Benzo[1,2-b:4,5-b′]dithiophene and benzo[1,2-b:4,5-b′]difuran based organic dipolar compounds for sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2014.04.043

Novel organic dyes that consist of either a benzo[1,2-b:4,5-b′] dithiophene or a benzo[1,2-b:4,5-b′]difuran core exhibited remarkable solar-to-energy conversion efficiency in dye-sensitized solar cells. The planar geometry of bridge moiety and its bulky substituents helped the dyes to form a high quality monolayer on the surface of titanium oxide. A typical device displayed photon-to-current conversion efficiency 60% in the region of 380-575 nm, a short-circuit photocurrent density 13.45 mA cm^-2, an open-circuit photovoltage 0.72 V, and a fill factor 0.63, corresponding to an overall conversion efficiency 6.12%. In a test of using deoxycholic acid as a co-absorbent, an improvement of quantum efficiencies 8.37% was observed for certain compounds. However, for others the quantum efficiency decreased in 6.60-7.91%. The latter result indicated that the quality of some films cannot be further improved by the addition of deoxycholic acid. The photophysical properties were analyzed with the aid of TDDFT.

Full text of DOI:10.1016/j.dyepig.2014.04.043

Accepted Manuscript
Benzo[1,2-b:4,5-b′]dithiophene and Benzo[1,2-b:4,5-b′]difuran Based Organic Dipolar
Compounds for Sensitized Solar Cells
Yan-Zuo Lin , Chia-Wei Yeh , Po-Ting Chou , Motonori Watanabe , Yu-Hsuang
Chang , Yuan Jay Chang , Tahsin J. Chow
PII:
S0143-7208(14)00181-8
10.1016/j.dyepig.2014.04.043
DYPI 4373
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 17 March 2014
Revised Date: 29 April 2014
Accepted Date: 30 April 2014
Please cite this article as: Lin Y-Z, Yeh C-W, Chou P-T, Watanabe M, Chang Y-H, Chang YJ, Chow TJ,
Benzo[1,2-b:4,5-b′]dithiophene and Benzo[1,2-b:4,5-b′]difuran Based Organic Dipolar Compounds for
Sensitized Solar Cells, Dyes and Pigments (2014), doi: 10.1016/j.dyepig.2014.04.043.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Benzo[1,2-b:4,5-b′]dithiophene and Benzo[1,2-b:4,5-b′]difuran
Based Organic Dipolar Compounds for Sensitized Solar Cells
a
b
c
d
a
Yan-Zuo Lin , Chia-WeiYeh ,Po-Ting Chou , MotonoriWatanabe ,Yu-Hsuang Chang ,
a,∗
b,c,∗
Yuan JayChang ,andTahsin J. Chow
a
Department of Chemistry, Tung Hai University, Taichung 40704, Taiwan
b
Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan
c
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
d
2
International Institute for Carbon-Neutral EnergyResearch (I CNER), Kyushu University, 744 Motooka,
Nishi-ku, Fukuoka 819-0395, Japan
Abstract: Novel organic dyes that consist of either a benzo[1,2-b:4,5-b′]dithiophene or a
benzo[1,2-b:4,5-b′]difuran core exhibited remarkable solar-to-energy conversion efficiency in dye-sensitized
solar cells. The planar geometry of bridge moiety and its bulky substituents helped the dyes to form a high
quality monolayer on the surface of titanium oxide. A typical device displayed photon-to-current conversion
-2
efficiency 60% in the region of 380~575 nm, a short-circuit photocurrent density 13.45 mA·cm , an
open-circuit photovoltage 0.72 V, and a fill factor0.63, corresponding to an overall conversion efficiency 6.12%.
In a test of using deoxycholic acid as a co-absorbent, an improvement of quantum efficiencies 8.37% was
observed for certain compounds. However, for others the quantum efficiency decreased in 6.60~7.91%. The
latter result indicated that the quality of some films cannot be further improved by the addition of deoxycholic
acid. The photophysical properties were analyzed with the aid ofTD-DFT.
Keywords. Dye-sensitized solar cells; Benzo[1,2-b:4,5-b′]dithiophene; Benzo[1,2-b:4,5-b′]difuran;
Triisopropylsilylethynyl (TIPSE); Deoxycholic acid(DCA); Intramolecular charge transfer
*Corresponding author. Tel: +886 4 23590248 ext 305; fax: +886 4 23590426 (Y. J. Chang). E-mail address:
1

