Photovoltaic properties of bis(octyloxy)benzo-[c][1,2,5]thiadiazole sensitizers based on an N,N-diphenylthiophen-2-amine donor

Article abstract of DOI:10.1039/c4tc00169a

Three D-A-π-A sensitizers (DOBT-IV to DOBT-VI) with N,N- diphenylthiophen-2-amine as the donor and bis(octyloxy)benzo-[c][1,2,5] thiadiazole (DOBT) as the auxiliary acceptor have been designed and synthesized. Their applications to dye-sensitized solar cells with I^-/I ₃^- and Co(ii)/(iii) electrolytes were measured and characterized. Via fine tuning of the π-bridge, the highest photoelectric conversion efficiency of 7.16% was obtained with J_sc = 16.88 mA cm^-2, V_oc = 0.662 V and FF = 64.03% for the DOBT-V based dye-sensitized solar cells using the I^-/I₃^- electrolyte under standard global AM1.5 solar conditions. A photoelectric conversion efficiency of 6.14% was obtained with J_sc = 11.35 mA cm^-2, V_oc = 0.760 V and FF = 71.16% with the Co(ii)/(iii) electrolyte under standard global AM1.5 solar conditions. The optical and electrochemical properties and the photovoltaic performance were evaluated and investigated using density functional theory calculations, a behavior study of the four performance parameters with dependence of the incident light intensity, electrochemical impedance spectroscopy and intensity-modulated photo-voltage spectrometry.

Full text of DOI:10.1039/c4tc00169a

Journal of
Materials Chemistry C
View Article Online
View Journal | View Issue
PAPER
Photovoltaic properties of bis(octyloxy)benzo-[c]
[1,2,5]thiadiazole sensitizers based on an N,N-
diphenylthiophen-2-amine donor†
Cite this: J. Mater. Chem. C, 2014, 2,
4063
Xiaoyu Zhang,a Long Chen,a Xin Li,b Jiangyi Mao,a Wenjun Wu,a Hans A˚grenb
a
*
and Jianli Hua
Three D–A–p–A sensitizers (DOBT-IV to DOBT-VI) with N,N-diphenylthiophen-2-amine as the donor and
bis(octyloxy)benzo-[c][1,2,5]thiadiazole (DOBT) as the auxiliary acceptor have been designed and
ꢀ
synthesized. Their applications to dye-sensitized solar cells with Iꢀ/I3 and Co(II)/(III) electrolytes were
measured and characterized. Via fine tuning of the p-bridge, the highest photoelectric conversion
efficiency of 7.16% was obtained with Jsc ¼ 16.88 mA cmꢀ2, Voc ¼ 0.662 V and FF ¼ 64.03% for the
DOBT-V based dye-sensitized solar cells using the Iꢀ/I3 electrolyte under standard global AM1.5 solar
ꢀ
conditions. A photoelectric conversion efficiency of 6.14% was obtained with Jsc ¼ 11.35 mA cmꢀ2, Voc
¼
0.760 V and FF ¼ 71.16% with the Co(II)/(III) electrolyte under standard global AM1.5 solar conditions. The
optical and electrochemical properties and the photovoltaic performance were evaluated and
investigated using density functional theory calculations, a behavior study of the four performance
parameters with dependence of the incident light intensity, electrochemical impedance spectroscopy
and intensity-modulated photo-voltage spectrometry.
Received 24th January 2014
Accepted 26th March 2014
DOI: 10.1039/c4tc00169a
www.rsc.org/MaterialsC
the sensitizers and corrosiveness towards most metals and seal-
Introduction
ing materials. To solve these problems, cobalt complexes have
ꢀ
emerged as alternatives to Iꢀ/I3 .5 As a result of their tuneable
and more positive redox potentials, they soon became the most
important of the redox couples. High open-circuit voltages can
usually be achieved via Co(II)/(III) electrolyte-based DSSC devices.
However, due to their larger size, heavier mass and slower
diffusion in the electrolyte, their mass transport is limited and
charge recombination is severe. Bulky sensitizers are used with
Co(II)/(III) electrolytes to retard charge recombination.6
As we enter the 21st century, the world is suffering from many
serious issues, one of which is the energy crisis. Worldwide
research and development in the eld of renewable energy are
booming. In just two decades, we have witnessed the develop-
ment of dye-sensitized solar cells (DSSCs),1 which are now
ourishing. A maximum photoelectric conversion efficiency
(PCE) of 15% has been achieved by perovskite-based DSSCs.2
For cells with organic sensitizers, an overall PCE of 13.0% has
been obtained using a zinc–porphyrin-based sensitizer and a
cobalt-based electrolyte without co-sensitization.3 To achieve
higher PCEs, the short-circuit current density (Jsc) and the open-
circuit voltage (Voc) need to be improved. Many strategies have
been put forward to attain high efficiencies.4
Another crucial factor is the sensitizer. Pure organic sensi-
tizers have drawn increasing attention due to the simplicity and
low cost of designing their molecular structure to nely adjust
properties such as their molecular geometries, molar extinction
coefficients, absorption properties and energy levels to achieve
higher efficiencies.7 In general, they have a donor–p-bridge–
acceptor (D–p–A) conguration with the advantage of intra-
molecular charge transfer (ICT) properties.8 Recent research has
showed that introducing an additional electron-withdrawing
unit between the donor and the p-bridge as an auxiliary
acceptor does not only tune the molecular energy levels, but also
red shis the charge-transfer absorption band, leading to an
improvement in the photovoltaic performance and stability.9
Many auxiliary acceptor units have been adopted successfully,
such as benzothiadiazole,10 benzotriazole,11 quinoxaline,12
diketopyrrolo-pyrrole,13 pyrido[3,4-b]pyrazine,14 and thieno-[3,4-
c]pyrrole-4,6-dione.15 Among these, benzothiadiazole shows
some advantages in light harvesting and efficiency, including
The electrolyte, which plays a signicant role in the photo-
voltaic performance of DSSCs, has attracted much attention. The
traditional electrolyte, the iodide/triiodide (Iꢀ/I3ꢀ) electrolyte,
suffers from problems such as competitive light absorption with
aKey Laboratory for Advanced Materials and Institute of Fine Chemicals East China
University of Science and Technology, Shanghai 200237, China. E-mail: jlhua@
ecust.edu.cn; Fax: +86-21-64252758
bDepartment of Theoretical Chemistry and Biology, School of Biotechnology, KTH
Royal Institute of Technology, Stockholm 10691, Sweden
† Electronic supplementary information (ESI) available: Cyclic voltammetry,
molecular orbital calculation results, the simulated parameters from EIS. See
DOI: 10.1039/c4tc00169a
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 4063–4072 | 4063

View Article Online
Journal of Materials Chemistry C
Paper
Scheme 1 Molecular structures of the dyes DOBT-IV to DOBT-VI.
