Dual-channel anchorable organic dye with triphenylamine-based core bridge unit for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2013.06.014

Dual-channel (DC) anchorable organic dye based on an acceptor-π-donor- core bridge donor-donor-π-acceptor structural motif, was synthesized and used as sensitizer for dye sensitized solar cells (DSSCs). DC dye linked the two donors using a core bridge don

Full text of DOI:10.1016/j.dyepig.2013.06.014

Dyes and Pigments 99 (2013) 599e606
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Dual-channel anchorable organic dye with triphenylamine-based core
bridge unit for dye-sensitized solar cells
Kang Deuk Seo a, Ban Seok You a, In Tack Choi a, Myung Jong Ju a, Mi You b,
,
Hong Seok Kang b, Hwan Kyu Kim a
*
a Global GET-Future Laboratory, Department of Advanced Materials Chemistry, Korea University, Sejong 339-700, South Korea
b Department of Nano & Advanced Materials, College of Engineering, Jeonju University, Chonju, Chonbuk 560-759, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Dual-channel (DC) anchorable organic dye based on an acceptorepedonor-core bridge donoredonorep-
Received 18 May 2013
Received in revised form
12 June 2013
Accepted 14 June 2013
Available online 2 July 2013
acceptor structural motif, was synthesized and used as sensitizer for dye sensitized solar cells (DSSCs). DC
dye linked the two donors using a core bridge donor such as triphenylamine unit, which makes two
channels maintaining a particular distance (>4 A). The inter-planar DC5 exhibited dye aggregation and
ꢀ
charge recombination due to strongly intermolecular
pep stacking between the channels of DC dyes
adsorbed on the TiO2 surface. As a result, a solar cell based on DC5 with a hole conductor coadsorbent of
HC-A1 showed better photovoltaic performance with a Jsc of 8.90 mA cmꢀ2, a Voc of 0.70 V, and an FF of
Keywords:
Dye-sensitized solar cell
Dual-channel
0.77, corresponding to an overall conversion efficiency (h) of 4.80%.
Published by Elsevier Ltd.
Core bridge donor
Triphenylamine
Dye aggregation
Coadsorbent
1. Introduction
self-quenching of excited states and hence inefficient electron in-
jection [14]. A common structural strategy for suppression of dye
aggregation is the introduction of a bulky group to the donor part
[15,16]. Another unique approach is bridging of two chromophores
into a spiro configuration similar to Ru-sensitizers [17]. To enhance
optical density and binding strength of the dye onto TiO2, of
particular interest is the synthesis of a dianchoring dye, which has
Dye-sensitized solar cells (DSSCs) are promising hybrid/organic
photovoltaic devices for high efficiency, low cost solar energy con-
version [1e4]. In these cells, the sensitizer is one of the key com-
ponents for high power-conversion efficiency. Recently, interest in
metal-free organic dyes as an alternative to noble metal com-
plexes has increased due to their many advantages, such as the di-
versity of molecular structures, high molar extinction coefficients,
simple synthesis, as well as low cost and negligible environmental
issues [5e8]. By employing a combination of a zinc porphyrin dye
(YD2-o-C8) and an organic dye (Y123) in conjunction with a
tris(2,20-bipyridine) cobalt(II/III) redox couple, Grätzel et al. re-
ported a 12.3% efficiency DSSCs [9]. Many organic dyes based on the
been found to enhance the photocurrent due to the extended
p-
conjugated framework and higher molar extinction coefficient.
Several dianchoring organic dyes have been synthesized for use in
DSSCs, and have demonstrated better cell performance than single
DepeA sensitizers with an improved photoresponse, photocurrent
and stability [18e22].
In this study, we present an extension of dianchoring organic
sensitizer, with the aim of further expanding the possibility of im-
proving the optical and energetic properties of the sensitizers. The
donore(
high DSSCs performance, have so far been designed and developed.
A single De eA sensitizer having a rod-like configuration may
cause undesirable dye aggregation and charge recombination [10e
13]. The intermolecular stacking of dye molecules can lead to
p-spacer)eacceptor (DepeA) system, exhibiting relatively
p
new approach, based on the acceptorepedonor-core bridge donore
donore -acceptor structural motif, includes the use of two donors,
p
p
-p
two acceptors and a core bridge donor as a spacer. We have adapted
diphenylamine as a donor group by using a core bridge donor such as
triphenylamine units. The dual-channel anchorable organic dye
which contained two separate light-harvesting units and two an-
choring group in one molecule has higher molar extinction coefficient
* Corresponding author. Tel.: þ82 44 860 1493; fax: þ82 44 867 5396.