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jaychang@thu.edu.tw.
Corresponding author. Tel: +886 2 27898552; fax: +886 2 27884179 (T. J. Chow). E-mail address:
chowtj@gate.sinica.edu.tw.
*
1. Introduction
The increased consumption of fossil fuels that results in higher environmental pollution has urged us to the
development of renewable energy sources. Solar energy is widely recognized as one of the best ways to solve
this problem. Dye-sensitized solar cells (DSSCs) are known to be capable of converting solar energy to
electricityeffectively. Devices can be produced inlarge-scale with reasonablylow-cost. The DSSCs utilizing the
ruthenium-based complexes, i.e., N3, N719, and black dye that reported by O’Regan and Grätzel, have
achieved a maximal power conversion efficiency over 11% [1-6]. Another recent report by Diau et al. using a
porphyrin dye has achieved a performance up to 13% [7]. In the fabrication of DSSC cells, the dyes are
absorbed on the surface of titanium oxide forming adensely packed monolayer that acts as a p-n junction in the
circuit. The morphologyof the organic monolayer plays a keyfactor for the performance of the solar cell.
Metal-free organic dyes are mostly molecules of linear shape, which consists of an electron donor (D) and
an electron acceptor (A) attached on the opposites ends of the molecule. The π-bridge connecting D and A is
usually a conjugated chromophore with substantial rigidity. It acts as part of the chromophore for harvesting
photons. Awide varietyof bridges have been reported in the literature, such as the derivatives of carbazole [8,9],
phenylene [10,11], thiophenylene [12-14], pyrrole [15], furan [16], fluorene [17,18], dithiensilole [19,20],
spirobifluorene [21-23], benzothiadiazole [24-28], benzotriazole [29,30], phenothiazine [31-37], quinoxaline
[38,39], and diketo-pyrrolopyrrole [40-42]. Compounds with multiple D and/or A moieties have also been
designed, such as the star-burst structures, that mayenhance the photo-electron injecting efficiency[43-48].
2

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According to previous experiences, π-chromophores with a planar shape are the most favourable for
forming a monolayer with better film morphology on the surface of TiO [49]. In this report we examine the
2
effectiveness of including either benzo[1,2-b:4,5-b′]dithiophene (BDS) or benzo[1,2-b:4,5-b′]difuran (BDF)
moieties in the π-chromophores of organic dyes [50,51]. The photovoltaic properties of BDS and BDF
derivatives have drawn considerable attention in the applications of organic optoelectonic materials. They have
been successfully applied onto a variety of devices, such as polymeric bulk hetereojuction (BHJ) solar cells
(PSCs). BDS moiety has been used in PSCs co-polymers successfully as an electron donor due to its narrow
energy gap. The device made with it displayed a significant high hole mobility [52-54]. The structureof BDF is
analogous to that of BDS, except the two thiophenyl groups are replaced by furanyl groups. Compared with
thiophenyl group, the furanyl group shows a weaker steric hindrance to adjacent units and thus leads to higher
flexibility on molecular geometry. The relatively higher electronegativity of furan will also change the chemical
propertyof BDF with respect to that of BDS [55,56].
2. Results and discussion
2.1 Synthesis Strategies and Chemical Characterization
The structures of four dyes BDS-TIPSE, BDS-OC6, BDF-TIPSE, and BDF-OC6 are shown in Fig. 1.
In all the structures, two side chains are added at the 4,8-positions of both BDS and BDF units. The side
chains are either a triisopropylsilylethynyl (TIPSE) or a hexyloxy (OC6) group. The synthesis of the dyes
commenced from either a thiophene-3-carboxylic acid or a 3-furoic acid as outline in Scheme 1. The
foregoing acids were first converted to the corresponding amides, then cyclized to quinones through
self-condensation reactions [57,58]. The side groups TIPSE and OC6 were introduced through a reductive
coupling to form compound 4 [59,60]. Compound 4 was then brominated by using n-BuLi and CBr to
4
3

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form 5 [61]. The bromine atoms on one side of the BDS or BDF moieties were replaced by a triarylamine
through a Suzuki- Miyaura coupling reaction to produce 6 in 54~65% yield [62]. The bromines on the
other side were replaced by a thiophene-2-carbaldehyde unit through a Stille coupling reaction, followed
by hydrolysis to yield 7 [63].
The formation of the final products wac achieved by a Knoevenagel condensation with a cyanoacetic
acid to build up the cyanoacrylic acid acceptor unit in yields of 65~82% [64]. All final products were
crystallized into deep coloured solids and their structures were confirmed by their spectroscopic
characteristics.
2.2 Photophysical properties
-5
The absorption spectra of dyes in dilute THF solution (3 × 10 M) were shown in Fig. 2. Each of these
compounds, except BDF-TIPSE, exhibits three major absorption bands, appearingat 279-295 nm, 355-382 nm,
and 476~485 nm. The first two regions were composed of localized aromatic π-π* transitions, and the last one
was assigned to a charge-transfer (CT) transition.The CT band of BDF is slightlyred-shifted with respect to that
of BDS. Ahigher absorption intensity is observed for the TIPSE-substituted compounds, while comparingwith
4
-1
-1
those of the OC6-substituted ones. For example, the extinction coefficient (ε) is 5.66 × 10 M cm for
4
-1
-1
BDS-TIPSE, yet 3.92 × 10 M cm for BDS-OC6 (Table 1). Such difference can be properly reflected on
TDDFT computational models. The calculated oscillator strengths are arranged in the order of BDS-TIPSE
(0.76) > BDS-OC6 (0.68) and BDF-TIPSE (0.84) > BDF-OC6 (0.62). The higher flexibility of hexyloxy
group may also provide more energy dissipation channels through thermal relaxation. It is worth noting that all
4
-1
-1
four dyes showed higher molar extinction coefficients (> 3.92 × 10 M cm ) than the well known dye N719
4
-1
-1
(
< 2 × 10 M cm ).
The dihedral angles between the BDS and BDF units and the adjacent aminophenyl and thiophenyl rings
displayed a good co-planarity, which is helpful forthe transport of electrons from the donor to the acceptor (Fig.
4

