyte in dry acetonitrile. Ferrocene was added to each sample solution at the end of the experiments, and the ferrocenium/ferrocene (Fcþ/Fc) redox couple was used as an internal potential reference. The potentials of dyes vs. normal hydrogen electrode (NHE) were calibrated by addition of 0.63 V to the potentials vs. Fcþ/Fc. The solutions were purged with argon and stirred for 15 min before the measurements. 2.3. General procedure for fabrication of the DSSCs devices The FTO glass (fluorine doped SnO2, sheet resistance 14 U sq1, transmission > 90% in the visible) was first cleaned in a detergent solution using an ultrasonic bath for 15 min, and then rinsed with water and ethanol. The washed FTO glass was immerged in 40 mM aqueous TiCl4 solution at 70  C for 30 min, then washed with water and ethanol, sintered at 450  C for 30 min. Titania paste was prepared from 12 g P25 (Degussa AG, Germany) following a literature procedure [2]. First the 20e30 nm particles sized TiO2 colloid was coated onto the above FTO glass by sliding glass rod method to obtain a TiO2 film of 14 mm thickness after drying. Subsequently, the 200 nm particles sized TiO2 colloid was coated on the electrode by the same method, resulting in a TiO2 light-scattering layer of 5 mm thickness. The double-layer TiO2-coated FTO glass were sintered at 450  C for 30 min, then treated with TiCl4 solution and calcined at 450  C for 30 min again. After cooling to 100  C, the TiO2 electrodes were soaked in a solution with 0.5 mM dyes and kept at room temperature under dark for 24 h. The electrode was then rinsed with toluene and dried. One drop of electrolyte solution was deposited onto the surface of the electrode and a Pt foil counter electrode was clipped onto the top of the TiO2 electrode to assemble a dye sensitized solar cells for photovoltaic performance measurements. The electrolyte consists of 0.5 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine (TBP) in 3methoxypropionitrile. A Pt foil used as counter electrode was clipped onto the top of the TiO2 working electrode. The photocurrentevoltage (JeV) characteristics were recorded on Keithley 2602 Source meter under 100 mW cm2 simulated air mass (AM 1.5) solar light illumination. The action spectra of monochromatic incident photo-current conversion efficiencies (IPCE) for the solar cells were also detected with a similar data acquisition system. 2.4. Synthesis The synthetic routes to the three dyes are shown in Scheme 1. The detailed synthetic procedures are as follows. 2.4.1. 2, 7-Bis(4-hexylthiophen-2-yl)-9H-fluoren-9-one (1) A mixture of 2, 7-dibromofluorenone (3.00 g, 8.88 mmol) tributyl(4-hexylthiophen-2-yl)stannane (9.00 g, 19.68 mmol), Pd(PPh3)4 (0.10 g, 0.09 mmol) and DMF (40 mL) was stirred and refluxed for 24 h under nitrogen. After cooling to room temperature, the mixture was extracted with dichloromethane, washed

833

with distilled water and dried over anhydrous MgSO4. The organic portion was combined and removed by rotary evaporation. The crude product was purified by silica gel column chromatography with petroleum ether/dichloromethane (6/1, v/v) as the eluent to provide compound 1 (3.64 g, yield: 80%). 1H NMR (400 MHz, CDCl3, d/ppm): 7.90 (s, 2 H), 7.70 (d, 2 H, J ¼ 7.2 Hz), 7.49 (d, 2 H, J ¼ 7.6 Hz), 7.25 (s, 2 H), 6.91 (s, 2 H), 2.62 (t, 4 H, J ¼ 7.6 Hz), 1.67e1.62 (m, 4 H), 1.37e1.28 (m, 12 H), 0.89 (t, 6 H, J ¼ 6.3 Hz). 13C NMR (100 MHz, CDCl3, d/ppm): 193.0, 144.4, 142.6, 142.5, 135.4, 135.0, 131.1, 125.0, 121.0, 120.5, 120.0, 31.7, 30.6, 30.3, 29.1, 22.6, 14.1. 2.4.2. 2-(2, 7-Bis(4-hexylthiophen-2-yl)-9H-fluoren-9-ylidene) malononitrile (2) A mixture of malononitrile (2.58 g, 39.0 mmol), compound 1 (2.00 g, 3.90 mmol) and pyridine (50 mL) was stirred at room temperature for 1 h. Then the solution was heated to 85  C for 24 h. The mixture of methanol (30 mL) and water (60 mL) was added to the solution and the reaction solution was cooled to 20  C. A black solid was obtained after suction filtration. The crude product was purified by silica gel column chromatography with petroleum ether/dichloromethane (1/2, v/v) as the eluent to provide compound 2 (1.76 g, yield: 80%). 1H NMR (400 MHz, CDCl3, d/ppm): 8.59 (s, 2 H), 7.68 (d, 2 H, J ¼ 7.3 Hz), 7.49 (d, 2 H, J ¼ 6.1 Hz), 7.22 (s, 2 H), 6.93 (s, 2 H), 2.62 (t, 4 H, J ¼ 7.5 Hz), 1.67e1.62 (m, 4 H), 1.37e1.28 (m, 12 H), 0.90 (t, 6 H, J ¼ 6.7 Hz). Elem. Anal. Calcd. for C33H36OS2(%): C, 77.30; H, 7.08. Found: C, 77.28; H, 7.02. 2.4.3. The general synthetic procedure of (3) In a 100 mL three-necked flask, compound 1 or 2 (3.00 mmol), 1, 2-dichloroethane (40 mL), dried DMF (0.31 mL, 4.00 mmol) and POCl3 (0.37 mL, 4.00 mmol) were added sequentially under nitrogen atmosphere. The reaction mixture was stirred at room temperature for 0.5 h and then heated up to reflux for another 24 h. After cooling to room temperature, saturated aqueous solution of NaOAc (20 mL) was added into the reaction mixture and stirred violently for 1 h. Then the reaction mixture was extracted with dichloromethane. The combined organic layer was washed with distilled H2O and brine, dried over anhydrous MgSO4. The solvents were removed by rotary evaporation, and the crude product was purified by silica gel column chromatography with petroleum ether/dichloromethane (1/1, v/v) as eluent to yield compound 3. 2.4.3.1. 2-(5-Formyl-4-hexylthiophen-2-yl)-7-(4-hexylthiophen-2yl)-9H-fluoren-9-one (3a). Yield: 65% (1.05 g). 1H NMR (400 MHz, CDCl3, d/ppm): 10.04 (s, 1 H), 7.97 (s, 1 H), 7.81e7.77 (m, 2 H), 7.62e 7.59 (m, 4 H), 7.32 (s, 1 H), 7.01 (s, 1 H), 2.98 (t, 2 H, J ¼ 7.3 Hz), 2.68 (t, 2 H, J ¼ 7.7 Hz), 1.73e1.62 (m, 4 H), 1.40e1.27 (m, 12 H), 0.90e 0.86 (m, 6 H). 2.4.3.2. 2-(2-(5-Formyl-4-hexylthiophen-2-yl)-7-(4-hexylthiophen2-yl)-9H-fluoren-9-ylidene) malononitrile (3b). Yield: 46% (0.82 g). 1 H NMR (400 MHz, CDCl3, d/ppm): 10.03 (s, 1 H), 8.70e8.62 (m, 2 H), 7.77e7.56 (m, 5 H), 7.29 (s, 1 H), 7.23 (s, 1 H), 2.97 (t, 2 H, J ¼ 7.4 Hz), 2.67 (t, 2 H, J ¼ 7.6 Hz), 1.71e1.64 (m, 4 H), 1.39e1.27 (m, 12 H), 0.95e0.90 (m, 6 H). 2.4.4. The general synthetic procedure of (4) Compound 3 (1.30 mmol) was dissolved in DMF (30 mL). Nbromosuccinimide (0.27 g, 1.50 mmol) was dissolved in DMF (10 mL) and added dropwise. And the reaction mixture was stirred at room temperature. After being stirred for 12 h, the reaction mixture was poured into water and extracted with dichloromethane, and the combined extracts were washed with distilled H2O and brine, dried over anhydrous MgSO4. The solvent was removed by rotary evaporation, and the crude product was purified