E-mail addresses: hsk@jj.ac.kr (H.S. Kang), hkk777@korea.ac.kr (H.K. Kim).
0143-7208/$ e see front matter Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.dyepig.2013.06.014

600
K.D. Seo et al. / Dyes and Pigments 99 (2013) 599e606
and increases the binding strength of dye on TiO2 film. Also, the
hexyloxy group was introduced into the triphenylamine moiety for
increase the electron-donating ability and decrease the dye aggrega-
tion, charge recombination. For comparison, the photophysical
yl)tributyl stannane [23] and D21L6 [24] were synthesized by
following the same procedures as described previously.
2.2. Measurements
properties and photovoltaic performances of a single DepeA struc-
tured D21L6 were also studied (see Scheme 1, Fig. 1, Tables 1 and 2).
1H NMR and 13C NMR spectra were recorded at room temper-
ature with Varian Oxford 300 spectrometers and chemical shifts
were reported in ppm units with tetramethylsilane as the internal
standard. FTIR spectra were taken on a JASCO, 4200 þ ATR Pro-450-
S spectrophotometer. UVevisible absorption spectra were obtained
in THF on a Shimadzu UV-2401PC spectrophotometer. Cyclic vol-
tammetry was carried out with a Versa STAT3 (AMETEK). A three-
electrode system was used and consisted of a reference electrode
(Ag/AgCl), a working electrode, and a platinum wire electrode. The
2. Experimental
2.1. Materials
All reactions were carried out under a nitrogen atmosphere.
Solvents were distilled from appropriate reagents. All reagents were
purchased from Aldrich. (50-(1,3-Dioxolan-2-yl)-2,20-bithiophen-5-
Br
HexO
OHex
HexO
HexO
Br
Br
, Pd(OAc) 2, DPPF, tert-BuOK
Pd(OAc) 2, DPPF, tert-BuOK
+
Toluene, 110 oC
Toluene, 110 oC
N
H
NH2
I
1
HexO
OHex
HexO
OHex
HexO
NH2
, Pd(OAc) 2, DPPF, tert-BuOK
N
2
N
3
1,4-DBB, Pd(OAc) 2, DPPF, STB
Toluene, 110 oC
HexO
OHex
Toluene, 110 oC
Br
Br
N
N
H
H
HexO
OHex
N
HexO
OHex
HexO
OHex
S
N
O
Sn
N
N
S
O
HexO
OHex
, Pd(PPh) 2Cl2
THF, reflux
N
N
S
S
S
S
Br
Br
4
CHO
CHO
5
HexO
OHex
N
OHex
HexO
HexO
OHex
N
N
N
O
O
NCCH 2COOH
AN/CHCl 3, 80oC
S
S
S
S
S
N
S
NC
NC
CN
COOH
COOH
COOH
COOH
D21L6
DC5
HC-A1
Scheme 1. Chemical structures and synthesis of DC5 dye.

K.D. Seo et al. / Dyes and Pigments 99 (2013) 599e606
601
Table 2
DC5
Dye-sensitized solar cell performance data of the DC5 dye.
0.6
0.5
0.4
0.3
0.2
0.1
0.0
D21L6
Dye
Coabs.
G
/10ꢀ7
Jsc (mA/cm2)
Voc (V)
FF
h (%)
(mol cmꢀ2
)
DC5
e
1.72
1.51
1.24
1.92
1.34
5.70
8.92
13.8
16.7
0.45
0.54
0.69
0.67
0.71
0.76
0.78
0.77
0.72
0.73
0.46
2.41
4.78
6.67
8.64
DCA 40 mM
HC-A1 1 mM
DCA 40 mM
e
D21L6
N719
TiO2 thickness 13,14 þ 2,sc,TiCl4; electrolyte condition 0.6 M DMPII, 0.5 M LiI, 0.05 M
I2, 0.5 M TBP in an acetonitrile solution; dye was dissolved in THF/MeOH.
0.45 mmol), bis(diphenylphosphinyl)ferrocene (DPPF) (0.52 g,
0.94 mmol), and sodium tert-butoxide (2.84 g, 29.6 mmol) was
heated in dry toluene at 110 ꢁC under an inert N2 atmosphere. After
12 h, the crude reaction mixture was treated with a saturated
ammonium chloride solution and extracted with dichloromethane.