expanding the absorption wavelengths into the NIR region,
improving electron distribution and increasing the photo-
stability of synthetic intermediates and target sensitizers.16
However, the low open current voltage limits its performance in
DSSCs.17 In our previously reported work, we introduced two
longer alkyl chains into benzothiadiazole units to construct
three triarylamine-based sensitizers (DOBT-I to DOBT-III).18 The
results showed that sensitizers using 5,6-bis(octyloxy)benzo[c]-
[1,2,5]thiadiazole had higher open-circuit voltages than that of a
benzothiadiazole-containing analogue.
With the intention of obtaining a higher short-circuit current
density, we replaced the triphenylamine unit with an N,N-
diphenylthiophen-2-amine unit, which has a better electron-
donating ability, to further broaden the absorption range.19 We
designed and synthesized three D–A–p–A sensitizers, DOBT-IV
to DOBT-VI (see Scheme 1) with N,N-diphenylthiophen-2-amine
as the donor, DOBT as the auxiliary acceptor, and benzene,
thiophene and furan rings used to ne tune the properties of
the sensitizers. The photovoltaic performance was measured
and analyzed. To enhance the Voc, we also used a Co electrolyte
in the devices. As expected, the Voc greatly increased, but the Jsc
decreased, resulting in a decreased PCE.
Scheme 2 Synthesis of the dyes DOBT-IV to DOBT-VI.
that the introduction of heterocyclic rings results in the emer-
gence of the new absorption band due to n–p* transitions. The
rst absorption band can be attributed to p–p* transitions
caused by the conjugation of the DOBT moiety and the
p-bridge, whereas the last absorption band can be assigned to
the ICT between the N,N-diphenylthiophen-2-amine donor and
the cyanoacetic acid acceptor. The main absorption peaks of
DOBT-IV, DOBT-V and DOBT-VI are at 483 nm, 525 nm and
531 nm, respectively. The 42–48 nm bathochromic shis of the
main absorption peaks of DOBT-V and DOBT-VI compared with
DOBT-IV can be ascribed to the much better planarity between
the DOBT moiety and the thiophene or furan moiety compared
with that between DOBT and benzene (Fig. S2†), which would
benet the ICT.
Synthetic procedures
The synthesis procedures for DOBT-IV to DOBT-VI are shown in
Scheme 2. Compound 3 was synthesized via the Suzuki coupling
reaction between 4,7-dibromo-5,6-bis(octyloxy)-benzo-[c]-[1,2,5]-
thiadiazole and tributyltin compound 2, which was derived
from N,N-diphenylthiophen-2-amine. Then (4-formylphenyl)-
boronic acid, (5-formylfuran-2-yl)boronic acid and (5-for-
mylthiophen-2-yl)boronic acid reacted with compound 3 to
produce the corresponding aldehyde precursors (compounds 4,
5, 6), followed by Knoevenagel condensation with cyanoacetic
acid to produce the target dyes (DOBT-IV to DOBT-VI). All the
intermediates and three target sensitizers were fully character-
To estimate the feasibility of electron injection into the
conduction band of TiO2 and dye regeneration, cyclic
1
ized by H-NMR, 13C-NMR and HRMS.
Results and discussion
Optical and electrochemical properties
The absorption spectra of the three dyes in CH2Cl2 solvent and
on 4 mm transparent TiO2 lms are shown in Fig. 1. It can be
seen that, in the CH2Cl2 solvent, both DOBT-V and DOBT-VI
exhibit three major absorption bands at 300–370 nm, 370–450
nm and 450–700 nm, whereas DOBT-IV only displays two major
absorption bands at 300–410 nm and 410–620 nm. It is clear
Fig. 1 Absorption spectra of DOBT-IV to DOBT-VI (a) in CH2Cl2 and
(b) on 4 mm transparent TiO2 films.
4064 | J. Mater. Chem. C, 2014, 2, 4063–4072
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
voltammetry measurements were carried out in CH2Cl2 solution ground state, and lmax, the excitation energy corresponding to
with 0.1 tetra-n-butylammonium hexauorophosphate the photo-induced ICT absorption band:
M
(TBAPF6) as the inert electrolyte with a three-electrode system
(working electrode Pt; counter electrode Pt wire; reference
electrode SCE; calibrated with ferrocene/ferrocenium as an
internal reference). The cyclic voltammograms of DOBT-IV to
DOBT-VI are shown in Fig. S1.† The redox potentials (EOX) of
DOBT-IV to DOBT-VI correspond to the highest occupied
molecular orbital (HOMO) energy levels, located at 0.71, 0.72
and 0.71 V vs. NHE. The nearly equal HOMO energy levels of
DOBT-IV to DOBT-VI indicate that the HOMO energy level
mainly depends on the donor moiety. All the electrochemical
properties of DOBT-IV to DOBT-VI are listed in Table 1. The
zero–zero transition energies (E0–0) of DOBT-IV to DOBT-VI are
2.11 eV, 1.90 eV and 1.92 eV, respectively; these were estimated
from the intersection of the normalized absorption and emis-
sion spectra via the equation E0–0 ¼ 1240/l. Calculated from
EHOMO ꢀ E0–0, the lowest unoccupied molecular orbital (LUMO)
energy levels are ꢀ1.40, ꢀ1.18 and ꢀ1.21 V vs. NHE. To be a
good sensitizer for DSSCs, the LUMO energy level of a dye has to
be at least 0.20 V higher than the conduction band energy level
(ECB) of TiO2 (ꢀ0.5 V vs. NHE) to achieve efficient injection of
the excited electrons into the conduction band, while its HOMO
energy level must be at least 0.15 V lower than the redox
potential of the electrolyte to guarantee an efficient regenera-
tion of the oxidized dyes by the redox species in the electrolyte.20
With LUMO values about 0.7–0.9 V more negative than the ECB
of TiO2, and HOMO values 0.3 V more positive than the redox
potential of Iꢀ/I3ꢀ (0.4 V vs. NHE) and 0.15 V more positive than
the redox potential of Co(II)/Co(III) (0.56 V vs. NHE), DOBT-IV to
DOBT-VI are theoretically qualied to be applied to DSSCs.