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3
). The absorption spectra of dyes, after they were chemisorbed onto the surface of TiO , showed a
2
hypsochromic shift with respect to those in THF solutions (Fig. S25). Such a phenomenon has been observed
previously, and was attributed to H-aggregation and/or the reduction of electron accepting ability of the
cyanoacrylate group comparing to a free carboxylic acid [65-68]. BDS-OC6 and BDF-OC6 showed slightly
large Stokes shifts with respect to BDS-TIPSE and BDF-TIPSE, revealing a larger conformational
re-adjustment upon photo-excitation (Fig. S26).
2.3 Electrochemical properties
The oxidation potentials (E ) were measured by cyclic voltammetry (CV) in THF, and the results were
ox
included in Table 1. The HOMO energy levels correspond to the first oxidation potentials of the organic dyes.
The LUMO level was estimated from the values of E and the zero-to-zero transition energy measured at the
ox
intersection of absorption and emission spectra. The HOMO-LUMO energy gap of BDF-series (2.25~2.29)
were smaller than that of BDS-series (2.30~2.31). The estimated LUMO levels of both dyes are sufficiently
higher than the level of conductive band of TiO (ca. -0.5 V vs. NHE), while their HOMO levels are sufficiently
2
－
－
3
lower than the level of electrolyte ion pair I /I (ca. 0.4 V vs. NHE) [69,70]. All these ensure an exothermic
flow of charges in the photo-electronic conversion process (Fig. 4). The relatively low-lying HOMO energy
levels of these dyes, e.g., 0.83 V forBDS-TIPSE, warrant efficient charge regeneration after charge separation.
Ahigh open circuit voltage (V ) is also expected bythe low HOMO levels.
oc
2.4 Theoretical calculation
The electronic configuration of the dyes was examined by using B3LYP/6-31G* hybrid functional implanted
in a Q-Chem 4.0 software [71]. For the excited states, a time-dependent density functional theory (TDDFT)
with the B3LYP functional was employed. The optimized molecular geometry of the dyes was shown in Fig. 3,
and the frontier orbitals were plotted byusing GaussView 4.1 (Fig. S28). The HOMO/LUMO energy levelsand
the corresponding energy gaps are listed in Table 1 (also Table S1). Among the four compounds, BDS-OC6
displayed a large dihedral angle between the plane of BDS and the adjacent aminophenyl ring (ca. 26º), while
5

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others form highly planar geometry along the main chromophores. For electronic distributions, the electron
density in the HOMOs is localized mainly on the triarylamine moiety, and those of the LUMOs at the
cyanoacrylic acid moiety (Fig. S28). In both MOs, the orbitals in D and Aare heavily coupled with theorbitals
in the central bridge linkage. The bridge moiety therefore acts not only as an effective relay for transporting
charges, but also participates in light harvesting. The compounds with TIPSE substituents, i.e., both
BDS-TIPSE and BDF-TIPSE, yield higher values of oscillator strength (0.76~0.84). The result is consistent
with the higher molar extinction coefficients observed in their absorption spectra. The difference in Mülliken
charge shift surrounding D, B and Asegments, before (S state) and after (S state) the photo-excitation, can be
0
1
clearlydepicted bythe magnitude of bar charts (Fig. S27).
2.5 DSSCs performance
The DSSCs devices made with the BDS and BDF dyes were fabricated according to a standard procedure.
-2
The parameters were measured under AM 1.5 solar light (100 mWꢀcm ), i.e., short circuit current (J ),
sc
open-circuit photovoltage (V ), fill factor (FF), and solar-to-electrical photocurrent density (η) are summarized
oc
in Table 2. The photocurrent-voltage (J-V) plots of four devices with E1 electrolyte are shown in Fig. 5. Two
kinds of electrolytes were used in order to achieve the best performance, i.e., system E1 was made of LiI (0.5
M), I (0.05 M), and TBP (4-tert-butylpyridine) (0.5 M) in MeCN, and system E2 was composed of
2
3-dimethylimidazolium iodide (DMII)(1.0 M) and guanidinium thiocyanate (0.1 M), in addition to LiI (0.05 M),
I2 (0.03 M), and TBP (0.5 M) in a mixed solvent of MeCN and valeronitrile (85:15, v/v). To avoid electrolyte
contacting directly with TiO , the electrolyte was changed from E1 to E2. An apparent improvement of V
2
oc
values were observed by using the type electrolyte E2, e.g. an increase of ca. 0.05~0.06 V was obtained with
respect to those using E1 (Table 2). The lower concentration of LiI in E2 raised the conduction fermi level of
TiO , therefore enhanced the V value [72,73]. The effect is supported by the higher resistance in the
2
oc
electrochemical impedance spectrum (EIS). The bulky side chains help prevent contact between the electrolyte
and the surface of TiO . The devices made with TIPSE-substituted compounds displayed larger V values
2
oc
(0.71~0.72 with E1) than those with OC6-substituted ones. The former also exhibited a reduced dark current,
6

