Influence of ethynyl position on benzothiadiazole based D-A-π-A dye-sensitized solar cells: Spectral response and photovoltage performance

Article abstract of DOI:10.1039/c6tc03418g

The ethynyl group has been widely employed in dye-sensitized solar cells (DSSCs) as an efficient π-spacer to prolong conjugation and promote electronic coupling at the interface of TiO₂ films. However, systematic studies of the ethynyl position on metal-free organic sensitizers remain relatively rare. Herein we report indoline-based organic dyes bearing an ethynyl group at different positions of D-A-π-A organic dyes. Based on the reference dye D1, we inserted an ethynyl unit in either the left or right side of benzothiadiazole to construct two novel dyes D2 and D3. It was found that inserting an ethynyl unit to the side of the anchoring group obtained a higher molar extinction coefficient with a red shift in the absorption band. Interestingly, D2 and D3 displayed a better photovoltaic performance with respect to D1. In particular, D3 exhibits an over 90 mV enhanced open-circuit voltage (V_OC) than D2 owing to a longer electron lifetime and slower charge recombination. With this incredible increase of V_OC, D3 bestows a high efficiency of 7.13% with respect to D1 and D2. Coadsorption strategies are exploited for further improving the cell behaviour. As a result, cosensitization with a long-wavelength-responsive dye WS-2 was demonstrated to efficiently compensate the light-harvesting, achieving an excellent efficiency of 9.83% in the iodine electrolyte. This work has paved a useful and practical way for molecular engineering in D-A-π-A metal-free organic dyes.

Full text of DOI:10.1039/c6tc03418g

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ARTICLE
Influence of ethynyl position on benzothiadiazole based D‐A‐‐A
dye‐sensitized solar cells: spectral response and photovoltage
performance†
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
a
a
a
DOI: 10.1039/x0xx00000x
Xiongrong Song, Weiwei Zhang, Xin Li, Huiyun Jiang, Chao Shen and Wei‐Hong Zhu*
www.rsc.org/
Ethynyl group has been widely employed in dye‐sensitized solar cells (DSSCs) as an efficient
‐spacer
to prolong the conjugation and promote electronic coupling at the interface of TiO films. However,
2
the systematic study of ethynyl position on metal‐free organic sensitizers remains relatively blank.
Herein we report indoline‐based organic dyes bearing an ethynyl group in different position of D‐A‐

‐A featured organic dyes. Based on the reference dye D1, we insert an ethynyl unit in either the left
or right side of benzothiadiazole to construct two novel dyes D2 and D3. As found, inserting an
ethynyl unit to the side of anchoring group obtains higher molar extinction coefficient with red shift
in absorption band. Interestingly, D2 and D3 display better photovoltaic performance with respect to
D1. Especially, D3 exhibits an over 90 mV enhanced open‐circuit voltage (VOC) than D2 owing to
longer electron lifetime and slower charge recombination. With this incredible increase of VOC, D3
bestows a high efficiency of 7.13% with respect to D1 and D2. Coadsorption strategy are exploited
for further improving the cell behaviour. As result, cosensitization with a long‐wavelength‐responsive
dye WS‐2 is demonstrated to efficiently compensate the light‐harvesting, achieving an excellent
efficiency of 9.83% in the iodine electrolyte. This work has paved a useful and practical way for
molecular engineering in D‐A‐

‐A featured metal‐free organic dyes.
1
Introduction
promote strong electronic coupling at the interface of TiO
2
films and broaden the light‐harvesting region, resulting in a
Recently, tremendous research has endeavored in exploiting
solar energy for the strategy of renewable energy sustainable
development. As an effective utilization of solar energy with
22‐31
better photovoltaic performances.
For example, Grätzel
et al. has reported two ethynyl‐containing porphyrin
sensitizers SM315 and SM371, especially achieving a record
1
low cost and facile fabrication, dye‐sensitized solar cells have
22
efficiency of 13% in 2014. Xie et al. also successfully inserted
2‐3
gained extensive attention. Here the structural engineering
of sensitizing dyes plays a vital role, which can rationally affect
the performance of fabricated solar cells by influencing the
molecular planarity, light‐harvesting ability and molecular
multiple triple bonds to porphyrin sensitizers, along with
cosensitization strategy to achieve a high efficiency of 10.45%
27b
in iodine electrolyte.
More recently, Wang et al. also
introduced ethynyl group to metal‐free donor‐acceptor dye,
4‐21
interaction.
26
realizing a remarkable power conversion efficiency of 12.5%.
However, the systematic study of ethynyl position on metal‐
Since Diau et al. reported “push‐pull” porphyrin dyes by
incorporating ethynylcoboxyphenyl as anchoring group,
ethynyl unit has been widely exploited to link with porphyrin
for constructing new kinds of sensitizers, which can well
32
free organic sensitizers remains relatively blank.
Alternatively, a molecular so‐called D‐A‐
incorporating an auxiliary acceptor into the bridge has been

‐A model by

demonstrated to be an effective way to decrease the optical
band gap, stabilize the electron‐rich donor, and improve the
aShanghai Key Laboratory of Functional Materials Chemistry, Key Laboratory for
Advanced Materials and Institute of Fine Chemicals, Collaborative Innovation
Center for Coal Based Energy (i‐CCE), School of Chemistry and Molecular
Engineering, East China University of Science and Technology, Shanghai 200237,
P. R. China. E‐mail: whzhu@ecust.edu.cn.
3
3
dye stability. With the introduction of auxiliary group like
benzothiadiazole (BTD), both of metal‐free organic dyes and
porphyrin dyes show significant promotion in photovoltaic
1
b,3a
b
performances.
With these in mind, we design three novel D‐A‐
dyes (Fig. 1). D1 is an indoline‐based D‐A‐ ‐A sensitizer
Division of Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal
Institute of Technology, SE‐10691 Stockholm, Sweden.