834

L. Wang et al. / Journal of Power Sources 246 (2014) 831e839

by silica gel column chromatography with petroleum ether/ dichloromethane (1/1, v/v) as eluent to yield compound 4. 2.4.4.1. 5-(7-(5-Bromo-4-hexylthiophen-2-yl)-9-oxo-9H-fluoren-2yl)-3-hexylthiophene-2-carbaldehyde (4a). Yield: 88% (0.71 g). 1H NMR (400 MHz, CDCl3, d/ppm): 10.04 (s, 1 H), 7.96 (s, 1 H), 7.80e 7.72 (m, 2 H), 7.58e7.53 (m, 3 H), 7.30 (s, 2 H), 2.97 (t, 2 H, J ¼ 7.4 Hz), 2.61 (t, 2 H, J ¼ 7.5 Hz), 1.86e1.61 (m, 4 H), 1.42e1.25 (m, 12 H), 0.92e0.89 (m, 6 H). Elem. Anal. Calcd. for C34H35BrO2S2(%): C, 65.90; H, 5.69; S, 10.35. Found: C, 65.98; H, 5.64; S, 10.31. MS (MALDI-TOF, m/z): calcd for C34H35BrO2S2, 619.67; found 619.40. 2.4.4.2. 2-(2-(5-Bromo-4-hexylthiophen-2-yl)-7-(5-formyl-4hexylthiophen-2-yl)-9H-fluoren-9-ylidene) malononitrile (4b). Yield: 82% (0.71 g). FT-IR (KBr, cm1): 2217 (nC^N), 1646 (nC]O). 1H NMR (400 MHz, CDCl3, d/ppm): 10.04 (s, 1 H), 8.71 (s, 1 H), 7.78e 7.48 (m, 5 H), 7.35e7.30 (m, 2 H), 2.98 (t, 2 H, J ¼ 7.7 Hz), 2.67 (t, 2 H, J ¼ 7.8 Hz), 1.71e1.61 (m, 4 H), 1.40e1.26 (m, 12 H), 0.95e0.90 (m, 6 H). Elem. Anal. Calcd. for C37H35BrN2OS2(%): C, 66.55; H, 5.28; N, 4.20. Found: C, 66.68; H, 5.19; N, 3.80. MS (MALDI-TOF, m/z): calcd. for C37H35BrO2S2, 667.72; found 667.29. 2.4.5. The general synthetic procedure of (5) A stirred mixture of compound 4 (0.80 mmol), N, N-bis (4methylphenyl)-4-aminophenylboronic acid (0.32 g, 0.80 mmol), Pd (PPh3)4 (100 mg), Na2CO3 (9.4 mL, 2 M), toluene (20 mL) and a few tetrabutylammonium bromide was refluxed for 24 h under nitrogen atmosphere. After cooling, water was added and the product was extracted with dichloromethane. The combined organic layer was washed with distilled H2O and brine, dried over anhydrous MgSO4. The solvents were removed by rotary evaporation, and the crude product was purified by silica gel column chromatography with petroleum ether/dichloromethane (1/1, v/v) as eluent to yield compound 5. 2.4.5.1. 5-(7-(5-(4-(Di-p-tolylamino)phenyl)-4-hexylthiophen-2-yl)9-oxo-9H-fluoren-2-yl)-3- hexylthiophene-2-carbaldehyde (5a). Yield: 72% (0.47 g). 1H NMR (400 MHz, CDCl3, d/ppm): 10.04 (s, 1 H), 7.96 (s, 1 H), 7.94 (s, 1 H), 7.79e7.72 (m, 2 H), 7.57e7.45 (m, 3 H), 7.30e7.29 (m, 3 H), 7.11e7.03 (m, 10 H), 2.97 (t, 2 H, J ¼ 7.6 Hz), 2.67 (t, 2 H, J ¼ 7.8 Hz), 2.33 (s, 6 H), 1.77e1.62 (m, 4 H), 1.43e1.26 (m, 12 H), 0.92e0.87 (m, 6 H). Elem. Anal. Calcd. for C54H53NO2S2(%): C, 79.86; H, 6.58.; N, 1.72. Found: C, 78.29; H, 6.54; N, 1.25. MS (MALDI-TOF, m/z): calcd. for C54H53NO2S2, 812.13; found 812.35.