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indicating a slower rate of charge recombination. The reason can be ascribed to an improved film morphology
that covers the surface of TiO better. Among all four compounds, BDS-TIPSE performed the best. The device
2
-2
made with BDS-TIPSE yielded the highest overall quantum efficiency of 6.12%, with J 13.45 mAꢀcm , V
sc
oc
0
.72 V, and FF 0.63 (Table 2). The TIPSE-substituted derivatives showed a higher photon-to-current conversion
efficiency (IPCE), in the region of 340~616 nm, than that of the OC6-substituted ones (Fig. 5). In the longer
wavelength region, however, the latter displayed aslightly better ICPE as a result of their absorption in the long
wavelength region (Fig. 2).
Electrochemical impedance spectroscopic (EIS) analysis was performed to further elucidate the photovoltaic
properties [74,75]. The EIS for the DSSCs made with these compounds were taken under a forward bias of
-0.73 V in the dark. In the Nyquist plot (Fig. 6), a major semicircle was observed in a frequency range of
2
0~100 Hz, which is related to the transport process of injected electrons at the interfaces between TiO and the
2
electrolyte/dye [76,77]. The charge recombination resistance (R ) can be deduced by fitting the curves using a
rec
Z-view software [78]. It corresponds to the charge recombination rate at the TiO surface of DSSC in the dark,
2
e.g., a larger R indicates a slower charge recombination and therefore a larger V values. The radius of the
rec
oc
semicircle of Nyquist plot increased when the electrolyte was changed from E1 to E2. The relative size of the
Nyquist semicircles is consistent with the relativeV values, i.e., BDS-TIPSE (0.72 V) ≈ BDF-TIPSE (0.71 V)
oc
>
BDF-OC6 (0.61 V) ≈ BDS-OC6 (0.60 V) in E1 electrolyte, and similarly BDS-TIPSE (0.77 V) ≈
BDF-TIPSE (0.77 V) >BDF-OC6 (0.67V) ≈ BDS-OC6 (0.66 V) in E2 electrolyte.
The film morphology of the dyes on the surface of TiO can be adjusted by the addition of deoxycholic acid
2
(DCA). For some dyes that are not able to form high quality films, the addition of DCA as a co-absorbent can
effectively improve the film morphology, and consequently reduce the rate of charge recombination [79-81]. In
this work, the amount of DCA has been added in two different concentrations, namely 1.0 mM and 10 mM.
The result showed that, for the TIPSE dyes, the addition of DCAdid not lead to any improvement. However for
the OC6 dyes, the effect was quite prominent, particularly when the concentration of DCAwas 1.0 mM (Fig. 7
and Table 3). The quantum efficiency of BDS-OC6 can be improved from 4.5% to 4.9%, i.e., an 8%
enhancement. For both derivativesBDS-OC6 and BDF-OC6, the values of both V and J increased upon the
oc
sc
7

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addition of DCA.
It is known that the addition of DCA may not help the materials whose film morphology is already of
sufficient quality [49]. Comparing with the OC6 dyes, the TIPSE dyes seems to form a monolayer with abetter
quality. Upon the addition of DCAas a co-absorbent, both J and V values of BDS-TIPSE and BDF-TIPSE
sc
oc
decreased. The J values decreased with increasing the concentrations of DCA from 1 to 10 mM, i.e., for
sc
-2
-2
BDS-TIPSE from 13.45 to 12.37 mAꢀcm , and for BDF-TIPSE from 13.49 to 12.41 mAꢀcm . An overall
reduction of quantum efficiencies in 6.60~7.91% were observed. The J-V curves of the dyes in the presence of
DCA are shown in Fig. 7. These phenomena reaffirm the high quality of surface morphology for
TIPSE-substituted dyes. The dyes are packed denser in the films as indicated by their higher loading amount.
(
Table 2). The larger size of TIPSE group helps tocover the surface of TiO more completely, therefore prevents
2
it from a direct contact with the electrolyte. The effect was evidenced by a slower charge recombination rate in
the EIS, and a higher value ofV (Table 2 and Fig. 6).
oc
3. Conclusion
Organic dipolar compounds containing either a BDS or a BDF moiety showed a high performance when
they are used as the sensitizer dyes in organic solar cells. The light harvesting quantum efficiency reaches to
4~6% depending on the substituents. The derivatives with TIPSE substituents performed better than those with
OC6 substituents. For the device made with BDS-TIPSE dye, the overall conversion efficiency was 6.12%,
-2
with a short-circuit photocurrent density (J ) 13.45 mA·cm , an open-circuit photovoltage (V ) 0.72 V, and a
sc
oc
fill factor (FF) 0.63. The maximal photon-to-current conversion efficiency (IPCE) was 60 % in the region of
80~575 nm. For the device made with BDS-OC6, the best conversion efficiency was 4.54 without DCA, and
.92% with the addition of DCA. The difference is believed to be a result of changing film morphologyon the
3
4
surface ofTiO2.
A comparison between BDS and BDF derivatives revealed that the former performed better than the latter.
-
The reason is ascribed to the relatively lower potential level of the HOMOs with respect to the redox level of I/
-
3
I as depicted in Fig. 4. The electron transfer process from the electrolyte to the HOMO proceeds more
8

