The effect of the number, position, and shape of methoxy groups in triphenylamine donors on the performance of dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2014.09.014

Four new organic photosensitizers (SH-11～14) that introduced methoxy groups as an additional donor were synthesized, and used in dye-sensitized solar cells. The dyes were designed in order to investigate the effects on the dye photophysical properties and the cell photovoltaic performance, by the number, position, and shape of methoxy groups introduced at the para-/ortho- and para-/meta- and para-(open or closed ring shape) positions on the triphenylamine donor. The introduction of methoxy groups led to bathochromic shift of the absorption spectrum, and enhancement of the molar extinction coefficient of the dyes. Their introduction decreased the amount of dye adsorbed on TiO₂surface, due to the increased steric hindrance. As their number increased, an open-circuit voltage value decreased. All things considered, the dye (SH-14) with closed ring shape shows the best conversion efficiency of 6.01% under AM 1.5G conditions (N719 dye's 7.59% under the same conditions).

Full text of DOI:10.1016/j.dyepig.2014.09.014

Dyes and Pigments 113 (2015) 390e401
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The effect of the number, position, and shape of methoxy groups in
triphenylamine donors on the performance of dye-sensitized solar
cells
Se Hun Kim a, 1, , Jun Choi b , Chun Sakong a , Jin Woong Namgoong 2, Woosung Lee c,
2
,
1
,
2
a,
a,
2
d
d
a, *, 2
Dong Hoe Kim , Boeun Kim , Min Jae Ko , Jae Pil Kim
a
Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea
Material R&D Team 4, LG Display Co., Ltd., Paju-si, Gyeongki-do 413-779, Republic of Korea
ICT Textile & Apparel R&BD Group, Korea Institute of Industrial Technology (KITECH), Ansan-si, Gyeonggi-do 426-910, Republic of Korea
Solar Cell Research Center, Materials Science and Technology Division, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 August 2014
Received in revised form
Four new organic photosensitizers (SH-11~14) that introduced methoxy groups as an additional donor
were synthesized, and used in dye-sensitized solar cells. The dyes were designed in order to investigate
the effects on the dye photophysical properties and the cell photovoltaic performance, by the number,
position, and shape of methoxy groups introduced at the para-/ortho- and para-/meta- and para-(open or
closed ring shape) positions on the triphenylamine donor. The introduction of methoxy groups led to
bathochromic shift of the absorption spectrum, and enhancement of the molar extinction coefficient of
4
September 2014
Accepted 6 September 2014
Available online 16 September 2014
2
the dyes. Their introduction decreased the amount of dye adsorbed on TiO surface, due to the increased
Keywords:
steric hindrance. As their number increased, an open-circuit voltage value decreased. All things
considered, the dye (SH-14) with closed ring shape shows the best conversion efficiency of 6.01% under
AM 1.5G conditions (N719 dye's 7.59% under the same conditions).
Dye-sensitized solar cell
Triphenylamine dye
Electron donor
Methoxy group
Bathochromic shift
Dihedral angle
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Recently, the highest efficiency of over 12% has been achieved from
DSSCs co-sensitized by zinc porphyrin sensitizers (YD2-o-C8 and
Since Gr a€ tzel et al. reported Ru-based photosensitizers in 1991,
dye-sensitized solar cells (DSSCs) have been considered as prom-
ising converters of solar energy, due to their low-cost fabrication,
environmental friendliness, and high power conversion efficiency
Y123) [4]. Porphyrin-based photosensitizer efficiencies have
sharply improved in recent years, by the intense efforts of countless
research scientists to achieve high efficiency [2c].
Although metal-free organic dye efficiencies are somewhat
lower than those of Ru complex photosensitizers and zinc
porphyrin photosensitizers, organic dyes as alternatives to metal
complex photosensitizers are still promising candidates for the
photosensitizers of DSSCs, due to their many advantages, such as
relatively low cost, simple synthetic route, high molar extinction
coefficient, diversity of molecular structure, and environmental
friendliness. Until now, organic dyes of diverse structures con-
taining carbazole [5], coumarin [6], fluorine [7], indoline [8], per-
ylene [9], phenothiazine [10], phenoxazine [11], squaraine [12],
tetrahydroquionoline [13], and triphenylamine [3d,14] have been
extensively studied. Among them, triphenylamine-based and
quinoxaline-based organic dyes showed higher efficiency
compared to those of different structures [3d,3e].
[1]. As one of the most important components in DSSCs, the
photosensitizer plays a crucial role in achieving high DSSC effi-
ciency. Thus, many photosensitizers, such as ruthenium complex
dyes, metal-free organic dyes containing diverse molecular struc-
tures, and organometallic dyes that employ porphyrins or phtha-
locyanines, have been extensively studied by researchers around
the world [2]. So far, Ru complex sensitizers (N3, N719, and black
dye) and organic dyes containing triphenylamine (TPA) moiety
have been reported to give high power conversion efficiencies of
over 11% and 10% under AM 1.5G irradiation, respectively [3].
*
Corresponding author. Tel.: þ82 2 880 7187; fax: þ82 2 880 7238.
E-mail address: jaepil@snu.ac.kr (J.P. Kim).
Both authors contributed equally to the research.
Tel.: þ82 2 880 7187; fax: þ82 2 885 1748.
TPA-based organic dyes are still under intense academic inves-
tigation, due to their prominent electron donation and hole-
1
2
http://dx.doi.org/10.1016/j.dyepig.2014.09.014
143-7208/© 2014 Elsevier Ltd. All rights reserved.
0

S.H. Kim et al. / Dyes and Pigments 113 (2015) 390e401
391
transport ability [15,2d]. To improve DSSC efficiency, photosensi-
tizers are required to have bathochromic shifts, broad absorption
spectra, and/or enhancement of molar extinction coefficients. To
used to study their interfacial charge transfer. The synthetic routes of
the new dyes are depicted in Scheme 1.
attain this end in most organic dyes (De
pushepull structure, two strategies were adopted: extending the
-conjugation length of molecule (De eA structure), and
introducing an additional donor (AD) to the main donor (D)
ADeDe eA or DeDe eA structure). However, organic dyes
having De eA structure can have close aggregation,
which can lead to self-quenching, and reduction of electron injec-
tion into TiO [16]. Compared to De eA structure, organic dyes
with ADeDe eA structure benefit from the lower possibility of
close aggregation. They can also have the advantages of
bathochromic shift of the absorption spectrum, and enhancement
of the molar extinction coefficient [17]. Therefore, organic dyes
with ADeDepeA structure have recently been studied by several
groups [17,18]. The aforementioned researches were oriented to the
effects of the introduction of sterically bulky moieties, such as
carbazole, diphenylamine group, alkoxy substituted phenyl group,
and heterocyclic ring, as additional donors at the para-position of
the TPA donor part. However, their introduction to the TPA donor
p
eA structure) having
2. Experimental section
p
p
e
p
2.1. Materials and regents
(
p
p
All reactions were performed under nitrogen atmosphere. 3,4-
Dimethoxyaniline, 6-amino-1,4-benzodioxane, 2,4-dimethoxyio
dobenzene, and 4-bromoaniline from TCI and 4-iodoanisole, KI, CuI,
1,10-phenanthroline, tetrakis(triphenylphosphine)palladium(0), 5-fo
rmyl-2-furan-boronic acid, cyanoacetic acid, and piperidine from
SigmaeAldrich were purchased and used as received without further
purification. All solvents (toluene anhydrous, tetrahydrofuran anhy-
drous, and acetonitrile anhydrous) were obtained from Sigma-
eAldrich and used as received. Other chemicals were reagent grade
and used without further purification.
p
e
p
pep
2
pep
p
pep
2.2. Analytical instruments and measurements
1H NMR and 13C NMR spectra were recorded on a Bruker Avance
300 and 500 MHz (Seoul National University National Center for
inter-University Research Facilities) with the chemical shifts against
TMS. Mass spectra were measured with a JEOL JMS-600W mass
spectrometer (Seoul National University National Center for inter-
University Research Facilities). Elemental analyses were carried
out with a Flash EA 1112 elemental (C,N,S) analyzer. UVevis spectra
part decreased the amount of dye adsorbed on TiO
the increased steric hindrance.
2
surface, due to
Therefore, to minimize this drawback, we designed and synthe-
sized four new TPA-based organic dyes (D1 [19] that has been previ-
ously published, SH-11~14 as shown in Fig. 1). The dyes have
ADeDe
donor (D), methoxygroups asanadditional electron donor (AD), furan
moiety as a -bridge ( ), and a cyanoacrylic acid group as an electron
peA structure, which has the TPA moiety as a main electron
and photoluminescence spectra were recorded on
a Hew-
p
p
lettePackard 8452A spectrophotometer and Shimadzu RF-5301PC
spectrofluorometer, respectively. Cyclic voltammetry spectra were
obtained using a three-electrode cell with a 273A (Princeton applied
research, Inc.). The measurement was carried out using an Ag wire
acceptor (A). The methoxy groups as an additional donor can lead not
only to bathochromic shift of the absorption spectrum and enhance-
ment of the molar extinction coefficient, but also to relatively smaller
sterichindrance, comparedtothe stericallybulkymoieties.The effects
of the number, position, and shape of the methoxy groups attached to
the TPA donor part were investigated by the dye photophysical and
electrochemical properties, and the cells photovoltaic performance.
Tothebestofourknowledge, thisisthefirstreportstudyingtheeffects
of the introduction of an additionaldonorof various number, position,
and shape to the TPA donor moiety for efficiency improvement. To
gain further insight into their effects, density functional theory (DFT)
calculations of all the dyes were carried out, using the B3LYP/6-
þ
(Ag/Ag ), a glassy carbon and a platinum wire as a reference, a
working and a counter electrode, respectively, in CH Cl solution
containing 0.1 M tetrabutylammonium tetrafluoroborate as a sup-
porting electrolyte. A standard ferrocene/ferrocenium (Fc/Fc )
redox couple was used to calibrate the oxidation peak. Photo-
currentevoltage (IeV) measurements were performed using a
Keithley model 2400 source measure unit. Incident photon-to-
current conversion efficiency (IPCE) was measured as a function of
wavelength from300 nm to 1000 nm using a specially designed IPCE
system for the dye-sensitized solar cell (PV measurements, Inc.).