2.4.6.1. 2-Cyano-3-(5-(7-(5-(4-(di-P-tolylamino)phenyl)-4hexylthiophen-2-yl)-9-oxo-9H- fluoren-2-yl)-3-hexylthiophen-2-yl) acrylic acid (dye 1). Yield: 75% (0.16 g), m.p. 206e208  C. FT-IR (KBr, cm1): 2219 (nC^N), 1679 (nC]O). 1H NMR (400 MHz, CDCl3, d/ ppm): 8.46 (s, 1 H), 7.98e7.73 (m, 4 H), 7.56e7.28 (m, 9 H), 7.11e7.05 (m, 7 H), 2.84 (t, 2 H, J ¼ 7.6 Hz), 2.65 (t, 2 H, J ¼ 7.6 Hz), 2.34 (s, 6 H), 1.85e1.68 (m, 4 H), 1.42e1.24 (m, 12 H), 0.98e0.85 (m, 6 H). 13C NMR (100 MHz, CDCl3, d/ppm): 191.8, 169.0, 163.3, 157.9, 151.8, 147.2, 144.9, 144.7, 144.2, 140.7, 138.1, 134.4, 134.3, 132.8, 132.1, 130.1, 130.0, 129.4, 129.0, 128.3, 126.8, 125.9, 125.0, 121.4, 120.9, 120.3, 115.0, 94.0, 31.6, 30.7, 30.5, 29.9, 29.4, 29.3, 29.2, 29.1, 22.7, 20.8, 14.2, 14.1. Elem. Anal. Calcd. for C57H54N2O3S2(%): C, 77.87; H, 6.19; N, 3.19. Found: C, 77.91; H, 6.07; N, 2.91. MS (MALDI-TOF, m/z): calcd for C57H54N2O3S2, 879.18; found 879.51. 2.4.6.2. 2-((5-(7-(5-(4-(Di-p-tolylamino)phenyl)-4-hexylthiophen-2yl)-9-oxo-9H-fluoren- 2-yl)-3-hexylthiophen-2-yl)methylene)malononitrile (dye 2). Yield: 70% (0.15 g), m.p. 185e187  C. FT-IR (KBr, cm1): 2192 (nC^N), 1636 (nC]O). 1H NMR (400 MHz, CDCl3, d/ ppm): 7.89 (s, 1 H), 7.80e7.43 (m, 6 H), 7.38e7.29 (m, 4 H), 7.15e 6.89 (m, 10 H), 2.64 (s, 4 H), 2.32 (s, 6 H), 1.78e1.65 (m, 4 H), 1.38e 1.25 (m, 12 H), 0.98e0.85 (m, 6 H). 13C NMR (100 MHz, CDCl3, d/ ppm): 190.8, 163.1, 159.2, 150.5, 147.6, 145.1, 144.7, 143.4, 139.4, 135.3, 135.0, 132.9, 132.8, 130.0, 129.7, 128.0, 127.5, 127.0, 126.9, 126.8, 125.0, 121.9, 120.9, 120.1, 109.9, 63.2, 31.9, 31.7, 31.1, 30.7, 30.5, 29.7, 29.5, 29.1, 22.6, 20.9, 14.1. Elem. Anal. Calcd. for C57H53N3OS2(%): C, 79.59; H, 6.21; N, 4.89. Found: C, 79.98; H, 5.63; N, 4.50. MS (MALDI-TOF, m/z): calcd for C57H53N3OS2, 860.18; found 860.30. 2.4.6.3. 2-Cyano-3-(5-(7-(5-(4-(di-p-tolylamino)phenyl)-4hexylthiophen-2-yl)-9- (dicyanomethylene)-9H-fluoren-2-yl)-3hexylthiophen-2-yl)acrylic acid (dye 3). Yield: 72% (0.17 g), m.p. 163e165  C. 1H NMR (400 MHz, CDCl3, d/ppm): 7.87 (s, 1 H), 7.69e 7.64 (m, 6 H), 7.55e7.53 (m, 3 H), 7.48e7.36 (m, 6 H), 7.09e7.05 (m, 5 H), 3.16 (s, 2 H), 2.66 (s, 2 H), 2.34 (s, 6 H), 1.79e1.63 (m, 4 H), 1.43e1.26 (m, 12 H), 1.01 (t, 6 H, J ¼ 6.6 Hz), 0.88 (t, 6 H, J ¼ 7.0 Hz). 13 C NMR (100 MHz, CDCl3, d/ppm): 160.1, 157.9, 148.5, 145.0, 144.9, 135.1, 133.0, 132.8, 132.2, 132.1, 131.8, 130.2, 130.0, 129.7, 128.6, 128.5, 127.3, 126.1, 125.1, 125.0, 124.8, 122.2, 121.9, 119.2, 117.8, 58.9, 31.7, 31.6, 30.0, 29.3, 29.2, 29.1, 29.0, 23.9, 22.6, 20.8, 19.7, 14.1, 13.6. Elem. Anal. Calcd. for C60H54N4O2S2(%): C, 77.72; H, 5.87; N, 6.04. Found: C, 76.54; H, 6.28; N, 5.13. MS (MALDI-TOF, m/z): calcd. for C60H54N4O2S2, 927.23; found 926.95. 3. Result and discussion