‐A featured
†Electronic Supplementary Information (ESI) available: Cyclic voltammograms,

calculated energy levels, and structural characterization of dyes D1, D2 and D3. See
DOI: 10.1039/x0xx00000x.
featuring BTD group as an auxiliary acceptor and benzoic acid
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as an anchoring group. The DSSCs fabricated with D1 displayed sensitized solar cells, the TiO
a power conversion efficiency (PCE, ) of 6.12%. Based on D1
we attempted to insert an ethynyl unit in different sides of cosensitization process, the TiO
BTD group to construct another two novel dyes named D2 and a solution of WS‐2 for 1 h, and then were immersed in a
D3 respectively. Both of them performed enhanced solution of D3 for 12 h, rinsed with ethanol and dried. The
efficiencies than D1. Interestingly, with a little change in liquid redox electrolyte was made according to reference.
molecular design, D3 based DSSCs exhibited a great The photocurrent‐photovoltage (I‐V) curves were measured
2
films were dipped in the dye
solution for 12 h, rinsed with ethanol and dried. For
View Article Online
DOI: 10.1039/C6TC03418G

,
2
films were firstly immersed in
,
3
5
enhancement in photovoltage compared to D2, which can be under AM 1.5G simulated solar light using Keithley 2400
ascribed to the longer electron lifetime of D3 based devices. Sourcemeter Instruments. The incident photon‐to‐charge
3
4‐38
Moreover, the cosensitization stragety
of D3 with a board carrier efficiencies (IPCEs) were measured using Newport‐
absorption spectra WS‐2 was adopted to further improve the 74125 system (Newport Instruments). Electrochemical
photovoltaic performances increasing from 7.13% to 9.83%.
impedance spectroscopy (EIS) was performed for DSSCs using
Zahner IM6e electrochemical workstation, with an
a
alternative sinusoidal perturbation of 5 mV and a frequency
range of 0.1 Hz‐100 kHz. The original spectra further analyzed
by Z‐View software.
S
N
N
N
COOH
D1
2.3 Materials and synthesis
Tetrahydrofuran (THF) was refluxed using sodium for
dehydration under argon atmosphere before use. All other
chemical reagents and solvents were commercially available
and used without further purification.
S
S
N
N
N
N
N
COOH
N
COOH
Synthesis of compound 1b. In a 250 mL three‐necked flask,
bis(pinacolato)diboron (8.3 g, 32.68 mmol), 1a (1.86 g, 8.65
D2
D3
Fig. 1 Chemical structures of dyes D1, D2 and D3. Note: Based on D1, an mmol), KOAc (2 g, 20 mmol) and Pd(dppf)Cl
2
(0.40 g, 0.54
ethynyl unit is inserted in different sides of BTD group for taking systematic mmol) were dissolved in DMSO (100 mL) under argon. After
insight into the ethynyl position effect on metal‐free organic sensitizers.
stirring at 50 °C for 8 h, the mixture was washed with H O,
2
2 4
dried over anhydrous Na SO , and evaporated under reduced
pressure, the residue was purified on a silica gel column to
1
2
Experimental
obtain white powder 1b (2.0 g, 88%). H NMR (400 MHz, CDCl
3
,
δ, ppm): 8.02 (d, J = 8.4 Hz, 2H), 7.87 (d, J = 8.4 Hz, 2H), 3.92 (s,
3H), 1.36 (s, 12H). C NMR (100 MHz, CDCl
34.67, 132.30, 128.60, 84.18, 52.16, 24.89.
2
.1 Characterization
1
3
1
1
3
, δ, ppm): 167.15,
H and C NMR spectra were obtained with a Bruker AM 400
spectrometer. HR‐MS spectra was carried out with a Waters
1
LCT Premier XE spectrometer. The UV‐Vis absorption spectra Synthesis of compound 1c. In a 250 mL three‐necked flask, 1b
were performed using a model CARY 100 spectrophotometer. (1.0 g, 3.8 mmol), 4,7‐dibromobenzo[c][1,2,5]thiadiazole (2.28
The cyclic voltammogram (CV) were tested in dichloromethane g, 7.7 mmol) and Pd(PPh ) (230 mg, 0.2 mmol) were dissolved
3
4
with a Versastat II electrochemical workstation (Princeton
Applied Research) based on a three electrode system, using Pt
as working electrode, Pt wire as counter electrode, and
saturated calomel reference electrode (SCE) as reference
electrode, respectively.
in THF (150 mL), and 2M K
stirring at 80 °C for 8 h, the mixture was extracted with CH
and removed under reduced pressure. The residue was
2
CO
3
(50 mL) was added. After
Cl
2
2
purified on a silica gel column to obtain faint yellow solid 1c
1
(
780 mg, 59 %). H NMR (400 MHz, CDCl
3
, δ, ppm): 8.20 (d, J =
2
.2 Device fabrication and measurements of solar cells
The fabrication of solar cells as well as the relevant
preparation of TiO photoanodes (a 6 m nanoporous layer
and a 8 m scattering layer) and counter electrodes were
following the previous work. Each dye dissolved in a mixture
solvent (CHCl :C OH = 1:1) was used as a dye sensitized
solution with a concentration of 3
8
.4 Hz, 2H), 7.99 (d, J = 8.0 Hz, 2H), 7.96 (d, J = 7.2 Hz, 1H), 7.64
1
3
(d, J = 7.6 Hz, 1H), 3.97 (s, 3H). C NMR (100 MHz, CDCl
3
, δ,
2

ppm): 166.76, 153.90, 152.87, 140.95, 132.82, 132.19, 130.11,

1
29.94, 129.16, 128.78, 114.25, 52.31. HRMS–ESI (m/z): [M +
1
0
+
H] Calcd. for (C14
H10BrN
2
O
2
S), 348.9646, found: 348.9645.
3
2 5
H
‐
4