2
2
þ
31G(d,p) level. Electrochemical impedance spectroscopy (EIS) was
Fig. 1. Molecular structures of D1, N719, SH-11, SH-12, SH-13, and SH-14.

392
S.H. Kim et al. / Dyes and Pigments 113 (2015) 390e401
Scheme 1. Synthesis of SH-11, SH-12, SH-13, and SH-14: (a) i) HCl, NaNO
acid, Pd(PPh , K CO , THF (d) cyanoacetic acid, piperidine, acetonitrile.
, H
2 2
2
O ii) KI, H O (b) 4-bromoaniline, CuI, 1,10-phenanthroline, KOH, toluene (c) 5-formyl-2-furan-boronic
3
)
4
2
3
2
.3. Fabrication of dye-sensitized solar cells and photovoltaic
photodiode G425 standard. IPCE values were measured under
halogen bias light at a low chopping speed of 10 Hz.
measurements
The nanocrystalline TiO
transparent layer (20 nm, synthesized) and a TiO
2
working electrode comprised a TiO
scattering layer
250 nm, G1). The Pt-coated counter electrode was prepared by a
reported procedure [20]. TiO electrodes were immersed in tert-
2
2.4. Synthesis of dyes
2
(
2.4.1. 4-Iodo-1,2-dimethoxybenzene (1)
2
A
mechanically stirred mixture of 3,4-dimethoxyaniline
ꢁ
butanoleacetonitrile (1:1) solution containing the dyes at 0.5 mM
for 40 h at ambient temperature. They were then washed with
ethanol and dried under a stream of nitrogen. The working and
(5.36 g, 35 mmol) and 36% HCl (50 mL) was cooled to ꢀ5 C. A
solution of NaNO (2.90 g, 42 mmol) in 20 mL H O was cooled to
2
2
ꢁ
ꢁ
0 C and slowly added while maintaining the temperature at ꢀ5 C.
ꢁ
counter electrodes were sealed with Surlyn (60
mm, Dupont) and
The final solution was stirred at 0 C for 30 min. A solution of KI
the electrolyte was injected through a hole in the counter electrode.
The electrolyte comprised 0.7 M 1-methyl-3-propylimidazolium
(11.62 g, 70 mmol) in 30 mL H O was added dropwise keeping the
temperature between 0 and 5 C. The resulting mixture was stirred
2
ꢁ
iodide (PMII), 0.2 M LiI (Aldrich), 0.05 M I
-tert-butylpyridine (TBP, Aldrich) in a mixed solvent of acetonitrile
and valeronitrile (v/v, 85/18). The active area of the dye-coated TiO
2
(Aldrich), and 0.5 M
overnight and extracted twice with diethyl ether. The organic layer
was consecutively washed with a concentrated NaHSO3 solution,
4
2
5% NaOH and water and dried over MgSO . The solvent was
4
2
film was 0.269e0.284 cm working area, measured by analyzing
images from a CCD camera (moticam 1000). TiO film thickness was
measured by an -step surface profiler (KLA tencor). Photo-
removed under reduced pressure and the crude product was pu-
2
rified by column chromatography using silica gel and dichlor-
omethaneehexane (3:1; v/v) as the eluent to give 1, a pale yellow
a
1
currentevoltage (IeV) measurements were performed using a
oil phase (6.91 g, 74.78%). H NMR (500 MHz, d -DMSO, ppm):
6
Keithley model 2400 source meter. A class-A solar simulator
d
7.23 (d, J ¼ 8.38 Hz, 1H), 7.19 (s, 1H), 6.78 (d, J ¼ 8.39 Hz, 1H), 3.75
(
Newport) equipped with 150 W Xe lamp was used as a light
(s, ꢀOCH , 3H), 3.73 (s, ꢀOCH , 3H).
3
3
source. Light intensity was adjusted with an NREL-calibrated Si
solar cell with KG-5 filter for approximating 1 sunlight intensity.
Photocurrentevoltage measurement of the dye-sensitized solar
cells was performed with an aperture mask. Incident photon-to-
current conversion efficiency (IPCE) was measured as a function
of wavelength from 300 to 1000 nm using a specially designed IPCE
system for dye-sensitized solar cells (PV measurements, Inc.). A
2
.4.2. 6-Iodo-1,4-benzodioxane (2)
A mechanically stirred mixture of 6-amino-1,4-benzodioxane
ꢁ
(
5.29 g, 35 mmol) and 36% HCl (50 mL) was cooled to ꢀ5 C. A
solution of NaNO
2
(2.90 g, 42 mmol) in 20 mL H
2
O was cooled to
ꢁ
ꢁ
0
C and slowly added while maintaining the temperature at ꢀ5 C.
ꢁ
The final solution was stirred at 0 C for 30 min. A solution of KI
11.62 g, 70 mmol) in 30 mL H O was added dropwise keeping the
7
5 W xenon lamp was used as a light source for generating
monochromatic beams. Calibration was performed using a silicon
photodiode, which was calibrated using NIST-calibrated
(
2
ꢁ
temperature between 0 and 5 C. The resulting mixture was stirred
overnight and extracted twice with diethyl ether. The organic layer
a

S.H. Kim et al. / Dyes and Pigments 113 (2015) 390e401
393
was consecutively washed with a concentrated NaHSO
3
solution,
25 mmol), 4-bromoaniline (1.72 g, 10 mmol), and 1,10-
phenanthroline (0.18 g, 1 mmol) in toluene (100 mL) were added
potassium hydroxide (5.61 g, 100 mmol) and copper(I) iodide
5
% NaOH and water and dried over MgSO . The solvent was
4
removed under reduced pressure and the crude product was pu-
rified by column chromatography using silica gel and dichlor-
(0.19 g, 1 mmol). Eluent: dichloromethaneehexane (1:1; v/v),
1
omethaneehexane (3:2; v/v) as the eluent to give 2, a pale yellow
product 6 (2.67 g, 60.6%). H NMR (500 MHz, d
6
-DMSO, ppm):
1
oil phase (8.64 g, 94.22%). H NMR (500 MHz, d
6
-DMSO, ppm):
d
7.33 (d, J ¼ 8.56 Hz, 2H), 6.83 (d, J ¼ 8.44 Hz, 2H), 6.71 (d,
d
7.18 (s, 1H), 7.13 (d, J ¼ 8.45 Hz, 1H), 6.68 (d, J ¼ 8.45 Hz, 1H), 4.22
J ¼ 8.56 Hz, 2H), 6.55e6.53 (m, 4H), 4.22 (s, ꢀOCH
2
CH
2
Oꢀ ꢂ 2, 8H).
13
(
s, ꢀOCH CH Oꢀ, 4H).
2
2
3
C NMR (126 MHz, CDCl , ppm): d 147.5, 143.9, 141.1, 140.1, 131.8,
122.9, 118.4, 117.7, 114.1, 112.9, 64.4, 64.3. HRMS (m/z): calcd. for
2
.4.3. 4-Bromo-N,N-bis(4-methoxyphenyl)aniline (3)
To a stirred solution of 4-iodoanisole (5.85 g, 25 mmol), 4-
bromoaniline (1.72 g, 10 mmol), and 1,10-phenanthroline (0.18 g,
mmol) in toluene (100 mL) were added potassium hydroxide
(C22
H
18BrNO
18BrNO
4
): 439.0419, found: 439.0420 [EIþ]. Anal. Cald for
(C22H
4
): C, 60.01; H, 4.12; N, 3.18. Found: C, 59.95; H, 4.23; N,
3.11.
1
(
5.61 g, 100 mmol) and copper(I) iodide (0.19 g, 1 mmol). The re-
2.4.7. 5-(4-(Bis(4-methoxyphenyl)amino)phenyl)furan-2-
carbaldehyde (7)
ꢁ
action mixture was heated under reflux for 24 h at 125 C. The crude
product was extracted into dichloromethane, and the organic layer
was washed with 1 N HCl solution, brine, and water. The organic
layer was dried with anhydrous MgSO
removed in vacuo. The residue was purified by column chroma-
Under N
2-furan-boronic acid (0.92 g, 6.6 mmol), Pd(PPh
0.3 mmol), 2 M aqueous solution of K CO (7.49 mL) in dry THF
(50 mL) was heated under reflux for overnight. The reaction
mixture extracted with CH Cl , water and brine. The organic layer
was dried with anhydrous MgSO and then the solvent was
2
atmosphere, a mixture of 3 (2.31 g, 6 mmol), 5-formyl-
3 4
)
(0.35 g,
4
and then the solvent was
2
3
tography using silica gel and dichloromethaneehexane (1:1; v/v) as
the eluent to give 3 (2.39 g, 62.2%). H NMR (500 MHz, d
2
2
1
6
-DMSO,
4
ppm):
H), 6.67 (d, J ¼ 9.0 Hz, 2H), 3.74 (s, ꢀOCH
126 MHz, CDCl , ppm): 156.0,147.9,140.6,131.7, 127.7,126.5, 121.9,
18BrNO ):
18BrNO ): C,
d
7.31 (d, J ¼ 9.0 Hz, 2H), 7.04e7.02 (m, 4H), 6.93e6.91 (m,
removed in vacuo. The residue was purified by column chroma-
13
4
(
1
3
6
3
ꢂ 2, 6H). C NMR
tography using silica gel and dichloromethaneemethanol (50:1; v/
1
3
d
v) as the eluent to give 7 (0.94 g, 39.2%). H NMR (500 MHz, d
6
-
14.8, 114.2, 112.4, 55.5. HRMS (m/z): calcd. for (C20
83.0520, found: 383.0516 [EIþ]. Anal. Cald for (C20
H
2
DMSO, ppm):
d
9.51 (s, ꢀCHO, 1H), 7.66 (d, J ¼ 8.86 Hz, 2H), 7.61 (d,
H
2
J ¼ 3.77 Hz, 2H), 7.12e7.10 (m, 4H), 6.97 (d, J ¼ 8.91 Hz, 4H), 6.78 (d,
13
2.51; H, 4.72; N, 3.65. Found: C, 62.39; H, 4.70; N, 3.52.
