Novel organic dye employing dithiafulvenyl-substituted arylamine hybrid donor unit for dye-sensitized solar cells

Organic Electronics 14 (2013) 2132–2138

Contents lists available at SciVerse ScienceDirect

Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Novel organic dye employing dithiafulvenyl-substituted arylamine hybrid donor unit for dye-sensitized solar cells Zhongquan Wan a, Chunyang Jia a,⇑, Yandong Duan b, Ximing Chen a, Yuan Lin b,⇑, Yu Shi a a State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, Chengdu 610054, PR China b CAS Key Laboratory of Photochemistry, Institute of Chemistry, BNLMS, Chinese Academy of Sciences, Beijing 100190, PR China

a r t i c l e

i n f o

Article history: Received 26 February 2013 Received in revised form 19 April 2013 Accepted 8 May 2013 Available online 22 May 2013 Keywords: Organic dye Dithiafulvenyl Triphenylamine Hybrid donor Dye-sensitized solar cells

a b s t r a c t In order to increase electron-donating ability of the donor part of the organic dye, two dithiafulvenyl (DTF) units were introduced into a triphenylamine unit to form dithiafulvenylsubstituted triphenylamine hybrid donor for dye-sensitized solar cells (DSSCs) for the first time. Novel donor–acceptor organic dye WD-10 containing this hybrid donor and 2-cyanoacetic acid acceptor has been designed, synthesized and applied in DSSCs. The influence of the substituent unit DTF in the dye on the device performance has been investigated. It was found that the dye with dithiafulvenyl-substituted triphenylamine hybrid donor gave higher photocurrent, open-circuit voltage, and efficiency value. The DSSC based on organic dye WD-10 displayed a short-circuit current (Jsc) of 9.58 mA cm 2, an open-circuit voltage (Voc) of 648 mV, and a fill factor (ff) of 0.71, corresponding to an overall conversion efficiency of 4.41%. An increase in g of about 79% was obtained from simple triphenylamine dye L0 to WD-10. The different photovoltaic behaviors of the solar cells based on the organic dyes were further elucidated by the electrochemical impedance spectroscopy. This work identifies that the introduction of DTF unit into the simple triphenylamine dye could significant improve the photovoltaic performance. Ó 2013 Published by Elsevier B.V.

1. Introduction Dye-sensitized solar cells (DSSCs), developed by Grätzel and coworkers, have attracted considerable attention of many research groups in the past two decades owing to their high efficiencies and low costs [1]. DSSCs typically contain four components: a mesoporous semiconductor metal oxide film, a dye, an electrolyte/hole transporter, and a counter electrode [2]. In these components, the dye is a crucial element, exerting significant influence on the conversion efficiency as well as the stability of the cells. DSSCs based on the ruthenium dyes have shown very impressive solar-to-electric power conversion efficiency. ⇑ Corresponding authors. Tel.: +86 28 83201991; fax: +86 28 83202569 (C. Jia), tel.: +86 10 82615031; fax: +86 10 82617315 (Y. Lin). E-mail addresses: [email protected] (C. Jia), [email protected] (Y. Lin). 1566-1199/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.orgel.2013.05.011

Recently, a new record efficiency (11.4%) of solar cell based on black dye with donor–acceptor type co-adsorbent was obtained [3]. However, the large-scale application of ruthenium dyes is limited because of costs and environmental issues. Porphyrin derivatives are the archetypal light collectors of photosynthetic organisms and they are naturally becoming the main focus of the design of new dyes for DSSCs. Indeed, Diau and co-workers developed very efficient push–pull porphyrin dyes, affording a record efficiency of 10.17% with an I3 =I -based electrolyte [4], and more recently a 12.3% efficiency for a cobalt electrolyte [5], which are the best performing sensitizers reported so far. However, the synthetic approach of porphyrin dyes is complicated and the yield is low. Based on the above reasons, a lot of efforts have been dedicated to the development of pure organic dyes which exhibit not only higher molar extinction coefficients, but also simple preparation and purification at lower cost.

Z. Wan et al. / Organic Electronics 14 (2013) 2132–2138 S S S S

S N

CN

S

N

CN

S

COOH

L0

S

COOH

WD-10

Scheme 1. Molecular structures of the organic dyes in this study.

