Influence of the benzo[d]thiazole-derived π-bridges on the optical and photovoltaic performance of D-π-A dyes

Article abstract of DOI:10.1016/j.dyepig.2012.11.004

New metal-free organic sensitizers containing a benzo[d]thiazole or phenyl unit as the π-conjugated system, a triphenylamine as an electron donor, and a cyanoacrylic acid moiety as an electron acceptor were synthesized and used for dye-sensitized solar cells. Photophysical and electrochemical properties of these dyes were investigated, and their performances as sensitizers in solar cells were measured. The introduction of a benzo[d]thiazole unit into the molecular structure resulted in a high incident photon-to-current conversion efficiency (more than 70%) from 340 nm to 600 nm. One solar cell containing a benzo[d]thiazole unit, produced a η of 5.85% (J_SC = 10.63 mA cm^-2, V_OC = 0.72 V, and ff = 0.77) under 100 mW cm ^-2 simulated AM 1.5 G solar irradiation.

Full text of DOI:10.1016/j.dyepig.2012.11.004

Dyes and Pigments 96 (2013) 619e625
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Influence of the benzo[d]thiazole-derived
p
-bridges on the optical
and photovoltaic performance of De
p
eA dyes
*
*
Zhenhua Ci, Xiaoqiang Yu , Ming Bao, Chaolei Wang, Tingli Ma
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116023, China
a r t i c l e i n f o
a b s t r a c t
Article history:
New metal-free organic sensitizers containing a benzo[d]thiazole or phenyl unit as the
p-conjugated
Received 17 September 2012
Received in revised form
30 October 2012
Accepted 2 November 2012
Available online 14 November 2012
system, a triphenylamine as an electron donor, and a cyanoacrylic acid moiety as an electron acceptor
were synthesized and used for dye-sensitized solar cells. Photophysical and electrochemical properties of
these dyes were investigated, and their performances as sensitizers in solar cells were measured. The
introduction of a benzo[d]thiazole unit into the molecular structure resulted in a high incident photon-
to-current conversion efficiency (more than 70%) from 340 nm to 600 nm. One solar cell containing
a benzo[d]thiazole unit, produced a
h
of 5.85% (JSC ¼ 10.63 mA cmꢀ2, VOC ¼ 0.72 V, and ff ¼ 0.77) under
Keywords:
Dye-sensitized solar cells
Organic dye
100 mW cmꢀ2 simulated AM 1.5 G solar irradiation.
Ó 2012 Elsevier Ltd. All rights reserved.
Triphenylamine
p-Conjugated bridge
Benzo[d]thiazole
Photon-to-current conversion efficiency
1. Introduction
thiophene [19e22], thienothiophene [23e27], dithienothiophene
[28,29], benzo[b]thiophene [30,31], or benzothiadiazole [32e35],
have been designed and synthesized as efficient sensitizers to
Dye-sensitized solar cells (DSSCs) have attracted significant
attention in recent years due to their high efficiency, low cost, and
facile fabrication [1e3]. DSSCs based on Ru(II)-polypyridyl complex
photosensitizers, such as N3, N719, and the black dye, have ach-
ieved remarkable conversion efficiency of up to 11% under AM 1.5
irradiation conditions [4e6]. However, the main drawbacks of the
Ru(II) complex sensitizers include the cost of ruthenium metal, the
requirement for cautious synthesis, and the tedious purification
process. Metal-free organic dyes have gained increasing attention
due to their unique advantages, such as high molar absorption
coefficient, ease of structure modification, and relatively low
material cost [7e10].
achieve high conversion efficiency in DSSC devices. The
gated bridge has a great influence on photoelectronic properties of
the De eA dyes. Thus, developing highly efficient De eA dyes
with novel -conjugated bridge applied in DSSCs is an area of
strong current research activity.
p-conju-
p
p
p
This study presents three new organic dyes (DBT1, DBT2, and
DPB) with a 2-methylbenzo[d]thiazole moiety or a phenyl group as
p-conjugated bridge, a simple triphenylamine moiety as electron
donor, and a cyanoacetic acid as electron acceptor, as shown in Fig.1.
The new sensitizers DBT1 and DBT2, with a 2-methylbenzo[d]
thiazole moiety as p-conjugated bridge have the following advan-
A common organic dye for DSSCs contains a structure of elec-
tages: (i) the use of a 2-methylbenzo[d]thiazole unit can avoid the
aggregation of the dye with its non-planar structure to achieve
a high short-circuit current density, (ii) the introduction of electron-
tron donor/acceptor (DeA) linked through a
p-conjugated bridge,
which is termed the De eA molecular structure [11]. Triphenyl-
p
amine derivatives have been widely used as the electron donor (D),
whereas a cyanoacrylic acid moiety acts as the electron acceptor (A)
rich benzo[d]thiazole unit as p-conjugated bridge connecting with
triphenylamine can extend the duration of the excited state.
in this structure. De
peA dyes based on triphenylamine moieties
with various -conjugated bridges, such as benzene [12e18],
p
2. Experimental section
2.1. General analytical measurements
* Corresponding authors. Tel.: þ86 411 84986181; fax: þ86 411 84986180.