The organic phase was washed with brine, dried over MgSO4, and the
solvent was evaporated under vacuum. The crude product was pu-
rified by column chromatography (silica gel, MCehexane 2:1) to give
1.91 g (52%) of compound 1. 1H NMR (300 MHz, DMSO-d6) com-
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. Absorption and emission spectra of DC5 dye with the concentration of
2 ꢂ 10ꢀ5 M in THF.
redox potential of dyes on TiO2 was measured in CH3CN with 0.1 M
pound:
4H), 6.77 (d, 4H), 6.87 (d, 4H), 7.48(s,1H); 13C NMR (300 MHz, DMSO-
d6) : 4.24, 12.4, 15.6, 19.1, 21.3, 58.0, 105.5, 108.2, 128.2, 142.4.
d 0.85 (t, 6H), 1.27e1.42 (m, 12H), 1.64e1.69 (m, 4H), 3.84 (t,
TBAPF6 with a scan rate between 50 mV sꢀ1
.
d
2.3. Density functional theory (DFT)/time-dependent DFT (TDDFT)
calculations
2.4.2. 3,5-Dibromo-N,N-bis(4-(hexyloxy)phenyl)benzenamine (2)
A mixture of compound 1 (1.50 g, 4.06 mmol), 1,3,5-tri-
bromobenzene (3.83 g, 12.2 mmol), Pd(OAc)2 (0.05 g, 0.22 mmol),
DPPF (0.22 g, 0.40 mmol), and sodium tert-butoxide (1.17 g,
12.2 mmol) were heated in dry toluene at 110 ꢁC under an inert N2
atmosphere. After 12 h, the crude reaction mixture was treated
with a saturated ammonium chloride solution and extracted with
dichloromethane. The organic phase was washed with brine, dried
over MgSO4, and the solvent was evaporated under vacuum. The
crude product was purified by column chromatography (silica gel,
MCehexane 1:8) to give 1.65 g (67%) of compound 2. 1H NMR
Structural optimization of dual-channel (DC) dye was done with
a PBE exchange-correlation function using the Vienna ab initio
simulation package (VASP) [25,26]. We used 400 eV as the cut-off
energy, and the conjugate gradient method was employed to
optimize the geometry until the force exerted on an atom was less
ꢀ
than 0.03 eV/A. The size of the super cell was maintained large
enough to guarantee the closest distance between atoms belonging
ꢀ
to the neighboring cells was larger than 16 A. More precisely,
structure optimizations were done for two configurations for the
dye, i.e., cis and trans configurations. In order to calculate the ab-
sorption spectra, we performed the TD-DFT calculations for more
stable one between the two configurations. Calculations were done
in the gas phase using the 6-31G(d,p) basis set in GAUSSIAN03
program [27]. We focused on transitions occurring in the range of
350e800 nm, specifically those whose oscillation strengths were
greater than 0.1. In order to treat the low wavelength excitations
correctly around 350 nm, we made extensive calculations up to 100
singlet / singlet transitions.
(300 MHz, CDCl3)
4H), 3.91 (t, 4H), 6.83 (m, 6H), 7.04 (m, 5H); 13C NMR (300 MHz,
CDCl3) : 14.2, 22.8, 25.9, 29.5, 31.8, 68.4, 115.7, 120.0, 123.1, 124.5,
d: 0.89 (t, 6H), 1.34e1.55 (m, 12H), 1.73e1.80 (m,
d
139.1, 151.3, 156.5.
