Investigation of benzo(1,2-b:4,5-b′)dithiophene as a spacer in organic dyes for high efficient dye-sensitized solar cell

Article abstract of DOI:10.1016/j.orgel.2015.06.033

Two benzo(1,2-b:4,5-b′)dithiophene (denoted as BDT) based organic dyes, Dye 1 and Dye 2, containing triphenylamine and carbazole in the molecular frameworks respectively, were synthesized, characterized and applied in dye-sensitized solar cells (DSSCs). The photo-physical, photovoltaic, and electrochemical properties of the two dyes were analyzed in this work. The two dyes exhibit strong charge transfer absorption bands in the visible region. The dyes were applied in dye sensitized solar cells obtaining 11.34 mA/cm², 0.75 V and 0.74, for the short-circuit photocurrent density (J_sc), open-circuit voltage (V_oc), and fill factor (FF) respectively, corresponding to an overall power conversion efficiency of 6.3%. These results revealed that BDT-based dyes are promising dyes for DSSCs.

Full text of DOI:10.1016/j.orgel.2015.06.033

Organic Electronics 25 (2015) 245–253
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Investigation of benzo(1,2-b:4,5-b0)dithiophene as a spacer in organic
dyes for high efficient dye-sensitized solar cell
c,
a,
Xiaole Zhou a,1, Xianghong Li b,1, Yuwen Liu a, Renjie Li a, , Kejian Jiang , Jiangbin Xia
⇑
⇑
⇑
a College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China
b Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central University for Nationalities, Wuhan 430074, China
c Laboratory of New Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
a r t i c l e i n f o
a b s t r a c t
Two benzo(1,2-b:4,5-b0)dithiophene (denoted as BDT) based organic dyes, Dye 1 and Dye 2, containing
triphenylamine and carbazole in the molecular frameworks respectively, were synthesized, characterized
and applied in dye-sensitized solar cells (DSSCs). The photo-physical, photovoltaic, and electrochemical
properties of the two dyes were analyzed in this work. The two dyes exhibit strong charge transfer
absorption bands in the visible region. The dyes were applied in dye sensitized solar cells obtaining
11.34 mA/cm2, 0.75 V and 0.74, for the short-circuit photocurrent density (Jsc), open-circuit voltage
(Voc), and fill factor (FF) respectively, corresponding to an overall power conversion efficiency of 6.3%.
These results revealed that BDT-based dyes are promising dyes for DSSCs.
Article history:
Received 22 June 2015
Accepted 22 June 2015
Keywords:
Dye-sensitization
Organic dye
Benzo(1,2-b:4,5-b0)dithiophene
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
example is the breakthrough observed with the application of
the novel low band gap polymer containing the benzo[1,2-b:4,5-
Dye-sensitized solar cell (DSSC) technology has attracted much
interest not only at academic level but also at industry level since
is the 3rd-generation solar cell technology closest to commercial-
ization [1–3]. DSSC devices comprise a transparent conducting
oxide made of fluorine doped tin oxide (FTO) on glass, a nanopor-
ous TiO2 film electrode which is sensitized with a dye, an elec-
trolyte and a back metal Pt counter electrode. Among these
components, the dye is one of the most important component
since it can be engineered to increase the power conversion effi-
ciency of the solar cell [3–6]. Research on novel dyes is nowadays
focused on obtaining high molar absorption coefficients, so the dye
can adsorb in the visible and near-infrared wavelengths and exhi-
bit enhanced light harvesting capacity [6–26]. Organic dyes are
very attractive molecules due to their lower cost, ease for structure
tunability and high molar extinction coefficients [15–26] if com-
pared to Ruthenium-based sensitizers [3–13]. New dyes with the
b0]dithiophene moiety (BDT) [27–30]. The BDT has been recognized
to favor light harvesting, thermal stability and high open-circuit
voltage in polymer heterojunction solar cells [27–30] and in this
work we want to explore the possibility to design new organic dyes
based on the BDT unit concept and to obtain more stable and rigid
p-conjugated condensed-polycyclic structures. Different from the
recently-reported BDT derivatives [22–24], we introduced not only
triphenylamine but also carbazole building blocks with the BDT
unit. We use ethylene linkage instead of the bulk thiophene rings.
Moreover, two long alkyl chains were further grafted on BDT to
suppress the aggregation of organic dyes [23] (see Scheme 1).
Herein, the two dyes were synthesized and characterized carefully.
The photo-physical and photo-electric properties of these two dyes
were systemically observed to get useful information for the
discovery of new dyes. Notably, the two new organic dyes with
triphenylamine and carbazole indeed have extended absorption
edge with maximum absorption band at 520 nm and 504 nm in
dichloromethane solutions and high molar absorption coefficients
over 25 ꢀ 103 Mꢁ1 cmꢁ1, respectively. Especially, the Dye 1-based
DSSCs showed a high conversion efficiency of up to 6.3% without
chenodeoxycholic acid (CDCA) as an anti-aggregation agent, while
reference ruthenium dye of N719 gives 8.8% of efficiency under the
same conditions.
donor-p-acceptor concept has been widely recognized in DSSCs
for enhancing light harvesting capacity [15–26]. This concept initi-
ated with the polymer heterojunction solar cells, and a successful
⇑
Corresponding authors at: Wuhan University, Department of Chemistry, China.