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efficientlyin the former.
4
.
Experimental Section
.1 General Information
All reactions and manipulations were carried out under a nitrogen atmosphere. Solvents were distilledfreshly
4
1
13
according to standard procedures. H and C NMR spectra were recorded on Brucker (AV 400/AV 500 MHz)
spectrometer in CDCl , and THF-d as a solvent. Chemical shifts are reported in scale downfield from the peak
3
8
for tetramethylsilane. Absorption spectra were recorded on a Jasco-550 spectrophotometer. Emission spectra
obtained from a Hitachi F-4500 spectrofluorimeter. The emission spectra in solution were measured in spectral
grade solvent by a 90° angle detection. The redox potentials were measured by using cyclic voltammetry on
CHI 620 analyzer. All measurements were carried out in THF solutions containing 0.1 M tetrabutylammonium
hexaflourophosphate (TBAPF ) as supporting electrolyte at ambient condition after purging 10 minutes with N .
6
2
The conventional three electrode configuration was employed, which consists of a glassy carbon working
+
electrode, a platinum counter electrode, and a Ag/Ag reference electrode calibrated with ferrocene/ferrocenium
+
(
Fc/Fc ) as an internal reference. The HOMO and LUMO values were calculated with reference to the
ferrocene oxidation potential byusing the following equations: HOMO = E + 4.8 eV; LUMO = HOMO – E ,
ox
0-0
where the HOMO of ferrocene was set at 4.8 eV. Mass spectra were recorded on a VG70-250S mass
spectrometer.
The chemicals, i.e., thiophene-3-carboxylic acid, thionyl chloride, diethylamine, 3-furoic acid,
triisopropylsilyl acetylene, tin(II) chloride (SnCl ), tetrakis(triphenylphosphine)palladium (0) (Pd(PPh ) ),
2
3 4
n-butyllithium (1.6 M in hexane), trans-dichlorobis(triphenylphosphine) palladium (II) (PdCl (PPh ) ),
2
3 2
N,N-dimethylformamide, 1-bromohexane, tetra-n-butylammonium bromide, CBr , tri-n-butyltin chloride,
4
9

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cyanoacetic acid, ammonium acetate, and acetic acid glacial, were purchased from ACROS, Alfa, Merck,
Lancaster, TCI, Sigma-Aldrich, Showa, separately, and purified as necessary. Chromatographic separations
were carried out by using silica gel from Merk, Kieselgel si 60 (40 - 63 ꢁm). The structure of all of
intermediates (2~7) was confirmed byNMR and other spectroscopic data(SI).
4
.1.1. (E)-2-cyano-3-(5-(6-(4-(naphthalen-1-yl(phenyl)amino)phenyl)-4,8-bis((triisopropylsilyl)ethynyl)benzo-
1,2-b:4,5-b′]dithiophen-2-yl)thio-phen-2-yl)acrylic acid (BDS-TIPSE).
Amixture of 7-SA (1.5 g, 1.6 mmol), cyanoacetic acid (205 mg, 2.4 mmol), and ammonium acetate (32 mg,
[
0.4 mmol) in acetic acid (15 mL) was placed in a three-necked flask under a nitrogen atmosphere and was
heated to 90-100 °C with stirring for 12 h. After cooling, the reaction was quenched by adding distilled water
(
30 mL), and extracted with CH Cl . The organic layer was dried over anhydrous MgSO and evaporated under
2
2
4
vacuum. The product was purified bysilica gel column chromatograph eluted byCH Cl /acetic acid (19/1). The
2
2
1
dark-red solid was isolated in 65% yield (1.06g, 1.04 mmol). Spectroscopic data of BDS-TIPSE: H NMR
(
500 MHz, THF-d ): δ 8.38 (s, 1H), 7.92-7.95 (m, 2H), 7.89 (d, 2H, J = 5.2 Hz), 7.85 (d, 1H, J = 8.0 Hz), 7.69
8
(s, 1H), 7.58 (d, 2H, J = 8.6 Hz), 7.50-7.54 (m, 2H), 7.46 (t, 1H, J = 8.3 Hz), 7.36-7.39 (m, 2H), 7.24 (t, 2H, J =
13
7
.8 Hz), 7.15 (d, 2H, J = 8.0 Hz), 6.97-7.02 (m, 3H), 1.25-1.26 (m, 42H); C NMR (100 MHz, THF-d ) 163.9,
8
150.7, 148.6, 148.3, 145.9, 145.8, 143.8, 141.8, 141.4, 141.3, 139.5, 139.4, 137.8, 137.6, 136.6, 132.3, 130.2,
129.4, 128.4, 128.2, 128.0, 127.5,127.2,127.1, 126.9, 124.8, 124.3, 124.0, 122.1, 121.3, 117.1, 116.5, 112.9,
111.7, 103.6, 103.4, 103.2, 103.1, 101.6, 57.9, 30.7, 19.2, 12.3; MS (FAB, 70 eV): m/z (relative intensity) 1020
+
(
(M+H) , 100); HRMS calcd for C H N O S Si : 1020.3668, found 1020.3699.
62
64
2
2 3
2
4.1.2. (E)-3-(5-(4,8-bis(hexyloxy)-6-(4-(naphthalen-1-yl(phenyl)amino)phenyl)benzo[1,2-b:4,5-b′]dithiophen-2-
1
0