J ¼ 8.88 Hz, 2H), 3.76 (s, ꢀOCH
ppm): 176.6, 160.3, 156.6, 151.4, 150.2, 139.9, 127.3, 126.4, 120.2,
119.0, 114.9, 105.8, 55.5. HRMS (m/z): calcd. for (C25 ):
399.1470, found: 399.1469 [EIþ]. Anal. Cald for (C25 21NO ): C,
75.17; H, 5.30; N, 3.51. Found: C, 75.05; H, 5.33; N, 3.58.
3
ꢂ 2, 6H). C NMR (126 MHz, CDCl
3
,
d
2.4.4. N-(4-Bromophenyl)-N-(2,4-dimethoxyphenyl)-2,4-
H21NO
4
dimethoxyaniline (4)
H
4
Product 4 was synthesized according to the procedure as
described above for synthesis of 3. To a stirred solution of 2,4-
dimethoxyiodobenzene (6.60 g, 25 mmol), 4-bromoaniline
2.4.8. 5-(4-(Bis(2,4-dimethoxyphenyl)amino)phenyl)furan-2-
(
1.72 g, 10 mmol), and 1,10-phenanthroline (0.18 g, 1 mmol) in
toluene (100 mL) were added potassium hydroxide (5.61 g,
00 mmol) and copper(I) iodide (0.19 g, 1 mmol). Eluent: dichlor-
carbaldehyde (8)
Product 8 was synthesized according to the procedure as
described above for synthesis of 7. A mixture of 4 (2.66 g, 6 mmol),
5-formyl-2-furan-boronic acid (0.92 g, 6.6 mmol), Pd(PPh
1
1
omethaneehexane (2:1; v/v), product 4 (2.97 g, 67.0%). H NMR
3 4
)
(
(
(
1
4
5
300 MHz, CDCl
s, 2H), 3.73 (s, ꢀOCH
126 MHz, CDCl , ppm):
3
, ppm):
d
7.13e7.06 (m, 4H), 6.45e6.36 (m, 4H), 6.27
(0.35 g, 0.3 mmol), 2 M aqueous solution of K CO (7.49 mL) in dry
THF (50 mL) was heated under reflux for overnight. Eluent:
dichloromethaneemethanol (50:1; v/v), product 8 (0.91 g, 33.0%).
2
3
13
3
ꢂ 2, 6H), 3.65 (s, ꢀOCH
3
ꢂ 2, 6H). C NMR
3
d
158.9,156.8,148.3,131.1,130.3,127.8,116.1,
1
09.2, 104.9, 100.1, 55.7, 55.5. HRMS (m/z): calcd. for (C22
4
H22BrNO ):
H NMR (500 MHz, CDCl
3
, ppm):
d
9.51 (s, ꢀCHO, 1H), 7.68 (d,
43.0732, found: 443.0731 [EIþ]. Anal. Cald for (C22
H
22BrNO ): C,
4
J ¼ 8.75 Hz, 2H), 7.55 (d, J ¼ 8.95 Hz, 2H), 7.47 (d, J ¼ 3.73 Hz, 1H),
6.58 (d, J ¼ 3.75 Hz, 1H), 6.54 (s, 2H), 6.49e6.46 (m, 4H), 3.82
9.47; H, 4.99; N, 3.15. Found: C, 60.06; H, 4.98; N, 2.98.
13
(
s, ꢀOCH
CDCl , ppm):
132.0, 131.9, 131.8, 130.9, 126.3, 120.6, 119.7, 118.0, 117.3, 112.2, 111.9,
109.8, 109.3, 106.0, 56.1, 56.0. HRMS (m/z): calcd. for (C27 ):
25NO ): C,
3
ꢂ 2, 6H), 3.74 (s, ꢀOCH
3
ꢂ 2, 6H). C NMR (126 MHz,
2
.4.5. N-(4-Bromophenyl)-N-(3,4-dimethoxyphenyl)-3,4-
3
d 176.6, 160.2, 149.9, 149.7, 146.2, 140.8, 140.1, 132.1,
dimethoxyaniline (5)
Product 5 was synthesized according to the procedure as
described above for synthesis of 3. To a stirred solution of 1 (6.60 g,
H25NO
6
459.1682, found: 459.1684 [FABþ]. Anal. Cald for (C27
H
6
2
5
mmol), 4-bromoaniline (1.72 g, 10 mmol), and 1,10-
70.58; H, 5.48; N, 3.05. Found: C, 70.48; H, 5.52; N, 3.00.
phenanthroline (0.18 g, 1 mmol) in toluene (100 mL) were added
potassium hydroxide (5.61 g, 100 mmol) and copper(I) iodide
2.4.9. 5-(4-(Bis(3,4-dimethoxyphenyl)amino)phenyl)furan-2-
(
0.19 g, 1 mmol). Eluent: dichloromethaneehexane (3:1; v/v),
carbaldehyde (9)
1
product 5 (2.84 g, 64.1%). H NMR (500 MHz, d
6
-DMSO, ppm):
7.32 (d, J ¼ 8.9 Hz, 2H), 6.93 (d, J ¼ 8.6 Hz, 2H), 6.73e6.70 (m, 4H),
.63 (d, J ¼ 8.5 Hz, 2H), 3.74 (s, ꢀOCH ꢂ 2, 6H), 3.64 (s, ꢀOCH ꢂ 2,
, ppm): 149.6, 147.7, 145.7, 140.8,
31.8, 122.6, 117.3, 112.9, 111.9, 109.3, 56.1, 55.9. HRMS (m/z): calcd.
for (C22 22BrNO
): 443.0732, found: 443.0736 [EIþ]. Anal. Cald for
22BrNO ): C, 59.47; H, 4.99; N, 3.15. Found: C, 59.67; H, 4.89;
N, 3.17.
Product 9 was synthesized according to the procedure as
described above for synthesis of 7. A mixture of 5 (2.66 g, 6 mmol),
5-formyl-2-furan-boronic acid (0.92 g, 6.6 mmol), Pd(PPh
(0.35 g, 0.3 mmol), 2 M aqueous solution of K CO (7.49 mL) in dry
THF (50 mL) was heated under reflux for overnight. Eluent:
d
6
6
1
3
3
3 4
)
13
H). C NMR (126 MHz, CDCl
3
d
2
3
H
4
dichloromethaneemethanol (100:1; v/v), product 9 (0.87 g, 31.6%).
1
(
C
22
H
4
H NMR (500 MHz, d
6
-DMSO, ppm):
d
9.50 (s, ꢀCHO, 1H), 7.67 (d,
J ¼ 8.81 Hz, 2H), 7.61 (d, J ¼ 3.72 Hz, 2H), 6.97 (d, J ¼ 8.60 Hz, 2H),
6
.83e6.80 (m, 4H), 6.73 (d, J ¼ 8.53 Hz, 2H), 3.76 (s, ꢀOCH
3
ꢂ 2, 6H),
13
2
.4.6. N-(4-Bromophenyl)-N-(2,3-dihydrobenzo[b][1,4]dioxin-6-
3.67 (s, ꢀOCH
3
ꢂ 2, 6H). C NMR (126 MHz, CDCl
3
, ppm): 176.6,
d
yl)-2,3-dihydrobenzo[b][1,4]dioxin-6-amine (6)
Product 6 was synthesized according to the procedure as
described above for synthesis of 3. To a stirred solution of 2 (6.55 g,
160.2, 151.5, 149.9, 149.7, 146.2, 140.8, 140.1, 132.1, 132.0, 131.9, 131.8,
128.6, 128.4, 126.3, 122.6, 119.7, 118.0, 112.2, 111.9, 109.8, 106.0, 56.1,
6
56.0. HRMS (m/z): calcd. for (C27H25NO ): 459.1682, found:

394
S.H. Kim et al. / Dyes and Pigments 113 (2015) 390e401
1
4
59.1688 [EIþ]. Anal. Cald for (C27
H
25NO
6
): C, 70.58; H, 5.48; N,
72.2%). H NMR (500 MHz, d
6
-DMSO, ppm):
d
7.97 (s, 1H), 7.72 (d,
3
.05. Found: C, 70.40; H, 5.47; N, 3.08.
J ¼ 8.90 Hz, 2H), 7.51 (d, J ¼ 3.75 Hz, 1H), 7.13 (d, J ¼ 3.80 Hz, 1H),
.98 (d, J ¼ 8.65 Hz, 2H), 6.83 (s, 2H), 6.79 (d, J ¼ 8.90 Hz, 2H), 6.75
(d, J ¼ 8.55 Hz, 2H), 3.76 (s, ꢀOCH ꢂ 2, 6H), 3.68 (s, ꢀOCH ꢂ 2,
, ppm): 167.8, 161.4, 150.2, 149.7,
147.1, 146.3, 139.8, 138.1, 126.6, 119.9, 119.5, 118.2, 116.2, 111.9, 109.9,
107.7, 94.3, 56.1, 56.0. HRMS (m/z): Calcd. for (C30 ):
526.1740, found: 526.1744 [FABþ]. Anal. Cald for (C30 ): C,
68.43; H, 4.98; N, 5.32. Found: C, 68.37; H, 4.94; N, 5.26.
