Effect of anchoring group in anthracene/ thiophene-bridged triphenylamine-based organic dyes for dye-sensitized solar cells

Article abstract of DOI:10.1166/jnn.2017.15105

Triphenylamine-based metal-free organic dyes (D1–D3) with different electron acceptors such as 2-cyanoacetic acid, rhodanine-3-acetic acid, or 5-oxo-1-phenyl-2-pyrazolin-3-carboxylic acid connected through anthracene and thiophene -spacers were synthesized and applied in dye-sensitized solar cells. The photophysical and electrochemical properties of these dyes were investigated and their performance as sensitizers in dye-sensitized solar cells was evaluated. Electrochemical studies showed that the lowest unoccupied molecular orbital energy levels can be tuned by introduction of various anchoring groups with different electron withdrawing abilities. The power conversion efficiencies of the dye-sensitized solar cells based on D1–D3 decreased as the electron withdrawing ability of their anchoring groups increased in the order D1 < D2 < D3. The D1-based device showed the highest power conversion efficiency (1.27%).

Full text of DOI:10.1166/jnn.2017.15105

Article
Journal of
Nanoscience and Nanotechnology
Vol. 17, 8053–8060, 2017
Copyright © 2017 American Scientific Publishers
All rights reserved
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Printed in the United States of America
Effect of Anchoring Group in Anthracene/
Thiophene-Bridged Triphenylamine-Based
Organic Dyes for Dye-Sensitized Solar Cells
Le Thi Thuy, Phuong Ho, Le Quoc Bao, Chau Thi Thanh Thuy, Rajesh Cheruku,
Hyun-Jun Jo, Suresh Thogiti∗, and Jae Hong Kim∗
Department of Chemical Engineering, Yeungnam University, 214-1, Dae-Hakro 280, Gyeongsan,
Gyeongbuk 712-749, Republic of Korea
Triphenylamine-based metal-free organic dyes (D1–D3) with different electron acceptors such
as 2-cyanoacetic acid, rhodanine-3-acetic acid, or 5-oxo-1-phenyl-2-pyrazolin-3-carboxylic acid
connected through anthracene and thiophene ꢀ-spacers were synthesized and applied in
dye-sensitized solar cells. The photophysical and electrochemical properties of these dyes were
investigated and their performance as sensitizers in dye-sensitized solar cells was evaluated. Elec-
trochemical studies showed that the lowest unoccupied molecular orbital energy levels can be tuned
by introduction of various anchoring groups with different electron withdrawing abilities. The power
conversion efficiencies of the dye-sensitized solar cells based on D1–D3 decreased as the electron
IP: 46.148.127.246 On: Sat, 22 Dec 2018 16:00:12
withdrawing ability of their anchoring groups increased in the order D1 < D2 < D3. The D1-based
Copyright: American Scientific Publishers
device showed the highest power conversion efficiency (1.27%).
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Keywords: Dye-Sensitized Solar Cells, Anchoring Group, Anthracene, Triphenylamine.
1. INTRODUCTION
impediments to the application of ruthenium and zinc-
based dye-sensitizers; moreover, the conversion efficien-
cies of these species are far below ideal. Therefore, the
development of dye-sensitizers still remains a challenge
for further improvement of DSSCs.22
Increased emission of greenhouse gases owing to combus-
tion of fossil fuels since the commencement of the indus-
trial revolution has caused the global climate to change
more drastically than recorded in history. One of the
most critical challenges in modern science is meeting
energy demands. In this regard, renewable energy sources
have continued to attract attention. Dye-sensitized solar
cells (DSSCs) are considered a possible answer to the
energy demands because of their high photovoltaic perfor-
mance, easy fabrication, and low cost of production com-
pared with other solar cells.1–3 Among the constituents of
DSSCs, the dye-sensitizer is a key element for achieving
high photovoltaic performance. Years of investigation have
led to the development of a plethora of dye-sensitizers,
including zinc-based porphyrin sensitizers, polypyridyl lig-
ated ruthenium-complexes, and organic dyes.4–21 However,
the cost of noble metals, complex and expensive purifi-
cation processes, and environmental issues are the main
Organic dye-sensitizers comprise
a
donor,
a
ꢀ-bridge/spacer, and an acceptor/anchoring unit, and this
structure (D-ꢀ-A) is related to the potent intramolecular
charge transfer (ICT), thereby resulting in facile charge
transfer from the excited dye sensitizer (via the anchoring
unit) to the semiconductor surface.23 For an excellent
dye-sensitizer, selecting a proper anchoring unit is highly
crucial as the anchoring unit determines the binding
strength of the dye-sensitizer on the semiconductor sur-
face and thus influences the charge transfer rate.24ꢁ25
In most organic dye-sensitizers, 2-cyanoacetic acid (CA)
is employed in the D-ꢀ-A structure as an anchoring unit.