ACCEPTED MANUSCRIPT
yl)thiophen-2-yl)-2-cyanoacrylic acid (BDS-OC6).
Compound BDS-OC6 was synthesized according to the same procedure asthat of BDS-TIPSE. Black solid
1
of BDS-OC6 was obtained in 82% yield. H NMR (400 MHz, THF-d ): δ 8.40 (s, 1H), 7.95-7.98 (m, 2H),
8
7.86-7.89 (m, 3H), 7.63-7.67 (m, 3H), 7.54-7.59 (m, 2H), 7.39 (t, 2H, J = 7.4 Hz), 7.25 (t, 2H, J = 7.9 Hz), 7.15
(d, 2H, J = 7.9 Hz), 7.02-7.04 (m, 3H), 4.37 (t, 2H, J = 6.4 Hz), 4.33 (t, 2H, J = 6.4 Hz), 1.91-1.95 (m, 4H),
13
1
.63-1.65 (m, 4H), 1.42-1.43 (m, 8H), 0.94-0.97 (m, 6H). C NMR (100 MHz, THF-d ) (100 MHz, THF-d )
8 8
1
64.3, 150.5, 149.2, 147.3, 146.6, 146.3, 146.1, 145.1, 144.4, 140.4, 138.6, 137.3, 137.0, 136.3, 135.9, 135.4,
32.8, 132.6, 131.1, 130.5, 129.8, 129.6, 128.5, 128.1, 127.7, 127.6, 127.4, 127.3, 125.3, 124.3, 124.0, 122.1,
20.4, 116.9, 115.5, 101.0, 75.3, 75.1, 33.0, 31.8, 31.0, 27.1, 24.0, 19.6, 19.5, 14.8; MS (FAB, 70 eV): m/z
1
1
+
(
relative intensity) 860 (M , 100); HRMS calcd for C H N O S : 860.2776, found 860.2800.
52 48 2 4 3
4.1.3. (E)-2-cyano-3-(5-(6-(4-(naphthalen-1-yl(phenyl)amino)phenyl)-4,8-bis((triisopropylsilyl)ethynyl)benzo-
[1,2-b:4,5-b′]difuran-2-yl)thiophen-2-yl)acrylic acid (BDF-TIPSE)
Compound BDF-TIPSE was synthesized according to the same procedure as that of BDS-TIPSE. Black
1
solid of BDF-TIPSE was obtained in 75% yield. H NMR (400 MHz, THF-d ): δ 8.39 (s, 1H), 7.91-7.94 (m,
8
3H), 7.85 (d, 1H, J = 8.2 Hz), 7.76 (d, 2H, J = 8.6 Hz), 7.73 (d, 1H, J = 4.0 Hz), 7.52 (t, 1H, J = 7.6 Hz),
7.44-7.48 (m, 2H), 7.35-7.39 (m, 2H), 7.23-7.27 (m, 2H), 7.14-7.16 (m, 3H), 6.99-7.03 (m, 3H), 1.24-1.28 (m,
13
4
2H); C NMR (100 MHz, THF-d ) 164.0, 159.7, 153.6, 153.5, 152.1, 150.9, 148.6,146.0, 143.9, 141.2, 139.3,
8
137.9, 136.7, 132.3, 131.9, 130.3, 129.5, 128.9, 128.3, 128.0, 127.5, 127.3, 127.2, 126.8, 126.0, 124.9, 124.4,
124.1, 123.0, 116.6, 105.5, 102.3, 101.1, 100.5, 100.0, 99.9, 99.6, 19.4, 19.3, 12.5, 12.4. MS (FAB, 70 eV): m/z
+
(
relative intensity) 989 ((M+H) , 100); HRMS calcd for C H N O SSi : 989.4204, found 989.4216.
62
64
2
4
2
11

ACCEPTED MANUSCRIPT
4.1.4. (E)-3-(5-(4,8-bis(hexyloxy)-6-(4-(naphthalen-1-yl(phenyl)amino)phenyl)benzo[1,2-b:4,5-b′]difuran-2-yl)-
thiophen-2-yl)-2-cyanoacrylic acid (BDF-OC6).
Compound BDF-OC6 was synthesized according to the same procedure asthat of BDS-TIPSE. Black solid
1
of BDF-OC6 was obtained in 77% yield. H NMR (400 MHz, THF-d ): δ 8.48 (s, 1H), 8.02 (d, 1H, J = 8.4
8
Hz), 7.99 (d, 1H, J = 4.4 Hz), 7.93 (d, 1H, J = 8.4 Hz), 7.84 (d, 1H, J = 8.4 Hz), 7.81 (d, 1H, J = 3.6 Hz), 7.72 (d,
2H, J = 8.4 Hz), 7.64 (s, 1H), 7.58 (t, 1H, J = 7.6 Hz), 7.52 (t, 1H, J = 7.2 Hz), 7.38-7.45 (m, 2H), 7.27-7.30 (m,
3H), 7.01-7.08 (m, 3H), 6.87 (d, 2H, J = 8.8 Hz), 4.42 (t, 2H, J = 6.4 Hz), 4.37 (t, 2H, J = 6.4 Hz), 1.69-1.75 (m,
13
4
H), 1.46-1.51 (m, 4H), 1.22-1.30 (m, 8H), 0.77-0.85 (m, 6H); C NMR (100 MHz, THF-d ) 162.8, 155.3,
8
148.4, 148.0, 146.6, 145.3, 141.8, 141.6, 141.2, 139.9, 139.4, 134.8, 134.6, 129.5, 129.0, 128.1, 126.8, 126.6,
126.2, 126.1, 125.8, 125.5, 125.2, 122.9, 122.6, 122.4, 122.3, 121.7, 119.5, 119.4, 115.8, 103.4, 98.8, 97.9, 72.5,
+
7
2.4, 30.6, 30.5, 29.2, 24.6, 24.5, 21.4, 13.2. MS (FAB, 70 eV): m/z (relative intensity) 828 (M , 100); HRMS
calcd for C H N O S: 828.3233, found 828.3232.
52 48 2 6
4.2 Fabrication of DSSCsand Characterization of DSSCs
The FTO conducting glass (FTO glass, fluorine dopedtin oxide over-layer, transmission >90% in the visible,
-1
sheet resistance 8 Ω square ), titania-oxide pastes of Ti-Nanoxide T/SP and Ti-Nanoxide R/SP were
2
purchased from Solaronix. Athin film of TiO (16~18 ꢀm thick) was coated on a 0.25 cm FTO glass substrate,
2
while the thickness was measured by Veeco Dektak 150. It was immersed in a CH Cl solution containing 3 ×
2
2
-4
1
0 M dye sensitizers for 12 h, then rinsed with anhydrous acetonitrile and dried. Another piece of FTO with
sputtering 100 nm thick Pt was used as a counter electrode. The active area was controlled at a dimension of
2
0
.25 cm by adhering 60 ꢀm thick polyester tape on the Pt cathode. The photoanode was placed on top of the
counter electrode and was tightly clipped together to form a cell. Electrolyte was then injected into the seam
1
2