Coumarin [6], squaraine [7], indoline [8], phenothiazine [9], triphenylamine [10], fluorene [11], carbazole [12] and tetrahydroquinoline [13] based organic dyes have been developed and showed good performance. Tetrathiafulvene (TTF) is a well-known electron-donating group, and the preparation of its derivatives has primarily been motivated for applications as optoelectronicmaterials [14]. Recently, Grätzel et al. introduced the use of exTTF as a donor unit in a sensitizer for the preparation of DSSCs that exhibit efficient photovoltaic conversion [15]. Dithiafulvene (DTF) can be regarded as a smaller version of the fulvene family characterized by a terminal electron-donating group, which could permit more concise synthesis, and effective charge separation [16]. So, Yang et al. designed metal-free organic sensitizer based on the dithiafulvenyl unit as an excellent electron donor. The g of the DTF-C3-sensitized solar cell has reached 8.29% [17]. These works identified the TTF or DTF unit can be as an electron donor for constructing Dp-A organic sensitizers of dye-sensitized solar cells. Based on the above description, we think that the DTF unit can be used as building block of electron donor part in organic dyes for efficient dye-sensitized solar cells. Based on these considerations, the DTF unit as building block was introduced into a triphenylamine unit to form dithiafulvenyl-substituted triphenylamine hybrid donor for DSSCs for the first time in this paper. A new donor– acceptor organic dye WD-10 containing this hybrid donor and 2-cyanoacetic acid acceptor has been designed, synthesized and applied in DSSCs (Scheme 1). The simple triphenylamine dye L0 for the purpose of comparison was also synthesized [18]. The influence of the substituent unit DTF in the dyes on the device performance has been investigated. It was found that the dye with DTF-substituted triphenylamine hybrid donor gave higher photocurrent, open-circuit voltage, and efficiency value. 2. Experimental details 2.1. Materials and equipments Triphenylamine was purchased from Astatech. Lithium iodide (LiI) was purchased from Acros. 2-Cyanoacetic acid, 4-tert-butylpyridine (TBP) and 3-Methoxypropionitrile (MPN) were purchased from Aldrich. 3-Hexyl-1-methylimidazolium Iodide (HMII) was prepared according to the literature [19]. Toluene was pre-dried over 4 Å molecular sieves and distilled under nitrogen atmosphere from

2133

sodium benzophenone ketyl immediately prior to use. The starting materials tris(4-formylphenyl)amine was synthesized according to the corresponding literature method [20]. 4,5-Bis(methylthio)-1,3-dithiol-2-one was synthesized according to the corresponding literature method [21]. All other solvents and chemicals were purchased from Aldrich and used as received without further purification. NMR spectra were obtained with a Brücker AM 400 spectrometer (relative to TMS). Mass spectra were recorded with a Waters LCT Premier XE spectrometer. The absorption spectra of the dyes in solution and adsorbed on TiO2 films were measured with a SHIMADZU (model UV2550) UV–vis spectrophotometer. The cyclic voltammograms of the dyes were obtained with a CH Instruments 660 C electrochemical workstation using a normal threeelectrode cell with a Pt working electrode, a Pt wire counter electrode, and a Ag/AgCl reference electrode. 2.2. Preparation of DSSCs TiO2 colloid for liquid-state DSSCs was prepared according to the literature [22]. The FTO glass substrates were immersed in 40 mM TiCl4 aq. at 70 °C for 30 min and washed with water and ethanol. The 12 lm thick mesoporous nano-TiO2 films composed of 15–20 nm anatase TiO2 particles were coated on the FTO glass plates by doctor blade. After drying the nanocrystalline TiO2 layer at 125 °C, a 4 lm thick second layer of 300–500 nm sized light scattering anatase particles (Shanghai Cai Yu Nano Technology Co., Ltd.) was deposited by doctor blade onto the first layer. The TiO2 electrodes were heated at 450 °C for 30 min. After the sintering, when the temperature cooled to about 90 °C, the electrodes were immersed in a dye bath containing 0.5 mM N3 in ethanol or 0.2 mM L0/WD-10 with 10 mM chenodeoxycholic acid in methanol and left overnight. The films were then rinsed in ethanol to remove excess dye. Solar cells were assembled, using a 25 lm thick thermoplastic Surlyn frame, with a platinized counter electrode. An electrolyte solution was then introduced through the hole predrilled in the counter electrode, and the cell was sealed with thermoplastic Surlyn covers and a glass coverslip. The liquid electrolyte employed was a solution of 0.3 M HMII, 0.5 M LiI, 0.05 M I2 and 0.5 M TBP in MPN. 2.3. Photovoltaic characterization The irradiation source for the photocurrent density– voltage (J–V) measurement is an AM 1.5 solar simulator (91160A, Newport Co., USA). The incident light intensity was 100 mW cm 2 calibrated with a standard Si solar cell. The tested solar cells were masked to a working area of 0.2 cm2. Volt–current characteristics were performed on a Model 2611 sourcemeter (Keithley Instruments, Inc., USA). A Keithley 2611 sourcemeter and a Model spectrapro 300i monochromator (Acton research, USA) equipped with a 500 W xenon lamp (Aosiyuan Technology & Science Co., Ltd., China) were used for photocurrent action spectrum measurements. Electrochemical impedance spectroscopy (EIS) data were obtained by using Solartron 1255B