10 M. For single dye
2
| J. Name., 2012, 00, 1‐3
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DOI: 10.1039/C6TC03418G
Scheme 1 Synthetic routes to D1, D2 and D3: (i) Pd(PPh
3
4 2 3
) , 2M K CO ; (ii) KOH, MeOH, THF, H
2
O; (iii) Pd(PPh
3 2
) Cl
2
, CuI, Et N.
3
Synthesis of compound 1e. To a solution of 1d (470 mg, 1.43 Synthesis of compound 2b. In a 250 mL three‐necked flask, 1c
mmol) in dry THF (6 mL) was added n‐BuLi (0.7 mL, 1.58 mmol) (694 mg, 1.99 mmol), CuI (64 mg, 0.34 mmol), Pd(PPh
dropwise at ‐78 °C under argon in dark. After stirring 30 min at (100 mg, 0.14 mmol) were dissolved in THF (30 mL) and Et
78 °C, B(OCH
3
)
2
Cl
2
3
N
‐
3
)
3
(0.3 mL, 2.1 mmol) was added. The reaction (10 mL) under nitrogen. Then trimethylsilylacetylene (0.8 mL, 5
mixture was stirred at the same temperature for 4 h, then mmol) was added. After stirring at 50 °C for 24 h, the solvent
gradually warmed up to room temperature and used for the was removed under reduced pressure, then KOH (150 mg, 2.7
next Suzuki coupling reaction without purification. The mmol), a few drops of methanol and 30 mL THF were mixed in
unpurified mixture was refluxed with 1c (566 mg, 1.62 mmol) it and stirred for 30 min at room temperature. Next the
adding Pd(PPh
3 4 2 3 2 2 2 4
) (92 mg, 0.08 mmol) and 2 M K CO aqueous organic layer was extracted with CH Cl and dried over Na SO .
solution (30 mL) in THF (100 mL) for 12 h. After cooling, water After the solvent was removed under reduced pressure, the
was added and the reaction mixture was extracted with CH
The combined organic layer was washed with H
dried over anhydrous Na SO
2
Cl
2
. residue was purified on a silica gel column to obtain 2b (310
1
2
O and brine, mg, yield 53%). H NMR (400 MHz, CDCl
, and evaporated under reduced 8.4 Hz, 2H), 8.03 (d, J = 8.4 Hz, 2H), 7.91 (d, J = 7.6 Hz, 1H), 7.74
3
, δ, ppm): 8.21 (d, J =
2
4
1
3
pressure. The residue was purified on a silica gel column to (d, J = 7.6 Hz, 1H), 3.97 (s, 3H), 3.66 (s, 1H). C NMR (100 MHz,
1
obtain red powder 1e (393 mg, 53%). H NMR (400 MHz, CDCl
3
, CDCl
3
, δ, ppm): 166.76, 155.43, 152.80, 141.06, 134.24, 133.82,
δ, ppm): 8.11 (d, J = 8.4 Hz, 2H), 7.98 (d, J = 8.4 Hz, 2H), 7.71 (d, 130.21, 129.25, 127.94, 115.51, 84.21, 79.25, 52.31. HRMS–ESI
+
J = 7.2 Hz, 2H), 7.65 (d, J = 7.6 Hz, 2H), 7.16 (d, J = 7.2 Hz, 2H), (m/z): [M + H] Calcd. for (C16
H
11
N
2
O
2
S), 295.0541, found:
7
3
1
.10 (d, J = 8.4 Hz, 2H), 6.95 (d, J = 8.4 Hz, 1H), 4.79 (m, 1H), 295.0538.
.88 (m, 4H), 2.27 (s, 3H), 1.97‐2.04 (m, 1H), 1.86‐1.90 (m, 2H), Synthesis of compound 2c. 1d (300 mg, 0.91 mmol), 2b (250
1
3
.69‐1.78 (m, 1H), 1.49‐1.64 (m, 2H). C NMR (100 MHz, CDCl
3
,
3 2 2
mg, 0.85 mmol), Pd(PPh ) Cl (100 mg, 0.14 mmol), and CuI (40
δ, ppm): 166.99, 154.24, 154.03, 148.67, 142.19, 140.11, mg, 0.21 mmol) were placed in a 100 mL three‐necked flask,
1
1
2
35.39, 134.75, 131.80, 129.84, 129.06, 128.97, 127.02, 126.04, flushed with nitrogen and then charged with dry THF (10 mL)
25.62, 120.42, 107.40, 69.34, 52.20, 45.45, 35.22, 33.73, and Et
3
N (2 mL). The mixture was stirred at 60 °C for 12 h.
+
9.71, 24.48, 20.83. HRMS–ESI (m/z): [M + H] Calcd. for Then, the solvent was removed under reduced pressure, and
(
C
32
H
28
N
3
O
2
S), 518.1902, found: 518.1902.
2 2 2 4
the residue was dissolved in CH Cl and dried over Na SO .
Synthesis of dye D1. A mixture of 1e (300 mg, 0.58 mmol) and After removing the solvent under reduced pressure, the
KOH (350 mg, 6.2 mmol) in THF (40 mL) and H
refluxed for 12 h under nitrogen. Then, the solvent was 20%). H NMR (400 MHz, CDCl
2
O (6 mL) was residue was purified on a silica gel column to obtain 2c (92 mg,
1
3
, δ, ppm): 8.20 (d, J = 8.6 Hz,
removed under reduced pressure. The residue was purified by 2H), 8.05 (d, J = 8.6 Hz, 2H), 7.82 (d, J = 7.4 Hz, 1H), 7.74 (d, J =
column chromatography on silica gel to obtain the product as 7.4 Hz, 1H), 7.42 (s, 1H), 7.37 (m, 1H), 7.19 (s, 4H), 6.82 (d, J =
1
a red powder (219 mg, yield 75 %). H NMR (400 MHz, DMSO, 8.4 Hz, 1H), 4.84 (m, 1H), 3.97 (s, 3H), 3.83 (m, 1H), 2.35 (s, 3H),
1
3
δ, ppm): 8.12 (d, J = 6.4 Hz, 4H), 7.99 (d, J = 6.0 Hz, 1H), 7.88 1.64‐2.10 (m, 6H). C NMR (100 MHz, CDCl
3
, δ, ppm): 166.88,
(
m, 2H), 7.80 (d, J = 8.0 Hz, 1H), 7.23 (m, 4H), 6.98 (d, J = 8.0 Hz, 155.30, 153.01, 149.09, 141.57, 139.55, 135.03, 132.42, 132.36,
1
H), 4.93 (m, 1H), 3.92 (m, 1H), 2.30 (s, 3H), 2.08 (m, 1H), 1.81 132.02, 131.60, 129.89, 129.87, 129.77, 129.12, 128.41, 128.38,
1
3
(
m, 3H), 1.64 (m, 1H), 1.40 (m, 1H). C NMR (100 MHz, DMSO, 120.90, 117.96, 110.91, 106.93, 99.21, 83.72, 69.32, 52.25,
δ, ppm): 153.36, 147.71, 139.51, 135.03, 133.40, 130.93, 45.13, 35.14, 33.58, 29.71, 24.37, 20.86. HRMS–ESI (m/z): [M +
+
1
1
2
5
29.78, 129.43, 129.27, 129.18, 128.94, 128.84, 126.48, 126.05, H] Calcd. for (C34
H
28
N
3
O
2
S), 542.1902, found: 542.1894.
25.42, 119.75, 106.80, 68.36, 44.58, 34.87, 33.11, 23.97, Synthesis of dye D2. A mixture of 2c (50 mg, 0.09 mmol) and
+
0.38. HRMS–ESI (m/z): [M + H] Calcd. for (C31
H
26
N
3
O
2
S), KOH (50 mg, 0.9 mmol) in THF (20 mL) and H
2
O (3 mL) was
04.1746, found: 504.1751.
refluxed for 12 h under nitrogen. Then, the solvent was
removed under reduced pressure. The residue was purified by
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column chromatography on silica gel to get the product as a unpurified mixture was refluxed with 3c (200 mg, 0.54 mmol)
View Article Online
1
DOI: 10.1039/C6TC03418G
2 3
3 4
red powder (25 mg, yield 54%). H NMR (400 MHz, DMSO, δ, adding Pd(PPh ) (0.1 g, 0.08 mmol) and 2M K CO aqueous
ppm): 8.13 (m, 4H), 7.95 (m, 2H), 7.36 (s, 1H), 7.30 (d, J = 8.0 solution (30 mL) as catalysts in THF (100 mL) for 12 h. After
Hz, 1H), 7.22 (m, 4H), 6.82 (d, J = 8.4 Hz, 1H), 4.93 (m, 1H), 3.84 cooling, water was added and the reaction mixture was
(
m, 1H), 2.30 (s, 3H), 1.96‐2.08 (m, 2H), 1.73‐1.81 (m, 2H), extracted with CH
2
Cl
2
. The combined organic layer was washed
1
3
1
1
1
1
2
.36‐1.47 (m, 2H). C NMR (100 MHz, DMSO, δ, ppm): 167.04, with H
2
O and dried over Na
2
SO
4
, and evaporated under
1
54.53, 152.33, 148.49, 140.59, 138.77, 135.27, 132.03, 131.80, reduced pressure to obtain red powder 3d (163 mg, 55%). H