There are a few reports on the effect of different anchoring
units on the D-ꢀ-A structure.
As we discussed above, the D-ꢀ-A system is the
basic feature for most organic dyes due to the effective
∗Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 11
1533-4880/2017/17/8053/008
doi:10.1166/jnn.2017.15105
8053

Effect of Anchoring Group in Anthracene/Thiophene-Bridged Triphenylamine-Based Organic Dyes for DSSCs
Thuy et al.
300 MHz spectrometer. The chemical shifts are reported
in parts-per-million, down-field from tetramethylsilane
(TMS) as an internal standard; measurements were per-
formed using appropriate deuterated solvents. The opti-
cal spectra of the dyes in solution were recorded with an
Agilent 8453 UV-vis spectrophotometer. The electrochem-
ical data were recorded using a CV-BAS-Epsilon instru-
ment. Cyclic voltammograms were obtained from a three
electrode cell using 0.1 M Bu4NPF6 chloroform solution
and a scan rate of 100 mV s−1; Pt wire was used as a
counter electrode and Ag/AgCl as the reference electrode.
Electrochemical impedance spectroscopy (EIS) was carried
out using a computer-controlled potentiostat (IVIUMSTAT,
IVIUM).
CN
S
N
COOH
D1
O
S
N
N
COOH
S
S
D2
2.1.1. 4-Bromo-N,N-Diphenylaniline (1)
O
In a three-necked, oven-dried, 150 mL round-bottom flask,
triphenylamine (3.000 g, 12.244 mmol) was dissolved in
50 mL of anhydrous chloroform, covered with aluminum
foil, and stirred at 25 ꢀC under inert atmosphere for
15 min. N -Bromosuccinimide (1.96 g, 11.121 mmol) was
then added in small portions, and the resulting solution
was stirred at 25 ꢀC for 5 h. The reaction mixture
was extracted three times with chloroform. The com-
bined organic fractions were washed with brine and dried
over anhydrous MgSO4. The solvent was removed under
reduced pressure and the residue was re-crystallized by
S
N
N
N
HOOC
D3
Figure 1. Chemical structures of dye sensitizers D1–D3.
intramolecular charge transfer property. Appropriate use
of electron-excessive ꢀ-bridge between D and A was
reported to be advantageous to red shift the charge
IP: 46.148.127.246 On: Sat,u2si2ngDeetcha2n0o1l8to16g:0iv0e:1a2 white powder (3.020 g, 76%).
transfer absorption band. Heo et al. reported anthracene
1
Copyright: American Scientific Publishers
H NMR (300 MHz, CDCl3ꢂ: 7.62–7.21 (m, 7H), 7.19–
mediated photosensitizers and achieved an efficiency of
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3.34%.26 Teng et al. utilized anthracene as an effec-
7.00 (m, 5H), 6.99–6.93 (m, 2H).
tive ꢀ-spacer between triphenylamine donor and typical
cyanoacetic acid acceptor. One of the dyes attained a
notable efficiency of 7.03%.15 Herein, for DSSC appli-
cations, we synthesize three organic dyes for which
the charge-transfer chromophoric system includes triph-
enylamine as the donor group with different acceptors.