E-mail addresses: yuxiaoqiang@dlut.edu.cn (X. Yu), tinglima@dlut.edu.cn
(T. Ma).
All chemicals were used as received form commercial sources
without purification. Solvents for chemical synthesis, such as
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.11.004

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Z. Ci et al. / Dyes and Pigments 96 (2013) 619e625
Fig. 1. Molecular structures of dyes DBT1, DBT2, and DPB.
dichloromethane, dimethylformamide, toluene, and tetrahydro-
furan were purified by distillation. 1H and 13C NMR spectra were
recorded on either Varian Inova-400 spectrometer (400 MHz for
1H; 100 MHz for 13C) or Bruker Avance II-400 spectrometer
(400 MHz for 1H; 100 MHz for 13C); CDCl3 and TMS were used as
solvent and internal standard, respectively. High resolution mass
spectra were recorded on either Q-TOF or GC-TOF mass
spectrometer.
product 3 as white solid (2.27 g, 78% yield). mp 173e175 ꢁC. 1H NMR
(400 MHz, CDCl3)
1H), 7.10 (dd, J ¼ 8.5, 2.4 Hz, 1H), 2.25 (s, 3H).
d
8.59 (br s, 1H), 7.57 (br s, 1H), 7.38 (d, J ¼ 8.8 Hz,
2.3.3. Synthesis of N-(2,5-dibromophenyl)ethanethioamide (4) [38]
A solution of compound 3 (2.93 g, 10 mmol) and hexame-
thyldisiloxane (HMDO) (2.92 g, 18 mmol) in toluene (50 mL) was
added to phosphorus pentasulfide (0.72 g, 3.25 mmol) at 60 ꢁC, and
the mixture was heated under reflux for 5 h. The crude product was
purified by column chromatography (petroleum ether/ethyl
acetate ¼ 10/1, v/v) to obtain the desired product 4 as yellowe
orange solid (2.66 g, 86% yield). mp 121e123 ꢁC. 1H NMR
2.2. Theoretical calculations
Gaussian 03 package was used for density functional theory
calculation [36]. The geometries and energies of DBT1, DBT2, and
DPB were determined using the B3LYP method with the 6-31G (d)
basis set.
(400 MHz, CDCl3)
J ¼ 8.6 Hz, 1H), 2.78 (s, 3H).
d
8.77 (s, 1H), 7.48 (d, J ¼ 8.5 Hz, 1H), 7.27 (d,
2.3.4. Synthesis of 4,7-dibromo-2-methylbenzo[d]thiazole (5)
2.3. Synthesis
A mixture of compound 4 (3.09 g, 10 mmol) and sodium
hydroxide (3.2 g, 80 mmol) in watereethanol (40 mLe2 mL) was
added dropwise to a solution of potassium ferricyanide (13.17 g,
40 mmol) in water (20 mL) and stirred at 95 ꢁC for 2 h. The reaction
mixture was cooled in an ice bath to yield the precipitate. Subse-
quently, the precipitate was filtered, washed with water, and
concentrated in vacuo. The crude product was purified by column
chromatography (petroleum ether/ethyl acetate ¼ 20/1, v/v) to
obtain the desired product 5 as pale yellow powder (1.46 g, 48%
yield). mp 153e155 ꢁC. IR (KBr): 3080, 2919, 1856, 1522, 1446, 1372,
The synthetic routes to DBT1, DBT2, and DPB dyes are shown in
Scheme 1.
2.3.1. Synthesis of 2,5-dibromonitrobenzene (2) [37]
A mixture of nitric acid (90%, 4.6 g, 70 mmol) and concentrated
sulfuric acid (75 mL) was added to
a solution of 1,4-
dibromobenzene (11.8 g, 50 mmol) in CH2Cl2 (30 mL) and
concentrated sulfuric acid (20 mL) using a dropping funnel, and
was allowed to react for 20 min at room temperature. The reaction
mixture was stirred for 30 min, quenched with a solution of 25% aq.
NaOH (3 mL), extracted with CH2Cl2 (30 mL), washed with water
(10 mL), and dried over MgSO4. Evaporation in vacuo afforded
compound 2 (13.7 g, 97% yield) as light yellow crystals. mp 83e
1339, 1170, 1113, 1087, 909, 798. 1H NMR (400 MHz, CDCl3)
d 7.52 (d,
J ¼ 8.4 Hz, 1H), 7.34 (d, J ¼ 8.4 Hz, 1H), 2.88 (s, 3H). 13C NMR
(100 MHz, CDCl3) d 168.7, 151.1, 139.2, 130.4, 128.4, 115.0, 112.9, 20.7.
HRMS (EI, m/z): calcd. for C8H5NSBr2, 304.8509 [M]þ; found,
304.8509.