31.31, 129.83, 129.47, 129.12, 128.68, 127.82, 120.51, 116.10, NMR (400 MHz, CDCl , δ, ppm): 8.07 (d, J = 8.4 Hz, 2H), 7.87 (d,
3
10.03, 106.53, 98.40, 84.06, 68.40, 44.26, 34.90, 32.88, 28.97, J = 7.5 Hz, 1H), 7.79 (s, 1H), 7.75 (m, 3H), 7.66 (d, J = 7.5 Hz,
3.90, 20.39. HRMS–ESI (m/z): [M + H]+ Calcd. for 1H), 7.22 (m, 4H), 7.01 (d, J = 8.4 Hz, 1H), 4.88 (t, J = 6.5 Hz, 1H),
(
C
33
H
26
N
3
O
2
S), 528.1743, found: 528.1746.
Synthesis of compound 3b. In a 250 mL three‐necked flask, 1.76‐1.86 (m, 1H), 1.59‐1.72 (m, 2H). C NMR (100 MHz, CDCl
methyl 4‐bromobenzoate (2.0 g, 9.3 mmol), CuI (90 mg, 0.5 δ, ppm): 166.57, 155.46, 153.30, 148.95, 139.92, 135.71,
mmol), Pd(PPh Cl (300 mg, 0.42 mmol) were dissolved in 135.46, 133.75, 131.99, 131.80, 129.86, 129.77, 129.55, 129.16,
THF (100 mL) and Et N (30 mL) under nitrogen. Then 127.71, 126.63, 125.62, 125.44, 120.54, 113.15, 107.35, 94.31,
3.95 (m, 4H), 2.35 (s, 3H), 2.04‐2.15 (m, 1H), 1.93‐1.98 (m, 2H),
13
3
,
3
)
2
2
3
trimethylsilylacetylene (3.2 mL, 20 mmol) was added. After 88.77, 69.35, 52.28, 45.39, 35.25, 33.68, 24.45, 20.85. HRMS–
+
stirring at 60 °C for 24 h, the solvent was removed under ESI (m/z): [M + H] Calcd. for (C34
H
28
N
3
O
2
S), 542.1902, found:
reduced pressure, then KOH (202 mg, 3.6 mmol), a few drops 542.1904.
of methanol and 30 mL THF were mixed in it, and stirred for 30 Synthesis of dye D3. A mixture of 3d (50 mg, 0.09 mmol) and
min at room temperature. Next the organic layer was KOH (50 mg, 0.9 mmol) in THF (20 mL) and H O (5 mL) was
extracted with CH Cl and dried over Na SO . After the solvent refluxed for 12 h under nitrogen. Then, the solvent was
2
2
2
2
4
was removed under reduced pressure, the residue was removed under reduced pressure. The residue was purified by
purified on a silica gel column to obtain grey powder 3b (454 column chromatography on silica gel to obtain the product D3
1
1
3
mg, yield 30%). H NMR (400 MHz, CDCl , δ, ppm): 8.00 (d, J = as a red powder (31 mg, yield 65 %). H NMR (400 MHz, DMSO,
13
8
.5 Hz, 2H), 7.55 (d, J = 8.5 Hz, 2H), 3.92 (s, 3H), 3.23 (s, 1H). C δ, ppm): 8.02 (dd, J
1
= 8.2 Hz, J
2
= 2.1 Hz, 3H), 7.90 (s, 1H), 7.84
NMR (100 MHz, CDCl
3
, δ, ppm): 166.43, 132.09, 130.12, 129.46, (m, 2H), 7.75 (d, J = 8.2 Hz, 2H), 7.24 (q, J = 8.4 Hz, 4H), 6.96 (d,
1
26.74, 82.79, 80.07, 52.30.
J = 8.4 Hz, 1H), 4.94 (m, 1H), 3.91 (m, 1H), 2.30 (s, 3H), 2.03‐
Synthesis
of
compound
3c.
4,7‐dibromoben‐ 2.15 (m, 1H), 1.77‐1.87 (m, 3H), 1.64‐1.66 (m, 1H), 1.36‐1.47
1
3
zo[c][1,2,5]thiadiazole (661 mg, 2.25 mmol), 3b (300 mg, 1.87 (m, 1H). C NMR (100 MHz, DMSO, δ, ppm): 166.73, 154.81,
mmol), Pd(PPh Cl (70 mg, 0.1 mmol), and CuI (22 mg, 0.1 152.43, 148.07, 139.31, 135.15, 134.62, 133.99, 131.52, 131.16,
mmol) were placed in a 100 mL three‐necked flask, flushed 129.81, 129.66, 129.11, 126.28, 126.03, 125.45, 119.94, 111.93,
3
)
2
2
with nitrogen and then charged with dry THF (20 mL) and Et
3
N
106.74, 93.86, 88.71, 68.40, 44.53, 34.89, 33.04, 23.94, 20.39.
+
(
2 mL). The mixture was stirred at 60 °C for 12 h. Then, the HRMS–ESI (m/z): [M + H] Calcd. for (C33
H
26
3
N O
2
S), 528.1746,
solvent was removed under reduced pressure, and the residue found: 528.1746.
was dissolved in CH Cl and dried over Na SO . After removing
2
2
2
4
the solvent under reduced pressure, the residue was purified
on a silica gel column to get yellow solid 3c (250 mg, 36%). H
1
Results and discussion
NMR (400 MHz, CDCl
J = 7.2 Hz, 1H), 7.71 (m, 3H), 3.95 (s, 3H). C NMR (100 MHz,
CDCl , δ, ppm): 166.44, 166.28, 133.18, 132.47, 131.96, 131.88,
3
, δ, ppm): 8.07 (d, J = 8.4 Hz, 2H), 7.87 (d,
The synthetic routes of dyes D1 to D3 are shown in Scheme 1.
1
3
The common intermediates 1c and 1d were synthesized
3
10,26,27
according to the reference.
Dye D1 was conveniently
1
5
3
30.29, 129.61, 129.58, 127.00, 116.14, 115.38, 95.76, 87.18,
obtained by the Suzuki‐Miyaura coupling reaction of 1c and 1d
,
+
79
2.33. HRMS–ESI (m/z): [M + H] Calcd. for (C16
72.9646, found: 372.9641; [M
H]+ Calcd. for
S), 374.9626, found: 374.9644.
Synthesis of compound 3d. To a solution of 1d (214 mg
2 2
H10 BrN O S),
and then hydrolysis of 1e. For the ethynyl‐containing dye D2
,
+
we firstly introduced the triple bond to the intermediate 1c via
the Sonogashira reaction, and obtained the intermediate 2b. It
was further reacted with 1d to give the dye precursor 2c, and
eventually gained dye D2 after hydrolysis. The synthetic
procedure of dye D3 was similar to that of dye D2. The only
change is that the insertion of triple bond is close to the
anchoring group, rather than the neighboring donor.
(
C
16
H
1081BrN
2
O
2
，
0.65
mmol) in dry THF (10 mL) was added n‐BuLi (0.4 mL, 0.8 mmol)
dropwise at ‐78 °C under argon in dark. After 30 min of stirring
at ‐78 °C, B(OCH ) (0.2 mL, 1.4 mmol) was added. The reaction
3 3
mixture was stirred at the same temperature for 4 h, then
gradually warmed up to room temperature and used for the
next Suzuki coupling reaction without purification. The
4
| J. Name., 2012, 00, 1‐3
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View Article Online
DOI: 10.1039/C6TC03418G
Table 1 Optical and electrochemical properties of dyes D1, D2 and D3.
λ
max
ε
λ
max on TiO
2
HOMO
[V] (vs. NHE)
E
[V]
0‐0
LUMO
[V] (vs. NHE)
Dye
‐1
‐1
a
[nm]b
c
d
d
[
nm]a
[M cm ]
D1
D2
D3
484
482
508
19900
22300
25700
462
453
474
0.87
0.91
0.88
2.17
2.18
2.07
‐1.30
‐1.27
‐1.19
a
b
c
Absorption peaks (λmax) and molar extinction coefficients (ε) in CH
2
Cl
2
. Absorption peaks on 3 μm TiO
2
films. The HOMO was measured in
CH Cl by cyclic voltammograms (CV), based on a three electrode system, using Pt as working electrode, Pt wire as counter electrode, and
2
2
saturated calomel reference electrode (SCE) as reference electrode, calibrated with ferrocene as an external reference and converted to NHE
d
by addition of 0.63V. E0−0 was estimated from the absorption thresholds of dyes adsorbed on 3 μm TiO
2
film, LUMO estimated by equation of
LUMO = HOMO ‐ E0−0
.
Fig. 2a depicted the UV‐vis absorption spectra of D1
,
D2 and due to the intermolecular interaction and deprotonation of
‐
5
D3 in CH
2
Cl
2
solution (3