6
2.4.10. 5-(4-(Bis(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)amino)
3
3
13
phenyl)furan-2-carbaldehyde (10)
Product 10 was synthesized according to the procedure as
described above for synthesis of 7. A mixture of 6 (2.64 g, 6 mmol),
6H). C NMR (126 MHz, CDCl
3
d
26 2 7
H N O
H N O
26 2 7
5
-formyl-2-furan-boronic acid (0.92 g, 6.6 mmol), Pd(PPh
3
)
4
(
0.35 g, 0.3 mmol), 2 M aqueous solution of K CO (7.49 mL) in dry
2
3
THF (50 mL) was heated under reflux for overnight. Eluent:
dichloromethaneemethanol (50:1; v/v), product 10 (1.18 g, 43.3%).
2.4.14. 3-(5-(4-(Bis(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)amino)
phenyl)furan-2-yl)-2-cyanoacrylic acid (SH-14)
Product SH-14 was synthesized according to the procedure as
described above for synthesis of SH-11. Carbaldehyde 10
(0.228 mg, 0.5 mmol), cyanoacetic acid (0.13 mg, 1.5 mmol) and
1
H NMR (500 MHz, d
6
-DMSO, ppm):
d
9.51 (s, ꢀCHO, 1H), 7.67 (d,
J ¼ 8.74 Hz, 2H), 7.61 (d, J ¼ 3.92 Hz, 1H), 7.04 (d, J ¼ 3.73 Hz, 1H),
6
4
d
.88 (d, J ¼ 7.37 Hz, 2H), 6.82 (d, J ¼ 8.76 Hz, 2H), 6.65e6.63 (m, 4H),
13
.24 (s, ꢀOCH
2
CH
2
Oꢀ ꢂ 2, 8H). C NMR (126 MHz, CDCl
3
, ppm):
176.6, 160.3, 151.5, 149.7, 144.0, 140.7, 140.4, 126.3, 120.6, 119.8,
piperidine (0.20 mL, 2 mmol) were added to anhydrous CH
3
CN.
1
19.2, 117.8, 114.9, 105.9, 64.4, 64.3. HRMS (m/z): calcd. for
Eluent: dichloromethaneemethanol (5:1; v/v), a red solid SH-14
(203 mg, 77.7%). H NMR (500 MHz, d -DMSO, ppm): d 7.98 (s, 1H),
6
1
(
(
C
C
27
H21NO
21NO
6
): 455.1369, found: 455.1372 [EIþ]. Anal. Cald for
27
H
6
): C, 71.20; H, 4.65; N, 3.08. Found: C, 71.18; H, 4.68; N,
7.72 (d, J ¼ 8.82 Hz, 2H), 7.51 (d, J ¼ 3.73 Hz, 1H), 7.13 (d, J ¼ 3.74 Hz,
3
.11.
1H), 6.88e6.6 (m, 2H), 6.79 (d, J ¼ 8.84 Hz, 2H), 6.66e6.64 (m, 4H),
1
3
4
d
.24 (s, ꢀOCH
2
CH
2
Oꢀ ꢂ 2, 8H). C NMR (126 MHz, CDCl
3
, ppm):
2.4.11. (E)-3-(5-(4-(Bis(4-methoxyphenyl)amino)phenyl)furan-2-
167.5, 161.9, 150.3, 147.0, 144.1, 140.9, 140.0, 126.7, 119.7, 119.4,
yl)-2-cyanoacrylic acid (SH-11)
117.9, 116.0, 115.2, 107.8, 93.3, 64.4, 64.3. HRMS (m/z): Calcd. for
Carbaldehyde 7 (200 mg, 0.5 mmol), cyanoacetic acid (0.13 g,
(C30
(C30
5.31.
H
H
22
N
N
2
O
O
7
): 522.1427, found: 522.1423 [FABþ]. Anal. Cald for
1
.5 mmol) and piperidine (0.20 mL, 2 mmol) were added to anhy-
drous CH CN. After the mixture was heated under reflux for 8 h, the
solution was extracted with CH Cl and 0.1 M HCl aqueous solution.
The organic phase was dried over anhydrous MgSO and the sol-
22
2
7
): C, 68.96; H, 4.24; N, 5.36. Found: C, 68.83; H, 4.19; N,
3
2
2
4
3. Results and discussion
vent was removed in vacuo. The crude product was purified by
column chromatography using silica gel and dichlor-
3.1. Synthesis
omethaneemethanol (5:1; v/v) as the eluent to give a red solid SH-
1
1
1
5
1 (175 mg, 75.0%). H NMR (500 MHz, d
6
-DMSO, ppm):
d
7.98 (s,
The synthetic routes and structures of the dyes with methoxy
groups are shown in Scheme 1 and Fig. 1. Synthesis of the dyes first
involved a Sandmeyer reaction of 3,4-dimethoxyaniline and 6-
amino-1,4-benzodioxane, which produced iodinated in-
termediates (1 and 2) for an Ullmann reaction [21]. Compounds 3~6
were derived from CeN coupling of the 4-iodoanisole, 2,4-
dimethoxyiodobenzene, 1, and 2 with 4-bromoaniline by the Ull-
mann reaction, respectively. Compounds 7~10 were subsequently
produced by Suzuki couplings between compounds 3~6 and 5-
formyl-2-furan-boronic acid, respectively. Finally, the organic
dyes (SH-11~14) were synthesized, by Knoevenagel condensation
reactions of the aldehydes (7~10) with cyanoacetic acid, in the
presence of piperidine and acetonitrile. The first intermediates (1
H), 7.71 (d, J ¼ 8.83 Hz, 2H), 7.51 (d, J ¼ 3.75 Hz, 1H), 7.14e7.12 (m,
H), 6.97 (d, J ¼ 8.88 Hz, 4H), 6.77 (d, J ¼ 8.85 Hz, 2H), 3.76
13
(
1
s, ꢀOCH
3
ꢂ 2, 6H). C NMR (126 MHz, CDCl
3
, ppm): d 168.4, 162.0,
56.8, 150.7, 146.9, 139.5, 138.3, 127.5, 126.8, 119.4, 118.7, 116.1, 114.9,
1
07.8, 93.3, 55.5. HRMS (m/z): Calcd. for (C28
22
H N
2
O
5
): 466.1529,
found: 466.1527 [FABþ]. Anal. Cald for (C28
22 2 5
H N O ): C, 72.09; H,
4
.75; N, 6.01. Found: C, 72.01; H, 4.88; N, 5.97.
2
.4.12. 3-(5-(4-(Bis(2,4-dimethoxyphenyl)amino)phenyl)furan-2-
yl)-2-cyanoacrylic acid (SH-12)
Product SH-12 was synthesized according to the procedure as
described above for synthesis of SH-11. Carbaldehyde 8 (230 mg,
.5 mmol), cyanoacetic acid (0.13 g, 1.5 mmol) and piperidine
0.20 mL, 2 mmol) were added to anhydrous CH CN. Eluent:
dichloromethaneemethanol (6:1; v/v), a red solid SH-12 (205 mg,
1
0
(
and 2), which are simple structures, were identified by H NMR. The
3
remainder of the intermediates and organic dyes were confirmed
1
13
by H NMR, C NMR, HRMS, and elemental analysis. D1 (reference
1
7
7.9%). H NMR (500 MHz, d
6
-DMSO, ppm):
d
7.63 (s, 1H), 7.53 (d,
dye) was synthesized, according to the published method [19].
J ¼ 8.73 Hz, 2H), 7.19 (d, J ¼ 8.61 Hz, 2H), 7.10 (d, J ¼ 3.42 Hz, 1H),
6
.83 (d, J ¼ 3.44 Hz, 1H), 6.69 (s, 2H), 6.55 (d, J ¼ 8.61 Hz, 2H), 6.28
ꢂ 2, 6H), 3.65 (s, ꢀOCH ꢂ 2,
-DMSO, ppm): 163.7, 160.0, 158.6,
3.2. Photophysical properties of the dyes in solution and on TiO
2
film
(
d, J ¼ 8.70 Hz, 2H), 3.78 (s, ꢀOCH
3
3
13
6
H). C NMR (126 MHz, d
6
d
1
1
55.9, 149.9, 145.8, 136.4, 133.6, 129.7, 127.0, 125.6, 125.4, 116.5,
16.3, 112.7, 106.7, 105.2, 99.3, 93.8, 55.1, 54.8. HRMS (m/z): Calcd.
The absorption spectra of the synthesized dyes in CH
2 2
Cl solu-
tion and on TiO films are shown in Fig. 2(a) and (b), respectively.
2
for (C30
H
26
N
2
O
7
): 526.1740, found: 526.1743 [FABþ]. Anal. Cald for
The dyes' absorption, emission and electrochemical properties are
listed in Table 1. The absorption spectra show two major bands,
below ca. 400 nm, and at ca. 400e600 nm. The absorption in the UV
(
C
H
30 26
N
2
O
7
): C, 68.43; H, 4.98; N, 5.32. Found: C, 68.35; H, 4.93; N,
5
.29.
region was attributed to localized pep* transitions; the band in the
2
.4.13. 3-(5-(4-(Bis(3,4-dimethoxyphenyl)amino)phenyl)furan-2-
visible region was ascribed to intra-molecular charge transfer (ICT)
from the TPA-based donor part to the acceptor [23].
yl)-2-cyanoacrylic acid (SH-13)
Product SH-13 was synthesized according to the procedure as
described above for synthesis of SH-11. Carbaldehyde 9 (230 mg,
The absorption maxima (lmax) of SH-11~14 with methoxy
groups on the TPA donor are at 504, 504, 504, and 502 nm,
respectively. The introduction of additional methoxy groups at
para-(SH-11), ortho- and para-(SH-12), or meta- and para-(SH-
13~14) positions on the TPA donor increased the electron donating
0
.5 mmol), cyanoacetic acid (0.13 g, 1.5 mmol) and piperidine
0.20 mL, 2 mmol) were added to anhydrous CH CN. Eluent:
dichloromethaneemethanol (6:1; v/v), a red solid SH-13 (190 mg,
(
3

S.H. Kim et al. / Dyes and Pigments 113 (2015) 390e401
395
0
0
0
0
0
0
.5
.4
.3
.2
.1
.0
maximum absorption and molar extinction coefficient of dyes are
closely related to the transition dipole moment, conjugation length,
SH-11
SH-12
SH-13
SH-14
D1(Ref.)