84 ꢁC. 1H NMR (400 MHz, CDCl3)
7.63 (m, 2H).
d
7.98 (d, J ¼ 2.3 Hz, 1H), 7.55e
2.3.5. Synthesis of 4-(7-bromo-2-methylbenzo[d]thiazol-4-yl)-N,N-
diphenylaniline (7a) and 4-(4-bromo-2-methylbenzo[d]thiazol-7-
yl)-N,N-diphenylaniline (7b) [39]
2.3.2. Synthesis of 2,5-dibromoacetanilide (3) [37]
A mixture of compound 2 (2.81 g, 10 mmol) and iron powder
(5.5 g, 100 mmol) in AcOH (50 mL) was stirred for 5 h at 70 ꢁC. The
mixture was washed with water and extracted with ethyl acetate
after cooling to room temperature. The organic layer was collected
and dried over anhydrous MgSO4. The solvent was removed under
reduced pressure to afford 2,5-dibromoaniline (2.408 g, 96% yield).
Acetyl chloride (0.83 g, 10.6 mmol) was slowly added to a solution
of the foregoing 2,5-dibromoaniline (2.408 g, 9.6 mmol) in pyridine
(20 mL), and the resulting mixture was heated under reflux for 1 h.
The reaction mixture was poured into water (100 mL) after cooling
to room temperature. The resulting precipitate was collected by
vacuum filtration, and then recrystallized from methanol to obtain
A mixture of compound 5 (0.307 g, 1 mmol), N,N-diphenyl-4-
aminophenylboronic acid (0.289 g, 1 mmol), Pd(PPh3)4 (0.116 g,
0.1 mmol), and Na2CO3 (0.106 g, 1 mmol) was dissolved in toluenee
THFeH2O (1 mLe1 mLe0.2 mL) and was heated under reflux for
24 h under nitrogen atmosphere. Water was added to quench the
reaction after the reaction was completed. The mixture was then
extracted with diethyl ether, dried over anhydrous MgSO4, and
evaporated under vacuum. The crude product was purified by
column chromatography (petroleum ether/ethyl acetate ¼ 20/1, v/
v) to afford two fractions. Fractions 1 product 7a (0.132 g, 28%
yield). mp 151e153 ꢁC. IR (KBr): 3053, 3032, 1589, 1509, 1490, 1459,
1277, 1170, 909, 835, 816, 805, 755, 696, 656, 535, 509. 1H NMR

Z. Ci et al. / Dyes and Pigments 96 (2013) 619e625
621
Scheme 1. Synthesis of the dyes (DBT1, DBT2, and DPB).
(400 MHz, d6-DMSO)
1H), 7.53 (d, J ¼ 8.1 Hz, 1H), 7.35 (dd, J ¼ 7.8, 8.0 Hz, 4H), 7.10e7.05
(m, 8H), 2.83 (s, 3H). 13C NMR (100 MHz, d6-DMSO)
167.2, 150.0,
d
7.76 (d, J ¼ 8.6 Hz, 2H), 7.68 (d, J ¼ 8.1 Hz,
on silica (petroleum ether/ethyl acetate ¼ 5/1, v/v) to obtain product
9a as yellow solid (0.452 g, 90%). mp 156e157 ꢁC. IR (KBr): 3433,
3032, 2922,1894,1659,1588,1511,1488, 1276, 877, 807, 753. 1H NMR
d
147.4, 139.1, 133.8, 131.8, 130.9, 130.1, 128.2, 127.7, 124.8, 123.8, 111.2,
20.6. HRMS (EI, m/z): calcd. for C26H19N2SBr, 470.0452 [M]þ; found,
470.0446.
(400 MHz, d6-DMSO)
d
9.99 (s, 1H), 8.17 (d, J ¼ 4.0 Hz, 1H), 7.94 (d,
J ¼ 8.0 Hz, 1H), 7.89 (d, J ¼ 4.0 Hz, 1H), 7.84 (m, 2.0 Hz, 2H), 7.72 (d,
J ¼ 8.0 Hz,1H), 7.36 (t, J ¼ 8.0 Hz, 4H), 7.12e7.06 (m, 8H), 2.87 (s, 3H).
Fraction 2 product 7b (24% yield). mp 149e151 ꢁC. IR (KBr):
3031, 2921, 1590, 1511, 1488, 1462, 1275, 1175, 921, 810, 751, 695,
13C NMR (100 MHz, d6-DMSO)
d 184.7,167.3,151.4,150.9,147.6,147.4,
142.9,139.2,135.5,134.6,1312.0,131.1,130.1,127.4,126.9,126.0,125.2,
125.0, 123.9, 122.6, 20.4. HRMS (EI, m/z): calcd. for C31H22N2OS2,
502.1174 [M]þ; found, 502.1175.
643, 581, 539. 1H NMR (400 MHz, d6-DMSO)
d
7.80 (d, J ¼ 8.0 Hz,
2H), 7.59 (d, J ¼ 8.7 Hz, 2H), 7.41e7.34 (m, 5H), 7.14e7.06 (m, 8H),
2.83 (s, 3H). 13C NMR (100 MHz, d6-DMSO)
168.9, 151.6, 148.1,
d
147.2,135.1,135.0,132.7,130.3,130.2,128.9,125.4,125.1,124.2,122.7,
114.1, 20.2. HRMS (EI, m/z): calcd. for C26H19N2SBr, 470.0452 [M]þ;
found, 470.0457.