10 M), and the corresponding carboxylic acid. The ICT bands of dyes D1 D2 and D3 showed
,
spectral parameters were summarized in Table 1. All three hypsochromic shift by 22, 29 and 34 nm, respectively. Besides,
dyes presented two similar absorption bands. The short D2 and D3 exhibit higher and broader absorption spectra
wavelength band around 340 nm could be ascribed to π−π* compared to D1, indicating that the insertion of an ethynyl
transition, while the long wavelength band at 500 nm was unit is beneficial to prolong conjugation and improve light‐
owing to the ICT transitions. Besides, upon insertion of triple harvesting capability after anchoring onto TiO films.
2
bond to the neighboring donor, D2 exhibited a slight
hypochromatic shift in absorption band with respect to D1
.
Remarkably, when incorporating an ethynyl unit to the right
side of auxiliary group, dye D3 exhibited a significant redshift
from 484 to 508 nm in absorption band, along with an obvious
LUMO
-1.27
-
1.5
-1.0
0.5
-
1.30
-
1.19
4
−1
−1
increment in molar extinction coefficient (2.57
 10 M cm
at 508 nm). Obviously, inserting an ethynyl unit to the side of
anchoring group could obtain higher molar extinction
coefficient with red shift in absorption band.
-0.5
-
TiO E
2
CB
0
0
1
1
.0
.5
.0
.5
0
.4
-
-
5
4
3
2
1
0
I3 / I
(a)
D1
D2
D3
0
.87
0.91
0.88
HOMO
D1
D2
D3
Fig. 3 Schematic diagram of the energy levels of TiO
D1, D2 and D3, and iodine redox couple.
2
conduction band, dyes
300
400
500
600
700
To evaluate the process of electron injection from the
excited dye to the conduction band of TiO and explore the
Wavelength (nm)
(
b)
2
0
0
0
0
0
.8
.6
.4
.2
.0
D1
D2
D3
ability of dye regeneration, cyclic voltammetry of three organic
dyes in a typical three‐electrode electrochemical cell were
carried out to investigate the highest occupied molecular
orbital (HOMO) energy levels (Fig. S1†). The calculated HOMO
level of dye D1 obtained from the first oxidation potentials is
0
.87 V vs. NHE (Fig. 3). After incorporating the ethynyl unit, the
HOMO level of D2 and D3 were both slightly shifted downward
to 0.91 and 0.88 V, which were obviously lying below the
iodide/triiodide (0.40 V vs. NHE), guaranteeing the efficient
dye regeneration. The band gap energies (E0−0) were estimated
from the wavelength at 10% maximum absorption intensity as
shown in Table 1. Interestingly, changing the ethynyl position
from donor side to acceptor side leads to a narrower band gap
of D3 (2.07 V) than D2 (2.18 V). Herein, the calculated LUMO
levels of D3 was ‐1.19 V, showing a relatively downward
compared to D1 (‐1.30 V) and D2 (‐1.27 V). Besides, all of the
4
00
500
600
700
Wavelength (nm)
solution (3  10‐5 M,
Cl
Fig. 2 Absorption spectra of dyes D1, D2 and D3 in CH
a) and anchored on 3 μm TiO film (b).
2
2
2
Upon adsorption onto a 3 μm transparent nanocrystalline
TiO films, the absorption spectra of all organic dyes (Fig. 2b)
were obviously hypsochromic shifted than those in solution
2
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Journal Name
LUMO levels can guarantee the ample electron injection from than that of D1, along with no obvious change VoiefwtAhrteicleinOintliinael
the excited dyes into the TiO
NHE).
2
conduction band (‐0.50 V vs response wavelength. Besides, D3 achieDvOeI:d10o.1v0e3r9/7C06%TC0in341t8hGe
range of 440‐600 nm, with a highest IPCE value of 80% at 480
To take insight into the effect of ethynyl unit to the nm. As a consequence, the improvement in JSC of D2 and D3
molecular structure and electron distribution, the density could be attributed to the broader absorption spectra and the
functional theory (DFT) calculations of all three dyes were enhanced absorption coefficients.
resorted to optimize the ground state geometries of the
3
9
molecules, using the hybrid B3LYP functional and the 6‐
1
5
0
5
0
(
a)
4
0
3
1G(d) basis set (Tables S1 and S2†). All calculations were
41
carried out using the Gaussian09 program package . As shown
in Fig. 4, the dihedral angles between donor and auxiliary
group (α) and between auxiliary group and phenyl unit (β) of
three dyes exhibited definitely different values, indicative of a
good molecular planarity. For dye D1, both of the dihedral
angles possess the largest values of 33.5° and 35.5°. With
insertion of triple bond to the neighboring donor, the
corresponding dihedral angle of α was 0.7°, distinctly
1
D1
D2
D3
0.0
0.2
0.4
0.6
0.8
Voltage (V)
100
becoming planar at the side of donor compared to D1
.
(b)
D1
D2
D3
Accordingly, the molecular structure of D3 showed similar
change, leading to a nearly 0° dihedral angle of β upon
incorporating the ethynyl unit to the neighboring side of
anchoring group. In addition, the computed spatial distribution
of the frontier molecular orbitals also shows the insertion
influence of ethynyl unit. It is found that both HOMO and
LUMO of D2 receive certain contributions to the ethynyl
moiety, as well as that of D3. This change in electron
distribution is beneficial to the transition process from HOMO
to LUMO.
80
6
0
0
4
20
0
300
400
500
600
700
Wavelength (nm)
Fig. 5 I‐V curves (a) and IPCE spectra (b) for the DSSCs fabricated on dyes D1,
D2, and D3 obtained under AM 1.5G simulated solar light (100 mW cm ).
‐2
Moreover, with insertion of triple bond to the side of
anchoring group, the DSSCs based on D3 brought forth a
distinct increase of 91 mV in VOC value than that of D2, which
incorporated ethynyl unit to the side of donor, resulting in a
relatively higher efficiency of 7.13%. It should be noted that,
with a little change in molecular engineering, the DSSCs based
D3 obtained
a
predominant contribution from the
enhancement in VOC compared to that of D2
.
Table 2 Photovoltaic parameters of DSSCs based on dyes D1
D3
, D2 and
.
Fig. 4 Dihedral angles and Contour plots of frontier molecular orbitals of dye
D1, D2 and D3.
Conditions
J
SC (mA
cm )
V
OC
FF
η
(%)
Amount of dye
‐2
‐2
(mV)
728
695
786
molecular / mol cm
1.55  10‐
7
D1
D2
D3
12.61
13.22
13.44
0.667 6.12
0.683 6.28
0.675 7.13
‐
7
7
The photocurrent–voltage (I–V) curves of DSSCs based on
2.58  10
2.33  10
‐
three dyes were measured under an irradiance of simulated
‐
2
AM1.5G sunlight (100 mW cm ) using iodide/triiodide
electrolyte (Fig. 5a), and the relevant parameters are
summarized in Table 2. D1 based DSSCs performed an
As well known, the variation of VOC is dependent upon the
conduction band (ECB) and the electron density
efficiency (