Anthracene and thiophene conjugate groups are inserted
between the donor and acceptor units as the ꢀ-spacers to
extend the ꢀ-conjugation, which is expected to enhance
the light-harvesting effect. CA, rhodanine-3-acetic acid
(RA) and 5-oxo-1-phenyl-2-pyrazolin-3-carboxylic acid
(OPCA) are employed as anchoring groups. Figure 1
shows the corresponding dye molecular structures.
The ultraviolet-visible (UV-vis), electrochemical, inci-
dent photon-to-current conversion efficiency (IPCE), and
current density–photovoltage (J–V ꢂ properties are also
analyzed.
2.1.2. N,N-Diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)Aniline (2)
Under inert atmosphere, a degassed solution of 1
(3.000 g, 9.250 mmol), bis(pinacolato)diboron (3.054 g,
12.255 mmol), KOAc (3.018 g, 32.225 mmol),
and Pd(dppf)2Cl2 (0.338 g, 0.05 mmol) in dry
dimethoxyethane (30 mL) was heated under reflux con-
ditions for 15 h. After this period, the mixture was
cooled to room temperature, filtered, and diluted with
CH2Cl2 (50 mL). The organic solution was washed with
H2O (2 × 30 mL) and brine, then dried (with anhydrous
MgSO4ꢂ and evaporated. The residue was separated by
column chromatography using hexane/CH2Cl2 (9/1 v/v)
to give compound 2 as a white solid product (2.170 g,
1
63%). H NMR (300 MHz, CDCl3ꢂ: 7.74–7.59 (d, 2H),
7.19–7.08 (d, 4H), 7.07–6.90 (t, 4H), 1.48–1.16 (t, 12H).
2.1.3. 5-(10-Bromoanthracen-9-yl)Thiophene-
2-Carbaldehyde (3)
2. EXPERIMENTAL DETAILS
2.1. Materials and Instruments
All reactions were carried out under nitrogen atmo-
sphere. The solvents were distilled using appropriate
reagents. All starting materials and reagents were pur-
chased from Sigma–Aldrich, TCI, and ACROS Co.
1H spectra were recorded on a Varian Mercury NMR
A 50 mL three neck round bottom flask was charged with
10-bromoanthracen-9-boronic acid (0.770 g, 2.000 mmol),
Pd(PPh3ꢂ4 (0.232 g, 10 mol%), tetrahydrofuran (THF;
20 mL) and 2 M aqueous K2CO3 (2 mL). The flask
was then purged with nitrogen gas for five evacuate/refill
cycles. 5-Bromo-2-thiophenecarboxaldehyde (0.460 g,
8054
J. Nanosci. Nanotechnol. 17, 8053–8060, 2017

Thuy et al.
Effect of Anchoring Group in Anthracene/Thiophene-Bridged Triphenylamine-Based Organic Dyes for DSSCs
2.400 mmol) was then added under inert atmosphere. The
tube was sealed and heated at 70 ꢀC with vigorous stirring
for 15 h. Upon cooling to ambient temperature, the organ-
ics were extracted three times with CH2Cl2. The combined
organic fractions were washed with brine and dried over
MgSO4. The solvent was removed under reduced pressure
and the residue was purified by silica gel column chro-
matography using hexane/CH2Cl2 (8/2, v/v) as the elu-
acid (0.036 g, 0.376 mmol) anchoring group was used to
afford D2 (0.115 g, 86%) as a dark yellow solid. 1H NMR
(300 MHz, DMSO-d6ꢂ: 8.36 (s, 1H), 8.12 (s, 1H), 7.81
(m, 3H), 7.67–7.55 (m, 3H), 7.47–7.14 (m, 15H), 4.75
(s, 2H).