EOX(ES) ¼ EOX(GS) ꢀ lmax
In this work, the hybrid B3LYP functional22,23 and the 6-31G*
basis set24 were adopted in the geometry optimizations of the
organic dyes and calculations of their EOX(GS) values. For lmax
,
time-dependent density functional theory calculations were
carried out with the hybrid BH and HLYP functional22,23,25 and
the 6-31G* basis set,24 which is a suitable choice for the study of
excitations of charge-transfer character. As suggested by Preat
et al.,21 the electron injection process was assumed to occur
before the relaxation in the excited state and the non-equilib-
rium polarizable continuum model26 was selected to model the
solvent effects of dichloromethane. All theoretical calculations
were carried out using the Gaussian 09 program package.22
The key computed parameters of the DSSCs are listed in
Table S1.† With the help of theoretical calculations, we were
able to gain an insight into the factors that may inuence the
performance of DSSC dyes. Firstly, the magnitude of EOX(GS) is
closely related to the energy level of the HOMO (Table S2†),
which is mainly localized on the donor part of the organic dye.
Therefore the choice of a suitable donor group is of great
importance in the design of DSSC dyes. We have shown that, in
DOBT-IV, DOBT-V and DOBT-VI, the donor part is almost
coplanar with the benzothiadiazole group, leading to a lied
HOMO energy level and hence a smaller EOX(GS), which is not of
benet for the regeneration of the dye cation radical. The choice
of the p-conjugation bridge between DOBT and cyanoacrylic
acid is also important. When a phenyl ring is used to connect
the benzothiadiazole and cyanoacrylic acid groups, the LUMO
energy level is lied, resulting in a larger HOMO–LUMO gap, a
larger ꢀDGinject and a smaller oscillator strength of the ICT
absorption band (e.g. DOBT-IV), which leads to a lower effi-
ciency of light harvesting. When a thiophene or furan ring is
used as the p-conjugation bridge, the HOMO–LUMO gap
decreases, leading to a decreased ꢀDGinject and an enhanced
oscillator strength. Fig. 2 shows the dependence of ꢀDGinject
and the oscillator strength of the ICT absorption band on the
HOMO–LUMO gap, from which it can be seen that ꢀDGinject is
positively and linearly correlated with the HOMO–LUMO gap
for DOBT-IV to DOBT-VI. Interestingly, the magnitude of the
oscillator strength is negatively and linearly correlated with the
HOMO–LUMO gap. Based on our previous work,27 we developed
Molecular orbital calculations
Density functional theory calculations were used to further
investigate intermolecular electron transfer. According to Preat
et al.,21 the free energy change for the electron injection process
can be expressed as:
DGinject ¼ EOX(ES) ꢀ ECB(TiO2)
where ECB(TiO2) is the reduction potential of the conduction
band of TiO2 (4.0 eV) and EOX(ES) is the oxidation potential of
the dye in the excited state, which is equal to the difference
between EOX(GS), the oxidation potential of the dye in the
Table 1 Optical properties and electrochemical properties of the dyes DOBT-IV to DOBT-VI
Dye
lmaxa/nm (3 ꢁ 10ꢀ4 Mꢀ1 cmꢀ1
)
lmaxb TiO2/nm
HOMOc/V (vs. NHE)
E0–0d/eVꢀ1
LUMOe/V (vs. NHE)
DOBT-IV
DOBT-V
DOBT-VI
483(1.01)
525(1.82)
531(1.34)
467
508
506
0.71
0.72
0.71
2.11
1.90
1.92
ꢀ1.40
ꢀ1.18
ꢀ1.21
Absorption maximum in CH2Cl2 solution. b Absorption maximum on TiO2 lm. c HOMO energy levels were measured in CH2Cl2 with 0.1 M tetra-
a
n-butylammonium hexauorophosphate (TBAPF6) as the electrolyte (working electrode Pt; reference electrode SCE; calibrated with ferrocene/
d
e
ferrocenium). E0–0 was estimated from the intercept of the normalized absorption and emission spectra of the dyes, E0–0 ¼ 1240/l. LUMO
energy levels were estimated by subtracting E0–0 from the HOMO energy level.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 4063–4072 | 4065

View Article Online
Journal of Materials Chemistry C
Paper
Fig. 2 Dependence of ꢀDGinject and the oscillator strength of the ICT
absorption band on the HOMO–LUMO gap.
Fig. 3 IPCE spectra of DSSCs based on DOBT-IV to DOBT-VI with
Co(II)/(III) and Iꢀ/I3ꢀ electrolytes.
two principles for the design of DSSC dyes: (1) a large EOX(GS);
and (2) the choice of an appropriate p-bridge, which ensures a
good balance between ꢀDGinject and the oscillator strength of
the ICT absorption band. From Fig. 2, it is clear that, although
none of DOBT-IV to DOBT-VI exhibits a good balance between
ꢀDGinject and the oscillator strength of the ICT absorption
band, DOBT-V is slightly better than the other two dyes. From
theoretical analysis we can predict that DOBT-VI might show
the worst performance as it has neither a large EOX(GS) nor a
good balance between ꢀDGinject and the oscillator strength of
the ICT absorption band, which has been proved by the results
of the DSSC characterizations.