E-mail addresses: rjli@whu.edu (R. Li), kjjiang@iccas.ac.cn (K. Jiang), jbxia@whu.
edu.cn (J. Xia).
1
These authors made an equal contribution.
http://dx.doi.org/10.1016/j.orgel.2015.06.033
1566-1199/Ó 2015 Elsevier B.V. All rights reserved.

246
X. Zhou et al. / Organic Electronics 25 (2015) 245–253
COOC4H9
HOOC
N
N
N
NCS
NCS
Ru
N
HOOC
COOC4H9
N719
Scheme 1. Chemical structures of Dye 1, Dye 2 and N719.
Glass, Japan) conducting electrodes were washed with soap and
water, followed by sonication for 10 min in acetone and
isopropanol, respectively. After drying, the electrodes were sub-
merged in a 40 mM aqueous solution of TiCl4 for 30 min at 75 °C,
2. Experimental details
2.1. Materials and reagents
and then washed with water and ethanol, respectively. An 11
lm
All the solvents and reagents, unless specify, were of analytical
grade and used as received without further purification.
Tetrahydrofuran (THF) and 1,2-dichloroethane (DCE) were dried
over CaH2 and freshly distilled prior to use. The dimethyl fumarate
(DMF) was dried over CaH2 and freshly distilled under reduced
pressure.
thick nanocrystalline TiO2 layer and a 6 m thick TiO2 light scatter-
l
ing layer (particle size, 400 nm, PST-400C) were coated on the
electrodes by the screen-printing method. The double-layer
TiO2-coated electrodes were heated at 500 °C for 30 min, then trea-
ted with a 40 mM of TiCl4 solution for 30 min at 75 °C and subse-
quent sintered at 500 °C for 30 min again. After cooling, the
electrodes were soaked in a 4-tert-butanol–acetonitrile–tetrahy
drofuran (1:1:0.2, v/v) solution with 0.2 mM dyes and 2 mM
3a,7a-dihydroxy-5b-cholic acid (chenodeoxycholic acid), which
were kept at room temperature under dark for 24 h. Herein, the
thickness of TiO2 films was measured using a profiler, Sloan,
Dektak3.
2.2. Analytical instruments
The 1H NMR and 13C NMR spectra were recorded on a Bruker
(AV 300 MHz or 400 MHz) spectrometer in CDCl3 and DMSO-d6
as solvents. The chemical shifts are reported in the d-scale down-
field from the peak for tetramethylsilicane. Absorption spectra
were recorded on a UV-2550 spectrophotometer. Emission spectra
were obtained on a LS 55 fluorescence spectrophotometer. The ele-
mental analyses were determined by Vario EL III O-Element
Analyzer system. Mass spectra were recorded on a VG70-250S
mass spectrometer. The redox potentials were measured by using
cyclic voltammetry on a CHI 620 analyzer. All measurements were
carried out in CH2Cl2 solutions containing 0.1 M tetrabutylammo-
nium hexafluorophosphate (TBAPF6) as the supporting electrolyte
under ambient conditions after purging with N2 for 10 min. 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 fer-
rocene/ferrocenium (Fc/Fc+) as an internal reference.
The dye-adsorbed TiO2 electrode washed with ethanol and
dried.
A platinum-coated counter electrode was prepared
according to a previous report [3], and two holes were drilled on
its opposite sides. The two electrodes were sealed together with
a 25 mm thick thermoplastic Surlyn frame. After that a drop of
electrolyte solution was then introduced through one of the two
holes in the counter electrode, and the holes were sealed with
the thermoplastic Surlyn. The electrolyte consisted of 0.68 M
dimethyl imidazolium iodide, 0.05 M iodine, 0.10 M LiI, 0.05 M
guanidinium thiocyanate, and 0.40 M tertbutylpyridine in the mix-
ture of acetonitrile and valeronitrile (85:15, v/v). All the devices
were prepared with a photoactive area of about 0.3 cm2, and a
metal mask of 0.165 cm2 was used to cover the device for photo-
voltaic property measurements.
2.3. Computational details
2.5. The detailed experimental procedures and characterization data
The hybrid density functional Becke–Lee–Young–Parr compos-
ite of exchange–correlation functional method was used for both
geometry optimizations and property calculations [31,32]. In all
cases, the 6-31G (d) basis set was used. Berny algorithm using
redundant internal coordinates was employed in energy minimiza-
tion and the default cutoffs were used throughout. Using the
energy-minimized structures generated in the previous step, nor-
mal coordinate analyses were carried out.
Thiophene-3-carbonyl chloride (1) [28], N,N-diethylthiophen
e-3-carboxamide (2), and benzo[1,2-b:4,5-b0]dithiophene-4,
8-dione (3), were synthesized according to the methods reported
in the literature [28] and characterized by 1H NMR. The synthetic
routes of these dyes were shown in Scheme 2, and the detailed
synthetic procedures are as follows.