2134

Z. Wan et al. / Organic Electronics 14 (2013) 2132–2138 S

S

S

S

O

S

S

S

S

a

b

N O

S

S O

S

N

S S

N

S

CN

S

O

S

COOH

WD-10

1

Reagents: (a) 4, 5-Bis (methylthio)-1,3-dithiol-2-one, P(OEt)3, toluene, N2, reflux; (b) 2-Cyanoacetic acid, piperidine, acetonitrile. Scheme 2. The synthetic procedure of the target organic dye.

2.4. Preparation of WD-10 The synthetic route of WD-10 is shown in Scheme 2. 2.4.1. 4-(Bis(4-((4,5-bis(methylthio)-1,3-dithiol-2ylidene)methyl)phenyl)amino)benzaldehyde (1) 4,5-Bis(methylthio)-1,3-dithiol-2-one (420.8 mg, 2 mmol) and Tris(4-formylphenyl)amine (329.3 mg, 1 mmol) were dissolved in 10 mL of boiling toluene under a nitrogen atmosphere, then P(OEt)3 (5 mL) was added. The resulting mixture was refluxed for 6 h. After cooling and addition of dichloromethane, the mixture was washed with brine and dried on magnesium sulfate. Solvents were removed by rotary evaporation and the residue was purified by silicon gel column chromatography with petroleum ether (60–90 °C): dichloromethane (1:1.5, v:v) as eluent to afford 1 as yellow viscous liquid (137 mg, yield 20%). 1H NMR (400 MHz, DMSO-d6) d: 8.17 (s, 1H), 7.11 (m, 12H), 6.38 (s, 2H), 2.43 (s, 6H), 2.41 (s, 6H). HRMS (ESI, m/z): [M + H]+ calcd for (C31H27NOS8): 685.9858; found: 685.9862. 2.4.2. (E)-3-(4-(Bis(4-((4,5-bis(methylthio)-1,3-dithiol-2ylidene)methyl)phenyl)amino)phenyl)-2-cyanoacrylic acid (WD-10) A CH3CN (10 mL) solution of 1 (137 mg, 0.2 mmol), 2cyanoacetic acid (34.4 mg, 0.4 mmol) and a few drops of piperidine was charged sequentially in a three-necked flask and heated to reflux for 10 h. After cooling to room temperature, solvents were removed by rotary evaporation, and the residue was purified by silica gel column chromatography with dichloromethane: methanol (10:1, v:v) as eluent to afford the dye WD-10 as a dark red solid (81 mg, yield 54%). 1H NMR (400 MHz, DMSO-d6) d: 13.61 (s, 1H), 8.15 (s, 1H), 7.94 (d, J = 9.2 Hz, 2H), 7.26 (d, J = 8.8 Hz, 4H), 7.18 (d, J = 8.8 Hz, 4H), 6.96 (d, J = 9.2 Hz, 2H), 6.74 (s, 2H), 2.46 (s, 6H), 2.42 (s, 6H). 13C NMR (100 MHz, DMSO-d6) d: 164.05, 142.61, 132.86, 130.35, 128.08, 126.78, 126.10, 122.33, 119.26, 116.93, 114.20, 18.31, 18.11. HRMS (ESI, m/z): [M + H]+ calcd for (C34H28N2O2S8): 752.9916; found: 752.9992.