2.4.3. N1,N1,N3,N5-Tetrakis(4-(hexyloxy)phenyl)benzene-1,3,5-triamine
(3)
A mixture of compound 2 (1.50 g, 2.49 mmol), 4-(hexyloxy)
benzenamine (1.05 g, 5.43 mmol), Pd(OAc)2 (0.03 g, 0.13 mmol),
DPPF (0.14 g, 0.25 mmol), and sodium tert-butoxide (0.72 g,
7.49 mmol) were heated in dry toluene at 110 ꢁC under an inert N2
atmosphere. After 12 h, the crude reaction mixture was treated
with a saturated ammonium chloride solution and extracted with
dichloromethane. The organic phase was washed with brine, dried
over MgSO4, and the solvent was evaporated under vacuum. The
crude product was purified by column chromatography (silica gel,
MCehexane 2:1) to give 1.10 g (53%) of compound 3. 1H NMR
2.4. Synthesis
2.4.1. Bis-(4-(hexyloxy) phenyl) amine (1)
A mixture of 4-(hexyloxy)benzenamine (3.00 g, 9.86 mmol), 1-
(hexyloxy)-4-iodobenzene (2.10 g, 10.9 mmol), Pd(OAc)2 (0.10 g,
Table 1
(300 MHz, CDCl3)
(m, 8H), 3.85e3.94 (m, 8H), 6.30 (s, 2H), 5.97 (s, 2H), 6.06 (s, 1H),
6.71(t, 8H), 6.93 (d, 4H), 7.04 (d, 4H); 13C NMR (300 MHz, CDCl3)
d: 0.89 (m, 12H), 1.34e1.45 (m, 24H), 1.70e1.81
Optical properties of DC5 dye.
Dye
Absorption
Emission
lmax/nm)
Eoxb/V
(vs. NHE)
E0e0c/V
ELUMOd/V
(vs. NHE)
d
:
lmax/(nm)a,
(
14.2, 22.8, 25.9, 29.5, 31.8, 68.4, 95.5, 100.7, 115.1, 121.7, 126.8, 135.7,
140.9, 146.3, 150.8, 154.5, 155.3.
3
(Mꢀ1 cmꢀ1
)
DC5
D12L6
440 (41700)
486 (27700)
645
597
0.94
0.96
2.26
2.27
ꢀ1.32
ꢀ1.31
2.4.4. N1,N3-Bis(4-bromophenyl)-N1,N3,N5,N5-tetrakis(4-(hexyloxy)
phenyl)benzene-1,3,5-triamine (4)
A mixture of compound 3 (1.00 g,1.21 mmol),1,4-dibromobenzene
(2.85 g, 12.1 mmol), Pd(OAc)2 (0.01 g, 0.06 mmol), DPPF (0.07 g,
0.13 mmol), and sodium tert-butoxide (0.35 g, 3.64 mmol) were
heated in dry toluene at 110 ꢁC under an inert N2 atmosphere. After
a
Absorption and emission spectra were measured in THF.
Oxidation potentials of dyes on TiO2 were measured in CH3CN with 0.1 M
b
TBAPF6 with a scan rate of 50 mV sꢀ1 (vs. NHE).
c
E0e0 was determined from the intersection of absorption and emission spectra
in CHCl3/MeOH.
d
LUMO was calculated by Eox ꢀ E0e0
.

602
K.D. Seo et al. / Dyes and Pigments 99 (2013) 599e606
12 h, the crude reaction mixture was treated with a saturated
ammonium chloride solution and extracted with dichloromethane.
The organic phase was washed with brine, dried over MgSO4, and the
solvent was evaporated under vacuum. The crude product was puri-
fied by column chromatography (silica gel, MCehexane 1:2) to give
with DCA (40 mM) or HC-A1 (1 mM) and kept at room temperature
overnight. Counter-electrodes were prepared by coating with a
drop of H2PtCl6 solution (2 mg of Pt in 1 mL of ethanol) on an FTO
plate. The dye-adsorbed TiO2 electrode and Pt counter electrode
were assembled in a sealed sandwich-type cell. One drop of elec-
trolyte was then introduced into the cell, which was composed of
0.6 M 1,2-dimetyl-3-propyl imidazolium iodide, 0.05 M iodine,
0.1 M LiI, and 0.5 M tert-butylpyridine in acetonitrile. The electro-
lyte was introduced into the inter-electrode space from the counter
electrode side through predrilled holes. The drilled holes were
sealed with a microscope cover slide and Surlyn to avoid leakage of
the electrolyte solution.
0.98 g (71%) of compound 4. 1H NMR (300 MHz, CDCl3)
d: 0.90 (m,
12H), 1.33e1.36 (m, 24H), 1.56 (m, 8H), 3.88 (m, 8H), 6.12 (s, 1H), 6.19
(s, 2H), 6.68e6.80(m, 12H), 6.91 (t, 8H), 7.16 (d, 4H); 13C NMR
(300 MHz, CDCl3) d: 14.5, 22.8, 25.9, 29.5, 31.80, 68.4, 113.2, 116.2,
125.4, 126.5, 126.9, 127.9, 132.3, 139.6, 140.3, 147.0, 148.6, 149.9, 155.2,
155.9.