Fig. S2† shows the frontier orbitals of DOBT-IV to DOBT-VI,
calculated at isodensity ¼ 0.020 a.u. The hydrogens are omitted
for clarity. The dihedral angles between the DOBT and the
donor moiety and those between the p-bridge are also shown in
Fig. S2.† We can see that the introduction of heterocyclic rings
largely reduced the dihedral angle between DOBT and the
p-bridge, from 42.7ꢂ for benzene as the p-bridge to 10.2ꢂ for
the thiophene moiety and 4.6ꢂ for furan moiety, improving the
planarity of the molecular structure. This can facilitate the
charge transfer, while at the same time increasing the likeli-
hood of dye aggregation and electron recombination.
increased to the NIR by replacing the triphenylamine with the
N,N-diphenylthiophen-2-amine unit. The ranges were further
broadened with respect to the absorption spectrum by
substituting the benzene ring with heterocyclic rings, which
would benet the photo-induced current density. However, the
IPCE values of DOBT-V are higher than those of DOBT-VI, which
might be due to the high molecular extinction coefficient of
DOBT-V (18 200 Mꢀ1 cmꢀ1), despite the similar planarity of
their molecular structures. The IPCE values of DSSCs with the
Co(II)/(III) electrolyte show a similar trend for the three dyes, but
ꢀ
are much lower than those of their counterparts with the Iꢀ/I3
electrolyte, indicating lower short current densities (Jsc). To bꢀe
more specic, the DOBT-IV-sensitized solar cell with the Iꢀ/I3
electrolyte had an IPCE >60% in the range 380–590 nm, with a
maximum value of 72%, whereas the solar cell with the Co(II)/
(III) electrolyte had an IPCE >45% in the range 360–570 nm, with
the highest value of 57% at 490 nm. This may explain the large
difference in Jsc values between the two electrolytes. Similarly, in
the range 370–620 nm, the IPCE values of the DOBT-V-sensi-
ꢀ
tized solar cell with the Iꢀ/I3 electrolyte are >60%, whereas
those with the Co(II)/(III) electrolyte are only >40%, with a
maximum value of 56%. However, the IPCE for the DOBT-VI-
sensitized solar cell is quite low, only reaching >30% from 340
to 620 nm, with the maximum value of 41% at 540 nm for the
Co(II)/(III) electrolyte and >50% from 360 to 620 nm with the
Photovoltaic performance of DSSCs
ꢀ
maximum value of 57% for the Iꢀ/I3 electrolyte.
The DSSCs based on DOBT-IV to DOBT-VI used two different
Table 2 gives the photovoltaic paꢀrameters of DSSCs based on
electrolytes. The electrolyte based on Iꢀ/I3ꢀ consisted of 0.05 M DOBT-IV to DOBT-VI withꢀthe Iꢀ/I3 and Co(II)/(III) electrolytes.
I2, 0.1 M LiI, 0.5 M 1,2-dimethyl-3-propylimidazolium iodide For DSSCs with the Iꢀ/I3 electrolyte, the Jsc value of DSSCs
and 0.75 M 4-tert-butylpyridine (tBP) in acetonitrile, whereas the based on DOBT-V is higher than that of DSSCs based on DOBT-
Co(II)/(III)-based electrolyte was composed of 0.22 M tris(2,20- IV and DOBT-VI. Given its broadened absorption range,
bipyridine)cobalt(II) bis(hexauorophosphate) ([Co(bpy)3](PF6)2), enhanced molar extinction coefficient and high IPCE values,
0.05
M
tris(2,20-bipyridine)cobalt(III)tris(hexauorophosphate) there is no doubt that DOBT-V has the best performance based
([Co(bpy)3] (PF6)3), 0.1 M LiClO4 and 0.2 M tBP in acetonitrile. on the short circuit current. In addition, the DSSCs display the
Fig. 3 shows the incident photon-to-electron conversion effi- same order of performance for other parameters: Voc (DOBT-IV)
ciency (IPCE) as a function of the incident wavelength for DSSCs > Voc (DOBT-V) > Voc (DOBT-IV). Detailed discussion of the Voc is
based on these dyes with the two electrolytes. Comparing the given in the following sections about electrochemical imped-
IPCE of DSSCs with the Iꢀ/I3 electrolyte with those in our ance spectroscopy (EIS) and intensity-modulated photo-voltage
ꢀ
previous work,18 the IPCE ranges of the three dyes were spectroscopy (IMVS). Finally, the highest PCE of 7.16% was
4066 | J. Mater. Chem. C, 2014, 2, 4063–4072
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
Table 2 Photovoltaic parameters of DSSCs based on DOBT-IV to
DOBT-VI with the Iꢀ/I3ꢀ and Co(II)/(III) electrolytes
Dye
Jsc (mA cmꢀ2
)
Voc (V)
FF (%)
PCE (%)
DOBT-IV-I
DOBT-V-I
DOBT-VI-I
DOBT-IV-Co
DOBT-V-Co
DOBT-VI-Co
15.05
16.88
15.37
9.96
11.35
8.37
0.686
0.662
0.629
0.774
0.760
0.713
64.53
64.03
64.02
73.44
71.16
71.06
6.66
7.16
6.19
5.66
6.14
4.24
obtained by DSSCs based on DOBT-V, with Jsc ¼ 16.88 mA cmꢀ2
,
Voc ¼ 0.662 V and FF ¼ 64.03% for the Iꢀ/I3 electrolyte. A
similar pattern was found in DSSCs with the Co(II)/(III) electro-
lyte and their highest PCE of 6.14% was also shown by DSSCs
based on DOBT-V, with Jsc ¼ 11.35 mA cmꢀ2, Voc ¼ 0.760 V and
FF ¼ 71.16% under standard global AM1.5 solar conditions.
Despite the advantages in Voc and FF, DSSCs based on DOBT-
IV to VI with the Co(II)/(III) electrolyte have much lower PCEs
than their counterparts with the Iꢀ/I3ꢀ electrolyte as a result of
the considerably lower Jsc. As we used much thicker TiO2 lms
for devices with the Iꢀ/I3ꢀ electrolyte, this result is no surprise.