2.3.8. Synthesis of 5-(7-(4-(diphenylamino)phenyl)-2-methylbenzo
[d]thiazol-4-yl)thiophene-2-carbaldehyde (9b)
The synthetic route of 9b is similar with 9a to give yellow solid in
82% yield. mp 154e156 ꢁC. IR (KBr): 3430, 3034, 2922, 1662, 1589,
2.3.6. Synthesis of 4’-bromo-N,N-diphenyl-[1,1’-biphenyl]-4-amine
(7c)
1512,1487,1273, 804, 697.1HNMR(400MHz, d6-DMSO)
d9.97(s,1H),
8.18 (d, J ¼ 4.0 Hz,1H), 8.16 (d, J ¼ 7.4 Hz, 1H), 8.08 (d, J ¼ 4.1 Hz,1H),
7.66 (d, J ¼ 8.7 Hz, 2H), 7.59 (d, J ¼ 8.0 Hz,1H), 7.37 (dd, J ¼ 7.8, 8.1 Hz,
4H), 7.15e7.07 (m, 8H), 2.92 (s, 3H). 13C NMR (100 MHz, d6-DMSO)
The synthetic route of 7c is similar with 7a to give orange solid
in 64% yield. mp 180e181 ꢁC. IR (KBr): 3446, 3064, 3035, 1589,1481,
1337, 1277, 814, 748, 695. 1H NMR (400 MHz, d6-DMSO)
d
7.52 (d,
J ¼ 8.6 Hz, 2H), 7.42 (dd, J ¼ 8.7, 1.3 Hz, 4H), 7.26 (dd, J ¼ 6.8, 1.8 Hz,
4H), 7.12e7.04 (m, 8H). 13C NMR (100 MHz, d6-DMSO)
147.6, 147.4,
d184.9,168.9,149.8,149.1,148.2,147.2,143.9,138.2,135.3,132.8,130.2,
129.1,127.6,125.7,125.2,124.9,124.6,124.3,122.6, 20.5. HRMS (EI, m/
d
z): calcd. for C31H22N2OS2, 502.1174 [M]þ; found, 502.1175.
139.2, 133.0, 132.2, 130.1, 128.6, 1278.0, 124.7, 123.8, 123.5, 120.7.
HRMS (EI, m/z): calcd. for C24H18BrN, 399.0623 [M]þ; found,
399.0617.
2.3.9. Synthesis of 5-(4’-(diphenylamino)-[1,1’-biphenyl]-4-yl)
thiophene-2-carbaldehyde (9c)
The synthetic route of 9c is similar with 9a to give yellow solid in
71% yield. mp 180e182 ꢁC. IR (KBr): 3443, 3031, 2957, 2924, 2794,
1666, 1591, 1519, 1491, 1450, 1326, 1274, 818, 800, 751. 1H NMR
2.3.7. Synthesis of 5-(4-(4-(diphenylamino)phenyl)-2-methylbenzo
[d]thiazol-7-yl)thiophene-2-carbaldehyde (9a)
A mixture of 7a (0.471 g, 1 mmol), 2-tributylstannyl-5-dioxolanyl
thiophene (0.49 g,1.1 mmol), and Pd(PPh3)4 (0.116 g, 0.1 mmol) in dry
DMF (10 mL) was heated at 80 ꢁC for 18 h under nitrogen atmo-
sphere. Water was added to quench the reaction when the reaction
was completed. The resulting mixture was then extracted with
diethyl ether, dried over anhydrous MgSO4, and evaporated under
vacuum to obtain the crude dioxolane derivative. The dioxolane
derivative was suspended in glacial acetic acid (5 mL) and heated at
50 ꢁC. Water (1 mL) was added after a clear solutionwas formed, and
temperature was maintained at 50 ꢁC for 5 h. The reaction was then
cooled by adding ice water (10 mL) into the mixture. Afterward, the
resulting orange precipitate was filtered with water and washed
with methanol. The residue was purified by column chromatography
(400 MHz, d6-DMSO)
d
9.92 (s, 1H), 8.06 (d, J ¼ 3.7 Hz, 1H), 7.86 (d,
J ¼ 8.1 Hz, 2H), 7.79 (d, J ¼ 3.6 Hz, 1H), 7.74 (d, J ¼ 8.2 Hz, 2H), 7.66
(d, J ¼ 8.4 Hz, 2H), 7.33 (dd, J ¼ 7.8, 7.5 Hz, 4H), 7.11e7.03 (m, 8H). 13
C
NMR (100 MHz, d6-DMSO)
d 184.4, 152.8, 147.7, 147.4, 142.3, 141.0,
139.7, 133.0, 131.4, 130.1, 128.0, 127.3, 127.2, 125.6, 124.8, 123.9,
123.4. HRMS (EI, m/z): calcd. for C29H21NOS, 431.1344 [M]þ; found,
431.1337.