2.4.5. 50,500-(4,40-(5-(Bis(4-(hexyloxy)phenyl)amino)-1,3-
phenylene)bis((4 (hexyloxy)phenyl)azanediyl)bis(4,1-phenylene))
di-2,20-bithiophene-5-carbaldehyde (5)
2.6. Photoelectrochemical measurements of DSSC
A mixture of compound 4 (0.23, 0.20 mmol), (50-(1,3-dioxolan-
2-yl)-2,20-bithiophen-5-yl)tributylstannane (0.32 g, 0.61 mmol),
Pd(PPh3)2Cl2 (0.02 g, 0.03 mmol), in 50 mL dry THF was refluxed
overnight under N2 atmosphere. The reaction was monitored by
thin layer chromatography. After complete consumption of starting
material, concentrated hydrochloric acid (0.5 mL) was added, the
mixture was stirred at room temperature for 10 min and extracted
with dichloromethane. The organic phase was washed with brine,
dried over MgSO4, and the solvent was evaporated under vacuum.
The crude product was purified by column chromatography (silica
gel, MC) to give 0.19 g (69%) of compound 5. 1H NMR (300 MHz,
Photoelectrochemical data were measured using a 1000 W
xenon light source (Oriel, 91193) that was focused to give 1000 W/
m2, the equivalent of one sun at AM 1.5G, at the surface of the test
cell. The light intensity was adjusted with a Si solar cell that was
double-checked with an NREL-calibrated Si solar cell (PV Mea-
surement Inc.). The applied potential and measured cell current
were measured using a Keithley model 2400 digital source meter.
The currentevoltage characteristics of the cell under these condi-
tions were determined by biasing the cell externally and measuring
the generated photocurrent. This process was fully automated us-
ing Wavemetrics software.
CDCl3) d 0.82e1.01 (m, 12H), 1.26e1.55 (m, 24H), 1.68e1.79 (m, 8H),
3.83 (t, 4H), 3.90 (t, 4H), 6.25 (s, 1H), 628 (s, 2H), 6.69 (d, 4H), 6.77
(d, 4H), 6.91 (t, 8H), 6.97 (m, 5H), 7.04 (d, 2H), 7.20(d, 2H), 7.26e7.29
(m, 6H), 7.33 (d, 4H), 7.64 (d, 2H), 9.84 (s, 2H); 13C NMR (300 MHz,
3. Results and discussion
The synthetic procedures of dual-channel (DC) dye containing
triphenylamine is depicted in Scheme 1. DC aldehyde derivative
was synthesized by the Stille coupling reaction, and the DC5
sensitizer was synthesized by the reaction of compound 5 with
cyanoacetic acid in the presence of a piperidine catalyst.
Fig. 1 shows the absorption and emission spectra of the DC5 dye
measured in THF solution; the photophysical properties are sum-
marized in Table 1. In the UVevis spectrum, the DC5 dye exhibited
two main prominent bands, appearing at 350e400 and 400e
500 nm, respectively. The former was ascribed to the localized ar-
CDCl3)
d 13.8, 14.3, 17.7, 22.8, 25.9, 27.1, 28.0, 29.5, 31.8, 68.4, 111.1,
115.1,115.3,121.6,122.9,123.6,125.9,126.2,126.4,127.4,133.7,139.3,
140.3, 141.2, 146.6, 147.6, 148.1, 148.4, 150.0, 155.2, 156.1, 182.5.