(
a)
andplanarity[24]. The molar extinction coefficients at
lmax of D1, SH-
1
1, SH-12, SH-13, and SH-14 were 28,600, 32,680, 39,200, 31,260, and
ꢀ1
ꢀ1
37,660 M cm , respectively. The molar extinction coefficients of
SH-11~14 were all larger than that of D1. This could be due to the
combined effects of the increased maximum absorption, and the
improvement of transition dipole moment by the introduction of
additional methoxy groups (Fig. 3). Among the dyes, SH-12 with
methoxy groups at ortho- and para-position on the TPA donor
showed the highest molar extinction coefficient. This could be
attributed to the smallest overall dihedral angles (
donor part, and that ( 4) between the TPA donor and the
despite the largest dihedral angles of phenyl ring planes (
F
1~3) of the TPA
-bridge,
5~6)
F
p
F
(Figs. 3 and 4). As for the shape of methoxy groups, the molar
extinction coefficient of SH-14 with closed ring system in the TPA
donor part was higher than that of SH-13 with open ring system. This
could be attributed to the improved stereoscopic planarity of the
closed ring in the TPA donor, compared to the open ring with pro-
truded methoxygroups. Asthe numberofmethoxygroupsincreased,
the molar extinction coefficient generally showed higher figures.
Consequently, the introduction of additional methoxy groups on
the TPA donor led to an increase of the electron donating ability of
the TPA donor part, and improved the transition dipole moment of
the whole molecule. Their introduction also led to the improve-
ment of the whole stereoscopic planarity of the molecule, in
comparison with D1. Therefore, the absorption maxima and molar
extinction coefficients of SH-11~14 have red-shifted and been
enhanced, compared to those of D1. This is highly desirable as a
3
00
350
400
450
500
550
600
650
Wavelength (nm)
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
SH-11
SH-12
SH-13
SH-14
(
b)
photosensitizer for large photocurrent generation. On TiO
2
films,
the absorption maxima ( max) of SH-11, SH-12, SH-13, and SH-14
l
are shown in Fig. 2(b). They exhibit large blue-shifts (42e48 nm),
due to H-aggregation of the dyes [13,25], or deprotonation of car-
boxylic acid upon adsorption onto the TiO surface [14b]. Such a
2
4
00
450
500
550
600
650
700
hypsochromic shift of the absorption spectra for the dyes has
generally been observed in other organic dyes, when dyes are
Wavelength (nm)
Fig. 2. UVevisible absorption spectra of (a) D1 and SH-11~14 in CH
1~14 on TiO films.
2
Cl
2
, and (b) SH-
attached to the TiO
2
surface.
1
2
3.3. Electrochemical properties of the dyes
ability of the TPA donor, and extended
p-electron delocalization
over the whole molecule, which red-shifted the absorption maxima
by 16e18 nm, compared with that of D1. However, the number,
position, and shape of methoxy groups attached to the TPA donor
had a marginal effect on the maximum absorption of the dyes.
The dye molar extinction coefficients at lmax were also affected by
the introduction of additional methoxy groups. In general, the
The first oxidation potential (Eox), corresponding to each dye's
HOMO level, was measured in CH Cl by cyclic voltammetry (CV).
2
2
The excited state oxidation potential (Eox* ¼ Eox e E0e0), corre-
sponding to each dye's LUMO level, was obtained by subtracting the
zerothezeroth energy (E0e0), estimated from the onset wavelength
of the absorption spectrum, from Eox. The electrochemical proper-
ties of the dyes are listed in Table 1. Their CV curves and HOMO and
LUMO level diagram are shown in Figs. 5 and 6, respectively. The
HOMO levels of D1, SH-11~14 were 0.84, 0.92, 0.94, 0.89, and 0.94 V,
respectively. They were more positive than that of iodineeiodide
(0.4 V vs. NHE) [26], and were sufficiently positive to regenerate the
Table 1
Optical and electrochemical properties of D1 and SH-11~14.
Dye
Absorptiona
Emissiona Oxidation potential datac
em/nm ox/V
0-0d/V
l
abs/nm
l
absb/nm
l
E
E
E
ox ꢀ E0e0/V
ꢀ
1
ꢀ1
ꢀ ꢀ
(
ε M cm
)
(on TiO
2
)
(vs. NHE)
(vs. NHE)
oxidized dyes from the redox electrolyte (I /I ). As methoxy groups
3
D1 (Ref.) 486 (28,600)
e
629
626
612
602
601
0.84
0.92
0.94
0.89
0.94
2.25
2.15
2.19
2.15
2.16
ꢀ1.41
ꢀ1.23
ꢀ1.25
ꢀ1.26
ꢀ1.22
were introduced on the TPA donor part, the HOMO levels of SH-
11~14 became slightly more positive than that of D1, which could
induce stronger driving force for more efficient dye regeneration.
The LUMO levels of D1, SH-11~14 were ꢀ1.41, e1.23, e1.25, e1.26,
and ꢀ1.22 V, respectively. They were sufficiently more negative
SH-11
SH-12
SH-13
SH-14
504 (32,680) 461
504 (39,200) 456
504 (31,260) 456
502 (37,660) 460
a
Absorption and emission spectra were measured in CH
2
Cl
2
solution at room
than the conduction band energy level (Ecb) of TiO
2
(ꢀ0.5 V vs. NHE)
ꢀ
5
temperature (1 ꢂ 10 M).
b
[26], to allow thermodynamically efficient injection of excited
electrons into the TiO electrode. The introduction of additional
2
Absorption spectra on TiO were obtained through measuring the dye absorbed
TiO
2
film in CH
2
Cl
2
.
2
c
Oxidation potentials of the dyes were measured in CH
2
Cl
2
containing 0.1 M
methoxy groups on the TPA donor part made both the HOMO and
LUMO levels more positive, in comparison with those of D1. The
potential increases in LUMO levels were higher than those in
HOMO levels, and the energy band gaps (Egap) of the HOMOeLUMO
levels were decreased. This may be attributed to increase of the
tetrabutylammonium tetrafluoroborate (TBABF
glassy carbon; counter electrode: Pt; reference electrode: Ag/Ag calibrated with
ferrocene/ferrocenium (Fc/Fc ) as an internal reference, and converted to NHE by
4
) as electrolyte (working electrode:
þ
þ
the addition of 630 mV [22]).
d
E0e0 was estimated from the onset wavelength of the absorption spectra.

396
S.H. Kim et al. / Dyes and Pigments 113 (2015) 390e401
Fig. 3. Dihedral angles of the TPA donor parts (F1~3), and that between the TPA donor and the p-bridge (F4) of the synthesized dyes.
electron donating ability of the TPA donor, and the improvement of
the stereoscopic planarity of the overall molecule by the change of
B3LYP/6-31G(d,p) level. The optimized structures, electron distri-
butions, and energy levels of the dyes are presented in Fig. 7.
the dihedral angles (
F
1~6) of the dye molecule (Figs. 3 and 4).
Dihedral angles of the TPA donor parts (
F
1~3 and
F5~6), and that
Consequently, the SH-11~14 became red-shifted, and might act as
more efficient photosensitizers for DSSCs, compared with D1.
between the TPA donor and the -bridge (F
p
4) in the dyes, are
depicted in Figs. 3 and 4. All the dye electron distributions in the
HOMO orbitals were mostly over the TPA donor part and the furan
ring. The size of electron distributions of the HOMO orbitals of SH-
3.4. Density functional theory (DFT) calculations
11, SH-13, and SH-14 were larger than that of D1. This may be
To gain further insight into the above results, density functional
theory (DFT) calculations of all the dyes were carried out, using the
because the electron donating ability of the TPA donor part was
improved, by adding methoxy groups as additional electron

S.H. Kim et al. / Dyes and Pigments 113 (2015) 390e401
397
Fig. 4. Dihedral angles (
F
5~6) between the planes of phenyl rings, and the plane of the N-benzene and furan ring.
donating groups. On the other hand, the size of electron distribu-
tions of the HOMO orbitals of SH-12 was relatively smaller,
compared to those of the other dyes. The electron distribution of
the HOMO orbital of SH-12 is mainly over the N-benzene ring of the
TPA donor connected to the furan ring and the furan ring itself, and
seems to be lower over the phenyl rings with methoxy groups, in
contrast to the other dyes. This may be due to the largest dihedral
angles (F5~6) between the planes (a, b, c) of phenyl rings, which
result in a relatively poor orbital overlap of the TPA donor part. This
calculation result can explain why the electrochemical property of
SH-12 is particularly different from those of the other dyes, which
will largely affect the cell performance of SH-12. All the dye elec-
tron distributions of the LUMO orbitals were localized towards the
cyanoacrylic acid and its adjacent furan ring. Therefore, the
HOMOeLUMO excitation induced by light irradiation could effec-
tively shift the electron distributions from the TPA donor part to the
acceptor group. Excited electrons could easily be injected from the
2
dye to the TiO surface by this electron separation. The molecular
þ
Fig. 5. CV curves of Fc/Fc , D1, SH-11, SH-12, SH-13, and SH-14 in CH
2
Cl
2
.
Fig. 6. Dye HOMO and LUMO levels diagram.

398
S.H. Kim et al. / Dyes and Pigments 113 (2015) 390e401
Fig. 7. (A) Plane figures of the dye optimized structures. (B) Dye energy levels and electron distributions through the frontier molecular orbitals of the HOMO and LUMO, calculated
with DFT at the B3LYP/6-31G(d,p) level.
structures of SH-11~14 showed various dihedral angles (
F
1~6) as
and furan ring have great relevance to the electron density of the
TPA donor part, and the inhibition of the close aggregation
between dye molecules [17]. The averages of the dihedral angles
methoxy groups on the introduced TPA donor part. The dihedral
pep
ꢁ
angles (
1
F
1) of SH-11~14 were ca. 117e118 , and decreased by ca.