2.5.1. 4,8-Bis(octyloxy)benzo[1,2-b:4,5-b’]dithiophene (4)
2.4. General procedure for fabrication and characterization of DSSCs
Compound 3 (660.8 mg, 3.0 mmol), zinc powder (579.4 mg,
9.0 mmol), together with 9 mL of water and 5 mL of THF were
put into a 100 mL flask. Then 1.8 g of NaOH was added in it. The
mixture was well stirred and heated to reflux for 1 h. After that,
1-bromo-octane (1.74 g, 9.0 mmol) and a catalytic amount of
The nanocrystalline TiO2 pastes (particle size, 20 nm) were pre-
pared using a previously reported procedure [3]. Herein, fluorine
doped thin oxide (FTO, 4 mm thickness, 10
X
sqꢁ1, Nippon Sheet

X. Zhou et al. / Organic Electronics 25 (2015) 245–253
247
C8H17
O
C8H17
S
O
O
O
O
CON(C 2H5)2
COCl
S
S
c
d
e
a
b
OHC
CHO
S
S
S
S
S
O
C8H17
C8H17
1
2
3
4
5
C8H17
O
O
C8H17
S
NC
N
O
O
S
COOH
N
PPh 3Br
CHO
S
N
S
C8H17
C8H17
Dye 1
6a
f
g
C8H17
C8H17
N
O
C8H17
S
NC
O
O
S
COOH
N
N
PPh 3Br
CHO
S
C8H17
C8H17
S
O
C8H17
C8H17
6b
Dye 2
CHO
CH 2OH
N
N
h
i
N
PPh 3Br
7
8
OH
PPh 3Br
OHC
j
k
l
m
N C8H17
NH
N C8H17
N
C8H17
C8H17
N
9
10
11
12
Scheme 2. The synthetic route of the dyes. The reaction conditions: (a) HN(C2H5)2, ice water; (b) n-BuLi, ice water, N2; (c) H2O/Zn/NaOH, reflux for 1 h; (d) n-C8H17Br, (n-
Bu)4NBr, reflux for 2 h; (e) n-BuLi, DMF; (f) K2CO3, 18-crown-6; (g) CNCH2COOH, piperidine; (h) NaBH4, EtOH; (i) PPh3ꢂHBr, CH2Cl2; (j) C8H17Br, (n-Bu)4NBr; (k) POCl3, DMF; (l)
NaBH4, EtOH; (m) PPh3ꢂHBr, CH2Cl2.
tetrabutylammonium bromide were added. When the solution was
turned yellow to deep red after refluxing for 2 h, an excess amount
of Zn powder was added in it. The mixture was poured into cold
water and extracted by 20 mL of dichloromethane for 3 times after
refluxing for 6 h. The organic layer was dried over anhydrous
Na2SO4. After removing solvent, the resulting solid was purified
by column chromatography on silica gel using
petroleum-dichloromethane (5:1, v/v) as eluent to give a white
crystal (1.13 g, yield: 84.3%). 1H NMR (CDCl3, 300 MHz), d (ppm):
7.46 (d, 2H), 7.36 (d, 2H), 4.30 (t, 4H), 1.88 (m, 4H), 1.57 (m, 4H),
1.38–1.26 (m, 16H), 0.90 (t, 6H). Anal. Calc. for C26H38O2S2: C,
69.64%; H, 7.95%. Found: C, 69.91%; H, 8.57%.
the reactant into 40 mL ice water, the solution was stirred for
another 2 h. After the mixture stirred for 1 h, the ice water was
extracted by CH2Cl2 for 3 times. The organic layer was dried over
anhydrous Na2SO4 and then the solvent was removed. The resulting
crude product was purified by column chromatography on silica gel
using petroleum-dichloromethane (1:2,v/v) as eluent to give orange
solid (366.6 mg, yield: 65.7%). 1H NMR (CDCl3, 300 MHz), d (ppm):
10.15 (s, 2H), 8.20 (s, 2H), 4.37 (t, 4H), 1.91 (m, 4H), 1.57 (m, 4H),
1.36–1.29 (m, 16H), 0.89 (t, 6H). 13C NMR (CDCl3, 300 MHz), d
(ppm): 183.89, 146.31, 143.67, 133.01, 130.50, 130.47, 73.97,
31.24, 29.88, 28.77, 28.67, 25.38, 22.08, 13.52. Anal. Calcd for
C
28H38O4S2: C, 66.89%; H, 7.62%. Found: C, 69.92%; H, 7.87%. MS
(MALDI-TOF): m/z 502.968 (M+H+).
2.5.2. 4,8-Bis(octyloxy)benzo-dithiophene-2,6-dicarbaldehyde (5)
Compound 4 (495.7 mg, 1.1 mmol) was dissolved in 15 mL of
anhydrous THF at ambient temperature under N2 atmosphere.
After cooling to ꢁ78 °C, 2.8 mL of n-butyllithium (6.72 mmol,
2.4 M) was added dropwise to the solution. The mixture was stirred
for 3 h and 2 mL of anhydrous DMF was then added. Before pouring
2.5.3. (4-(Di-p-tolylamino)phenyl)methanol (7)
4-(Di-p-tolylamino)benzaldehyde (1.2 g, 4.0 mmol) and NaBH4
(90.8 mg, 2.3 mmol) were dissolved in 20 mL dry ethanol under
N2 atmosphere. Then the mixture was maintained at 60 °C for
2.5 h. When the reaction was over, an excess of water was added

248
X. Zhou et al. / Organic Electronics 25 (2015) 245–253
to remove the unreacted NaBH4. The resulting mixture was
extracted by CH2Cl2 and then the solvent was removed to obtain
the pure product 7. It was no further purified and dissolved in
20 mL of CH2Cl2 for the next step.