2.4.6. (2E,20E)-3,30-(50,50’-(4,40-(5-(Bis(4-(hexyloxy)phenyl)amino)-
1,3-phenylene)bis((4-(hexyloxy)phenyl)azanediyl)bis(4,1-
phenylene))bis(2,20-bithiophene-50,5-diyl))bis(2-cyanoacrylic acid)
(DC5)
Compound 5 (0.07 mg, 0.05 mmol), dissolved in CHCl3 (20 mL)
and acetonitrile (20 mL), was condensed with 2-cyanoacetic acid
(0.01 g, 0.12 mmol) in the presence of piperidine (0.01 mL,
0.12 mmol). The mixture was refluxed for 12 h. After cooling the
solution, the organic layer was removed in vacuo. Dark red solid of
DC5 was obtained by silica gel chromatography (MC/MeOH ¼ 4:1);
omatic
pep* transition, and the latter was that of the charge-
transfer character, which was from the donor part to the acceptor
group. The absorption spectrum of the DC5 dye showed maximum
absorption at 440 nm, which was 18 nm blue-shifted in contrast to
D21L6 dye. But, the molar absorption coefficient ( ) of the DC5 dye
3
Yield was 78%. 1H NMR (300 MHz, DMSO-d6)
d
0.86e0.88 (m, 12H),
(
(
3
¼ 41,700 Mꢀ1 cmꢀ1) was much higher than that of D21L6
max
1.10e1.59 (m, 24H), 1.97(m, 8H), 3.79e3.86 (m, 8H), 5.98 (m, 3H),
3
¼ 27,700 Mꢀ1 cmꢀ1), indicating that the DC5 dye could have
max
6.78e7.02 (m, 18H), 7.38 (m, 12H), 7.68 (m, 4H), 8.08 (s, 2H); 13C
better light-harvesting ability. The dianchoring mode of DC5 dye
was further confirmed by ATR-FTIR spectroscopy (Fig. 2). The C]O
stretching band at 1730 cmꢀ1 assigned to the carboxylic acid groups
of free DC5 dye was found to have disappeared in the IR spectrum of
DC5 dye adsorbed on TiO2 film, indicating the involvement of both
carboxylic acid groups in adsorption on the TiO2 surface; this result
is similar to the observations made by Heredia et al. [17] and
Abbotto et al. [18]
NMR (300 MHz, DMSO-d6)
d 13.8, 14.3, 17.7, 22.8, 25.9, 27.1, 28.0,
29.5, 31.8, 68.4, 111.1, 115.1, 115.3, 121.6, 122.9, 123.6, 125.9, 126.2,
126.4, 127.4, 133.7, 139.3, 140.4, 141.2, 146.2, 147.3, 148.3, 148.5,
150.4, 155.7, 156.1, 163.5; UVevis (THF, nm): lmax (log
(41700). PL (THF, nm): lmax 645; MS (MALDI-TOF): Calcd. for
90H91N5O8S4, 1498.97; found, 1498.62.
3
) 440
C
2.5. Fabrication and testing of DSSC
The electrochemical properties were investigated by cyclic vol-
tammetry (CV) to obtain the HOMO and LUMO levels of the DC5
dye. The cyclic voltammograms were obtained from a three-
electrode cell in 0.1 M TBAPF6 in CH3CN at a scan rate of
50 mV sꢀ1, using a dye coated TiO2 electrode as the working elec-
trode, a Pt wire counter-electrode, and an Ag/AgCl (saturated KCl)
reference electrode (þ0.197 V vs. NHE) which was calibrated with
ferrocene. All of the measured potentials were converted to the
NHE scale. The band gap was estimated from the absorption edges
of the UVevis spectra and LUMO energy levels were derived from
the HOMO energy levels and the band gap. HOMO values (0.94 V vs.
FTO glass plates (Pilkington) were cleaned in a detergent solu-
tion using an ultrasonic bath for 1 h, then rinsed with water and
ethanol. The FTO glass plates were immersed in an aqueous solu-
tion of 40 mM TiCl4 at 70 ꢁC for 30 min and then washed with water
and ethanol. The first TiO2 layer with a thickness of 12 mm was
prepared by screen-printing TiO2 paste (Solaronix, 13 nm anatase),
and the second scattering layer containing 400 nm sized anatase
particles was deposited by screen printing. The TiO2 electrodes
were immersed into the dye solution (0.3 mM) in THF/EtOH (1:2)

K.D. Seo et al. / Dyes and Pigments 99 (2013) 599e606
603
DFT and TD-DFT calculations. First, our optimization results indicate
that the cis configuration is more stable than the trans configuration
by 0.06 eV. Therefore, we can conclude that the DC5 dye predomi-
nantly adopt the cis configuration, and the TD-DFT calculations were
performed for the configuration. The closest distances between the
100
98
96
ꢀ
two m-phenylene units were 6.6 and 15.1 A for the cis and trans
configurations, respectively (Fig. S3). The
f
angles were ꢀ10.2ꢁ and
94
47.1ꢁ for cis and trans configurations, respectively. Such an inter-
Power
planar conformation leads to intermolecular
teractions between channels of DC5 dye adsorbed on the TiO2 sur-
face. It is well-known that the stacking of the inter-planar
pep stacking in-
2200
2000
1800
1600
1400
1200
1000
Wavenumver (cm-1)
pep
molecules may facilitate the formation of detrimental excimeric
species and lead to fluorescence quenching in the solid state [29].