However, what we are really concerned about is whether the
larger size, heavier mass and slower diffusion of Co complexes
pose a problem for their mass transport. The dependence of the
photocurrent density behaviour with incident light intensity
was therefore measured to determine whether mass transport
limitation occurred in our DSSC systems. The I–V curves of
ꢀ
Fig. 4 I–V cꢀurves of DSSCs based on DOBT-IV to DOBT-VI with Co(II)/
(III) and Iꢀ/I3 electrolytes under different incident light intensities (0
Sun, 0.1 Sun, 0.3 Sun, 0.5 Sun, 0.6 Sun, 0.8 Sun, 1.0 Sun).
ꢀ
DSSCs based on DOBT-IV to VI with Co(II)/(III) and Iꢀ/I3 elec-
trolytes were measured under different incident light intensities
(Fig. 4). In addition, the behavior of other parameters, such as
Voc, FF and the PCE, was studied to give an overview of how the
performance varies with the incident light intensity (Fig. 5).
According to Tsao et al.,28 a linear increase in Jsc with light
intensity can indicate no mass transport problem in the system.
From the photocurrent behaviour, we found that the photo-
currents of DSSCs with the Co(II)/(III) and Iꢀ/I3ꢀ electrolytes are
both in a direct ratio with the incident light density (Ii). The
similar trends of their photocurrent behavior and the incident
light density indicate that the mass transport limitation of the
Co(II)/(III) redox couples does not seem to be a problem in our
system. In addition, for DSSCs with the Iꢀ/I3ꢀelectrolyte, the Jsc
of DSSCs based on DOBT-V are much higher than that of DSSCs
based on DOBT-IV and DOBT-VI, between which there is almost
no difference. On the other hand, for DSSCs with the Co(II)/(III)
electrolyte, the Jsc decreases in the order Jsc (DOBT-V) >
Jsc(DOBT-VI) > Jsc (DOBT-IV).
the Co(II)/(III) electrolyte and their counterparts with the Iꢀ/I3
ꢀ
electrolyte are larger than 0.1 V, which is surprising because our
sensitizers do not have blocking groups such as alkyl chains on
the donor moiety, but still perform well with the Co(II)/(III)
As for the photo-voltage behavior, in general, the photo-
voltage increases slowly with light intensity. As a result of the
benet of the more positive redox potential of Co complexes,
high open current voltages were obtained. At an illumination
intensity of 1 Sun, Voc values of 0.774, 0.760 and 0.713 V were
obtained for DSSCs based on DOBT-IV to DOBT-VI, compared
with the corresponding 0.686, 0.662 and 0.ꢀ629 V for DSSCs
based on DOBT-IV to DOBT-VI with the Iꢀ/I3 electrolyte. It is
Fig. 5 Behavior of performance parameters with dependence of
incident light intensity of DSSCs based on DOBT-IV to DOBT-VI with
notable that the gaps between the photo-voltages of DSSCs with Co(II)/(III) and Iꢀ/I3ꢀ electrolytes.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 4063–4072 | 4067

View Article Online
Journal of Materials Chemistry C
Paper
electrolyte. This means the alkyl chains on the auxiliary accep-
The electron lifetimes were calculated according to the Bode
tors could retard the charge recombination to some extent, but plots using se ¼ 1/(2pf), in which f represents the frequency of
not enough. The charge recombination in DSSCs with the Co(II)/ the highest peak. With the Co(II)/(III) electrolyte, the electron
ꢀ
(III) electrolyte is still much more severe than that in the Iꢀ/I3
lifetimes for DSSCs based on DOBT-IV to DOBT-VI are 8.06, 5.13
electrolyte, which can be proved not only by EIS, but also by and 0.82 ms, respectively. In contrast, the electron lifetimes for
IMVS, as discussed in the following sections.
DSSCs based on DOBT-IV to DOBT-VI with the Iꢀ/I3ꢀ electrolyte
With respect to FF, there are different changing patternꢀs are much higher, reaching 16.58, 11.70 and 5.35 ms, respec-
between DSSCs with the Co(II)/(III) and those with the Iꢀ/I3
tively. The simulated results obtained usingꢀthe equivalent
ꢀ
electrolyte. For DSSCs with the Iꢀ/I3 electrolyte, the FF values circuit for R(QR)(QR) for DSSCs with the Iꢀ/I3 and Co(II)/(III)
decrease gradually with the incident light intensity, while the FF electrolytes are given in Table S3.†
values of DSSCs with the Co(II)/(III) electrolyte increased
The electron lifetime sn, which represents the response time
dramatically in the range 0.1 Sun to 0.3 Sun, then gradually constant for recombination, is the product of a recombination
decreased with increasing light intensity in the order: FF resistance and a capacitance.29 It can be written as sn ¼ RrecC,
(DOBT-IV) > FF (DOBT-V) > FF (DOBT-IV). The reason for this is where C is the capacitance at the TiO2/dye/electrolyte interface.
not yet clear. Despite this, the overall PCEs at different light The relationship between the electron lifetime in DSSCs and
intensities vary in a similar way, climbing up and down twice, applied biases is given in Fig. 7, which was obtained from
with peaks at the incident light intensities of 0.3162 Sun and Nyquist plots measured under different biases in the dark.
0.7943 Sun. All the systems exhibit the same order of PCE According to Fig. 7, the lifetime gures for DSSCs based on
(DOBT-V) > PCE (DOBT-IV) > PCE (DOBT-VI).
DOBT-IV are the longest in both electrolytes. In contrast, DSSCs
based on DOBT-VI always have the shortest eꢀlectron lifetime.
The electron lifetimes in DSSCs with the Iꢀ/I3 electrolyte are
higher than those with the Co(II)/(III) electrolyte. This strongly
suggests that there are serious charge recombination problems
in the Co(II)/(III) electrolyte system and DSSCs based on DOBT-VI.