2.1.7. (E)-4-((5-(10-(4-(Diphenylamino)phenyl)
anthracen-9-yl)thiophen-2-yl)methylene)-5-Oxo-1-
Phenyl-4,5-Dihydro-1H-Pyrazole-3-Carboxylic
Acid (D3)
The procedure used for synthesis of compound D1 was fol-
lowed. The 5-oxo-1-phenyl-2-pyrazolin-3-carboxylic acid
(0.076 g, 0.376 mmol) anchoring group was used to
afford D3 (0.073 g, 54%) as a red solid. 1H NMR
(300 MHz, DMSO-d6ꢂ: 9.39 (s, 1H), 8.42 (s, 1H), 8.10-
7–7.90 (m, 7H), 7.89–7.75 (m, 9H), 7.65–7.00 (m, 12H).
1
ent to give a yellow powder (0.420 g, 57%). H NMR
(300 MHz, acetone-d6ꢂ: 10.14 (s, 1H), 8.7–8.62 (d, 2H),
8.25 (d, 2H), 7.88–7.77 (m, 4H), 7.68–7.57 (t, 2H), 7.53–
7.46 (d, 1H).
2.1.4. 5-(10-(4-(Diphenylamino)phenyl)anthracen-9-yl)
Thiophene-2-Carbaldehyde (4)
A mixture of compound 3 (0.918 g, 2.500 mmol), com-
pound 2 (0.900 g, 2.500 mmol) and K2CO3 (1.029 g,
8.75 mmol) in toluene/ethanol (15/5 mL) was combined
with Pd(PPh3ꢂ4 (0.347 mg, 0.3 mmol) by addition of the
latter under inert atmosphere. After stirring for 24 h at
110 ꢀC, water (10 mL) and dichloromethane (30 mL) were
added. The organic layer was separated, and the aqueous
layer was extracted with dichloromethane (2 × 10 mL).
The organic layer and the dichloromethane extracts were
combined and dried (over anhydrous MgSO4ꢂ, and then
filtered. The organic solvent was completely removed by
rotary evaporation. The solid residue was purified by col-
2.1.8. Assembly and Characterization of DSSCs
Transparent conducting glass substrates were cleaned
sequentially with ethanol, deionized (DI) water, and ace-
tone under ultrasonication. Nanocrystalline TiO2 paste was
synthesized using ethyl cellulose (Aldrich), lauric acid
(Fluka, City, State, Country), and terpineol (Aldrich). TiO2
particles (ca. 20–30 nm in diameter) were used. The pre-
pared TiO2 paste was doctor-bladed onto the pre-cleaned
ꢀ
glass substrates, followed by drying at 70 C for 30 min
ꢀ
and 30 min of calcination at 500 C. A scattering layer
IP: 46.148.127.246 On: Sat, 22 Dec 2018 16:00:12
umn chromatography using hexane/CH2Cl2 (8/2, v/v) as
consisting of rutile TiO2 particles (250 nm in a size) was
Copyright: American Scientific Publishers
the eluent to give a pale yellow solid (0.916 g, 70%).
1H NMR (300 MHz, CDCl3ꢂ: 10.08 (s, 1H), 8.21–7.97
(d, 1H), 7.90–7.75 (m, 5H), 7.50–7.40 (t, 5H), 7.39–7.20
(m, 10H), 7.15–7.02 (t, 3H).
deposited on the mesoporous TiO films. These layers
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2
were dipped into an aqueous solution of TiCl4 (0.04 M)
ꢀ
at 70 C for 30 min. The sensitizers were dissolved in dry
ꢀ
chloroform (0.2 mM) at 25 C and stirred for 24 h. The
annealed TiO2 electrodes were dipped in the dye solutions
for 18 h.
2.1.5. (Z)-2-Cyano-3-(5-(10-(4-(diphenylamino)phenyl)
anthracen-9-yl)thiophen-2-yl)Acrylic Acid (D1)
Pt counter electrodes were prepared by thermal reduc-
tion of the films dip-coated in H2PtCl6 (7 × 10−3 M) in
Compound 4, 5-(10-(4-(diphenylamino)phenyl)anthracen-
9-yl)thiophene-2-carbaldehyde (0.100 g, 0.188 mmol) was
dissolved in acetic acid (15 mL) and combined with
cyanoacetic acid (0.032 g, 0.376 mmol) in the presence of
ammonium acetate (0.029 g, 0.376 mmol). The reaction
mixture was then refluxed for 5 h under inert atmosphere.