) of 6.12%, with a short‐circuit current (JSC) of 12.6 position of TiO
2
−
2
mA cm , an open‐circuit voltage (VOC) of 728 mV and a fill in the semiconductor. The electron density is strongly on
factor (FF) of 0.67. As expected, the slightly higher JSC values of account of the rate of charge recombination, which can usually
3.22 and 13.44 mA cm were obtained with dyes D2 and D3 evaluated by the electron lifetimes. To analysis the variances
−
2
1
than that of D1, probably due to the larger conjugation in of three dye‐based sensitizers in the voltage as well as further
molecular structure upon inserting ethynyl unit. As shown in investigate interfacial charge transfer and recombination
the incident photon‐to‐current conversion efficiency (IPCE) process, we performed electrochemical impedance analysis
spectra (Fig. 5b), D2 and D3 exhibited slightly broader plateau (EIS) in the dark with a series of bias voltage. The values of
6
| J. Name., 2012, 00, 1‐3
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chemical capacitance (C
shown in Fig. 6a, which is relevant to the shift of conduction more effectively suppress the process ofDcOhIa:r1g0e.10r3e9c/oCm6TbCi0n3a4t1io8Gn,
band of TiO . It could be found that for all three dyes, the resulting in obviously higher electron density in TiO
logarithm of C increases linearly with the bias voltage in the conduction band. As a result, it renders the superior
same order of magnitude. Here, the same level C values photovoltage performance of D3 based cells. Generally
meant that the positions of TiO conduction band of D2 and D3 speaking, after inserting an ethynyl group on the side of
based sensitizers were little fluctuation after inserting the anchoring group, D3 exhibited extraordinary open circuit