(CDCl3, 300 MHz), d (ppm): 10.01 (s, 1H), 8.14 (s, 1H), 7.36–7.31
(m, 3H), 7.26 (s, 1H), 7.15 (s, 1H), 7.11–6.94 (m,10H), 4.35 (t,
2H), 4.25 (t, 2H), 2.33 (s, 6H), 1.90 (m, 4H), 1.57 (m, 4H), 1.37–
1.25 (m, 16H), 0.89 (t, 6H). MS (MALDI-TOF): m/z 772.774 (M+).
2.5.4. (4-(Di-p-tolylamino)benzyl)triphenylphosphonium bromide (8)
PPh3ꢂHBr (1.38 g, 4.0 mmol) was added into the CH2Cl2 solution
of 7 and the mixture refluxed for 4 h. Then the solvent was
removed by rotary evaporation. The product was purified by col-
umn chromatography using CH2Cl2 and CH2Cl2–CH3OH (10:1,
v/v) as eluent continuously. The compound 8 was then obtained
as a solid (2.24 g, yield: 89.1%). 1H NMR (CDCl3, 300 MHz), d
(ppm): 7.74–7.63 (m, 15H), 7.04 (d, 4H), 6.91–6.75 (m, 6H), 6.73
(d, 2H), 5.23 (d, 2H), 2.29 (s, 6H).
2.5.9. 2-Cyano-3-(6-(4-(di-p-tolylamino)styryl)-4,8-bis(octyloxy)-
benzo[1,2-b:4,5-b’]dithiophen-2-yl) acrylic acid (Dye 1)
A 20 mL of acetonitrile solution containing compound 6a
(1.54 g, 2.0 mmol), cyanoacetic acid (171 mg, 2.12 mmol), and
0.20 mL of piperidine were charged sequentially in
a
three-necked flask and heated to reflux overnight under N2 atmo-
sphere. After cooling at room temperature, solvents were removed
by rotary evaporation and the residue was absorbed on silica gel
and purified by column chromatography using CH2Cl2–CH3OH
(10:1, v/v) as eluent to yield Dye 1 as a dark solid (153.6 mg, yield:
46.2%). 1H NMR (CDCl3, 300 MHz), d (ppm): 8.44 (s, 1H), 8.07 (s,
1H), 7.31 (m,2H), 7.12–6.91 (m, 13H), 4.33 (t, 2H), 4.24 (t, 2H),
2.33 (s, 6H), 1.90 (m, 4H), 1.56 (m, 4H), 1.37–1.25 (m, 16H), 0.89
(t, 6H); 13CNMR (CDCl3, 100 MHz), d (ppm): 189.71, 166.39,
161.13, 158.93, 145.99, 144.77, 129.99, 127.82, 127.64, 125.16,
121.58, 99.99, 98.18, 82.67, 75.26, 74.27, 60.55, 56.88, 54.29,
52.84, 52.37, 31.85, 30.49, 29.97, 29.42, 29.30, 26.03, 22.66,
20.85, 20.71, 14.10, 13.72; MS (MALDI-TOF): m/z 839.766
(M+H+); Calcd. For C52H58N2S2O4: C, 74.43%; H, 6.97%; N, 3.34%;
Found: C, 73.96%; H, 6.79%; N, 3.59%.
2.5.5. 9-Octyl-9H-carbazole (9)
Carbazole (1.67 g, 10 mmol), 1-bromo-n-octane (2.41 g,
12.5 mmol) and tetrabutyl ammonium bromide (TBAB, 111.3 mg,
0.3 mmol) were added into a 100 mL flask. Then, 20 mL of 50%
NaOH aqueous solutions and 5 mL of toluene were added into
the flask. The reactants were stirred at 80 °C for 30 min. After cool-
ing to room temperature, the product was extracted by CH2Cl2 for
3 times and yellow solid was obtained by removing the solvent.
Herein, compound 9 was finally obtained as a white crystal
(1.468 g, 52%) by recrystallized in ethanol and used directly in next
step.
2.5.10. 6-(2-(9-Octyl-9H-carbazol-3-yl)vinyl)-4,8-bis(octyloxy)benzo-
[1,2-b:4,5-b0]dithioph-ene-2-carbaldehyde (6b)
2.5.6. 9-Octyl-9H-carbazole-3-carbaldehyde (10)
POCl3 (1.55 g, 9.86 mmol) was slowly added into dry DMF
(1.98 g, 27.0 mmol) under N2 atmosphere at 0 °C. Then the mixture
was allowed to warm at room temperature for 1 h. The dry
1,2-dichloroethane (10 mL) solution of 9 was added dropwise into
the above-mentioned solution at 0 °C. After keeping at 0 °C for 1 h,
the reactants were heated at 90 °C and stirred for 8 h. When the
reaction was over, the product was poured into cold water, and
the pH of the aqueous solution should be adjusted to 9–10. Then,
crude yellow oil was obtained by extracting with CH2Cl2. After
purified it by column chromatography using petroleum ether:
CH2Cl2 (3:2, v/v) as eluent, compound 10 was obtained as a light
yellow oil (1.1 g, yield: 73.3%). 1H NMR (CDCl3, 300 MHz), d
(ppm): 10.10 (s, 1H), 8.62 (s, 1H), 8.16 (d, 1H), 8.00 (d,1H),
7.57–7.30 (m, 4H), 4.34 (t, 2H), 1.87 (t, 2H), 1.35–1.24 (m, 10H),
0.86 (t, 3H).