After cooling to room temperature, the reaction mixture
was poured into crushed ice and the solid obtained was
washed thoroughly with water to remove the excess acetic
acid and cyanoacetic acid. The solid was washed with hex-
ane to afford a yellow colored solid D1 (0.045 g, 40%).
1H NMR (300 MHz, DMSO-d6ꢂ 8.27 (s, 1H), 8.00 (s, 1H),
7.89–7.72 (t, 5H), 7.48–7.34 (m, 8H), 7.33–7.20 (t, 7H),
7.19–7.10 (m, 3H).
ꢀ
2-propanol at 400 C for 20 min. The dye-adsorbed TiO2
and Pt counter electrodes were sandwiched between a
60×10−6 m thick Surlyn (Dupont 1702) layer, which was
used as a bonding agent and spacer. An I−/I3− redox cou-
ple composed of 0.6 M 1,2-dimethyl-3-propylimidazolium
iodide (DMPII), 0.06 M NaI, 0.04 M I2, and 0.4 M
4-tert-butylpyridine in dried CH3CN was then introduced
through a hole on the Pt counter electrode. The com-
plete device structure was shown in Figure 2. The active
area of the dye-adsorbed TiO2 films was estimated using
a digital microscope camera with image-analysis-software
(Moticam 1000). The photovoltaic I–V characteristics of
the prepared DSSCs were measured under 1 sunlight inten-
sity (100 mW cm−2, AM 1.5), which was verified with an
AIST-calibrated Sisolar cell (PEC-L11, Peccell Technolo-
gies, Inc., City, State, Country). The incident photon-to-
current efficiencies (IPCEs) were plotted as a function of
the light wavelength using an IPCE measurement instru-
ment (PEC-S20, Peccell Technologies, Inc.).
2.1.6. (Z)-2-(5-((5-(10-(4-(Diphenylamino)phenyl)
anthracen-9-yl)thiophen-2-yl)methylene)-4-oxo-
2-thioxothiazolidin-3-yl)Acetic Acid (D2)
The procedure used for synthesis of compound D1 was
followed for synthesis of D2, where the rhodanine-3-acetic
J. Nanosci. Nanotechnol. 17, 8053–8060, 2017
8055

Effect of Anchoring Group in Anthracene/Thiophene-Bridged Triphenylamine-Based Organic Dyes for DSSCs
Thuy et al.
different acceptor in these D-ꢀ-A chromophores may pro-
vide useful information about the importance of accep-
tor units proximal to the anchoring unit in modifying
the photophysical, electrochemical, and photovoltaic per-
formance. Herein, three anthracene/triphenylamine-based
organic dyes, D1, D2, and D3, having different accep-
tors, such as 2-cyanoacetic acid (CA), rhodanine-3-acetic
acid (RA) and 5-oxo-1-phenyl-2-pyrazolin-3-carboxylic
acid (OPCA), were prepared as sensitizers for TiO2-
based DSSCs. Scheme 1 shows the route for preparation
of the new sensitizers. Firstly, simple triphenylamine
was brominated using N -bromosuccinimide in chlo-
roform, followed by Suzuki-Miyaura borylation using
bis(pinacalato)diboron to obtain intermediate 2. In another
step, we started with 10-bromoanthracen-9-boronic acid,
which was subjected to Suzuki coupling using 5-bromo-
2-thiophenecarboxaldehyde to obtain intermediate 3. The
intermediates 2 and 3 were then subjected to Suzuki
coupling to obtain the aldehyde intermediate 4. Finally,
intermediate 4 was subjected to Knoevenagel conden-
sation using 2-cyanoacetic acid, rhodanine-3-acetic acid,
and 5-oxo-1-phenyl-2-pyrazolin-3-carboxylic acid, respec-
tively, to obtain the final target dye-sensitizers D1, D2,
and D3.