2.4.5.2. 2-(2-(5-(4-(Di-p-tolylamino)phenyl)-4-hexylthiophen-2-yl)7-(5-formyl-4-hexylthiophen-2-yl)-9H-fluoren-9-ylidene) malononitrile (5b). Yield: 70% (0.48 g). 1H NMR (400 MHz, CDCl3, d/ppm): 10.04 (s, 1 H), 8.04e7.64 (m, 6 H), 7.55e7.54 (m, 3 H), 7.48e7.47 (m, 5 H), 7.09e7.05 (m, 6 H), 2.96 (t, 2 H, J ¼ 7.4 Hz), 2.65 (t, 2 H, J ¼ 7.5 Hz), 2.33 (s, 6 H), 1.68e1.61 (m, 4 H), 1.41e1.26 (m, 12 H), 1.01e0.90 (m, 6 H). Elem. Anal. Calcd. for C57H53N3OS2 (%): C, 79.59; H, 6.21; N, 4.89; S, 7.46. Found: C, 79.84; H, 6.18; N, 4.13. MS (MALDI-TOF, m/z): calcd. for C57H53N3OS2, 860.18; found 860.56. 2.4.6. The general synthetic procedure of sensitizers Under nitrogen atmosphere, a mixture of compound 5 (0.25 mmol), 2-cyanoacetic acid or malononitrile (2.5 mmol), piperidine (0.5 mL), and CH3CN (30 mL) was refluxed at 80  C for 24 h. After cooling, most of solvent was removed under vacuum and the residue was dropped into a mixture of petroleum ether and HCl (0.1 M) to form a precipitation. The crude product was further purified by silica gel column chromatograph eluted with dichloromethane/methanol (15/1, v/v) to obtain the dye.

3.1. Design and synthesis of the sensitizers Initially, fluorenone and cyanoacetic acid units were chose as A0 and A, respectively, to build a typical DeA0 epeA structural Dye 1. To investigate the effects of acceptors on the optoelectronic properties of this kind of dyes, then, malononitrile was used to replace cyanoacetic acid as A to create Dye 2. Finally, with the aim of increasing electron-withdrawing ability of A0 , fluorenone moiety was further modified by substituting dicyanovinylene for carbonyl group used in Dye 3. The three triphenylamine-based DeA0 epeA dyes were synthesized according to the synthetic routes in Scheme 1. These dyes were synthesized by the similar stepwise synthetic protocol. Firstly, the conjugated bridge was formed by the incorporation of fluorenone unit and hexylthiophene via the Stille coupling reaction with tributyl(4-hexylthiophen-2-yl)stannane. The introduction of long alkyl chains into sensitizers can not only inhibit the dye aggregation [18,24] but also improve the solubility, which is of great

L. Wang et al. / Journal of Power Sources 246 (2014) 831e839

Dye 1 Dye 2 Dye 3

Absorbance

0.5 0.4 0.3 0.2 0.1

400

500

Dye 1 Dye 2 Dye 3

0.8 0.6 0.4 0.2

0.0 250 300 350 400 450 500 550 600 650 700

0.0 300

1.0 Normalized absporption (a.u.)

0.6

835

600

Wavelength (nm)

Wavelength (nm)

benefit to the synthesis and purification. Fluorenone-containing compound 1 was transferred conveniently to compound 2 with more strong electron-withdrawing ability. Compound 3a,b was obtained by the followed Vilsmeier reaction. For further grafting the triphenylamine donor unit, simple bromination and Suzuki coupling reactions were treated on the reactive hexylthiophene unit, to produce the aldehyde precursor 5a,b. Finally, Knoevenagel condensation reactions of 5a,b with excess of cyanoacetic acid or malononitrile afforded the three target dyes in acetonitrile using piperidine as the catalyst. The structures of all important new compounds were characterized unambiguously with FT-IR, 1H NMR, 13C NMR, MALDI-TOF MS spectroscopy and elemental analysis.