2.3.10. Synthesis of 2-cyano-3-(5-(4-(4-(diphenylamino)phenyl)-2-
methylbenzo[d]thiazol-7-yl)thiophen-2-yl) acrylic acid (DBT1)
A mixture of 9a (1 mmol, 0.503 g), cyanoacetic acid (1.1 mmol,
0.094 g), ammonium acetate (0.25 mmol, 0.02 g), and acetic acid
(5 mL) was heated to reflux for 5 h under nitrogen atmosphere. The

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Z. Ci et al. / Dyes and Pigments 96 (2013) 619e625
mixture was subsequently cooled to room temperature, poured
into ice water, and extracted with ethyl acetate. The organic layer
was collected, dried over anhydrous MgSO4, and evaporated under
vacuum. The crude product was purified by column chromatog-
raphy on silica (DCM/ethanol ¼ 20/1, v/v) to obtain DBT1 as a red
solid (0.453 g, 76%). mp 238e240 ꢁC. IR (KBr): 3447, 2211, 1636,
1591, 1508, 1490, 1438, 1358, 1279, 1177, 753, 697. 1H NMR
workstation. The irradiation source for the photocurrentevoltage
(JeV) measurement was an AM 1.5 solar simulator (16S-002,
Solar Light Co. Ltd., USA). The incident light intensity was
100 mW cmꢀ2 calibrated with a standard silicon solar cell. The
currentevoltage curve was obtained by linear sweep voltammetry
(LSV) method using an electrochemical workstation (LK9805,
Lanlike Co. Ltd., China). The incident photon-to-current conversion
efficiency (IPCE) measurement was performed by a Hypermono-
light (SM-25, Jasco Co. Ltd., Japan).
(400 MHz, d6-DMSO)
d
8.59 (s, 1 H), 8.13 (d, J ¼ 4.0 Hz, 1H), 7.94 (d,
J ¼ 8.0 Hz, 1H), 7.90 (d, J ¼ 4.0 Hz, 1H), 7.84 (d, J ¼ 8.0 Hz, 2H), 7.72
(d, J ¼ 8.0 Hz, 1H), 7.35 (t, J ¼ 8.0 Hz, 4H), 7.12e7.06 (m, 8H), 2.88 (s,
3H). 13C NMR (100 MHz, d6-DMSO)
d 167.2, 151.4, 147.5, 147.4, 139.7,
3. Results and discussion
132.1, 131.1, 130.1, 126.9, 126.2, 125.4, 124.9, 123.9, 122.7, 30.9. HRMS
(ES, m/z): calcd. for C34H22N3O2S2, 568.1153 [M]þ; found, 568.1161.
3.1. Synthesis
2.3.11. Synthesis of 2-cyano-3-(5-(7-(4-(diphenylamino)phenyl)-2-
methylbenzo[d]thiazol-4-yl)thiophen-2-yl) acrylic acid (DBT2)
The synthetic route of DBT2 is similar with DBT1 to give red
solid in 71% yield. mp 240e242 ꢁC. IR (KBr): 3448, 2214, 1613, 1593,
1517,1490,1433,1384,1332,1282,1220,1177, 809, 752, 697. 1H NMR
DBT1, DBT2, and DPB dyes were synthesized using the stepwise
synthetic procedure (Scheme 1). The nitration reaction of 1,4-
dibromobenzene (1) produced 2,5-dibromonitrobenzene (2). The
reduction of the nitro group was realized following a standard
protocol using iron powder in acetic acid to afford 2,5-
dibromoaniline. Benzanilide (3) was prepared from 2,5-
dibromoaniline by reaction with acetylchloride, which was then
efficiently transformed into thiobenzanilide (4) by reaction with
Lawesson’s reagent. Regiospecific cyclization of 4 using potassium
ferricyanide and aqueous sodium hydroxide produced 4,7-
dibromo-2-methylbenzo[d]thiazole (5). The Suzuki cross-coupling
reaction of 5 or 1 with arylboronic acid 6 afforded the corre-
sponding products 7ae7c [41]. The Stille cross-coupling reaction of
7ae7c with organotin compound 8, followed with deprotection
reaction to afford the free aldehydes 9ae9c. Finally, the aldehydes
were condensed with cyanoacetic acid to obtain the target
compounds DBT1, DBT2, and DPB via Knöevenagel reaction, in the
presence of ammonium acetate [42].
(400 MHz, d6-DMSO)
d
8.52 (s, 1H), 8.21 (d, J ¼ 4.2 Hz, 1H), 8.14 (d,
J ¼ 8.1 Hz, 1H), 8.07 (d, J ¼ 4.3 Hz, 1H), 7.66 (d, J ¼ 8.7 Hz, 2H), 7.60
(d, J ¼ 8.0 Hz, 1H), 7.37 (dd, J ¼ 8.0, 7.9 Hz, 4H), 7.15e7.07 (m, 8H),
2.89 (s, 3H). 13C NMR (100 MHz, d6-DMSO)
d 168.7, 149.6, 148.2,
147.2, 135.4,132.2,132.0,130.2,129.1,129.1,127.8,125.6,125.2,125.1,
124.3, 122.7, 29.5. HRMS (ES, m/z): calcd. for C34H22N3O2S2,
568.1153 [M]þ; found, 568.1160.