Figure 2. Schematic diagram of the DSSC device.
3. RESULTS AND DISCUSSION
3.1. Design and Synthesis
In organic dyes, the introduction of a powerful electron
withdrawing moiety near the anchoring unit is required to
skew the electron density away from the donor segment,
thereby lowering the energy bandgap, which shifts the
optical spectra to the lower energy regime and is expected
to aid electron transport to the TiO2 semiconductor. This
situation has been observed for diverse organic D-ꢀ-A
dyes.27–29 In most organic D-ꢀ-A dyes, the presence of
cyanoacetic acid (which is prone to cis-trans isomerization
upon photo-irradiation) near the anchoring region results
in enhanced recombination currents.30 Elimination of such
anchoring units could enhance the planarity of the chro-
mophore and might aid in diminishing the recombination.
3.2. Optical Properties
The UV-vis absorption spectra of the dyes (D1–D3) in
IP: 46.148.127.246 On: Sat, 22 Dec 2018 16:00:12
Hence, substituting the cyanoacetic acid acceptor with a
solution and on the photoelectrode film (Figs. 3(a and b))
Copyright: American Scientific Publishers
Delivered by Ingenta
O
O
NBS, Chlorofform
5h, RT
Bis(pinacolate)diboron, KOAc
Pd(dppf)Cl2, DMF, 15h, 85C
r
N
B
B
N
N
1
2
+
O
r
B
H
H
O
O
O
H
S
S
Pd(PPh3)4, Na2CO3
THF, 24h
H
+
r
B
B
r
B
3
Pd(PPh3)4, Na2CO3
THF, 24h
O
S
H
CA or RA or OPCA, piperidine
Acetonitrile, 4h, 60C
N
D1 or D2 or D3
4
Scheme 1. Route for preparation of dye sensitizers D1–D3.
8056
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Thuy et al.
Effect of Anchoring Group in Anthracene/Thiophene-Bridged Triphenylamine-Based Organic Dyes for DSSCs
(a)
the replacement of cyanoacetic acid with rhodanineacetic
acid having more extended conjugation and a strongly
50000
D1
D2
withdrawing acceptor. Similarly, the replacement of CA
D3
40000
30000
20000
10000
0
in D1 with another acceptor group, OPCA, to give D3
resulted in spectral broadening and an increased ꢃ of
22675 M−1 cm−1 with an absorption maximum at 404 nm.
This decrease in the energy gap is highly advantageous
for enhancing the light harvesting ability of the sensitizers.
Notably, D2 and D3 exhibited broader peaks and higher ꢃ
values than obtained for D1. Figure 3(b) shows the absorp-
tion spectra of the corresponding dyes on TiO2 film. The
absorption spectra of all three dyes were broadened com-
pared to the corresponding solution spectra (Fig. 3(a));
this is attributed to electronic coupling of the dyes with
the TiO2 surface and possibly some dye aggregation. This
phenomenon has been observed for many organic dyes on
TiO2 films. From Figure 3(a), E0−0 values of 266, 2.43,
and 2.19 eV were calculated for D1–D3, respectively.
300
400
500
600
Wavelength (nm)
(b)
4
3
2
1
0
D1
D2
D3
3.3. Electrochemical Properties
The cyclic voltammograms of D1–D3 were acquired in a
0.1 M n-Bu4NPF6 solution (Fig. 4). The data were used
to calculate the energy of the highest occupied molec-
ular orbital (HOMO) and the lowest unoccupied molec-
ular orbital (LUMO) to investigate the regeneration of
the dyes from the redox electrolyte and electron transfer
from the excited dye-sensitizer to the conduction band of
IP:54006.148.127.620406 On: Sat, 22 Dec 2018 16:00:12
300
400
the TiO2 semiconductor, respectively. Generally, the for-
Copyright: American Scientific Publishers
Wavelength (nm)
mal oxidation potentials correspond to the HOMO level
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of the dye-sensitizer and the LUMO values are calcu-
lated from the difference between Eox and E0−0, where
the E0−0 values are determined from the onset absorp-
tion spectra (Fig. 3(a)). The HOMO, LUMO, and E0−0
values are summarized in Table I. The HOMO levels of
the dyes were in the range of 1.14–1.16 V; these val-
ues are more positive than that of the I−/I3− redox elec-
trolyte (∼0.4 V vs. NHE).28 For comparison, optical and
electrochemical data of structurally similar dye TC401
Figure 3. UV-vis absorption spectra of dye sensitizers D1–D3 (a) in
solution (dichloromethane); (b) on TiO2 film.