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
~2 , compared to that of D1 (ca. 119 ). The dihedral angles (
F
2~3)
(
F
5~6) of D1 and SH-11~14 were 46.15 , 50.2 , 75.65 , 49.75 , and
ꢁ
on an axis of SH-11~14 were greatly influenced by the introduction
49.65 , respectively. SH-11, SH-13, and SH-14 showed similar
dihedral angles (
dihedral angles (
ꢁ
ꢁ
ꢁ
ꢁ
of methoxy groups, which decreased by ca. 5.5 (SH-11), ca. 24.0
F
5~6; ca. 50 ); whereas, SH-12 showed the largest
ꢁ
ꢁ
(
(
SH-12), ca.4.2 (SH-13), and ca. 3.9 (SH-14), in comparison to that
F
5~6; ca. 76 ), due to the steric hindrance induced
ꢁ
ca. 31.8 ) of D1. SH-11, SH-13, and SH-14 showed similar dihedral
by the methoxy groups at the ortho-position adjacent to the N-
benzene ring. This increase of the dihedral angles ( 5~6) can
inhibit close aggregation between the dye molecules, and may
decrease the amount of dye adsorbed to the TiO surface. This result
explains the amount of dye adsorbed to the TiO surface (Table 2).
ꢁ
angles (
F
2~3; ca. 26e28 ); whereas, SH-12 showed the largest
F
change of the dihedral angles (
F
2~3), due to the steric hindrance
pep
induced by methoxy groups at the ortho-position adjacent to the N-
benzene ring. The TPA donor parts of all the dyes had noncoplanar
2
2
propeller structure that could suppress dye aggregation. The
As a result, the number, position, and shape of methoxy groups
introduced on the TPA donor part affected the electron distribu-
tions and the dihedral angles (F1~6) of the dye molecule, which
ꢁ
dihedral angles (
F
4) of SH-11~14 were ca. 0~2 , and similar to that
ꢁ
(ca. 1.2 ) of D1, respectively. This indicates that all the dyes have
coplanar structure from the TPA donor part, furan, and to the
acceptor group, which can form an efficient conjugation, and hence
favor the electron transport process. The dihedral angles (F5~6)
may have contributed to the bathochromic shift of the absorption
spectrum, the enhancement of the molar extinction coefficient, and
the decrease of the amount of dye adsorbed.
between the planes of phenyl rings and the plane of the N-benzene
3
.5. Photovoltaic properties of DSSCs
Table 2
a
Photovoltaic performance data of D1, SH-11, SH-12, SH-13, SH-14, and N719.
DSSCs were fabricated using the dyes as photosensitizers, and
Dyeb
J
sc/mA cm
ꢀ2
V
oc/mV FF/%
h
/%
Area (cm2) Dye amount
c
their photovoltaic properties were measured under AM 1.5G irra-
diation (100 mW cm ). The effects on cell performance by the
ꢀ
2
ꢀ2
(
mol cm
)
2.73 ꢂ 10ꢀ
7
7
7
7
7
D1 (Ref.) 10.41
704.3
691.6
578.5
676.3
687.5
783.3
75.11 5.51 0.269
67.37 5.57 0.275
67.29 3.85 0.274
68.66 5.29 0.275
71.18 6.01 0.284
71.26 7.59 0.279
number, position, and shape of methoxy groups attached to the TPA
moiety were assessed, by measuring their conversion efficiencies,
relative to the D1 as a reference. The capabilities of all the dyes as
possible photosensitizers were compared with N719 cell perfor-
mance (Table 2). Fig. 8(a) and (b) shows the incident photon-to-
current conversion efficiency (IPCE) spectra and photo-
currentevoltage (JeV) curves, respectively. All the dyes could
effectively convert visible light to photocurrent at ca. 300e700 nm.
The IPCE value of the cell with D1 as a reference exceeded 80% from
ꢀ
ꢀ
ꢀ
ꢀ
SH-11
SH-12
SH-13
SH-14
N719
11.96
9.89
11.39
12.29
13.60
2.16 ꢂ 10
1.43 ꢂ 10
2.31 ꢂ 10
2.35 ꢂ 10
e
a
Measured under AM 1.5 irradiation G (100 mW cmꢀ1), with 0.269e0.284 cm2
working area.
b
Dyes were maintained at 0.5 mM in tert-butanoleacetonitrile (1:1) solution,
with 10 mM CDCA as co-adsorbent. Electrolyte comprised 0.7 M 1-methyl-3-
2
propylimidazolium iodide (MPII), 0.2 M LiI, 0.05 M I , 0.5 M TBP in acetoni-
4
00 to 540 nm, with the highest value of 86.3% at 490 nm. The IPCE
trileevaleronitrile (v/v, 85/15) for organic dyes.
c
value of the cells with SH-11 exceeded 80% from 430 to 550 nm,
with the highest value of 84.2% at 500 nm. The IPCE values of the
cells with SH-13 (open ring system) and SH-14 (closed ring system)
2
The amount of dyes adsorbed on TiO was measured by a colorimetric method,
using 0.1 M NaOH aqueous-DMF (1:1) mixed solutions to wash the dyes off the TiO
film.
2

S.H. Kim et al. / Dyes and Pigments 113 (2015) 390e401
399
exceeded 80% from 480 to 500 nm and from 430 to 540 nm, with
the highest values of 80.6% and 83.4% at 490 nm, respectively. In
contrast with the above dyes, the IPCE value of the cells with SH-12
did not show over 80% in the range, and the highest value was 79.0%
at 490 nm. Contrary to expectations, SH-12 with the highest molar
part, their introduction led to bathochromic shift and enhancement
of the molar extinction coefficient by the increase of electron
donating ability, improving the Jsc values. However, SH-12 resulted
in the lowest Jsc value, due to the largest dihedral angles (F5~6),
which decreased the electron density of the TPA donor part, by
forming a relatively poor orbital overlap between the two planes.
The Jsc value was also likely to be affected by the amount of dye
extinction coefficient and
among them, which may be due to a poor orbital overlap by the
largest dihedral angles ( 5~6) [11b]. Although the IPCE value of the
lmax showed the lowest IPCE value
F
2
adsorbed to the TiO surface, which was influenced by the steric
cell with N719 as the other reference also did not show over 80% in
the range, its IPCE spectrum had the broadest region, and the
highest value was 76.1% at 500 nm. Except for SH-12, other dyes
hindrance induced by the introduction of methoxy groups on the
TPA donor part (Table 2).
The amount of dye adsorbed increased in the order of SH-
ꢀ7
(
SH-11 and SH-13~14) can convert visible light to photocurrent
more efficiently than D1 at the whole wavelength range (ca.
00e700 nm). These results can be closely linked to the Jsc values of
the dyes.
The Jsc values of all the dye cells ranked SH-12 < D1 < SH-
3 < SH-11 < SH-14 (SH-12, 9.89; D1, 10.41; SH-13, 11.39; SH-11,
12 ≪ SH-11 ꢃ SH-13 z SH-14 < D1 (SH-12, 1.43 ꢂ 10 ; SH-11,
ꢀ
7
7
ꢀ7
ꢀ7
2.16 ꢂ 10 ; SH-13, 2.31 ꢂ 10 ; SH-14, 2.35 ꢂ 10 ; D1,
ꢀ
ꢀ2
3
2.73 ꢂ 10 mol cm ). This order coincides with the tendency of
decreasing steric hindrance by dihedral angles ( 5~6), which
F
signify the twisted degree of the phenyl rings on the backbone
plane (b) of dyes (Fig. 4). This indicates that the increase of steric
1
ꢀ2
11.96; SH-14, 12.29 mA cm ), and are similar to the result of IPCE
hindrance could decrease the amount of dye adsorbed to the TiO
surface, by suppressing close aggregation between the dye
molecules. As a result, all things considered, SH-14 with a high
molar extinction coefficient, small dihedral angles ( 5~6), and a
2
values. Except for SH-12, the Jsc values of the other dyes (SH-11 and
SH-13~14) showed higher Jsc values than that of D1. Though the
pep
dihedral angles (
F
5~6) of SH-11 and SH-13~14 increased a little by
F
the introduction of additional methoxy groups at the TPA donor
relatively high amount of dye adsorbed, showed the largest Jsc
value. Though SH-12 had a high molar extinction coefficient and
red-shifted, the dye showed the lowest Jsc value, due to the least
amount of dye adsorbed, and relatively poor orbital overlap by the
100
SH-11
SH-12
SH-13
SH-14
D1
largest dihedral angles (
The oc values ranked SH-12 (578.5 mV)
(676.3 mV) < SH-14 (687.5 mV) ꢃ SH-11 (691.6 mV) < D1
704.3 mV). The Voc values of all the dyes decreased, as the number
of methoxy groups increased. This may be due to the decease of the
amount of dye adsorbed, and the formation of iodine-electron
donor complexes. The decease of the amount of dye adsorbed
F5~6) of the TPA donor part.
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
(a)
V
≪
SH-13
(
N719
was mainly attributed to the lower packing density of dyes on the
e
3
TiO
2
surface, which could result in higher I
concentration in the
vicinity of the TiO
2
surface, and increase the possibility of electron
recombination, compared to D1 [17]. In addition, it has been re-
ported that iodine has the tendency to form iodine-electron donor
complexes with atoms that contain lone-pair electrons, such as
oxygen, nitrogen, and sulfur [11b]. Therefore, SH-11~14 is more
likely than D1 to form dyeeiodine complexes, due to the presence
3
00 350 400 450 500 550 600 650 700 750 800
of the additional methoxy groups. Accordingly, the presence of the
Wavelength (nm)
e
additional methoxy groups could naturally increase I
tion near the TiO
3
concentra-
2
surface. Consequently, as the number of methoxy
1
1
1
1
6
groups increases, more dyeeiodine complexes will form, and sub-
sequently result in enhanced electron recombination, in compari-
son to D1. D1 without methoxy groups showed the highest Voc
value; and SH-11 with two methoxy groups at para-position also
showed a higher Voc value than SH-12~14 with four methoxy
groups. Dyes with four methoxy groups showed different Voc
values, depending on the position and shape of the methoxy
groups. The Voc value of SH-12 was far lower than those of SH-
SH-11
(
b)
SH-12
SH-13
SH-14
D1
4
2
0
8
6
4
2
0
N719
1
3~14, which is mainly attributed to the decease of the amount of
e
dye adsorbed, and the improvement of I
3
accessibility by the in-
crease of dihedral angles ( 5~6) induced by the steric hindrance of
F
methoxy groups at ortho-position adjacent to the N-benzene ring.