3.2. Photophysical properties Fig. 1 depicts the ultravioletevisible (UVevis) absorption spectra of the three dyes in dilute CHCl3 (105 mol L1) solutions, and the corresponding data are collected in Table 1. One strong absorption band occurs at 368e428 nm with absorption peaks (lmax) at 427 nm for Dye 1, 368 nm for Dye 2 and 369 nm for Dye 3 assigned to the pep* transitions of the conjugated aromatic moieties. The weak shoulders observed at around 500 nm for Dye 1 and Dye 3 corresponded to intramolecular charge transfer (ICT) absorption from D (triphenylamine) to A (cyanoacetic acid) [35]. Han et al. also reported similar weak absorption features with absorption band at around 500 nm for fluorenone-based D-A-p-A type dyes [32]. It is clear that the lmax of pep* transitions in Dye 1 is remarkably red-shifted compared to Dye 2 and Dye 3, indicating there is a suitable combination of A and A0 in Dye 1 molecule. On the other hand, compared with Dye 1 and Dye 3 with cyanoacetic acid moieties as A, Dye 2 with malononitrile as A lost a weak ICT absorption band. This phenomenon can be ascribed to the different

Table 1 Maximum absorption of the three dyes. a

b

1

Dye

lmax /nm (3 /M

Dye 1 Dye 2 Dye 3

303 (46900) e 303 (24500)

a b c d

cm

1

)

d

c

lmax /nm 427 (56300) 368 (35600) 369 (22900)

2

G (mol cm ) 439 365 371

6.76  108 1.04  109 2.83  108

Maximum absorption in CHCl3 solution (105 M). 3 is the molar extinction coefficient at lmax of maximum absorption. Maximum absorption on TiO2 film. Amount of the dyes adsorbed on TiO2 films.

Fig. 2. Normalized absorption spectra of the three dyes adsorbed on TiO2 surface.

electron-acceptor ability of the acceptors. Obviously, cyanoacetic acid moiety as the acceptor (Dye 1 and Dye 3) has more strong electron-withdrawing ability than that of malononitrile moiety (Dye 2), which enhances the ICT between the triarylamine donating unit and the acceptor moiety (A), and as a result to exhibit the ICT band in the long wavelength. In other word, too strong electronwithdrawing ability of A0 (e.g. malononitrile for Dye 2) is unfavorable to ICT between donating unit and the end acceptors (A), which can be explained entirely in the following quantum chemical calculations. As depicted in Table 1, the molar extinction coefficients (3 ) of the ICT bands of the three dyes (22.9e56.3  103 M1 cm1) are higher than that of the standard N719 sensitizer (14.0  103 M1 cm1) [2], indicating a better ability of light harvesting of these new DeA0 epe A organic dyes than that of classical organometallic dye. Moreover, the 3 of the corresponding absorption peaks (lmax) increases in the order of Dye 3 < Dye 2 < Dye 1, which suggests that the electronwithdrawing ability of both A (cyanoacetic acid vs. malononitrile) and A0 (fluorenone vs. 9-(dicyanomethylene)fluorene) have an obviously influence on the light harvesting ability of the dyes. The absorption spectra of the three dyes absorbed onto the surface of TiO2 displayed broader and a slight red shift of 12 nm (Dye 1) and 2 nm (Dye 3) or blue shift of 3 nm (Dye 2) with respect to those in solutions, respectively (Fig. 2). It is presumed that the red and blue shifts of the absorption spectra may be contributed to

25

Dye 1 Dye 2 Dye 3

20 15

Current (10-3 mA)

Fig. 1. Absorption spectra of the three dyes in CHCl3 solutions (105 M) at 25  C.

Dye 1 (1.19 V) Fc (0.83 V)

10 5 0 -5 -10 -15 0.4

0.6

0.8

1.0

1.2

1.4

Potential (V) Fig. 3. Cyclic voltammograms of the three dyes: working electrode, Pt ring; auxiliary electrode, Pt wire; reference electrode, Hg/Hg2Cl2; scanning rate is 100 mV s1.

836

L. Wang et al. / Journal of Power Sources 246 (2014) 831e839

the formation of aggregates [36e38] on the TiO2 surface. Such a slight shift indicates that the aggregates of dyes are not obvious which may be explained that the introduction of the long alkyl of thiophenyl unit inhibits the aggregates to some extent [22,37]. It is worth mentioning that both in solution and on TiO2, the absorption peaks of Dye 1 are located at longer wavelengths (lmax in CHCl3 427 nm; on TiO2 439 nm) than those of fluorenone containing analogous dye (HIQF2) (lmax in DMF 393, 477 nm; on TiO2 385 nm) [32]. This result reveals that the introduction of the additional thiophene unit in the dye backbone is beneficial to improve the absorption properties. 3.3. Electrochemical properties

injection into the conduction bands of TiO2 could thermodynamically accept electrons from the iodide ions. On the other hand, the LUMO levels of Dye 1, Dye 2, and Dye 3 are 0.92, 1.56, and 1.05 V (vs. NHE), respectively. Noticeably, the obviously negative shift of the reduction potentials in Dye 2 and Dye 3 compared to that of Dye 1 implies that the improvement of electron-withdrawing strength of both A0 and A can enormously declined the LUMO levels of the DeA0 epeA dyes. The sufficiently negative LUMO levels of these dyes compared to the conduction band energy of TiO2 (Ecb) being 0.5 V vs. NHE implies possibility of electron injection from the excited dye into the conduction band of TiO2. 3.4. Molecular orbital calculations