2.3.12. Synthesis of 2-cyano-3-(5-(4’-(diphenylamino)-[1,1’-biphe
nyl]-4-yl)thiophen-2-yl) acrylic acid (DPB)
The synthetic route of DPB is similar with DBT1 to give red solid
in 81% yield. mp 248e249 ꢁC. IR (KBr): 3431, 3031, 2974, 2219, 1684,
1583, 1488, 1422, 1365, 1327, 1282, 1224, 821, 802, 750, 696. 1H NMR
(400 MHz, d6-DMSO)
d
8.43 (s, 1H), 7.97 (d, J ¼ 4.0 Hz, 1H), 7.83 (d,
J ¼ 8.3 Hz, 2H), 7.78 (d, J ¼ 4.1 Hz, 1H), 7.75 (d, J ¼ 8.4 Hz, 2H), 7.65
3.2. Absorption properties in solutions
(d, J ¼ 8.6 Hz, 2H), 7.33 (dd, J ¼ 7.8, 7.8 Hz, 4H), 7.11e7.03 (m, 8H). 13
C
NMR (100 MHz, d6-DMSO)
d 164.2, 151.8, 147.7, 147.4, 145.6, 140.8,
The optical and electrochemical properties of the three dyes are
summarized in Fig. 2 and Table 1. Fig. 2 shows that all dyes in THF
solutions gave two distinct absorption bands: one relatively weak
band in the near-UV region (300 nme310 nm) that corresponds to
140.7, 135.3, 133.0,131.3, 130.1, 128.0,127.3, 127.1, 125.4,124.8, 123.9,
123.4, 117.7. HRMS (EI, m/z): calcd. for C32H22N2O2S, 498.1402 [M]þ;
found, 498.1404.
the
pep* electron transition and the other one has a strong
2.4. Fabrication of DSSCs
absorption in the visible region (420 nme460 nm), which can be
assigned to an intramolecular charge transfer (ICT) between the
triarylamine donating unit and the cyanoacrylic acid anchoring
moiety, thereby producing an efficient charge-separated state.
Compared with DPB, the maximum absorptions of DBT1 and DBT2
The screen-printable TiO2 pastes were prepared according to the
procedure developed by Ma’s group [40]. A screen-printing tech-
nique was used to fabricate TiO2 films. First, The paste was
deposited on a fluorine-doped tin oxide conductive glass (FTO,
Asahi Glass Co., Ltd.; sheet resistance: 10 ohm/sq). Second, the film
was sintered at 450 ꢁC for 30 min in atmospheric air, and then
immersed in 40 mM TiCl4 solution for 30 min at 70 ꢁC, rinsed with
water and ethanol, and sintered at 500 ꢁC for 30 min. The film was
dipped into DBT1, DBT2, and DPB dye solutions (0.4 mM in THF) for
18 h after cooling to 80 ꢁC. Finally, dye-sensitized TiO2 photo-
40
35
DBT1
DBT2
DPB
30
electrodes (thickness: 12 mm) were obtained. The organic electro-
25
20
15
10
5
lyte was composed of 0.06 M LiI, 0.03 M I2, 0.1 M guanidinium
thiocyanate, 0.6 M 1-propyl-3-methylimidazolium iodide (PMII),
and 0.5 M tert-butyl-pyridine in acetonitrile. The active area of
DSSCs was 0.16 cm2. DSSC devices were assembled with counter
electrodes (thermally platinized FTO) using a thermoplastic frame
(Surlyn, 60 mm thick).
2.5. Measurements
0
300
400
500
600
Absorption and emission spectra were recorded with HP8453
(USA) and PTI700 (USA), respectively. Electrochemical measure-
ment was carried out on a BAS100W (USA) electrochemistry
Wavelength (nm)
Fig. 2. Absorption spectra of DBT1, DBT2, and DPB in THF at ambient temperature.

Z. Ci et al. / Dyes and Pigments 96 (2013) 619e625
623
Table 1
Optical and electrochemical properties of DBT1, DBT2, and DPB dyes.
3.4. Electrochemical properties
The redox behavior of DBT1, DBT2, and DPB dyes were studied
by cyclic voltammetry (in Fig. 4) to investigate the ability of electron
transfer from the excited dye molecules to the conductive band of
TiO2. The cyclic voltammograms of these dyes were measured in
a solution of 0.1 M n-Bu4NPF6 in THF. A three-electrode cell con-
taining a Pt-coil working electrode, a Pt wire counter electrode, and
a Ag/AgCl reference electrode was employed. The ferrocene/ferri-
cenium (Fc/Fcþ) redox couple was used as an internal reference.