3. Results and discussion 3.1. Absorption properties in solutions and on TiO2 films Fig. 1 shows the UV–vis spectra of the organic dyes measured in methanol solutions. The corresponding spectroscopic parameters extracted are summarized in Table 1. The UV–vis spectra of L0 and WD-10 exhibit absorption maxima at 407 and 398 nm with molar absorption coefficients of 16,230 and 36,840 M 1 cm 1, respectively. In addition, two shoulder peaks appear at 335 and 443 nm with molar absorption coefficients of 17,950 and 20,350 M 1 cm 1 in absorption spectra of WD-10. The shoulder peaks in the spectra of WD-10 can be ascribed to the fused DTF unit. According to kmax and molar extinction coefficients of these organic dyes, it is clear that the better ability for light harvesting of WD-10 than that of L0, which would lead to the higher photocurrent of organic dye WD-10. Fig. 2 shows the absorption spectra of the dyes on 3 lm thick TiO2 films after 12 h adsorption. Compared to the spectra in methanol solution, a red-shift and broadening of the absorption spectra was observed in all dyes on TiO2 surface, which can be attributed to the formation of J-type aggregate [23]. The broadened absorption spectra caused by dye aggregates on TiO2 surface would benefit the photoelectrical conversion efficiency of the dye-sensitized solar cells [24].

Molar extinction coefficient / M-1 cm-1

frequency analyzer and Solartron SI 1287 electrochemical interface system with a frequency range of 0.1 Hz to 100 kHz.

37500

L0 WD-10

30000 22500 15000 7500 0 300

350

400

450

500

550

600

Wavelength (nm) Fig. 1. Absorption spectra of the dyes in methanol.

650

2135

Z. Wan et al. / Organic Electronics 14 (2013) 2132–2138 Table 1 UV–vis and electrochemical data of the dyes. Dye

kmaxa (nm) eb (M

1

L0 WD-10

407 (16230) 335 (17950), 398 (36840), 443 (20350)

cm

1

)

kmaxc (nm)

kemd (nm)

Eoxe (V) (vs. NHE)

E0

435 410

545 549

1.07 1.20

2.62 2.61

0

f

(eV)

Eredg (V) (vs. NHE) 1.55 1.41

a

Absorption spectra were measured in methanol (2.5  10 5 M). The molar extinction coefficient at kmax of the absorption spectra. c Absorption maximum on TiO2 film. d Emission spectra were measured in methanol. e Eox was measured in methanol with 0.1 M n-Bu4NPF6 as electrolyte (scanning rate: 100 mV s 1, working electrode and counter electrode: Pt wires, and reference electrode: Ag/AgCl), potentials measured vs. Ag/AgCl were converted to normal hydrogen electrode (NHE) by addition of +0.2 V [26]. f E0 0 values were calculated from the intersection of the normalized absorption and emission spectra. g Ered was calculated from Eox–E0 0. b

2.0 L0 WD-10

Absorbance

1.6

1.2

0.8

0.4

0.0 400

450

500

550

600

650

700

Wavelength (nm) Fig. 2. Absorption spectra of TiO2 electrodes sensitized by the dyes.

Fig. 3. Schematic energy levels of the dyes based on absorption and electrochemical data.

3.2. Electrochemical properties To evaluate the possibility of electron transfer between the TiO2/dye/redox electrolyte systems, cyclic voltammetry (CV) were performed in methanol solution, using 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte. The oxidation potentials of the organic dyes were determined from the peak potentials by CV. The oxidation potential vs. NHE (Eox) corresponds to the highest occupied molecular orbital (HOMO), while the lowest unoccupied molecular orbital (LUMO), could be calculated from Eox–E0 0 [25]. As shown in Table 1 and Fig. 3, the HOMO levels are more positive than the iodine/iodide redox potential value (0.4 V vs. NHE), indicating that the oxidized dye molecules formed after electron injection into the conduction band of TiO2 could be regenerated by the reducing species in the electrolyte solution. The LUMO levels of the dyes are sufficiently more negative than the CB level of the TiO2 electrode ( 0.5 V vs. NHE), which implies that electron injection from the excited dye into the conduction band of TiO2 is energetically permitted. Therefore, these organic dyes could be used as sensitizers in DSSCs. 3.3. Theoretical calculations All the calculations were performed with the Gaussian 03 [27]. Full ground state geometry optimization of