To evaluate the thermodynamic possibility of electron transfer from the excited dye molecule to the conductive band of TiO2 and the dye regeneration, cyclic voltammetry (CV) method was performed in acetonitrile solution, using 0.1 M Bu4NPF6 as a supporting electrolyte (Fig. 3). The reference electrode is saturated calomel electrode (SCE) calibrated with Fcþ/Fc as an internal reference. Table 2 summarizes all the electrochemical properties of the three dyes. The first oxidation potential (Eox) vs. normal hydrogen electrode (NHE), which corresponds to the highest occupied molecular orbital (HOMO vs. NHE), was calibrated by addition of 630 mV to the potential (vs. SCE) vs. Fc/Fcþ by CV. For example, it can be seen that the oxidation potential peaks of Dye 1 and Fc/Fcþ are 1.19 V and 0.83 V from the CV of Dye 1, respectively. Then the Eox of Dye 1 (0.99 V) can be obtained from the formula (1.19 Ve0.83 V) þ 0.63 V (Fig. 3). And the reduction potential vs. NHE (Ered), which corresponds to the lowest unoccupied molecular orbital (LUMO vs. NHE), can be obtained from the first oxidation potential (Eox) and the E0e 0 value determined from the edge of absorption spectra of the dyes on TiO2 film (labs edge), namely, Eox  E0e0. As shown in Table 2, the HOMO levels of Dye 1, Dye 2, and Dye 3 are 0.99, 1.09, and 1.06 V (vs. NHE), respectively. The HOMO level of Dye 1 is 100 and 70 mV higher than those of Dye 2 and Dye 3, respectively, indicating a better delocalization degree of the donor part in the former, which is a further support for the better donore acceptor interaction in Dye 1 due to the matching A and A0 . Such a lift in the HOMO level can decrease the HOMOeLUMO bandgap and red-shift the absorption band; meanwhile, it makes the HOMO level closer to the potential of the redox couple in a solar-cell electrolyte, thus making it unfavorable for fast dye regeneration [24]. In view of the same triarylamine donor, we presume that the difference of HOMO levels for these dyes should be attributed to the acceptors A and A0 . It means that we can conveniently tune the HOMO levels through the change of the acceptors in such DeA0 epe A configuration dyes. Such a regular phenomenon seems to be helpful to molecular design and energy levels tuning for novel De A0 epeA dyes. More importantly, all these HOMO levels are sufficiently more positive than the iodine/iodide redox potential value (0.42 V vs. NHE), implying the oxidized dyes formed after electron

Table 2 Electrochemical data of the three dyes.a Dye

labs edge (nm)

E0e0 (V)

Eox (V vs. NHE)

Ered (V vs. NHE)

Dye 1 Dye 2 Dye 3

652 467 588

1.91 2.65 2.11

0.99 1.09 1.06

0.92 1.56 1.05

a E0e0 values were estimated from the edge of absorption spectra of the dyes on abs TiO2 film (labs edge): E0e0 ¼ 1240/ledge. The first oxidation potential (vs. NHE), Eox was measured in acetonitrile and calibrated by addition of 0.63 V to the potential vs. Fc/ Fcþ. The reduction potential, Ered, was calculated from Eox  E0e0.

To further investigate the electron distribution for the frontier molecular orbitals and the electronic transition processes upon photo-excitation, density functional theory (DFT) calculations were performed on the three dyes using the DMol3 program package [21,39]. In particular we used PBE as exchange-correlation functional and DND 4.4 as basis set. The frontier molecular orbital profiles (HOMO and LUMO) are shown in Table 3. It is obvious that the electron density of HOMO is localized mainly on the donor part (triarylamine and neighboring thiophene) for all the three dyes. The DMol3 program may be not as accurate as Gaussian program we used before [16,18,22], but it also shows some information we need from the frontier molecular orbital profiles (Table 3). The results imply that delocalization of the HOMO on the donor moiety may facilitate reduction of the oxidized dye by reaction with I, making the dyes suitable for efficient solar cells. On the other hand, the electron density of the LUMOs for Dye 1 and Dye 2 are primarily located at the fluorenone and cyanoacrylic acid/malononitrile acceptor units and to some extent at the neighboring thiophene rings. Thus, HOMOeLUMO excitation would move the photoexcited electrons on the p-conjugation from the donor unit to the cyanoacrylic acid moiety via the additional acceptor-fluorenone group, which also is advantageous to efficient charge separation and electron injection. Moreover, since it is more weakly electrophilic than the cyano group, the electron withdrawing ketone unit of the fluorenone does not limit electron injection. It is noteworthy that the LUMO of Dye 3 was located predominantly on the 9-(dicyanomethylene)fluorene moiety and hardly extended to the cyanoacetic acid unit, due to the too strong electron-withdrawing ability of 9-(dicyanomethylene)fluorene moiety, finally resulting in low electron injection efficiency. Thus, excitation from the HOMO to the LUMO could not lead to efficient photoinduced electron transfer from the electron donating triarylamine moiety to the TiO2 film via the terminal cyanoacrylic acid, which means Dye 3 would show an inferior efficiency. 3.5. Photovoltaic performance of DSSCs based on the three dyes The preliminary photovoltaic properties of the solar cells constructed from these organic dye-sensitized TiO2 electrodes were measured under simulated AM 1.5 irradiation (100 mW cm2). DSSCs were fabricated using these dyes as the sensitizers, TiO2 particles on FTO, and the electrolyte composed of 0.5 M LiI, 0.05 M I2, and 0.5 M 4-tert-butyl-pyridine (TBP) in 3-methoxy-propionitrile. Fig. 4 shows incident photon-to-electron conversion efficiency (IPCE) as a function of the wavelength for the sandwiched DSSCs based on the three dyes. As shown in Fig. 4, the DSSCs based on Dye 1 show higher IPCE values in the spectra range of 300e800 nm than those of Dye 2 and Dye 3, producing a maximum IPCE of ca. 65% at 375 nm. Moreover, the IPCE of this dye maintained a high plateau of about 50% throughout much of the visible region, which implies the

L. Wang et al. / Journal of Power Sources 246 (2014) 831e839

837

Table 3 Frontier molecular orbitals of the HOMO and LUMO calculated with DFT. Dye