Product 6b was synthesized according to the procedure of the
synthesis of 6a. The crude compound was purified by column chro-
matography using CH2Cl2–CH3OH (10:1, v/v) as eluent to give red
oil. The resulting product was pure enough to be used in the next
procedure. Therefore, no further characterization was preceded.
2.5.11. 2-Cyano-3-(6-((E)-2-(9-octyl-9H-carbazol-3-yl)vinyl)-4,8-
bis(octyloxy)benzo[1,2-b:4,5-b’]-dithiophen-2-yl)acrylic acid (Dye 2)
Product Dye 2 was synthesized and purified according to the
procedure of Dye 1, giving dark red solid. It is noted that all the
peaks of Dye 2 in 1HNMR spectrum were broaden and no obvious
coupling splits were observed. 1H NMR (d6-DMSO, 300 MHz), d
(ppm): 8.50 (s,1H), 8.40 (s, 1H), 8.32 (s, 1H), 8.13 (s, 1H), 7.72 (s,
1H), 7.63–7.46 (m, 5H), 7.22 (m, 2H), 4.31 (m, 4H), 4.24 (s, 2H),
1.81 (m, 6H), 1.53 (s, 4H), 1.32–1.16 (m, 27H), 0.85–0.81 (m, 9H).
13CNMR (d6-DMSO, 100 MHz), d (ppm): 211.24, 157.97, 143.27,
131.79, 128.55, 127.54, 99.99, 94.52, 88.63, 74.38, 74.09, 56.49,
42.82, 31.73, 31.72, 31.61, 30.42, 29.28, 29.23, 29.14, 29.05,
28.96, 26.91, 25.99, 25.88, 22.57, 22.56, 22.45, 19.01, 14.40,
14.38, 14.32; MS (ESI-MS negative mode): m/z 843.44
([MꢁH+]ꢁ); Calcd. For C52H64N2S2O4: C, 73.89%; H, 7.63%, N,
3.31%; found: C, 73.60%; H, 7.79%; N, 3.59%.
2.5.7. ((9-Octyl-9H-carbazol-3-yl)methyl)triphenylphosphonium
bromide (12)
Herein, compound 11 was obtained as crude product by the
reduction reaction of 10 and NaBH4. Then it was used without fur-
ther purification to synthesize compound 12 according to the pro-
cedure for the synthesis of 8. The compound 12 was obtained as a
yellow solid (1.03 g, yield: 56.58%). 1H NMR (CDCl3, 300 MHz), d
(ppm): 7.79–7.72 (m, 10H), 7.65–7.62 (m, 6H), 7.51–7.35 (m,
4H), 7.18–7.13 (m, 2H).
3. Results and discussion
2.5.8. 6-(4-(Di-p-tolylamino)styryl)-4,8-bis(octyloxy)benzo[1,2-b:4,5-
3.1. Synthesis and characterization
b’]dithiophene-2-carbaldehyde (6a)
Compound
5
(214.3 mg, 0.426 mmol), anhydrous K2CO3
The molecular structures and synthetic routes of the two
organic dyes Dye 1 and Dye 2 are shown in Schemes 1 and 2,
respectively. The two dyes were synthesized by the stepwise syn-
thetic protocol. The main material, compound 5, prepared from
compound 4, was initially obtained by the vilsmeier reaction using
POCl3-DMF system [20]. However, many by-products were found
in the final product which made the purification difficult. Thus,
we employed the BuLi-DMF system, obtaining compound 5 with
a satisfied yield of 66%. We attribute this high yield to the low tem-
perature at which the reaction took place at ꢁ78 °C.
(121.6 mg, 0.881 mmol), and 18-Crown-6 (8.5 mg, 0.032 mmol)
in a three-necked flask were dried in vacuum for 30 min and fol-
lowed by adding 20 mL of dry DMF. After that, the compound 8
(271.2 mg, 0.431 mmol) was dissolved in 5 mL of dry DMF and
added dropwise to the above solution with stirring under N2 atmo-
sphere. The mixture was stirred for another 3 h at ambient temper-
ature and then poured into ice-water (40 mL). The precipitate was
filtered off and purified by silica gel column chromatography with
CH2Cl2-petroleum ether (2:1) as eluent to yield red oil. 1H NMR

X. Zhou et al. / Organic Electronics 25 (2015) 245–253
249
As shown in Scheme 2, the extension of p-conjugated aryl chain
in compound 5 was carried out by the Wittig reaction obtaining
the 4-(di-p-tolylamino)styryl or 2-(9-octyl-9H-carbazol-3-yl)vinyl
moiety (compound 6). It is observed that compound 6 should keep
one (should keep one what? substituents? ‘‘one of the two sub-
stituents’’) leaving the aldehyde group. Therefore, different con-
centration of t-BuOK and BuLi were used as base in the reaction.
The compound 6 was then easily obtained and used in the next
reaction [19]. Finally, the Knoevenagel condensation reactions of
aldehyde derivatives 6a and 6b with cyano-acetic acid resulted
in the obtention of Dye 1 and Dye 2 in the presence of piperidine.