ACCEPTED MANUSCRIPT
between two electrodes. Two kinds of electrolytes were used in order to achieve the best performance, i.e., E1
system was made of LiI (0.5 M), I (0.05 M), and TBP(4-tert-butylpyridine) (0.5 M) in MeCN, and E2 system
2
was composed of 3-dimethylimidazolium iodide (DMII)(1.0 M) and guanidinium thiocyanate (0.1 M), in
addition to LiI (0.05 M), I (0.03 M), and TBP (0.5 M) in a mixed solvent of MeCN and valeronitrile (85:15,
2
-4
v/v). Devices made of a commercial dye N719 under the same condition (3 × 10 M, Solaronix S.A.,
Switzerland) was used as a reference.
-2
The cell parameters were obtained under an incident light with intensity 100 mWꢀcm measured by a
thermopile probe (Oriel 71964), which was generated by a 300 W Xe lamp (Oriel 6258) passing through an
AM 1.5 filter (Oriel 81088). The current-voltage parameters of DSSCs were recorded by a
potentiostat/galvanostat model CHI650B (CH Instruments, USA). The light intensity was further calibrated by
an Oriel reference solar cell (Oriel 91150) and adjusted to be 1.0 sun. The monochromatic quantum efficiency
was recorded through a monochromator (Oriel 74100) at short circuit condition. Electrochemical impedance
spectra of DSSCs were recorded byan Impedance/Gain-Phase analyzer CHI650B (CH Instruments, USA). The
total power conversion efficiency (η) was expressed by the equation: η = (J V FF)/P, where Pis the power of
sc oc
incident light. The incident photo-to-electron conversion efficiency (IPCE) can be expressed by the equation:
IPCE = (1240 J )/λP. [82]
sc
4.3 Theoretical calculation
All organic dyes were optimized by using B3LYP/6-31G* hybrid functional. Geometry optimizations were
performed to locate the minima on the potential energy surface, in order to predict the equilibrium structure of a
given molecule. For the excited states, a time-dependent density functional theory (TDDFT) with the B3LYP
functional was employed. All analyses were performed under Q-Chem 3.0 software. The frontier orbital plots
1
3