HOMO

LUMO

Dye 1

Dye 2

Dye 3

dye would show a relatively large photocurrent in DSSCs. In addition, the IPCE spectrum for DSSC based on Dye 1 is broader toward the red region than those of Dye 2 and Dye 3, which is in accordance with their absorption spectra on the TiO2 films (Fig. 2). It should be noted that Dye 1 still show weak IPCE response at the range of long wavelength region (650e800 nm), we believe that this feature should be ascribed two reasons: one is the weak absorption band of the dyes on TiO2 films at the range of 650e800 nm (see Fig. 2), which is consistent with our and the other reports [22,40]; the other is the effect of light scattering by TiO2 nanoparticles, which increases the photocurrents for the weak absorption in that region [41]. More importantly, the Jsc values calculated by integrating the IPCE data (Fig. 4) with the AM 1.5G spectrum were 12.01, 4.46 and 3.15 mA cm2 for Dye 1, Dye 2 and Dye 3 devices, respectively, were in agreement with the directly

measured Jsc values, considering that <5%e10% error would take place during the IPCE measurement. To investigate the effect of different A units (cyanoacrylic acid vs. malononitrile) on the absorptive abilities and explain the Jsc of the three dyes, we employ the thin films to evaluate the total amounts of the dyes adsorbed on the TiO2 films, which were dissolved from the dye-coated TiO2 films into DMF/H2O ¼ 4:1 with 0.1 M NaOH. The surface density (G) values of the dyes absorption in DMF are listed in Table 1. The adsorbed amounts of three dyes are 6.76  108, 1.04  109, and 2.83  108 mol cm2 for Dye 1, Dye 2, and Dye 3, respectively. The G is in the order of Dye 1 > Dye 3 > Dye 2. Because G is related to the molecular size and the thickness of dye aggregates [21], which correlates directly to the absorptive abilities of the three dyes and may influence the photovoltaic performance of solar cells. For Dye 3, the absence of anchoring group in the acceptor makes it to possess the smallest absorbed amount. Therefore, the result clearly showed that Dye 1 has more strong

70

Current density (mA/cm2)

50

IPCE (%)

14

Dye 1 Dye 2 Dye 3

60

40 30 20 10

Dye 1 Dye 2 Dye 3

12 10 8 6 4 2

0 300

400

500

600

700

Wavelength (nm) Fig. 4. IPCE plots for the DSSCs based on the three dyes.

800

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

Voltage (V) Fig. 5. The JeV characteristics for DSSCs based on the three dyes.

0.7

L. Wang et al. / Journal of Power Sources 246 (2014) 831e839

Jsc (mA cm2)

Voc (V)

FF

PCE (%)

Dye 1 Dye 2 Dye 3

13.39 4.73 3.13

0.66 0.54 0.54

0.60 0.71 0.73

5.33 1.81 1.25

absorptive ability than the other two dyes, indicating a higher Jsc would be obtained for DSSCs based on Dye 1. It is in agreement with the IPCE spectra. Fig. 5 presents current densityevoltage (JeV) characteristics of devices based on the three dyes. The detailed photovoltaic performances are listed in Table 4. Compared to Dye 2 and Dye 3, the Dye 1sensitized cell exhibited better photovoltaic performances with a short-circuit photocurrent density (Jsc) of 13.39 mA cm2, an opencircuit voltage (Voc) of 0.66 V, and a filled factor (FF) of 0.60, corresponding to a power conversion efficiency (PCE) of 5.33% (Table 4). The dramatically increased Jsc value of Dye 1 compared with other two dyes mainly derived from its better light harvesting ability (broad absorption spectrum and high molar extinction coefficient), reflecting in its better IPCE spectrum (Fig. 4) and higher G value (Table 1). Additionally, it should be noted that the Jsc value of Dye 1 is also higher than that of the reported analogous dye (HIQF2) containing fluorenone unit (Jsc ¼ 11.46 mA cm2) [32], which can be readily understood because of the improved absorption property of the former as mentioned before. The poor Jsc of Dye 2 should be ascribed to the weak anchoring ability of malononitrile moiety compared with cyanoacrylic acid, which acts as anchoring group for attachment to the nanocrystalline TiO2 in the solar cell set-up. As regards the Dye 3, the inefficient photoinduced electron transfer from the electron donating triarylamine moiety to the TiO2 film via the terminal cyanoacrylic acid should be responsible for the lowest Jsc. As well known, the driving force to electron injection is dependent upon the LUMO energy level of sensitizers. The potential difference between the LUMO of the dye and the Ecb of TiO2 has a remarkable effect on the electron injection. Previous research has demonstrated that the difference value greater than 0.20 eV was indispensable for efficient electron-injection [42]. However, if the difference value is too great, it will inevitably cause a waste energy in the electron injection driving force, which will finally damage the photovoltaic performance including the Voc. Moreover, the improved Voc of Dye 1 can be also ascribed to the relatively long electron lifetime (see the following discussion of EIS). 3.6. Electrochemical impedance spectroscopy studies Electrochemical impedance spectroscopy (EIS) is a powerful technique of characterizing the important interfacial charge transfer processes in a DSSC. The EIS was measured in the dark to elucidate correlation of Voc with those dyes. The Nyquist plots of DSSCs with three dyes are shown in Fig. 6. Three semicircles are observed in the Nyquist plots. As mentioned in the previous report, the small and large semicircles respectively located in the high- and middle-frequency regions, are assigned to the redox reaction at the platinum counter electrode and the electron transfer at the TiO2/ dye/electrolyte interface, respectively [14]. Another small semicircle, which should have appeared at the low-frequency region, is overlapped by the middle-frequency large semicircle. Therefore, the large semicircle located in the middle-frequency region represents the resistance of charge transfer from TiO2 to the electrolyte. The larger middle-frequency semicircles for Dye 2 and Dye 3 based DSSCs imply that it is more difficult for charge transfer from TiO2 to electrolyte for the two devices. At the same time, DSSCs based on Dye 1 produced higher Voc than Dye 2 and Dye 3, which