These two dyes were purified by column chromatography before
measurements of the physical and electrochemical properties as
well as solar cell devices fabrication.
distinct absorption bands: one in the low-energy region
(<400 nm) corresponding to the
p⁄ transitions of the conjugated
p–
molecules, and a second one in the visible region (400–650 nm),
assigned to an intramolecular charge transfer (ICT) between the
electron donating unit and the cyano-acrylic acid anchoring moiety
through the cross-conjugated bridge. It can be seen that the
maximum absorptꢁio1n in visible region is centered at 520 nm
(
(
e
e
= 39,500 Mꢁ1 cm
)
for
Dye
1
and
500 nm
= 27,100 Mꢁ1 cmꢁ1) for Dye 2, respectively. As depicted in
) of the ICT bands of
Table 1, the molar extinction coefficient (
e
Dye 1 is higher than that of Dye 2, indicating a stronger ability of
light harvesting of Dye 1. However, they exhibited weak emission
in dichloromethane solutions and strong emission in ethanol
solution around 570 nm, when excited at 460 nm. As shown in
Fig. 2, the maximum emission of Dye 2 (560 nm) is blue-shifted
in comparison with Dye 1 (578 nm), which indicates that Dye 2
needs more energy to be photoactivated from the ground state to
the excited state. These results are in well-agreement with their
corresponding absorption spectra. The normalized absorption and
emission spectra of Dye 1 and Dye 2 in ethanol solutions are shown
in Fig. 2. The E0–0 (zero–zero transition energies) were estimated
from the intersection points of normalized absorption and emis-
sion spectra as 2.38 and 2.50 eV for Dye 1 and Dye 2, respectively
[36,37].
The absorptions of the two dyes on TiO2 films were also inves-
tigated. As seen in Fig. 3, the absorption spectra of the dyes on the
surface of the mesoporous TiO2 films were broadened and
red-shifted in about ca. 18–24 nm with respect to those in
CH2Cl2 solution (see Table 1). Another noteworthy feature in the
spectra of the dyes adsorbed on TiO2 surface, is that the addition
of CDCA as co-adsorbent leads to relative narrower absorption
wavelength range and much smaller adsorption amounts in both
Dye 1 and Dye 2 (see Figs. S1, S2 and Table 2). These results indi-
cate that the aggregation of the dyes is not taking place, this is
3.2. Photophysical and electrochemical properties
The visible absorption spectra of the two dyes in different sol-
vents are shown in Fig. 1. Analysis of the dyes by UV–Vis revealed
a blue-shift in the maximum absorption band when the dyes were
dissolved in solvents with increasing polarity. This blue-shift may
be attributed to a solvatochromic effect or to the tendency of the
molecules to form aggregates. It is observed that the addition of
CDCA as anti-aggregate agent results in the blue-shift of the
absorption peaks of dyed-TiO2 (see Figs. S1 and S2). These results
demonstrates that the two long alkyl chains can effectively sup-
press dye aggregation [23,33]. On the other hand, it appeared that
the polar solvents can interact with the carboxylic acid unit more
effectively and led to weaken the strength of the O–H bond. This
may diminish the electron-accepting ability of the acceptor moiety,
which in turn can cause a reduction in donor–acceptor interaction
in the dyes and as a consequent the blue shift in the absorption
bands is observed [34–35]. Next, the absorption bands in CH2Cl2
solutions were studied. In CH2Cl2 solution, they exhibit two
1.2
1.2
CH2Cl2
ethanol
CH3CN
DMF
THF
Dye 1
CH2Cl2
Dye 2
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
ethanol
CH3CN
DMF
THF
0.4
0.2
0.0
300
400
500
600
700
300
400
500
600
700
Wavelength / nm
Wavelength / nm
Fig. 1. The UV–Vis absorption spectra of Dye 1 and Dye 2 in different solvents (c = 2 ꢀ 10ꢁ5 mol dmꢁ3).
Table 1
Calculated (DFT/B3LYP) and experimental parameters of two dyes.
HOMO/LUMOa (eV)
Band gapa
kabs nm,
e
/Mꢁ1 cmꢁ1
kabs nm
Eox
HOMO/LUMOe (eV)
E0–0
E⁄g
b
c
d
f
Dye
Dye 1
Dye 2
ꢁ4.99/ꢁ2.76
ꢁ5.23/ꢁ2.75
2.23
2.48
520(39500)
500(27100)
538
524
0.96
1.09
ꢁ4.91/ꢁ2.53
ꢁ5.00/ꢁ2.52
2.38
2.50
ꢁ1.42
ꢁ1.41
a
b
c
DFT/B3LPY calculated values.
Absorptions measured in CH2Cl2.
Absorptions measured on TiO2 film.
d
Oxidation potentials of dyes in CH2Cl2 containing 0.1 M Bu4NPF6. (Vs Ag+/Ag) with ferrocene/ferrocenium (Fc/Fc+) as an internal reference and converted to NHE by
addition of 450 mV.
e
HOMO and LUMO calculated by EHOMO = ꢁ(Eox,onset + 4.8 ꢁ E+Fc/Fc) eV, ELUMO = EHOMO + EOpt
.
f
E0-0 determined from the intersection of absorption and emission in ethanol.
g
E⁄ calculated by Eox ꢁ E0–0
.