) of these identical devices were ethynyl unit to the neighboring side of anchorinVgiewgArroticulepOnclainne
2
2


2
ethynyl unit compared to that of D1
.
voltage than D1 and D2, which can be an effective method to
improve solar cell performance in future design. Herein, the
HOMO levels may not be the major reason inducing the
variation of VOC.
1E-3
(a)
D1
D2
D3
2
1
1
1
1
0
8
6
4
2
(a)
1E-4
10
8
D3
WS-2
6
4
2
0
D3+CDCA
WS-2+D3
0
.58 0.60 0.62 0.64 0.66 0.68 0.70 0.72
Potential (V)
0.0
0.2
0.4
0.6
0.8
(
b) 10000
Voltage (V)
D1
D2
D3
1
00
(b)
8
6
4
2
0
0
0
0
0
1
000
D3
WS-2
D3+CDCA
WS-2+D3
100
0
.58 0.60 0.62 0.64 0.66 0.68 0.70 0.72
Potential (V)
3
00 400 500 600 700 800
Wavelength (nm)
Fig. 7 I‐V plots for DSSCs based on D3, D3+CDCA, and WS‐2+D3 (a), IPCE
spectra for DSSCs based on D3, WS‐2, D3+CDCA, WS‐2+D3 (b)
(c)
D1
D2
D3
1
As mentioned above, D3 showed a narrow IPCE response
0
.1
leading to a low J values along with the cutoff wavelength at
SC
7
00 nm, and a moderate dip in the IPCE spectrum is observed
0
.01
around 400 nm, which is mainly resulted from the effects of
the low absorbance of dyes in this region and the competitive
absorption of triiodide in the electrolyte. Given that D3
exhibited a potentially high photvoltage, D3 based DSSCs is
expected to be optimized when the IPCE spectrum would be
improved, especially at long wavelength region. We further
exploited chenodeoxycholic acid (CDCA) as a coadsorbent with
D3 to improve the photovoltaic performance. Indeed, the
relevant solar cells achieved higher JSC with a slight decrease of
1E-3
0
.58 0.60 0.62 0.64 0.66 0.68 0.70 0.72
Potential (V)
Fig. 6 TiO capacitance (a), recombination resistance (b), and calculated
2
electron lifetime (c) as a function potential based on D1, D2 and D3.
On the other hand, the recombination resistance (RCT) of all
three sensitizers are presented in Fig. 6b. At a fixed potential,
the RCT increased as D2
V
; SC
OC, resulting in a higher efficiency of 8.22% (VOC: 775 mV J :
‐
2
1
5.1 mA cm ). Its IPCE spectrum also showed an obvious
< D1 < D3, which were in great
increment in the range of 350‐630 nm compared to individual
dye (Fig. 7b). This improvement could be attributed to the
influence of CDCA, which can reduce the dye aggregation and
the intermolecular interaction to some extent. Besides, the
amount of adsorbed D3 almost has no significant change after
coadsorption with CDCA (Table 3).
agreement with their relevant tendency of VOC values. The
electron lifetimes as a function of potential were also
measured to further elucidate the degree of electron
recombination at the TiO
obtained from the equation of
2
electrolyte interface, which could be
= C CT. As shown in Fig. 6c,


R
D3 based DSSCs exhibited longer lifetime in all region of tested
potential than that of D2, which implies that the insertion of
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SC values, resulting in a performance of 8.22%. The
View Article Online
DOI: 10.1039/C6TC03418G
cosensitization strategy of WS‐2/D3 also achieved an obvious
Table 3 Photovoltaic parameters of DSSCs based on D3+CDCA
,
WS‐
enhancement compared to individual dye, reaching an
2
+D3 and WS‐2
.
‐
2
optimized efficiency of 9.83% (JSC = 18.30 mA cm , VOC = 737
mV, and FF = 0.73). Our results indicate that insertion of an
ethynyl group to the proper position can effectively broaden
Conditions
J
SC (mA
V
OC
FF
η
(%)
Amount of dye
molecular/ mol cm
‐
2
‐2
cm )
15.06
18.30
17.51
(mV)
775
737
649
‐
7
D3+CDCA
WS‐2+D3
WS‐2
0.704 8.21
0.729 9.83
0.720 8.18
2.3210
‐7
‐7
the spectra response and develop high efficient D‐A‐
organic sensitizers, paving a practically molecular engineering
in development of D‐A‐ ‐A featured metal‐free organic dyes.
‐A
0.4510 +1.8710
2.1010‐7