WD-10 before and after binding to (TiO2)9 in vacuum was carried out using B3LYP hybrid functional alone with 6–31G for C, H, O, N, S atoms, effective core potential (ECP) LANL2DZ and its accompanying basis set for Ti atom. Fig. 4 shows the frontier molecular orbitals of free dye. For the dye WD-10, the highest occupied molecular orbital (HOMO) is spread over DTF-TPA hybrid unit, whereas the lowest unoccupied molecular orbital (LUMO) is mainly localized on the linker and 2-cyanoacetic acid. The anchoring group (–COOH) of the dye has sizable contribution to the LUMO, which could lead to a strong electronic coupling with TiO2 surface and boost the electron injection efficiency. Thus, the HOMO ? LUMO excitation induced by light irradiation could move the excited electron distribution from the DTF-TPA unit to the 2-cyanoacetic acid, and injected into the conduction band of TiO2 through the anchoring group. The HOMO and LUMO of dye-(TiO2)9 complex was also studied. There was electron distribution mostly delocalized on dye molecule in the HOMO, whereas the LUMO showed injected electron delocalized dominantly on TiO2 surface. These results indicated that efficient electron injection from the LUMO of dyes to the CB of TiO2 substrate can be performed through a carboxylic acid acceptor group, which implying that intermolecular charge transfer (CT) transition between dye and (TiO2)9 cluster occurred. These results show that the direct charge transfer is the dominant

2136

Z. Wan et al. / Organic Electronics 14 (2013) 2132–2138

Optimized geometry

HOMO

LUMO

WD-10

WD-10-(TiO2)9

Fig. 4. Frontier orbitals of the free dye and dye–(TiO2)9 complex.

80

2.0

Absorbance

IPCE (%)

60 50

1.2 0.8 0.4 0.0 350

40

10

L0 WD-10

1.6

Jsc (mA / cm2)

70

400

450

500

550

600

Wavelength (nm)

30 L0 WD-10

20

8 6 4

L0 WD-10

2

10 0 0 400

0 450

500

550

600

650

700

750

800

Wavelength (nm) Fig. 5. IPCE spectra of DSSCs with the dyes. Absorption spectra of TiO2 electrodes sensitized by the dyes (inset).

mechanism for the charge-injection process within the dye-(TiO2)9 complex. 3.4. Photovoltaic performances of DSSCs Fig. 5 shows the IPCEs for DSSCs based on the organic dyes L0 and WD-10. The IPCE spectra changing tendency of the organic dyes are in accordance with their UV–vis absorption spectra on the TiO2 film. However, the onsets of the IPCE spectra for L0 and WD-10 are 580 and 720 nm, respectively, which are significantly broadened, as compared with those of their UV–vis absorption spectra on the TiO2 film. In the DSSC based on L0, the IPCE values relatively exceed 40% from 400 nm to 484 nm (with the highest value of 56.1% at 435 nm). The IPCE values of DSSC based on WD-10 exceeded 40% from 400 to 580 nm (with the highest value of 65.3% at 490 nm).

100

200

300

400

500

600

700

Voltage (mV) Fig. 6. J–V characteristics measured at an irradiation of 100 mW cm simulated AM1.5 sunlight.

2

In comparison with L0, WD-10 gives the higher and broader IPCE values, which implies that the dye would show a relatively large photocurrent in DSSC. The higher IPCE values of WD-10 might attribute to the higher molar extinction coefficient. These results are consistent with the absorption spectra in solutions and on TiO2 films of the dyes. The photocurrent density–photovoltage (J–V) curves of the DSSCs based on these dyes performed under simulated AM 1.5 solar irradiation (100 mW cm 2) are shown in Fig. 6. The photovoltaic properties of these dye-based DSSCs are summarized in Table 2. The solar cell based on L0 shows an efficiency of 2.47%, with a short-circuit photocurrent density (Jsc) of 5.48 mA cm 2, an open-circuit photovoltage (Voc) of 617 mV, and a fill factor (ff) of 0.73. Two DTF units were introduced into L0 to construct WD10, the performance of solar cell based on WD-10 significant improved (g = 4.41%, Jsc = 9.58 mA cm 2, Voc = 648 mV, and ff = 0.71). Under the same measurement

2137

Z. Wan et al. / Organic Electronics 14 (2013) 2132–2138 Table 2 Photovoltaic performances of DSSCs based on the dyes. Jsc (mA cm

L0 WD-10 N3

5.48 9.58 16.63

2

)

Voc (mV)

ff

g (%)

se (ms)

617 648 665

0.73 0.71 0.66

2.47 4.41 7.30

12.41 19.21

conditions, the solar cell based on dye N3 generated an efficiency of 7.30% (Jsc = 16.63 mA cm 2, Voc = 665 mV, and ff = 0.66). According to Fig. 6 and Table 2, it is clear that the photovoltaic performances of the DSSCs can be evidently affected by the DTF unit in the organic dyes. In comparison with L0, the Jsc and Voc value of WD-10 is improved by introducing DTF unit into L0. These observations indicate the short circuit photocurrent as well as the open circuit voltage increase significantly with the introducing the DTF unit.