250
X. Zhou et al. / Organic Electronics 25 (2015) 245–253
(NHE), respectively. The oxidation potentials are higher than iodi-
Dye 1
Dye 2
ne/iodide redox potential (+0.4 V vs NHE), ensuring that there is
enough driving force for efficient regeneration of the dye through
the recapture of the injected electrons by the dye cation radical.
On the other hand, the first redox behavior was used to estimate
excited-state redox potentials of the two dyes by subtracting E0–0
from Eox. The excited-state redox potentials of Dye 1 and Dye 2
are ꢁ1.42 and ꢁ1.41 V vs NHE, which are lower than the conduc-
tion band edge of TiO2 (ꢁ0.5 V vs NHE). This fact indicates that
electrons from TiO2 undergo no reductive reaction with ground
state dyes, and carrier separation occurs fast from the exited dyes
on TiO2. This result indicates that the redox energy levels of Dye 1
and Dye 2 makes these dyes suitable as sensitizers in DSSCs.
0.9
0.6
0.3
0.0
300
400
500
600
700
800
Wavelength /nm
Fig. 2. Normalized electronic absorption and emission spectra of Dye 1 and Dye 2 in
ethanol solutions. The emission spectra were obtained by excitation at 460 nm.
3.3. Density functional theory (DFT)
To gain further insight into the molecular structure and frontier
molecular orbitals of the two dyes, the geometries of the dyes are
optimized by density functional theory (DFT) calculations at the
B3LYP/6-31G(d) level. In principle, the different substitutes can
modify the molecular orbital energy levels. As seen from Fig. 4,
the HOMO of the asymmetrical thiophene derivatives are essen-
tially isolated from the HOMO-1, which lie at 0.66 eV and 0.55 eV
below their respective HOMO levels. The LUMO levels with the
LUMO + 1 less than 1.06 eV and 1.11 eV above the individual
1.0
Dye1
Dye2
0.8
0.6
0.4
0.2
0.0
LUMO are also isolated. Therefore, Dye
1 shows narrower
HOMO–LUMO gap than that of Dye 2, which is a benefit for har-
vesting light with longer wavelength. This result is consistent with
the above UV–Vis absorption spectra. On the other hand, because
LUMO energies of the two dyes are higher than the TiO2 conduc-
tion band (ꢁ3.90 eV), the electron injection process is energetically
favorable compared with the conduction band edge energy level of
TiO2 electrode. And their HOMO energies are lower than Iꢁ/I3ꢁ
potential (ꢁ4.85 eV) [33], indicating that their ground-state sensi-
tizers regeneration are energetically favorable in DSSCs. The calcu-
lated HOMO–LUMO energies (Table 1) are fully consistent with
electrochemical measurements, showing a HOMO (LUMO) onset
at ꢁ4.91/ꢁ5.00 (ꢁ2.53/ꢁ2.52) eV for Dye 1 and Dye 2, respectively,
in good agreement with the calculated values at ꢁ4.99/ꢁ5.23
(ꢁ2.76/ꢁ2.75) eV.
400
500
600
700
800
Wavelength / nm
Fig. 3. The UV–Vis absorption spectra of Dye 1 and Dye 2 without CDCA absorbed
on 10 m-thick TiO2 film.
l
probably due to the introduction of the bulk moieties and long
alkyloxy chains on the BDT unit that inhibits the formation of
the aggregates. However, the adsorption amounts of Dye 1 on
TiO2 films were much smaller than those of Dye 2, while Dye 1
exhibited higher absorption than that of Dye 2 in the range of
360–600 nm, indicating that Dye 1 has better light harvesting
capability on TiO2 films. Once again, these results are in
well-agreement with the absorption spectra obtained in solution.
The electrochemical properties of Dye 1 and Dye 2 were further
analyzed by cyclic voltammetry (CV) measurements, which were
carried out in a 0.1 M solution of Bu4NPF6 in CH2Cl2. The reported
potentials were calibrated against the ferrocene/ferrocenium
(Fc/Fc+) couple, used as the internal standard. The redox potentials
and HOMO, LUMO levels [38] calculated from the oxidation poten-
tials are summarized in Table 1. The first oxidation peaks for Dye 1
and Dye 2 are +0.96 and +1.09 V vs normal hydrogen electrode
Moreover, Dye 1 and Dye 2 exhibited a similar localization of
HOMO and LUMO (see Fig. 5). For Dye 1, the HOMO is delocalized
over the entire molecule except the tail of the thiophene unit. The
LUMO is localized on BDT group and the acrylic acid, whereas
the LUMO + 1 is localized on the BDT group with ethylene. The
LUMO + 2 is localized on triphenylamine and LUMO is a p⁄ orbital
delocalized across the thiophene frame moving to the carboxyl
group. Since dyes are adsorbed onto TiO2 films through the car-
boxylic acid, the presence of a LUMO on the acrylic acid indicates
that the photo-excited electrons can transfer from the thiophene
skeleton to the carboxyl group, which is a benefit to the injection
of the photo-excited electrons to the TiO2 CB [34]. The similar dis-
tribution observed for Dye 2 indicated that the molecule may also
provide efficient electron injection into the TiO2 CB.
Table 2
Photovolatic performances of DSSCs sensitized by Dye 1, Dye 2 and N719.