Although the utilization of CDCA can improve the cell
performance by suppressing dye aggregations, there is no
Acknowledgements
dramatic promotion in photovoltaic efficiency, especially the This work was supported by NSFC for Creative Research
relatively narrow IPCE response still restricted the Groups (21421004) and Distinguished Young Scholars
performances of D3‐based solar cells. Here, another strategy (21325625), NSFC/China, Oriental Scholarship, Fundamental
Research Funds for the Central Universities (WJ1416005),
Scientific Committee of Shanghai (15XD1501400), and
Programme of Introducing Talents of Discipline to Universities
of cosensitization with a broad spectral response dye for
pursuing better cell performances was carried out. According
3
3c
to our previous work, WS‐2 (Fig. S2†) exhibited efficient
IPCE values above 70% from 400 to 680 nm with the cutoff
wavelength at 800 nm, which is apparently compensated with
dye D3. Besides, D3 showed an extremely high open‐circuit
voltage, which can well improve the poor performance in
(
B16017).
Notes and references
photovoltage of WS‐2
.
1
(a) B. O'Regan and M. Gratzel, Nature, 1991, 353, 737; (b) A.
Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson,
Chem. Rev., 2010, 110, 6595; (c) Y. Lin, Y. Li and X. Zhan,
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W. Wong, H. Wu, L. Chen, S. Su and Y. Cao, Adv. Mater., 2011,
23, 4636.
For the cosensitized cells of WS‐2 and D3, the IPCE spectra
showed above 80% from 450 to 650 nm with the cutoff
wavelength extending to 800 nm. The adsorption amount for
‐
7
WS‐2 and D3 in the cosensitized condition are 0.45

10 and
‐
7
‐2
1
.87

10 mol cm , respectively. After cosensitization, the
2
3
(a) S. Zhang, X. Yang, Y. Numata and L. Han, Energy Environ.
‐
7
‐7
Sci., 2013,
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
10 to 1.87  10 mol
‐
2
3, 1170; (c) Y. F. Li, Sci. China Chem., 2015,
cm , but a considerable amount of WS‐2 was adsorbed, which
resulted in a significant enhancement in IPCE spectra.
Furthermore, the VOC of cosensitized cells became 737 mV, an
obvious increase relative to the single WS‐2 (653 mV). Finally,
as shown in Fig. 7a and Table 3, the strategy of cosensitization
with WS‐2 resulted in a significant enhancement in JSC, and
showed an excellent efficiency of 9.83% in the iodine
electrolyte.
5
8
7
W. G. Diau, Chem. Soc. Rev., 2013, 42, 291; (c) I. Chung, B.
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Conclusions
In summary, we have synthesized three novel D‐A‐‐A
featured organic dyes, and taken systematic insight into the
effect of ethynyl positions on photovoltaic performances.
Compared to D1, dye D2 incorporating an ethynyl group
between the donor group indoline and auxiliary group showed
little change to absorption and limited enhancement in molar
extinction coefficients, eventually had no significant promotion
in photovoltaic performance. However, upon inserting an
ethynyl group between the auxiliary group and benzene ring,
dye D3 successfully red‐shifted the absorption band, and
7
8
9
(a) Y. Hu, A. Ivaturi, M. Planells, C. L. Boldrini, A. O. Biroli and
N. Robertson, J. Mater. Chem. A, 2016, 4, 2509.
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(a) H. Jia, M. Zhang, W. Yan, X. Ju and H. Zheng, J. Mater.
Chem. A, 2016; (b) J. Ni, Y. Yen and J. T. Lin, J. Mater. Chem. A,
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Huang, T. Lee, J. T. Lin, C. Tsai and K. Chen, J. Mater. Chem.,
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Deng, RSC Adv., 2015, , 37574.
4
, 6553; (c) S. Chaurasia, C. Liang, Y. Yen and J. T. Lin, J.
3
, 9765; (d) H. Chou, Y. Chen, H.
2
5
enhanced the molar extinction coefficients, finally achieving a 10 (a) W. Zhang, Y. Z. Wu, H. Zhu, Q. Chai, J. Liu, H. Li, X. Song
better efficiency of 7.13%. Strikingly, D3 exhibited excellent
and W. H. Zhu, ACS Appl. Mater. Interfaces, 2015, , 26802;
b) B. Liu, B. Wang, R. Wang, L. Gao, S. Huo, Q. Liu, X. Li and
7
(
VOC values (786 mV) than D1 and D2, indicating that this triple
W. H. Zhu, J. Mater. Chem. A, 2013, , 804.
2
bond position could effectively retard the electron recapture,
which was proved by measurements of EIS. The addition of
coadsorbent CDCA with dye D3 exhibited dramatically higher
1
1 S. Li, K. Jiang, K. Shao and L. Yang, Chem. Commun., 2006,
792.
2
8
| J. Name., 2012, 00, 1‐3
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1
2 (a) J. Yang, P. Ganesan, J. Teuscher, T. Moehl, Y. J. Kim, C. Yi, 27 (a)Y. Xie, Y. Tang, W. Wu, Y. Wang, J. Liu, X. Li, HV.ieTwiaAnrticalenOdnWline.
P. Comte, K. Pei, T. W. Holcombe, M. K. Nazeeruddin, J. Hua,
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Journal of Materials Chemistry C
Page 10 of 10
ARTICLE
Journal Name
View Article Online
Influence
of
ethynyl
position
on
DOI: 10.1039/C6TC03418G
benzothiadiazole based D‐A‐‐A dye‐
sensitized solar cells: spectral response and
photovoltage performance
Xiongrong Song, Weiwei Zhang, Xin Li, Huiyun Jiang,
Chao Shen and Wei‐Hong Zhu*
Three novel D‐A‐‐A featured organic dyes has been
designed for systematic insight into the effect of ethynyl
positions on photovoltaic performances. Insertion of triple
bond close to anchoring group exhibits better photovoltaic
performance with strikingly higher open‐circuit voltage.
With the efficient cosensitization method, the optimized
device achieved an excellent efficiency of 9.83%.
1
0 | J. Name., 2012, 00, 1‐3
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