The examined highest occupied molecular orbital (HOMO) levels
and the lowest unoccupied molecular orbital (LUMO) levels were
collected, as shown in Table 1. HOMO values of (1.12 Ve1.16 V vs.
NHE) were more positive than the Iꢀ/I3ꢀ redox couple (0.4 V vs.
NHE), suggesting that the oxidized dyes can thermodynamically
accept electrons from Iꢀ ion in iodide/triiodide electrolyte for
regeneration. Electron injection from the excited sensitizers to
the conduction band of TiO2 should be energetically favorable
because of the more negative LUMO values (ꢀ1.38 V to ꢀ1.50 V
vs. NHE) compared with the conduction band edge energy level
of the TiO2 electrode (at approximately ꢀ0.5 V vs NHE) [43].
These results clearly demonstrate that the novel dyes are poten-
tially efficient dyes for DSSCs. Table 1 also shows that the intro-
duction of the benzo[d]thiazole bridge can change the HOMOe
LUMO energy gaps of the dyes more narrowly.
Dye
Absorptiona
Emissiona Oxidation potential
lmax ε at lmax
lmax
Eox [V]b
E0e0 [V]c ELUMO [V]
[nm] [Mꢀ1 cmꢀ1
]
[nm]
(versus NHE) (Abs/Em) (versus NHE)
DBT1 420 30,902
DBT2 423 37,531
DPB 416 37,148
592
573
583
1.13
1.16
1.12
2.51
2.55
2.62
ꢀ1.38
ꢀ1.39
ꢀ1.50
a
Absorption and emission spectra were measured in THF, with a concentration of
1.0 ꢂ 10ꢀ3 M at room temperature.
b
The oxidation potential of the dyes was measured under the following condi-
tions: working electrode, Pt; electrolyte, 0.1
M tetrabutylammonium hexa-
fluorophosphate, n-Bu4NPF6 in THF; scan rate, 0.1 V/s. Potentials measured vs Feþ/Fe
were converted to NHE by addition of þ0.63 V [44].
c
The E0e0 energies were estimated from the intercept of the normalized
absorption and emission spectra.
were slightly distant due to the expansion of the
system due to the introduction of the benzo[d]thiazole unit into the
-conjugated bridge. Noticeably, the molar extinction coefficients
of the three dyes were higher than that of the N719 dye
(14.0 Mꢀ1 cmꢀ1 ꢂ 103 Mꢀ1 cmꢀ1) [5], indicating a good light har-
vesting ability.
p-conjugation
p
3.3. Molecular orbital calculations
The molecular geometries and electron distributions of DBT1,
DBT2, and DPB were obtained using the density function theory
(DFT) calculations with Gaussian 03 program. The calculations
were performed with the B3LYP exchange correlation functional
under 6-31G (d) basis. The results are shown in Fig. 3. The electron
distribution before the light irradiation locates mainly on the donor
units; whereas it moves to the acceptor units close to the anchoring
groups after light irradiation, which favors the electron injection
from the dye molecules to the conduction band edge of TiO2.
3.5. Photovoltaic performance
The action spectrum, the incident photon-to-electron conver-
sion efficiency (IPCE) as a function of wavelength, was measured to
evaluate the photoresponse of the photoelectrode in the whole
spectral region. DBT1, DBT2, and DPB sensitizers were used to
manufacture solar cell devices. Fig. 5 shows the IPCE obtained with
0.06 M LiI, 0.03 M I2, 0.1 M guanidinium thiocyanate, 0.6 M 1-
propyl-3-methylimidazolium iodide (PMII), and 0.5 M tert-butyl-
Fig. 3. Frontier HOMO and LUMO orbitals of DBT1, DBT2, and DPB dyes.

624
Z. Ci et al. / Dyes and Pigments 96 (2013) 619e625
Table 2
Photovoltaic performance of DSSCs sensitized with the as-synthesized dyes
compared with the N719 dye.a
20
10
0
DBT1
DBT2
DPB
Dye
CDCA
JSC/mA cmꢀ2
VOC/mV
ff
h/%
DBT1
0 mM
0.8 mM
0 mM
0.8 mM
0 mM
10.63
11.01
10.04
10.43
8.38
0.72
0.71
0.70
0.71
0.68
0.68
0.70
0.77
0.78
0.77
0.76
0.76
0.75
0.76
5.85
6.08
5.43
5.63
4.24
4.82
7.16
DBT2
DPB
0.8 mM
0 mM
9.44
N719
13.54
a
The DSSCs had an active area of w0.16 cm2 and used an electrolyte composed of
0.06
M LiI, 0.03 M I2, 0.1 M guanidinium thiocyanate, 0.6 M 1-propyl-3-
methylimidazolium iodide (PMII), and 0.5 M tert-butyl-pyridine in acetonitrile.
-10
0.5
1.0
1.5
pyridine in acetonitrile as redox electrolyte. The three dyes can
efficiently convert visible-light to photocurrent in the region of
300 nme600 nm. A solar cell based on DBT1 showed the highest
IPCE value of 83% at 440 nm. In addition the cell exhibited
a broad IPCE spectrum with high IPCE values (>70%) from
340 nm to 500 nm. The IPCE spectrum of the DSSCs based on
DBT2 was similar to that based on DBT1. However, the IPCE
spectrum of DPB was slightly lower, with a maximum IPCE of
76% at 420 nm.