220 200 180 160 140 120 100 80 60 40 20 0 -20

Dye 1 Dye 2 Dye 3

30 25 20 15 10 5 0

Dye 1

30 40 50 60 70 80 90 Z' (ohm)

50

100 150 200 250 300 350 400 450 500

Z' (ohm) Fig. 6. EIS Nyquist plots for DSSCs based on the three dyes measured in the dark.

can be explained by the electron lifetime. Two peaks in Fig. 7 located at the high-frequency (right) and middle-frequency (left) respectively correspond to the small semicircle (left) and large semicircle (right) in the Nyquist plots (Fig. 6) [43]. The reciprocal of the peak frequency for the middle-frequency peak is regarded as the electron lifetime since it represents the charge transfer process at the TiO2/dye/electrolyte interface. It is evident that the electron lifetime for the DSSCs based on Dye 1 is longer than those for DSSC based on Dye 2 and Dye 3, indicating the lowest charge recombination rate in Dye 1 based DSSC. The retarded charge recombination rate constant will reduce electron loss at open circuit. When more charge is accumulated in TiO2, Fermi level moves upward and Voc gets larger. This therefore explained the Voc increase from Dye 1 to Dye 2 and Dye 3. Introduction of dicyanomethylene unit into the dye’s conjugated backbone may be strengthen the intermoleuclar pep stacking. As a consequence, the charge recombination rate increases, thus resulting in a lower Voc for dicyanomethylene containing dyes Dye 2 and Dye 3. To further design organic dyes with not only good absorption properties but also slower charge recombination for high Voc, detailed study on charge recombination mechanism is needed. 4. Conclusions Three triarylamine-based DeA0 epeA dyes containing fluorenone or 9-(dicyanomethylene)fluorene as the additional

50

Dye 1 Dye 2 Dye 3

40

-Theta (deg.)

Dye

-Z'' (ohm)

Table 4 Photovoltaic performance of DSSCs based on the three dyes.

-Z'' (ohm)

838

30 20 10 0 1

10

100

1000

10000

100000

Frequency (Hz) Fig. 7. EIS Bode plots for DSSCs based on the three dyes measured in the dark.

L. Wang et al. / Journal of Power Sources 246 (2014) 831e839

acceptor A0 and malononitrile or cyanoacrylic acid as the acceptor A were designed and synthesized as the sensitizers for DSSCs applications. The effects of acceptors A0 and A on the photophysical, electrochemical and photovoltaic properties were investigated in detail. It was found that both A0 and A have great influence on the absorption spectra and HOMO and LUMO energy levels. Molecular orbital calculations showed that the HOMOeLUMO excitation could not readily move the electron distribution from the triarylamine donor to the cyanoacrylic acid anchoring group via p-conjugated bridges if the electron-withdrawing ability of the additional acceptor is too strong (e.g. 9-(dicyanomethylene)fluorene), further leading to inefficient electron injection and transfer. Thus, both our experimental and theoretical results indicate that introducing a proper electron-withdrawing additional acceptor into the DeA0 epeA configuration is a reasonable strategy. As a good example, Dye 1 with fluorenone as a suitable additional acceptor has achieved a high PCE up to 5.33%. In contrast, Dye 3 with a too strong electron-withdrawing 9-(dicyanomethylene) fluorepne unit as the additional acceptor only gave a PCE of 1.25%. Our results show that it is crucial importance to rational select the two kinds of acceptors in DeA0 epeA structural dyes. Moreover, such a regular phenomenon seems to be helpful to molecular design and energy levels tuning for high-performance DeA0 epeA dyes in the future. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21004050, 51003089), the General Program of the Education Department of Hunan Province (No. YB2011B030), and Undergraduate Investigated-Study and Innovated Experiment Plan of Xiangtan University. References [1] B. O0 Regan, M. Grätzel, Nature 353 (1991) 737. [2] Md. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Müller, P. Liska, N. Vlachopoulos, M. Grätzel, J. Am. Chem. Soc. 115 (1993) 6382. [3] M.K. Nazeeruddin, S.M. Zakeeruddin, R. Humphry-Baker, M. Jirousek, P. Liska, N. Vlachopoulos, V. Shklover, C.-H. Fischer, M. Grätzel, Inorg. Chem. 38 (1999) 6298. [4] M.K. Nazeeruddin, P. Péchy, T. Renouard, S.M. Zakeeruddin, R. HumphryBaker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia, G.B. Deacon, C.A. Bignozzi, M. Grätzel, J. Am. Chem. Soc. 123 (2001) 1613. [5] F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing, R. Humphry-Baker, P. Wang, S.M. Zakeeruddin, M. Grätzel, J. Am. Chem. Soc. 130 (2008) 10720. [6] Y.M. Cao, Y. Bai, Q.J. Yu, Y.M. Cheng, S. Liu, D. Shi, F.F. Gao, P. Wang, J. Phys. Chem. C 113 (2009) 6290. [7] C.Y. Chen, S.J. Wu, C.G. Wu, J.G. Chen, K.C. Ho, Angew. Chem. Int. Ed. 45 (2006) 5822. [8] Z.-S. Wang, Y. Cui, Y. Dan-oh, C. Kasada, A. Shinpo, K. Hara, J. Phys. Chem. C 111 (2007) 7224. [9] W.H. Zhu, Y.Z. Wu, S.T. Wang, W.Q. Li, X. Li, J. Chen, Z.S. Wang, H. Tian, Adv. Funct. Mater. 21 (2011) 756.

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