In very good agreement with the experimental data shown in
Fig. 1, we first find two absorption maxima. We were able to
confirm that absorption peaks below 400 nm are not observed if
only 40 transitions are examined. The major absorption appears at
488 nm, which is red-shifted by 48 nm with respect to the exper-
imental data. Now, we delve into the nature of transitions
responsible for the observed absorption maxima. Table S1 shows
that six absorptions at 462, 465, 474, 490, 498, and 509 nm, which
are responsible for the absorption peak at 488 nm, can be largely
ascribed to five charge transfer transitions, i.e., HOMO 7 /
LUMOþ1, HOMOꢀ6 / LUMO, HOMOꢀ2 / LUMOþ3, HOMOꢀ1 /
LUMOþ2, and HOMOꢀ1 / LUMOþ3 transitions. (See Fig. 3 for the
isodensity plots of the four molecular orbitals.) Note that the
(HOMOꢀ7, HOMOꢀ6) are doubly degenerate, which is also true for
(LUMO, LUMOþ1) and (LUMOþ2, LUMOþ3).
100
98
96
Dye/TiO
2
2200
2000
1800
1600
1400
1200
1000
Wavenumber (cm-1)
Fig. 2. ATR-FTIR spectra of DC5 dye.
NHE) were more positive than the Iꢀ/I3ꢀ redox couple (0.4 V vs.
NHE). Electron injection from the excited dyes to the conduction
band of TiO2 should be energetically favorable because of the more
negative LUMO values (ꢀ1.32 V vs. NHE) compared to the con-
duction band edge energy level of the TiO2 electrode [28].
Here, structural optimization of dual-channel (DC) dye was also
done with a PBE exchange-correlation function using the Vienna ab
initio simulation package (VASP). We summarize our results for the
To effectively suppress dye aggregation for DSSCs, in our pre-
vious study [30e35], coadsorbents such as DCA and HC-A1 were
used. Both Voc and Jsc were dramatically enhanced by the coad-
sorption of the multi-functional coadsorbent HC-A1 with the dye,
due to the efficient retardation of charge recombination arising
Fig. 3. Isodensity plots of the HOMOꢀ7 (a), HOMOꢀ6 (b), HOMOꢀ2 (c), LUMOþ1, (d), LUMOþ2 (e) and LUMOþ3 (f) of the DC5 dye.

604
K.D. Seo et al. / Dyes and Pigments 99 (2013) 599e606
10
from the prevention of
pep stacking, light-harvesting in shorter
wavelength regions, and a hole conducting function through a
redox cascade process [30]. Fig. 4 shows the spectra for the effi-
ciency of conversion of an incident photon to current (IPCE) for the
DSSCs with dye only and dye/coadsorbent, respectively. The onset
wavelengths of the IPCE spectra for DSSCs based on DC5 were about
700 nm. For DSSCs based on DC5 and DC5/DCA, the IPCE spectra
exhibit the sharp peak below 400 nm, which could correspond to
the absorption of TiO2 film [28]. The IPCE values of higher than 50%
were observed in the range of 350e600 nm with a maximum value
of 60% at 490 nm for the DSSC based on DC5 with HC-A1. The IPCE
response of the DC5/HC-A1-sensitized DSSC was remarkably
increased compared to that with DCA. We suggest that intermo-
8
6
4
2
0
DC5
DC5 + DCA
DC5 + HC-A1
lecular pep stacking between channels of DC5 dye adsorbed on the
TiO2 surface was effectively reduced by the incorporation of coad-
sorbents, resulting in the enhanced light harvesting.
The photovoltaic performances of the DC- and D21L6-
sentsitized DSSCs are summarized in Table 2. Under standard
global AM 1.5 solar conditions, the DC5 sensitized cell (without
coadsorbents) gave a short circuit photocurrent density (Jsc) of
1.34 mA cmꢀ2, an open circuit voltage (Voc) of 0.45 V, and a fill factor
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Potential (V)
Fig. 5. Photocurrent-voltage characteristics of representative TiO2 electrodes sensi-
tized with DC5 dye.