Electrochemical impedance spectrometry
EIS was used to help understand the differences in Voc. Fig. 6
shows typical Nyquist and Bode plots, which were obtained by
applying a ꢀ0.70 V bias in the dark in the frequency range 0.1–
100 Hz. Rs, RCE and Rres, which represent the series impedance,
the charge transfer impedance at the interface of the Pt/elec-
trolyte and the interface of the TiO2/dye/electrolyte, are in
accordance with the intercept on the horizontal axis and the
radii of the rst and second semicircles in the Nyquist plot,
respectively. In general, the large Rres value will retard charge
recombination at the interface of the TiO2/dye/electrolyte,
leading to an increasing concentration of electrons in the
semiconductor. This results in the rise of the Fermi energy level
of TiO2, which will enhance Voc. It is thus easy to understand the
high Voc of DSSCs based on DOBT-IV as this has the largest Rres
value, no matter which electrolyte is used.
Intensity-modulated photo-voltage spectrometry
As the current in EIS is supplied by an external voltage
source, we also used IMVS to further investigate the band
edge movement and recombination kinetics in DSSCs, in
which the current is generated by the adsorbed dye on the
TiO2 surface.30
Using a small sinusoidal perturbation of the light intensity,
the periodic response of the photo-voltage was measured. As the
IMVS response is determined by the electron lifetime under
open circuit conditions, it gives a reliable insight into electron
recombination and changes in Voc. Fig. 8 shows that the DSSCs
with the Co(II)/(III) electrolyte have much shorter electron life-
times than those with Iꢀ/I3ꢀ electrolyte, further conrming the
results of EIS. Under the open current condition, there are
larger dark currents in the DSSCs with the Co(II)/(III) electrolyte,
which are unfavorable and lower the Fermi level (EF,n) in the
dark. They also follow the order sn (DOBT-IV) > sn (DOBT-V) > sn
(DOBT-VI), which is in accordance with their Voc.
Fig. 6 Nyquist and Bode plots of DSSCs based on DOBT-IV to DOBT- Fig. 7 Electron lifetime as a funcꢀtion of applied bias in DSSCs based on
VI with Iꢀ/I3ꢀ and Co(II)/(III) electrolytes, all measured applying a bias of DOBT-IV to DOBT-VI with Iꢀ/I3 and Co(II)/(III) electrolytes, obtained
ꢀ0.70 V in the dark.
by EIS under various biases in the dark.
4068 | J. Mater. Chem. C, 2014, 2, 4063–4072
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
Experimental section
Instruments and characterization
13
1H-NMR and C-NMR spectra were obtained using a Brucker
AM 400 spectrometer. Mass spectra were measured using an ESI
instrument. A Varian-Cary 500 spectrophotometer was utilized
to measure the UV-visible spectra. A Versastat II electrochemical
workstation (Princeton Applied Research) was used to obtain
the cyclic voltammograms of the dyes with a three-electrode
system in which a Pt working electrode, a Pt wire counter
electrode and a regular calomel reference electrode in saturated
KCl solution were used. The lm thickness was measured using
a step proler (Veeco Dektak 150).
¨
Fig. 8 Electron lifetime derived from IMVS of DSSCs based on DOBT-
IV to DOBT-VI with the Iꢀ/I3ꢀ and Co(II)/(III) electrolytes.
Given the results of EIS and IMPS, we may speculate about
the reason why there is an enhanced Voc with the Co(II)/(III)
electrolyte, but a shorter electron lifetime than in devices with
The photovoltaic characterization was performed using a
450 W xenon lamp (Oriel), a Schott K113 Tempax sunlight lter
the Iꢀ/I3 electrolyte. It is known that Voc ¼ EF,n ꢀ Eredox
,
ꢀ
¨
(PrazisionsGlas & Optik GmbH) and a source meter (Keithley
where the EF,n is the Fermi energy level of TiO2 and Eredox is
the redox potential of the redox shuttles in the electrolyte.
Although the EF,n of the semiconductor layer in DSSCs with
2400), which applies the potential bias and measures the photo-
generated current. A set of circular variable neutral density
lters (Unice E-O Services Inc., Taiwan) was used to adjust the
light intensity. The incident light intensities used were 1.0, 0.8,
0.6, 0.5, 0.3 and 0.1 Sun. IPCE was obtained using a SR830 lock-
in amplier, a 300 W xenon lamp (ILC Technology) and a
Gemini-180 double monochromator (Jobin-Yvon Ltd). The EIS
measurements of the DSSCs were made using a Zahner IM6e
impedance analyzer (ZAHNER-Elektrik GmbH & CoKG, Kro-
nach, Germany). The frequency range was 0.1 Hz to 100 kHz.
The applied voltage bias was from ꢀ0.60 to ꢀ0.85 V. The
magnitude of the alternating signal was 5 mV. IMPS was carried
out using the Zahner IM6e impedance analyzer and a light-
emitting display array (l ¼ 457 nm, blue light). The frequency
range was 0.1 Hz to 100 kHz.
ꢀ
the Co(II)/(III) electrolyte are more positive than with the Iꢀ/I3
electrolyte as a result of the shorter electron lifetimes, which
indicate lower electron concentrations in TiO2, the differ-
ences between them are fairly small. Considering that the
Eredox of Co(II)/(III) redox couples (0.56 V vs. NHE) is much
more positive than that of the Iꢀ/I3 electrolyte (0.4 V vs.
ꢀ
NHE), the gaps between EF,n and Eredox can still be widened,
resulting in the large improvement in Voc
.