In shape, SH-14 with a closed ring system of methoxy groups
had slightly higher Voc value, compared with SH-13 with an open
ring system of methoxy groups. This means that methoxy groups in
e
3
the closed ring blocked the approach of I
more effectively,
0.0
0.1
0.2
0.3
0.4
V
0.5
0.6
0.7
0.8
0.9
compared to methoxy groups in the open ring. Although the
amount of SH-11 adsorbed was relatively lower than those of SH-
(V)
oc
1
3~14, the Voc value of SH-11 was a bit higher than those of SH-
Fig. 8. (a) Incident photo-to-current conversion efficiency (IPCE) spectra for DSSCs
based on SH-11, SH-12, SH-13, SH-14, D1, and N719. (b) Photocurrentevoltage curves
for DSSCs based on SH-11, SH-12, SH-13, SH-14, D1, and N719, under illumination of
13~14, which might be because the number of methoxy groups has
a greater impact on Voc value, compared with the amount of dye
adsorbed. FF values slightly decreased on introducing methoxy
ꢀ
2
AM 1.5G simulated sunlight (100 mW cm ).

400
S.H. Kim et al. / Dyes and Pigments 113 (2015) 390e401
groups on the TPA donor part. The decreased order coincides with
the increased order of dihedral angles ( 5~6). These results indi-
cate that FF values could be affected by the shape of
photosensitizer.
F
(a)
D1, SH-11, SH-12, SH-13, SH-14, and N719 dyes'
h values were
5
.51%, 5.57%, 3.85%, 5.29%, 6.01%, and 7.59%, respectively, under AM
ꢀ2
1.5G irradiation (100 mW cm ) (Table 2). Through the introduc-
tion of methoxy groups on the TPA donor part, values of SH-11
h
and SH-14 improved, due to the larger increase of Jsc values than the
decrease of Voc and FF values, compared to D1. Although SH-13
showed the higher Jsc value than D1, SH-13 dye
to the larger decrease of Voc and FF value. SH-12 showed the lowest
value, because of its small adsorption on the TiO surface, and
relatively poor orbital overlap by the largest dihedral angles ( 5~6).
h value was less, due
h
2
F
Consequently, all things considered, the best overall conversion
efficiency was achieved by the cell fabricated with SH-14
2
(h
¼ 6.01%, Jsc ¼ 12.29 mA/cm , Voc ¼ 687.5 mV, FF ¼ 71.18% under
AM 1.5G irradiation), reaching about 79.2% of N719 dye's 7.59%.
3.6. Electrochemical impedance spectroscopy (EIS)
Electrochemical impedance spectroscopy (EIS) analysis [27] was
(b)
employed to investigate the electron recombination and interfacial
charge transfer process in DSSCs. The EIS spectra for the DSSCs
based on SH-11~SH-14 were performed under forward bias
(
ꢀ0.65 V) in the dark. The Nyquist and Bode phase plots of DSSCs
are shown in Fig. 9(a) and (b), respectively. The major semicircles
and frequency peaks in the Nyquist and Bode phase plots indicate
2
the charge transfer resistance at the TiO /dye/electrolyte interface.
The radii of the major semicircles in the Nyquist plots increase in
the order of SH-12 < SH-13 < SH-14 < SH-11, indicating that the
electron recombination resistance increases from SH-12, SH-13,
SH-14 to SH-11. The major frequency peaks in the Bode phase plots
lower in the order of SH-12 > SH-13 > SH-14 > SH-11, indicating
that the electron lifetime increases in the order of SH-12 < SH-
13 < SH-14 < SH-11. A larger electron recombination resistance and
longer electron lifetime indicate improved suppression of back
e
Fig. 9. EIS spectra of DSSCs based on SH-11, SH-12, SH-13, and SH-14 measured
at ꢀ0.65 V forward bias in the dark: (a) Nyquist, and (b) Bode phase plots.
reactions between the injected electrons (e ) and the electrolyte
e
3
(
I
); they lead to improvement of the Voc value, due to the reduced
electron recombination rate [28]. The ranking of the cell electron
recombination resistances and electron lifetimes measured by EIS
coincided with that of the cell Voc values. Consequently, these re-
sults indicate that as the number of methoxy groups and dihedral
The Jsc values of other dyes (SH-11 and SH-13~14) except SH-12
generally increased, as the methoxy groups were introduced.
Among the organic dyes, SH-14 with a high molar extinction co-
efficient, small dihedral angles (F5~6), and a relatively high amount
of dye adsorbed, showed the highest Jsc value.
The Voc values of all the dyes decreased, as the number of
methoxy groups increased. This may be due to decease of the
amount of dye adsorbed by the increase of overall steric hindrance
and the formation of iodine-electron donor complexes by the lone-
pair electrons in the methoxy groups.
angles (F1~6) of a dye increase, the electron recombination resis-
tance decreases and electron lifetime shortens, due to the
e
improvement of accessibility between the injected electrons (e )
e
3
and the electrolyte (I
. Conclusion
New triphenylamine-based organic dyes with ADeDe
).
4
As a result, the highest solar energy-to-electricity conversion
peA
efficiency was achieved with SH-14, which had the maximum
h
structure (SH-11~14) were designed and synthesized, to study the
effects of the number, position, and shape of methoxy groups
attached to the TPA donor on the dye photophysical and electro-
chemical properties and the cell photovoltaic performance in
DSSCs.
The introduction of methoxy groups as an additional donor on
the TPA donor led to bathochromic shift of the absorption spectrum
and enhancement of the molar extinction coefficient of the dyes,
due to increase of the electron donating ability of the TPA donor
2
value of 6.01% (Jsc ¼ 12.29 mA/cm , Voc ¼ 687.5 mV, FF ¼ 71.18%)
under AM 1.5G irradiation. These results indicate that dye Jsc, Voc
and values are greatly influenced by the number, position, and
,
h
shape of additional methoxy groups attached to the TPA donor, and
that novel TPA donor of closed ring shape, such as SH-14, can be
promising electron donor for higher performance of DSSCs. We will
be able to achieve better performance, by introducing appropriate
p-bridges (eAe
pe) that improve stereoscopic planarity in
ADeDeAe eA structure.
p
part, and the extension of p-electron delocalization over the whole
molecule. Their introduction also led to decrease of the amount of
dye adsorbed, due to the increased steric hindrance afforded by
protruding methoxy groups toward the backbone plane (b) of the
molecule.
References
[
1] O'Regan B, Gr €a tzel M. A low-cost, high-efficiency solar cell based on dye-
sensitized colloidal TiO films. Nature 1991;353:737e40.
2

S.H. Kim et al. / Dyes and Pigments 113 (2015) 390e401
401
[
[
2] (a) Qin Y, Peng Q. Ruthenium sensitizers and their applications in dye-
(c) Liang Y, Peng B, Liang J, Tao Z, Chen J. Triphenylamine-based dyes bearing
functionalized 3,4-propylenedioxythiophene linkers with enhanced perfor-
mance for dye-sensitized solar cells. Org Lett 2010;12:1204e7;
(d) Jiang X, Karlsson KM, Gabrielsson E, Johansson EMJ, Quintana M,
Karlsson M, et al. Highly efficient solid-state dye-sensitized solar cells based
on triphenylamine dyes. Adv Funct Mater 2011;21:2944e52;
(e) Sakong C, Kim SH, Yuk SB, Namgoong JW, Park SW, Ko MJ, et al. Influence
of solvent and bridge structure in alkylthio-substituted triphenylamine dyes
on the photovoltaic properties of dye-sensitized solar cells. Chem Asian J
2012;7:1817e26;
(f) Tian H, Yang X, Chen R, Zhang R, Hagfeldt A, Sun L. Effect of different dye
baths and dye-structures on the performance of dye-sensitized solar cells
based on triphenylamine dyes. J Phys Chem C 2008;112:11023e33;
(g) Ning Z, Tian H. Triarylamine: a promising core unit for efficient photo-
voltaic materials. Chem Commun 2009:5483e95.
sensitized solar cells. Int J Photoenergy 2012:1e21;
(
b) Zhang S, Yang X, Numata Y, Han L. Highly efficient dye-sensitized solar
cells: progress and future challenges. Energy Environ Sci 2013;6:1443e64;
c) Li L-L, Diau EW-G. Porphyrin-sensitized solar cells. Chem Soc Rev 2013;42:
91e304;
d) Liang M, Chen J. Arylamine organic dyes for dye-sensitized solar cells.
(
2
(
Chem Soc Rev 2013;42:3453e88.
3] (a) Gr a€ tzel M. Dye-sensitized solar cells. J Photochem Photobiol C 2003;4:
1
45e53;
b) Gr a€ tzel M. Conversion of sunlight to electric power by nanocrystalline dye-
sensitized solar cells. J Photochem Photobiol A Chem 2004;164(3):3e14;
c) Park N-G, Kim K. Transparent solar cells based on dye-sensitized nano-
crystalline semiconductors. Phys Status Solidi A 2008;205(8):1895e904;
d) Zeng WD, Cao Y, Bai Y, Wang Y, Shi Y, Zhang M, et al. Efficient dye-
(
(
(
sensitized solar cells with an organic photosensitizer featuring orderly con-
jugated ethylenedioxythiophene and dithienosilole blocks. Chem Mater
[15] Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H. Dye-sensitized solar cells.