To further elucidate the photovoltaic results and obtain more interfacial charge transfer information in DSSCs sensitized by L0 and WD-10, electrochemical impedance spectroscopy (EIS) was also performed in the dark under a forward bias of 0.60 V [28]. The Nyquist and Bode plots for L0 and WD-10 sensitized cells are shown in Fig. 7a and b, respectively. Nyquist plots have two semicircles. The larger semicircle at lower frequencies corresponded to the charge transfer processes at the TiO2/dye/electrolyte interface, while the smaller semicircle at higher frequencies corresponded to the charge transfer processes at the Pt/electrolyte interface. The two cells showed minimal differences in the smaller semicircles at higher frequencies due to the same Pt counter electrode and electrolyte; however, the difference between the cells in the larger semicircles at lower frequencies was significant. As can be seen from Fig. 7a that the radius of the larger semicircle increases in the order of WD-10 > L0, indicating that the charge recombination resistance is increased from L0 to WD-10. This is to some extent consistent with the order of decreasing Voc: WD-10 (648 mV) > L0 (617 mV). The higher Voc of WD-10 can be further explained by electron lifetime, calculated through the relation se = 1/ (2pf) (f is the peak frequency of lower-frequency range in EIS Bode plot). For the two devices, the peak frequency of lower-frequency range decreased in the order of L0 > WD-10, and the electron lifetime was enhanced in reverse with the calculated values of 12.41 and 19.21 ms, respectively. Thus, WD-10 has the longer electron lifetime, indicative of a more effective suppression of the back reaction between the injected electrons and the electrolyte, resulting in the improvement of the Voc due to reduced charge recombination rate. The combination of the higher Voc and Jsc values (vide supra) affords the better photovoltaic performance of the solar cell based on WD-10. The EIS research results indicate that the introduction of the DTF unit in simple triphenylamine dye could improve photovoltaic performance. The results of EIS are in

L0 WD-10

15 10 5 0 20

30

40

50

60

z' / ohm 25

b

L0 WD-10

20

Theta / deg

3.5. Electrochemical impedance spectroscopy analysis

a

20

-Z'' / ohm

Dye

25

15 10 5 0 0.1

1

10

100

1000

10000 100000

Frequency / Hz Fig. 7. EIS spectra of DSSCs based on the dyes measured at forward bias in the dark: (a) Nyquist and (b) Bode phase plots.

0.60 V

good agreement with photovoltaic results of the DSSCs based on the organic dyes.

4. Conclusions In this work, the DTF unit was introduced into a triphenylamine unit to form DTF-substituted triphenylamine hybrid donor for organic dye in DSSCs. One new organic dye WD-10 containing this hybrid donor and 2-cyanoacetic acid acceptor has been designed, synthesized and applied in DSSCs. The influence of the substituent DTF unit in organic dyes on the device performance has been investigated. It was found that the dye with dithiafulvenylsubstituted triphenylamine hybrid donor gave higher photocurrent, open-circuit voltage, and efficiency value. The better performance was found in organic dye WD-10 (g = 4.41%, Jsc = 9.58 mA cm 2, Voc = 648 mV, and ff = 0.71), in which two DTF units were introduced into a triphenylamine unit. The DSSC based on WD-10 has both higher Voc and Jsc values than that of L0 because of the more efficient dark current suppression and light harvesting. The results from this work strongly indicate that the introduction of DTF units into the simple triphenylamine dye is a promising way to improve the performance in triphenylamine dye-based DSSCs. Further structural optimization, such as broadening the absorption spectra and tuning the energy