(FF) of 0.76, corresponding to an overall conversion efficiency (h) of
0.46% due to heavy aggregation. On the other hand, the DC1/DCA-
sensitized DSSC, compared to the DC5-sensitized DSSC, Jsc
increased from 1.34 to 5.70 mA cmꢀ2, 0.45e0.54 V for Voc, and
To further characterize the open circuit voltage effect with
respect to the coadsorbents used, we measured electrochemical
impedance spectroscopy (EIS) under dark conditions. Their EIS
parameters calculated from the equivalent circuit are summarized
in Table 3. Typical Nyquist and Bode plots for the DSSCs based on
the DC5 dye, DC5/DCA, and DC5/HC-A1 are shown in Fig. 6. The
DC5/HC-A1-sensitized DSSC revealed relatively high charge trans-
fer resistance compared with the dye only and DCA, while the
frequency was apparently shifted to the lower frequency region.
The maximum frequencies (umax) in the middle frequency region of
the Bode plots of the DC5/HC-A1, DC5/DCA and DC5 sensitized
DSSCs were 5.2, 10.1 and 144 Hz, respectively. Since umax is inversly
0.46e2.41% for h. As shown in Fig. 4, Jsc increased in the same order
as the integrated photocurrent from the IPCE spectrum. Also, the
DC5/HC-A1-sensitized DSSC, compared to the DC5/DCA-sensitized
DSSC, had an increased Jsc from 5.70 to 8.92 mA cmꢀ2, 0.54e
0.69 V for Voc, and 2.41e4.78% for
h. It was observed that the
coadsorption of either DCA or HC-A1 simultaneously improved Jsc
and Voc, but coadsorption with HC-A1 enhanced Jsc and Voc more
significantly than with DCA. The HC-A1 has multiple functions,
such as the light-harvesting function as a short-wavelength light
absorption dye molecule to increase Jsc, the prevention effect of the
associated with electron lifetime (
umax indicates a reduced rate of the charge-recombination process
of the DSSCs. The calculated values of DC/HC-A1-based device
s
),
s
¼ 1/(2
pf) [36], a decrease in
pep stacking of organic dye to enhance Voc by reducing the charge
recombination [30e32]. On the other hand, the D21L6/DCA-
s
sensitized DSSC gave a Jsc of 13.5 mA cmꢀ2, Voc of 0.67 V, and a FF
(8.69 ms) was slower than those of the DC only and DC/DCA-based
devices (2.29 ms, 6.23 ms), which was consistent with the sequence
of open circuit voltage values in the devices, and which might also
be due to the significant increased in capacitance (Cm) at the
interface of the counter electrode/dye/electrolyte solution (Table 3).
Cm of DC/HC-A1-based device was enhanced due to the decrease
electron loss by efficient prevention of charge recombination and
of 0.72, corresponding to
DC structure leads to heavier aggregation and charge recombina-
tion than the single De eA structured D21L6 due to intermolec-
h of 6.67%. These results indicate that the
p
ular
p
ep
stacking between adsorbed channels on the TiO2 surface
(Fig. 5).
70
DC5
pep stacking. From the EIS results, it is clear that the charge
DC5 + DCA
DC5 + HC-A1
recombination rate was the rate-determining step of the DSSC in
terms of device performance, since the DC5-DSSCs with coad-
sorbents decreased the charge recombination rate and resulted in
an improvement in device performance, in that order. From these
results, we can conclude that charge recombination of the DSSCs
with dye only is faster than that of the DSSCs with coadsorbents
(DCA and HC-A1). This insulating molecular layer effectively in-
hibits back electron transfer from TiO2 to Iꢀ3 ions and results in a
60
50
40
30
20
10
0
higher Voc
.
Table 3
Numerical values of the electrochemical impedance spectra for DSSCs based on DC5
dye.
Dye
DC5
Coadsorbent
Rct
(
U
)
Cm (mF)
s (ms)
ea
303.8
64.5
44.6
0.08
0.97
1.95
2.29
6.23
8.69
DCA 40 mMb
300
400
500
600
700
HC-A1 1 mMc
Wavelength (nm)
a
Calculated from the EIS spectrum at a forward bias of w0.45 V.
Calculated from the EIS spectrum at a forward bias of w0.55 V.
Calculated from the EIS spectrum at a forward bias of w0.7 V.
b
c
Fig. 4. Typical action spectra of incident photon-to-current conversion efficiencies
(IPCE) obtained for nanocrystalline TiO2 solar cells sensitized by DC5 dye.

K.D. Seo et al. / Dyes and Pigments 99 (2013) 599e606
605
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2013.06.014.
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by the Korea Government Ministry of Trade, Industry and Energy.
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