Dye
Jsc (mA/cm2)
Voc (V)
FF
g
(%)
Adsorption amounts (mol Lꢁ1 cmꢁ2
)
Dye 1
Dye 1 with CDCA
Dye 2
Dye 2 with CDCA
N719
11.34
9.50
8.21
7.67
17.20
0.75
0.77
0.67
0.71
0.75
0.74
0.71
0.76
0.74
0.69
6.30
5.21
4.18
4.03
8.85
2.67 ꢀ 10ꢁ5
9.85 ꢀ 10ꢁ6
4.36 ꢀ 10ꢁ5
2.38 ꢀ 10ꢁ5

X. Zhou et al. / Organic Electronics 25 (2015) 245–253
251
100
N719
dye 1
dye 2
80
60
40
20
0
300
400
500
600
700
800
Wavelength / nm
Fig. 6. Photocurrent action spectra of TiO2 film electrodes sensitized by Dye 1, Dye
2 and N719.
Jsc between them. The higher Jsc for Dye 1 is consistent with its
higher and broader IPCE spectrum as compared with that for Dye
2. The electron lifetime (s) in the TiO2 film by impedance spectra
Fig. 4. Molecular orbital energy levels of the asymmetrical thiophene derivatives.
measurement is explained (see Supporting information Fig. S3).
The information on electron lifetime can be extract by using the
3.4. DSC performance
equation of se = 1/xmid = 1/2pfmid [39,40]. It is observed that
dye-1 and dye-2 exhibit the maximum fmid of 1.0 Hz, 1.78 Hz,
respectively. These results show that dye-1 presents the longest
se of 159 ms which is longer 80% than that of dye-2 with 89 ms,
and may explain the larger output of Voc. The comparison between
Dye 1 and Dye 2 clearly reflected the importance of molecular con-
figuration for the solar cell devices performance.
The Fig. 6 shows the action spectra in the form of monochro-
matic incident photon-to-current conversion efficiency (IPCE).
The Dye 1 based device clearly exhibited a stronger and broader
response in the entire visible spectral region, as compared to that
of the Dye 2 based device. The former showed plateaus of over
70% from 400 to 530 nm with the highest value of 82% at
483 nm, while Dye 2 presented a relatively narrow response
spectrum with the highest value of 74% at 436 nm. The higher
IPCE values for Dye 1 in the whole spectrum imply efficient charge
transfer upon photo-excitation. The difference between IPCE spec-
tra is in good agreement with their difference in absorption spectra
as shown in Fig. 1.
The chenodeoxycholic acid (CDCA) was used in this work for the
sensitization process to suppress the aggregation of dye molecules
and improve solar cell performance. The co-adsorption of dyes on
the TiO2 surface should improve both the photocurrent and the
photo-voltage of DSSC cells. However, our results show that the
presence of CDCA as co-adsorbent improved slightly the open cir-
cuit voltage, and that the short circuit photocurrent density was
dramatically decreased. As it is well known, a co-adsorbents would
influence the intermolecular interaction among dye molecules and
the extent of dye adsorption on the TiO2 surfacel. In the case of
applying the CDCA co-adsorbent, lower fill factor and lower
power conversion efficiencies were observed. This could be attrib-
uted to the dye aggregation which was not effectively avoided by
the use of CDCA, resulting on less dye adsorbed on the TiO2
(as shown in Table 2). On the other hand, triphenylamine and
N-octylcarbazole as donor group have relative big space block to
reduce the aggregation of dye molecules [41]. It can also be playing
other important roles as anti-aggregation of long alkyoxyl chains
grafted on BDT units [23,33].
The Fig. 7 shows the short circuit photocurrent density (Jsc),
open circuit voltage (Voc), fill factor (FF), and overall conversion
efficiencies (
g) of the two dyes-sensitized solar cells under AM
1.5 G simulated solar light at a light intensity of 100 mW cmꢁ2
,
and the data are summarized in Table 2. It can be seen that the
Dye 1 based device gave a short circuit photocurrent density (Jsc)
of 11.34 mA cmꢁ2, an open circuit voltage (Voc) of 750 mV and fill
factor (FF) of 0.74, corresponding to an overall conversion effi-
ciency of 6.30%, while the Dye 2 based device gave a Jsc of
8.21 mA cmꢁ2, Voc of 670 mV, FF of 0.76 and overall conversion
efficiency of 4.18% under the same testing conditions. The large dif-
ference in the efficiency comes mainly from the differences in the
Fig. 5. Calculated molecular orbital energy levels of the asymmetrical thiophene derivatives as Dye 1 (a) and Dye 2 (b).

252
X. Zhou et al. / Organic Electronics 25 (2015) 245–253
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Fig. 7. The photocurrent density–voltage curve of N719, Dye 1 and Dye 2 based
DSSC.
4. Conclusions
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We successfully synthesized two novel organic dyes by
introducing promising benzo(1,2-b:4,5-b0)dithiophene units as
spacers between the donor and the acceptor. The photochemical
and photoelectric properties of these two dyes were observed in
detail. Dye 1 containing triphenylamine group showed relative
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Acknowledgements
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This work was supported by the Natural Science Foundation of
China (21371138, 21301196) and the Funds for Creative Research
Groups of Hubei Province (2014CFA007), the Fundamental
Research Funds for the Central Universities (2042015kf0180) of
China and Large-scale Instrument and Equipment Sharing
Foundation of Wuhan University.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.orgel.2015.06.
033.
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