were acquired, given that the overlap between the spectral
profile of the dyes and the solar emission spectrum has
a significant impact of the cell photocurrent. The corre-
sponding data are listed in Table I.
The three dyes had a strong absorption band in the vis-
ible region, corresponding to intramolecular charge trans-
fer between the donor units and acceptor groups (Fig. 3
and Table I). As shown in Figure 3, D1 exhibited an
absorption maximum at 400 nm with a molar extinc-
tion coefficient (ꢃꢂ of 16455 M−1 cm−1. Compared to the
absorption maximum of D1, the absorption pattern of D2
was bathochromically shifted by 14 nm, with an absorption
maximum at 414 nm (ꢃ = 42675 M−1 cm−1ꢂ because of
D1
6
D2
D3
4
2
0
Table I. Photophysical and electrochemical data for dye sensitiz-
ers D1–D3.
ꢄmax (nm)
ꢃmax (M−1 cm−1
ꢂ
Sol. TiO2 Eox (V) Eo∗x (V) E0−0 (V)
–2
Dye
D1
D2
16455
42675
22675
12100
400 402
414 446
404 400
402 400
1ꢅ14
1ꢅ15
1ꢅ16
1ꢅ17
−1ꢅ52
−1ꢅ28
−1ꢅ03
−1ꢅ48
2ꢅ66
2ꢅ43
2ꢅ19
2ꢅ65
0.0
0.3
0.6
0.9
1.2
1.5
Potential (V)
D3
TC40115
Figure 4. Cyclic voltammograms of dye sensitizers D1–D3.
J. Nanosci. Nanotechnol. 17, 8053–8060, 2017
8057

Effect of Anchoring Group in Anthracene/Thiophene-Bridged Triphenylamine-Based Organic Dyes for DSSCs
Thuy et al.
were shown in Table I.15 This suggests that the oxidized
4
D1
D2
D3
dye can be regenerated via electron donation from the
I− ions in the I−/I13 redox electrolyte. Further, the calcu-
3
lated LUMO values (Table I) were more negative than
the conduction band edge of the TiO2 semiconductor
(∼−0ꢅ5 V vs. NHE),28 suggesting that the excited elec-
2
trons are efficiently injected into the conduction band of
TiO2. The strong electron-withdrawing acceptors such as
RA and OPCA could positively shift the reduction poten-
1
tial of D2 and D3 compared to that of D1 having the
less electron-withdrawing CA acceptor unit. This could
lead to lowering of the LUMO level of the dye, result-
ing in a lower thermodynamic driving force for electron
injection.
0
0.00
0.15
0.30
0.45
0.60
Photovoltage (V)
3.4. Photovoltaic Properties
Figure 6. J–V curves for the DSSCs based on dye sensitizers D1–D3.
Figure 5 presents the incident photon-to-current conversion
efficiency (IPCE) of the devices based on dyes D1–D3.
D1 with CA as the acceptor showed a stronger solar cell
response in the 350–550 nm spectral region, compared to
that of the other dyes with RA and OPCA as acceptors.
The device with D1 showed plateaus of more than 40%
from 350–470 nm with the highest value at 418 nm and an
onset at 550 nm. The onset of the IPCE for D2 and D3 was
extended to 600 nm, which corresponds to a bathochromic
shift of 50 nm compared to that of D1. The difference
between the IPCE spectra of the studied dyes is in good
Table II lists the corresponding photovoltaic parame-
ters. For comparison, the photovoltaic performance of the
structurally similar dye TC401 included in Table II.15
The device based on D1 showed better photovoltaic perfor-
mance with a photo-current density (Jsc) of 3.12 mA cm−2
,
an open circuit photovoltage (Voc) of 0.607 V, and a fill
factor (FF) of 66.88, corresponding to a power conversion
efficiency (ꢆꢂ of 1.27%.