2138

Z. Wan et al. / Organic Electronics 14 (2013) 2132–2138

levels, is very likely to generate more efficient sensitizers and this work is currently underway in our laboratory. Acknowledgments We thank the National Natural Science Foundation of China (Grant Nos. 20873015, 21272033), the Fundamental Research Funds for the Central Universities (Grant No. ZYGX2010J035), the Innovation Funds of State key Laboratory of Electronic Thin Films and Integrated Device (Grant No. CXJJ201104) and Beijing National Laboratory for Molecular Sciences (BNLMS) for financial support. References [1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737. [2] M. Grätzel, Acc. Chem. Res. 42 (2009) 1788. [3] L.Y. Han, A. Islam, H. Chen, C. Malapaka, B. Chiranjeevi, S.F. Zhang, X.D. Yang, M. Yanagida, Energy Environ. Sci. 5 (2012) 6057. [4] Y.C. Chang, C.L. Wang, T.Y. Pan, S.H. Hong, C.M. Lan, H.H. Kuo, C.F. Lo, H.Y. Hsu, C.Y. Lin, E.W.G. Diau, Chem. Commun. 47 (2011) 8910. [5] A. Yella, H.W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, E.W.G. Diau, C.Y. Yeh, S.M. Zakeeruddin, M. Grätzel, Science 334 (2011) 629. [6] (a) K. Hara, K. Sayama, Y. Ohga, A. Shinpo, S. Suga, H. Arakawa, Chem. Commun. (2001) 569; (b) K.D. Seo, I.T. Choi, Y.G. Park, S. Kang, J.Y. Lee, H.K. Kim, Dyes Pigments 94 (2012) 469. [7] (a) L. Etgar, J.H. Park, C. Barolo, V. Lesnyak, S.K. Panda, P. Quagliotto, S.G. Hickey, M.K. Nazeeruddin, A. Eychmuller, G. Viscardi, M. Grätzel, RSC Adv. 2 (2012) 2748; (b) K. Funabiki, H. Mase, Y. Saito, A. Otsuka, A. Hibino, N. Tanaka, H. Miura, Y. Himori, T. Yoshida, Y. Kubota, M. Matsui, Org. Lett. 14 (2012) 1246. [8] (a) B. Liu, W.Q. Li, B. Wang, X.Y. Li, Q.B. Liu, Y. Naruta, W.H. Zhu, J. Power Sources 234 (2013) 139; (b) M. Matsui, T. Inoue, M. Ono, Y. Kubota, K. Funabiki, J.Y. Jin, T. Yoshida, S. Higashijima, H. Miura, Dyes Pigments 96 (2013) 614; (c) S. Higashijima, Y. Inoue, H. Miura, Y. Kubota, K. Funabiki, T. Yoshida, M. Matsui, RSC Adv. 2 (2012) 2721; (d) B. Liu, Q.B. Liu, D. You, X.Y. Li, Y. Naruta, W.H. Zhu, J. Mater. Chem. 22 (2012) 13348. [9] (a) W.J. Wu, J.B. Yang, J.L. Hua, J. Tang, L. Zhang, Y.T. Long, H. Tian, J. Mater. Chem. 20 (2010) 1772; (b) Z.Q. Wan, C.Y. Jia, J.Q. Zhang, Y.D. Duan, Y. Lin, Y. Shi, J. Power Sources 199 (2012) 426; (c) M.H. Tsao, T.Y. Wu, H.P. Wang, I.W. Sun, S.G. Su, Y.C. Lin, C.W. Chang, Mater. Lett. 65 (2011) 583; (d) Z. Iqbal, W.Q. Wu, D.B. Kuang, L.Y. Wang, H. Meier, D.R. Cao, Dyes Pigments 96 (2013) 722. [10] (a) K. Pei, Y.Z. Wu, W.J. Wu, Q. Zhang, B.Q. Chen, H. Tian, W.H. Zhu, Chem. Eur. J. 18 (2012) 8190; (b) H.X. Shang, K.J. Jiang, X.W. Zhan, Org. Electron. 13 (2012) 2395; (c) T. Kono, T.N. Murakami, J. Nishida, Y. Yoshida, K. Hara, Y. Yamashita, Org. Electron. 13 (2012) 3097; (d) Y.Z. Wu, W.H. Zhu, Chem. Soc. Rev. 42 (2013) 2039.