ACCEPTED MANUSCRIPT
were drawn by using GaussView 4.0. All calculations were performed using the GaussView 4.0 suite of
programs.
Acknowledgments
Financial supports from the National Science Council of Taiwan and Academia Sinica are gratefully
acknowledged. (NSC 101-2113-M-035-001-MY2) Special thanks to Professor Hsu CP. in the Institute of
Chemistry, Academia Sinica, for the assistance on Quantum chemistrycomputations.
Appendix. Supplementary data
1
13
The H and C NMR spectra of all compounds, absorption spectraand emission spectra in THF solution, the
absorption spectra on TiO film, TDDFT calculated orbitals, Mulliken charges, low energy transitions, CV
2
spectra, HOMO/LUMO level, and EIS spectra. Supplementary data associated with this article can be found in
the online version, at doi: xxx.
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Table 1. Calculated and experimental parameters of all dyes.
a
a
b
b
c
e
f
g
dye
HOMO/LUMO
Energy
f
λabs nm
λabsnm
λ
em nm
HOMO/
E
ox
E
0-0
E
red
a
-1
-1
-1
-1
d
(
eV)
gap
(S1)
(ε/M cm )
(film)
445
464
449
472
(ε/M cm )
635
LUMO (eV)
(V)
(V)
(V)
BDS-TIPSE
BDS-OC6
-5.02/-2.86
-4.99/-2.75
-4.99/-2.81
-4.77/-2.73
2.16 0.7693 476 (56600)
2.24 0.6862 477 (39200)
2.18 0.8410 483 (67300)
2.04 0.6231 485(45800)
-5.33/-3.02
-5.21/-2.91
-5.32/-3.03
-5.13/-2.88
0.83 2.31 -1.48
0.71 2.30 -1.59
0.82 2.29 -1.47
0.63 2.25 -1.62
647
BDF-TIPSE
BDF-OC6
637
659
f : Oscillator strength for the lowest energy transition; ε: absorption coefficient; Eox: oxidation potential; E0-0: 0-0 transition energy measured at the intersection of absorption and
emissionspectra.
a
b
c
-3
4 9 4 6
TDDFT/B3LYPcalculated values. Absorptions and emission were measured in THF. Oxidation potentials of dyes (10 M) in THF containing 0.1 M (n-C H ) NPF with a
-1
+
d
e
f
scan rate of50 mVꢀs (vs. Fc / Fc). LUMO calculated by HOMO+E0-0. Eox calculated by HOMO+ 4.5 (eV) (vs NHE). E0-0 determined fromtheintersection ofabsorption
g
andemissioninTHF. Ered calculatedbyEox −E0-0
.
Table 2. Photovoltaic Parameters of Devices underAM 1.5
b
dye
E1/
E2
E1
E2
E1
E2
E1
E2
E1
E2
E1
E2
Jsc
Voc
FF
η
Dye loading
-2
-7
2
(mAꢀcm )
13.45
12.09
12.14
10.78
13.49
11.00
10.91
7.94
(V)
(%)
6.12
6.05
4.54
4.38
5.94
5.56
4.06
3.28
6.69
7.18
(10 mol/cm )
BDS-TIPSE
BDS-OC6
BDF-TIPSE
BDF-OC6
N719
0.72
0.77
0.60
0.66
0.71
0.77
0.61
0.67
0.70
0.72
0.63
0.65
0.62
0.62
0.62
0.65
0.61
0.62
0.60
0.62
4.4
2.8
5.0
3.2
---
16.07
15.99
Jsc:short-circuitphotocurrentdensity;Voc:open-circuitphotovoltage;FF:fillfactor;η:totalpowerconversionefficiency.
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4

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2 2
Electrolyte 1 (E1): LiI (0.5 M), I (0.05 M), and TBP (0.5 M) in MeCN. Electrolyte 2 (E2): 3-dimethylimidazolium iodide (1.0 M), LiI (0.05 M), I (0.03 M), guanidinium
thiocyanate(0.1M),andTBP(0.5M)inMeCN:valeronitrile(85:15,v/v). PerformanceofDSSCmeasuredina0.25cm workingareaonanFTO(8ꢂ/square)substrate.
a
b
2
Table 3. Photovoltaic Parameters of Devices made with different concentration ofDCA.
a
-2
b
Dye
DCA(mM)
J (mAꢀcm )
Voc (V)
0.72
0.69
0.69
0.60
0.625
0.63
0.71
0.705
0.70
0.61
0.63
0.63
FF
η (%)
sc
BDS-TIPSE
0
1
13.45
12.88
12.37
12.14
12.91
12.30
13.49
13.08
12.41
10.91
11.59
10.93
0.63
0.66
0.67
0.62
0.61
0.61
0.62
0.63
0.63
0.61
0.57
0.60
6.12
5.85
5.72
4.54
4.92
4.72
5.94
5.81
5.47
4.06
4.16
4.13
10
BDS-OC6
0
1
10
BDF-TIPSE
BDF-OC6
0
1
10
0
1
10
Jsc:short-circuitphotocurrentdensity;Voc:open-circuitphotovoltage;FF:fillfactor;η:totalpowerconversionefficiency.
a
-4
b
2
Concentrationofdyeis3×10 MinCH
2 2
Cl . PerformanceofDSSCmeasuredina0.25cm workingareaonanFTO(8Ω/square)substratewithelectrolyte1(E1)underAM
1.5condition.
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Figure Captions
Scheme 1. Synthesis route of organic dyes. i) SOCl , diethylamine, 90 ºC; ii)n-BuLi, THF, 0 ℃; iii)zinc dust,
2
NaOH(20%), 1-bromohexane, tetra-n-butylammonium bromide, 80 ºC; triisopropylsilyl acetylene, n-BuLi,
SnCl , AcOH, 60 ºC, THF; iv) n-BuLi, CBr , THF, -78 ºC; v)N-(4-bromophenyl)-N-phenylnaphthalen-1-amine,
2
4
Pd(PPh ) , K CO , THF/toluene (1/2) , 80 ºC; vi) (5-(1,3-dioxolan-2-yl)thiophen-2-yl)tributylstannane,
3
4
2
3
PdCl (PPh ) , DMF, 90 ºC, AcOH/THF/H O (4/2/1), 60 ºC; viii) cyanoacetic acid NH OAc, AcOH, 90~100 ºC.
2
3 2
2
4
Fig. 1. Dye structures incorporating a BDS or BDF unit inthe center of their backbones.
Fig. 2.Absorption spectra of the dyes inTHF.
Fig. 3. Optimized molecular structure of the dyes.
Fig. 4. CV spectra and Energy levels of HOMO and LUMO of dyes.
Fig. 5. J-V curves and IPCE plots of the four dyes with E1 electrolyte.
Fig. 6. EIS Nyquist plots of dyes at -0.73 Vbias in the dark (i.e., minus imaginary part of the impedance –Z”vs.
the real part of the impedance Z′ when sweeping the frequency).
Fig. 7. J-V curves of the dyes with different concentrations of DCA.
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