Upon replacing the CA acceptor with the RA accep-
tor, the power conversion efficiency of the D2-based
IP: 46.148.127.246 On: Sat, 22 Dec 2018 16:00:12
device declined to 0.91% due to a drop of the Jsc from
agreement with their optical absorption spectra (Fig. 3).
Copyright: American Scientific Publishers
3.12 mA cm−2 to 2.84 A cm−2. Previously, molecular
modeling studies showed that the acceptor CA induces a
vertically oriented dye geometry when it is attached to
a semiconductor surface, whereas the donor part of the
RA acceptor-based dye was aligned in close proximity
to the semiconductor surface at a tilted angle. Hence, it
was concluded that the dye cation with the RA group as
an acceptor unit was prone to undergo more inner-path
recombination with the injected electrons.31–33 The Jsc and
efficiency decreased further to 1.78 mA cm−2 and 0.70%,
respectively, for the device with D3, where the CA accep-
tor group was replaced with the OPCA acceptor. This
might be partially attributed to the poor electron injec-
tion efficiency, as evidenced in the above section. On the
other hand, compared to the D1-based device with a Voc of
0.607 V, the D2 and D3-based devices exhibited consider-
ably lowered Voc values of 0.485 and 0.541 V, respectively,
possibly due to the tilted orientation on the semiconductor
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The relatively higher IPCE values for D1 led to higher
Jsc values compared with those of D2 and D3. Although
D2 and D3 have a broader and higher absorption in all
regions, in comparison with dye D1, the DSSCs based on
D2 and D3 had a lower IPCE. This might be caused by
the lower electron injection caused by the lower LUMO
levels.
Figure 6 presents the current density–photovoltage
(J–V ) characteristics of the devices using D1–D3 and
60
D1
D2
D3
45
30
15
0
Table II. Photovoltaic data for the DSSCs based on dye sensitizers
D1–D3 and reference N719.
Dye
Jsc (mA cm−2
ꢂ
Voc (V)
FF (%)
ꢆ (%)
N719
D1
D2
15ꢅ35
3ꢅ19
2ꢅ84
1ꢅ78
5ꢅ39
0ꢅ670
0ꢅ607
0ꢅ484
0ꢅ541
678
66ꢅ62
66ꢅ88
66ꢅ13
73ꢅ33
79ꢅ7
6ꢅ84
1ꢅ27
0ꢅ91
0ꢅ70
2ꢅ91
300
400
500
600
Wavelength (nm)
D3
TC40115
Figure 5. IPCE spectra for the DSSCs based on dye sensitizers D1–D3.
8058
J. Nanosci. Nanotechnol. 17, 8053–8060, 2017

Thuy et al.
Effect of Anchoring Group in Anthracene/Thiophene-Bridged Triphenylamine-Based Organic Dyes for DSSCs
The overall power conversion efficiency of the DSSC
D1
–50
–40
–30
–20
–10
based on D1 with the CA anchoring group (1.27%) was
higher than that of the other dyes. Overall, the data demon-
strate that the introduction of different anchoring groups
D2
D3
with different withdrawing abilities has a significant effect
on the power conversion efficiency of DSSCs.
Acknowledgments: This research was supported by
Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Min-
istry of Education (2016R1D1A3B01011514).
0
0
References and Notes
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40
60
80
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4. CONCLUSION
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and OPCA) bridged by anthracene- and thiophene-
containing ꢀ-spacers were synthesized for application in
dye-sensitized solar cells. Specific analyses of the relation-
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Received: 26 November 2016. Accepted: 9 March 2017.
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8060
J. Nanosci. Nanotechnol. 17, 8053–8060, 2017