Conclusions
In the work reported here, we designed and synthesized three
D–A–p–A sensitizers (DOBT-IV to DOBT-VI) with bis(octyloxy)
benzo-[c][1,2,5]thiadiazole as the auxiliary acceptor. We used
N,N-diphenylthiophen-2-amine as the donor to increase the
Fabrication of DSSCs
absorption range and to obtain high Jsc values. This goal was For the DSSCs with Iꢀ/I3ꢀ electrolyte, three layers of Dyesol 90-T
fullled and, via ne tuning of the p-bridge, the highest PCE of TiO2 paste and a scattering layer were screen-printed onto the
7.16% was achieved with Jsc ¼ 16.88 mA cmꢀ2, Voc ¼ 0.662 V and FTO glass before sintered gradually up to 500 ꢂC. For the DSSCs
ꢀ
FF ¼ 64.03% for the DSSCs based on DOBT-V using the Iꢀ/I3
with the Co(II)/(III) electrolyte, pre-treatment of the FTO glass
electrolyte. To address the problem of low Voc, a Co(II)/(III) was carried out by immersing the FTO glass into 40 mM TiCl4
electrolyte was applied. The Voc values were signicantly aqueous solution at 70 ꢂC for 30 min three times before sin-
enhanced, but, owing to the large decrease in Jsc, the perfor- tering at 450 ꢂC for 30 min. Then one layer of Dyesol 90-T TiO2
mance was not further enhanced and the highest PCE was paste and a scattering layer were screeꢂn-printed onto the FTO
6.14% with Jsc ¼ 11.35 mA cmꢀ2, Voc ¼ 0.760 V and FF ¼ 71.16% glass and sintered gradually up to 500 C. Aer post-treatment
for the DSSCs based on DOBT-V. Density functional theory with 40 mM TiCl4 aqueous solution ꢂat 70 ꢂC for 30 min, the
calculations were used to explain the differences in optical and photo-anodes were calcinated at 450 C for 30 min and cooled
ꢀ4
ꢂ
electrochemical properties among the three dyes induced by down to 80 C before a 3 ꢁ 10 M dye bath in CH3CH2OH–
changing the p-bridge. A behavior study of the four perfor- CHCl3 (7 : 3 v/v) solution. The dye bath lasted for 14 h for DSSCs
mance parameters provided an overview of their variation with with the Iꢀ/I3ꢀ electrolyte and for 6.5 h for DSSCs with the Co(II)/
ꢀ
incident light intensity when the Co(II)/(III) and Iꢀ/I3 electro- (III) electrolyte. Aer rinsing with ethanol, the dye-sensitized
lytes were used. EIS and IMVS were used to prove the more photo-anodes were sealed with platinized counter electrodes
severe charge recombination in DSSCs with the Co(II)/(III) elec- using 25 mm thick Surlyn (Dupont) as a binder and a spacer. The
ꢀ
trolyte than with the Iꢀ/I3 electrolyte and the order of Voc(- electrolytes were introduced to the cells via two pre-drilled holes
ꢀ
DOBT-IV) > Voc(DOBT-V) > Voc(DOBT-VI). The results show that in the counter electrodes. The composition of the Iꢀ/I3 elec-
the alkyl chains on the auxiliary acceptors could retard charge trolyte is as follows: 0.05 M I2, 0.1 M LiI, 0.5 M 1,2-dimethyl-3-
recombination to some extent. Further improvement could propylimidazolium iodide and 0.75 M 4-tert-butylpyridine (tBP)
focus on adding blocking groups such as alkyl chains to the in acetonitrile. The composition of the Co(II)/(III) electrolyte is as
donor moiety to enhance their performance in DSSCs.
follows: 0.22 M tris(2,20-bipyridine)cobalt(II) bis(hexauoro-
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 4063–4072 | 4069

View Article Online
Journal of Materials Chemistry C
Paper
phosphate) ([Co(bpy)3](PF6)2), 0.05 M tris(2,20-bipyridine)cobalt- mixture was cooled down to room temperature and the solvent
(III)tris(hexauorophosphate) ([Co(bpy)3](PF6)3), 0.1 M LiClO4 evaporated. Aer extraction with CH2Cl2 and water, the organic
and 0.2 M tBP in acetonitꢀrile. The thicknesses of the TiO2 lm layers were combined and dried with anhydrous MgSO4. The
for DSSCs with the Iꢀ/I3 and the Co(II)/(III) electrolyte were solvent was removed under reduced pressure. The residue was
8.3 and 4.8 mm, respectively. The active area of all DSSCs puried by chromatography with PE–CH2Cl2 (1 : 1, v/v) as the
was 0.25 cm2.
eluent to obtain 200 mg of red solid. The yield was 67%. 1H-NMR
(400 MHz, CDCl3): d 10.10 (s, 1H), 8.41 (d, J ¼ 4.4 Hz, 1H), 8.02
(d, J ¼ 8.4 Hz, 2H), 7.91 (d, J ¼ 8 Hz, 1H), 7.25–7.33 (m, 8H), 7.06–
7.11 (m, 2H), 6.75 (d, J ¼ 4 Hz, 2H), 4.12 (q, J ¼ 6.8 Hz, 2H), 3.82
(q, J ¼ 6.8 Hz, 2H), 1.77–1.85 (m, 2H), 1.46–1.53 (m, 2H), 1.25–
1.34 (m, 20H), 0.84–0.91 (m, 6H).
5-(7-(5-(Diphenylamino)thiophen-2-yl)-5,6-bis(octyloxy)benzo-
[c]-[1,2,5]thiadiazol-4-yl)furan-2-carbaldehyde (5). Compound 5
was synthesized in a similar way to compound 4 by replacing
(4-formylphenyl)boronic acid with (5-formylfuran-2-yl)boronic
acid. The residue was puried by chromatography with PE–
CH2Cl2 (1 : 1, v/v) as the eluent to obtain 224 mg of purplish red
solid. The yield was 70%. 1H-NMR (400 MHz, CDCl3): d 9.76
(s, 1H), 8.48 (d, J ¼ 4 Hz, 1H), 7.63 (d, J ¼ 4 Hz, 1H), 7.44 (d, J ¼ 4
Materials and reagents
Fluorine-doped SnO2 conducting glass (FTO glass, transparency
>90%, sheet resistance 15 U sqꢀ1) was obtained from Geao
Science and Educational Co. Ltd (China). Acetonitrile, tetra-n-
butyl ammonium hexauorophosphate (TBAPF6), 4-tert-butyl-
pyridine and LiI were bought from Fluka and iodine (99.999%)
was purchased from Alfa Aesar. All chemicals and reagents were
purchased from suppliers and used without further purica-
tion. Tetrahydrofuran (THF) was dried with sodium before use.