Chem Soc Rev 2010;110:6595e663.
2
010;22:1915e25;
[16] (a) Gather MC, Bradley DDC. An improved optical method for determining the
order parameter in thin oriented molecular films and demonstration of a
highly axial dipole moment for the lowest energy pep* optical transition in
poly(9,9-dioctylfluorene-co-bithiophene). Adv Funct Mater 2007;17:479e85;
(b) Fink RF, Seibt J, Engel V, Renz M, Kaupp M, Lochbrunner S, et al. Exciton
(
e) Yang J, Ganesan P, Teuscher J, Moehl T, Kim YJ, Yi C, et al. Influence of the
donor size in Dꢀ ꢀA organic dyes for dye-sensitized solar cells. J Am Chem
p
Soc 2014;136:5722e30.
[
4] Yella A, Lee H-W, Tsao HN, Yi C, Chandiran AK, Nazeeruddin MdK, et al.
Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte
exceed 12 percent efficiency. Science 2011;334:629e34.
5] (a) Koumura N, Wang Z-S, Miyashita M, Uemura Y, Sekiguchi H, Cui Y, et al.
Substituted carbazole dyes for efficient molecular photovoltaics: long electron
lifetime and high open circuit voltage performance. J Mater Chem 2009;19:
trapping in
p-conjugated materials: a quantum-chemistry-based protocol
applied to perylene bisimide dye aggregates. J Am Chem Soc 2008;130:
12858e9.
[
[17] Tan L-L, Chen H-Y, Hao L-F, Shen Y, Xiao L-M, Liu J-M, et al. Starburst triar-
ylamine based dyes bearing a 3,4-ethylenedioxythiophene linker for efficient
dye-sensitized solar cells. Phys Chem Chem Phys 2013;15:11909e17.
[18] (a) Ning Z, Zhang Q, Wu W, Pei H, Liu B, Tian H. Starburst triarylamine based
dyes for efficient dye-sensitized solar cells. J Org Chem 2008;73:3791e7;
(b) Hagberg DP, Jiang X, Gabrielsson E, Linder M, Marinado T, Brinck T, et al.
Symmetric and unsymmetric donor functionalization. comparing structural
and spectral benefits of chromophores for dye-sensitized solar cells. J Mater
Chem 2009;19:7232e8;
4
(
829e36;
b) Sudyoadsuk T, Pansay S, Morada S, Rattanawan R, Namuangruk S,
Kaewin T, et al. Synthesis and characterization of DeDe eA-type organic
p
dyes bearing carbazoleecarbazole as a donor moiety (DeD) for efficient dye-
sensitized solar cells. Eur J Org Chem 2013;23:5051e63;
(
c) Cai S, Tian G, Li X, Su J, Tian H. Efficient and stable DSSC sensitizers based
on substituted dihydroindolo[2,3-b]carbazole donors with high molar
extinction coefficients. J Mater Chem A 2013;1:11295e305.
(c) Zhang M-D, Pan H, Ju X-H, Ji Y-J, Qin L, Zheng H-G, et al. Improvement of
dye-sensitized solar cells' performance through introducing suitable hetero-
cyclic groups to triarylamine dyes. Phys Chem Chem Phys 2012;14:2809e15;
(d) Bonnier C, Machin DD, Abdi OK, Robson KCD, Koivisto BD. The effect of
donor-modification in organic light-harvesting motifs: triphenylamine donors
appended with polymerisable thienyl subunits. Org Biomol Chem 2013;11:
7011e5.
[
6] (a) Wang Z-S, Cui Y, Dan-oh Y, Kasada C, Shinpo A, Hara K. Thiophene-func-
tionalized coumarin dye for efficient dye-sensitized solar cells: electron life-
time improved by coadsorption of deoxycholic acid.
007;111(19):7224e30;
b) Wang Z-S, Cui Y, Hara K, Dan-oh Y, Kasada C, Shinpo A. A high-light-
J Phys Chem C
2
(
harvesting-efficiency coumarin dye for stable dye-sensitized solar cells. Adv
Mater 2007;19(8):1138e41.
[19] Lin JT, Chen P-C, Yen Y-S, Hsu Y-C, Chou H-H, Yeh M-CP. Organic dyes con-
taining furan moiety for high-performance dye-sensitized solar cells. Org Lett
2009;11:97e100.
[
[
[
7] Kim S, Lee JK, Kang SO, Ko J, Yum J-H, Fantacci S, et al. Molecular engineering
of organic sensitizers for solar cell applications. J Am Chem Soc 2006;128(51):
1
6701e7.
[20] Park SW, Son K-I, Ko MJ, Kim K, Park N-G. Effect of donor moiety in organic
sensitizer on spectral response, electrochemical and photovoltaic properties.
Synth Met 2009;159:2571e7.
[21] Santos PF, Reis LV, Duarte I, Serrano JP, Almeida P, Oliveira AS, et al. Synthesis
and photochemical evaluation of iodinated squarylium cyanine dyes. Helv
Chim Acta 2005;88:1135e43.
8] Ito S, Miura H, Uchida S, Takata M, Sumioka K, Liska P. High-conversion effi-
ciency organic dye-sensitized solar cells with a novel indoline dye. Chem
Commun 2008;41:5194e6.
9] Li C, Yum J-H, Moon S-J, Herrmann A, Eickemeyer F, Pschirer NG, et al. An
improved perylene sensitizer for solar cell applications. ChemSusChem
2
008;1(7):615e8.
[22] Hagberg DP, Edvinsson T, Marinado T, Boschloo G, Hagfeldt A, Sun L. A novel
organic chromophore for dye-sensitized nanostructured solar cells. Chem
Commun 2006:2245e7.
[23] Thomas KRJ, Hsu Y-C, Lin JT, Lee K-M, Ho K-C, Lai C-H, et al. 2,3-Disubstituted
thiophene-based organic dyes for solar cells. Chem Mater 2008;20:1830e40.
[24] (a) In: Gordon PF, Gregory P, editors. Organic chemistry in colour. Berlin
Heidelberg: Springer-Verlag; 1987. p. 23e261;
[
10] (a) Kim SH, Kim HW, Sakong C, Namgoong J, Park SW, Ko MJ, et al. Effect of
five-membered heteroaromatic linkers to the performance of phenothiazine-
based dye-sensitized solar cells. Org Lett 2011;13:5784e7;
(
b) Kim SH, Sakong C, Chang JB, Kim B, Ko MJ, Kim DH, et al. The effect of N-
substitution and ethylthio substitution on the performance of phenothiazine
donors in dye-sensitized solar cells. Dyes Pigments 2013;97:262e71;
(
c) Hua Y, Chang S, Huang D, Zhou X, Zhu X, Zhao J, et al. Significant
(b) Tsai M-S, Hsu Y-C, Lin JT, Chen H-C, Hsu C-P. Organic dyes containing 1H-
improvement of dye-sensitized solar cell performance using simple
phenothiazine-based dyes. Chem Mater 2013;25:2146e53.
phenanthro[9,10-d]imidazole conjugation for solar cells. J Phys Chem C
2007;111:18785e93.
[
11] (a) Tian H, Bora I, Jiang X, Gabrielsson E, Karlsson KM, Hagfeldt A, et al.
Modifying organic phenoxazine dyes for efficient dye-sensitized solar cells.
J Mater Chem 2011;21:12462e72;
[25] (a) Tian H, Yang X, Chen X, Pan Y, Li L, Hagfeldt A, et al. Phenothiazine de-
rivatives for efficient organic dye-sensitized solar cells. Chem Commun 2007:
3741e3;
(
b) Karlsson KM, Jiang X, Eriksson SK, Gabrielsson E, Rensmo H, Hagfeldt A,
(b) Hara K, Wang Z-S, Sato T, Furube A, Katoh R, Sugihara H, et al. Oligo-
thiophene-containing coumarin dyes for efficient dye-sensitized solar cells.
J Phys Chem B 2005;109:15476e82.
et al. Phenoxazine dyes for dye-sensitized solar cells: relationship between
molecular structure and electron lifetime. Chem Eur J 2011;17:6415e24.
12] Qin C, Wong W-Y, Han L. Squaraine dyes for dye-sensitized solar cells: recent
advances and future challenges. Chem Asian J 2013;8:1706e19.
[
[
[26] Hagfelt A, Gr €a tzel M. Light-induced redox reactions in nanocrystalline sys-
tems. Chem Rev 1995;95:49e68.
13] Chen R, Yang X, Tian H, Sun L. Tetrahydroquinoline dyes with different
spacers for organic dye-sensitized solar cells. J Photochem Photobiol A Chem
[27] (a) Bisquert J. Chemical capacitance of nanostructured semiconductors: its
origin and significance for nanocomposite solar cells. Phys Chem Chem Phys
2003;5:5360e4;
2007;189:295e300.
[
14] (a) Liu W-H, Wu I-C, Lai C-H, Lai C-H, Chou P-T, Li Y-T, et al. Simple organic
molecules bearing a 3,4-ethylenedioxythiophene linker for efficient dye-
sensitized solar cells. Chem Commun 2008;41:5152e4;
(b) Wang Q, Moser J-E, Gr a€ tzel M. Electrochemical impedance spectroscopic
analysis of dye-sensitized solar cells. J Phys Chem B 2005;109:14945e53.
[28] Kuang D, Uchida S, Humphry-Baker R, Zakeeruddin SM, Gr a€ tzel M. Organic
(
b) Lin L-Y, Tsai C-H, Wong K-T, Huang T-W, Hsieh L, Liu S-H, et al. Organic
dye-sensitized ionic liquid based solar cells: remarkable enhancement in
performance through molecular design of indoline sensitizers. Angew Chem
Int Ed 2008;47:1923e7.
dyes containing coplanar diphenyl-substituted dithienosilole core for efficient
dye-sensitized solar cells. J Org Chem 2010;75:4778e85;