[11] (a) S. Kim, J.K. Lee, S.O. Kang, J. Ko, J.H. Yum, S. Fantacci, F.D. Angelis, D.D. Censo, M.K. Nazeeruddin, M. Grätzel, J. Am. Chem. Soc. 128 (2006) 16701; (b) S. Ko, H. Choi, M.S. Kang, H. Hwang, H. Ji, J. Kim, J. Ko, Y. Kang, J. Mater. Chem. 20 (2010) 2391. [12] (a) C.J. Chen, J.Y. Liao, Z.G. Chi, B.J. Xu, X.Q. Zhang, D.B. Kuang, Y. Zhang, S.W. Liu, J.R. Xu, J. Mater. Chem. 22 (2012) 8994; (b) H. Lai, J. Hong, P. Liu, C. Yuan, Y.X. Li, Q. Fang, RSC Adv. 2 (2012) 2427; (c) S. Jungsuttiwong, R. Tarsang, T. Sudyoadsuk, V. Promarak, P. Khongpracha, S. Namuangruk, Org. Electron. 14 (2013) 711. [13] Y. Hao, X.C. Yang, J.Y. Cong, A. Hagfeldt, L.C. Sun, Tetrahedron 68 (2012) 552. [14] (a) A. Gorgues, P. Hudhomme, M. Sallé, Chem. Rev. 104 (2004) 5151; (b) J.L. Segura, N. Martín, Angew. Chem. Int. Ed. 40 (2001) 1372. [15] S. Wenger, P.A. Bouit, Q.L. Chen, J. Teuscher, D.D. Censo, R. HumphryBaker, J.E. Moser, J.L. Delgado, N. Martín, S.M. Zakeeruddin, M. Grätzel, J. Am. Chem. Soc. 132 (2010) 5164. [16] (a) M. Blanchard-Desce, I. Ledoux, J.M. Lehn, J. Malthete, J. Zyss, J. Chem. Soc. Chem. Commun. (1998) 737; (b) F. Meyers, J.L. Bredas, J. Zyss, J. Am. Chem. Soc. 114 (1992) 2914. [17] K.P. Guo, K.Y. Yan, X.Q. Lu, Y.C. Qiu, Z.K. Liu, J.W. Sun, F. Yan, W.Y. Guo, S.H. Yang, Org. Lett. 14 (2012) 2214. [18] D.P. Hagberg, T. Marinado, K.M. Karlsson, K. Nonomura, P. Qin, G. Boschloo, T. Brinck, A. Hagfeldt, L.C. Sun, J. Org. Chem. 72 (2007) 9550. [19] P. Bonhôte, A.P. Dias, N. Papageorgiou, K. Kalyanasunda-ram, M. Grätzel, Inorg. Chem. 35 (1996) 1168. [20] Y.X. Wang, M.K. Leung, Macromolecules 44 (2011) 8771. [21] K.B. Simonsen, N. Svenstrup, J. Lau, O. Simonsen, P. Mørk, G.J. Kristensen, J. Becher, Synthesis (1996) 407. [22] J. Liu, H.T. Yang, W.W. Tan, X.W. Zhou, Y. Lin, Electrochim. Acta 56 (2010) 396. [23] Z.S. Wang, K. Hara, Y. Dan-oh, C. Kasada, A. Shinpo, S. Suga, H. Arakawa, H. Sugihara, J. Phys. Chem. B 109 (2005) 3907. [24] M. Guo, P. Diao, Y.J. Ren, F.S. Meng, H. Tian, S.M. Cai, Sol. Energy Mater. Sol. Cells 88 (2005) 23. [25] J. Shi, J.N. Chen, Z.F. Chai, H. Wang, R.L. Tang, K. Fan, M. Wu, H.W. Han, J.G. Qin, T.Y. Peng, Q.Q. Li, Z. Li, J. Mater. Chem. 22 (2012) 18830. [26] C. Paven-Thivet, A. Ishikawa, A. Ziani, L. Gendre, M. Yoshida, J. Kubota, F. Tessier, K. Domen, J. Phys. Chem. C 113 (2009) 6156. [27] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Jr. Montgomery, T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian 03, Revision B.05, Gaussian, Inc., Wallingford, CT, 2004. [28] (a) J. Bisquert, Phys. Chem. Chem. Phys. 5 (2003) 5360; (b) F. Fabregat-Santiago, J. Bisquert, L. Cevey, P. Chen, M. Wang, S.M. Zakeeruddin, M. Grätzel, J. Am. Chem. Soc. 131 (2009) 558.