Fig. 6 shows the currentevoltage (JeV) curves of the DSSCs
based on dyes DBT1, DBT2, and DPB under standard global AM
1.5 G solar irradiation, and the results are shown in Table 2. The
DSSCs based on dye DBT1 showed the best performance, with an
open-circuit voltage of 0.72 V, a short-circuit photocurrent density
of 10.63 mA cmꢀ2, and a fill factor of 0.77, yielding an overall
Potential / V (vs Ag/AgCl)
Fig. 4. Cyclic voltammogram of DBT1, DBT2, and DPB in THF.
90
80
70
60
50
40
30
20
10
0
DBT1
DBT2
DPB
conversion efficiency (h) of 5.85%. For a fair comparison, the N719-
sensitized TiO2 solar cell showed an efficiency of 7.16%, with a JSC of
13.54 mA cmꢀ2, a VOC of 0.70 V and a fill factor of 0.76. The
conversion efficiency of DBT1 (
h
¼ 5.85%) reached 80% of the N719
cell efficiency (
h
¼ 7.16%). Compared with the DPB based cells, the
DBT1 and DBT2 based cells showed higher JSC values, reflecting
their better sunlight-harvesting ability due to the benzo[d]thiazole
structure as a
Chenodeoxycholic acid (CDCA) is used as coadsorbent to
dissociate the -stacked dye aggregates and improve the electron-
p-spacer.
300
350
400
450
500
550
600
650
700
Wavelength (nm)
p
Fig. 5. IPCE spectra for DSSCs based on the as-synthesized dyes.
injection yield, thus affording higher JSC value. For the DSSCs
sensitized by the three dyes, the JSC value of dye DPB with CDCA
greatly increased (1.06 mA cmꢀ2) than that of DBT1 and DBT2
(<0.4 mA cmꢀ2) as shown in Table 2. The results indicate that the
use of a 2-methylbenzo[d]thiazole unit can avoid the aggregation of
the dyes with its non-planar structure to achieve a high short-
circuit current density.
16
12
8
DBT1
DBT2
DPB
4. Conclusion
N719
In summary, three new organic sensitizers (DBT1, DBT2, and
DPB) containing benzo[d]thiazole or benzene structures as a
p-
conjugated spacer were designed and synthesized for dye-
sensitized solar cells. The introduction of benzo[d]thiazole unit to
tune the HOMOeLUMO levels increased the short circuit photo-
currents (JSC) and resulted in higher efficiencies. The cells based on
4
DBT1 and DBT2, with benzo[d]thiazole structures as p-conjugated
spacers exhibited 5.85% and 5.43% conversion efficiencies, respec-
tively, which are higher than that of DPB with the benzene moiety.
Our results indicate that the benzo[d]thiazole moiety can be
0
advantageously incorporated into the dye-sensitizer as a
to improve cell conversion efficiency. Further improvement of the
power conversion efficiency of De eA dyes can be investigated by
-conjugated spacer based on different
p-spacer
0.0
0.2
0.4
0.6
0.8
Voltage (V)
p
using benzo[d]thiazole as a
p
Fig. 6. Currentepotential (JeV) curve for the DSSCs based on DBT1, DBT2, DPB, and
N719.
donors.

Z. Ci et al. / Dyes and Pigments 96 (2013) 619e625
625
[23] Li G, Jiang KJ, Li YF, Li SL, Yang LM. Efficient structural modification of
triphenylamine-based organic dyes for dye-sensitized solar cells. J Phys Chem
C 2008;112:11591e9.
Acknowledgments
We are grateful to the National Natural Science Foundation of
China (No. 21002010 and 20923006) for their financial support.
This work was also supported by the Fundamental Research Funds
for the Central Universities (DUT12LK44 and DUT10RC(3)28).
[24] Xu MF, Li RZ, Pootrakulchote N, Shi D, Guo J, Yi ZH, et al. Energy-level and
molecular engineering of organic D-
p-A sensitizers in dye-sensitized solar
cells. J Phys Chem C 2008;112:19770e6.
[25] Wang MK, Xu MF, Shi D, Li RZ, Gao FF, Zhang GL, et al. High-performance
liquid and solid dye-sensitized solar cells based on a novel metal-free organic
sensitizer. Adv Mater 2008;20:4460e3.
[26] Zhang GL, Bai Y, Li RZ, Shi D, Wenger S, Zakeeruddin SM, et al. Employ
a bisthienothiophene linker to construct an organic chromophore for efficient
and stable dye-sensitized solar cells. Energy Environ Sci 2009;2:92e5.
[27] Zhang GL, Bala H, Cheng YM, Shi D, Lv XJ, Yu QJ, et al. High efficiency and
stable dye-sensitized solar cells with an organic chromophore featuring
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