Synthesis
N,N-Diphenyl-5-(tributylstannyl)thiophen-2-amine (2). N,N- Hz, 1H), 7.27–7.34 (m, 8H), 7.10 (q, J ¼ 7.2 Hz, 2H), 6.73 (d, J ¼
Diphenylthiophen-2-amine (2.51 g, 10 mmol) was dissolved in 4.4 Hz, 1H), 4.06–4.15 (m, 4H), 1.88–1.96 (m, 2H), 1.76–1.84
10 ml of anhydrous THF. The solution was cooled down to (m, 2H), 1.23–1.38 (m, 20H), 0.86–0.91 (m, 6H).
ꢀ78 ꢂC under argon and then 1.6 M n-butyllithium in n-hexane
5-(7-(5-(Diphenylamino)thiophen-2-yl)-5,6-bis(octyloxy)benzo-
(6).
(7.5 ml, 12 mM) was added dropwise and stirred for 1 h. Aer [c]-[1,2,5]thiadiazol-4-yl)thiophene-2-carbaldehyde
reaction at room temperature for 30 min, the mixture was Compound 6 was synthesized in a similar way to compound 4 by
cooled to ꢀ78 ꢂC and stirred for another 30 min. Tributyltin replacing (4-formylphenyl)boronic acid with (5-formylthiophen-
chloride was quickly injected into the mixture and stirred for 2 h 2-yl)boronic acid. The residue was puried by chromatography
ꢂ
at ꢀ78 C, followed by another 2 h at room temperature. Salt with PE–CH2Cl2 (1 : 1, v/v) as the eluent to obtain 160 mg of
water was then added to quench the reaction. The crude purplish red solid. The yield was 53%. 1H-NMR (400 MHz,
product was extracted with CH2Cl2 and dried with anhydrous CDCl3): d 10.00 (s, 1H), 8.66 (d, J ¼ 4 Hz, 1H), 8.44 (d, J ¼ 4.4 Hz,
MgSO4. Then 20 ml of dimethylformamide (DMF) was added to 1H), 7.85 (d, J ¼ 4 Hz, 1H), 7.27–7.34 (m, 8H), 7.10 (q, J ¼ 7.2 Hz,
produce a 0.5 mM mlꢀ1 brown solution.
5-(7-Bromo-5,6-bis(octyloxy)benzo[c][1,2,5]thiadiazol-4-yl)-N,N- 7.2 Hz, 2H), 1.87–1.95 (m, 2H), 1.77–1.85 (m, 2H), 1.25–1.43 (m,
diphenylthiophen-2-amine (3). A mixture of 4,7-dibromo-5,6- 20H), 0.86–0.91 (m, 6H).
2H), 6.74 (d, J ¼ 4.4 Hz, 1H), 4.14 (q, J ¼ 7.2 Hz, 2H), 4.06 (q, J ¼
bis(octyloxy)benzo[c][1,2,5]thiadiazole (275 mg, 0.5 mM) and tet-
(E)-2-Cyano-3-(4-(7-(5-(diphenylamino)thiophen-2-yl)-5,6-
rakis(triphenylphosphine) palladium [Pd(PPh3)4] (20 mg, 0.018 bis(octyloxy)benzo[c][1,2,5]thiadiazol-4-yl)phenyl)acrylic acid
mM) in 20 ml of DMF was heated and reuxed under argon for 15 (DOBT-IV). A mixture of compound 4 (128 mg, 0.17 mM), cya-
min. Aer compound 2 in DMF (270 mg, 0.5 mM) had been noacetic acid (102 mg, 1.2 mM), ammonium acetate (120 mg)
added, the mixture was reuxed for 20 h before cooling down to and acetic acid (14 ml) was reuxed under argon for 20 h before
room temperature, at which time the solution was red. The cooling down to room temperature. The mixture was then
mixture was poured into salt water and stirred to precipitate the poured into salt water to precipitate the crude product before
crude product, which was then ltered and washed with salt extraction with CH2Cl2 and water. The combined organic phase
water, followed by extraction with CH2Cl2 and water. The was dried with anhydrous MgSO4 and the solvent was removed
combined organic phase was dried with anhydrous MgSO4 and under reduced pressure. The crude product was puried by
the solvent was removed under reduced pressure. The crude chromatography with CH2Cl2–CH3CH2OH (100 : 6, v/v) as the
product was puried by chromatography with petroleum ether eluent to obtain 114.6 mg of dark red solid. The yield was 83%.
(PE)–CH2Cl2 (5 : 1, v/v) as the eluent to obtain 145 mg of red solid. 1H-NMR (400 MHz, DMSO-d6): d 8.38 (s, 1H), 8.24 (s, 1H), 8.09
The yield was 40%. 1H-NMR (400 MHz, CDCl3): d 8.25 (d, J ¼ 4 Hz, (d, J ¼ 3.6 Hz, 1H), 7.80–7.77 (m, 2H), 7.34–7.41(m, 4H), 7.19 (d,
1H), 7.20–7.25 (m, 4H), 7.16–7.19 (m, 2H), 6.98–7.03 (m, 2H), 4.06 J ¼ 7.2 Hz, 4H), 7.14–7.11 (m, 2H), 6.73–6.69 (m, 1H), 4.04 (m, J
(q, J ¼ 6.8 Hz, 2H), 3.97 (q, J ¼ 7.2 Hz, 2H), 1.77–1.87 (m, 2H), ¼ 4.4 Hz, 2H), 3.77 (m, J ¼ 4.4 Hz, 2H), 1.69 (d, J ¼ 3.2 Hz, 2H),
1.65–1.74 (m, 2H), 1.18–1.31 (m, 20H), 0.80–0.84 (m, 6H).
1.43–1.41 (m, 2H), 1.32–1.21 (m, 20H), 0.78–0.85 (m, 6H). 13C-
4-(7-(5-(Diphenylamino)thiophen-2-yl)-5,6-bis(octyloxy)benzo- NMR (100 MHz, DMSO-d6), 163.4, 153.7, 152.9, 151.4, 151.1,
[c]-[1,2,5]thiadiazol-4-yl)benzaldehyde (4). 2 M K2CO3 (aqueous, 149.6, 149.5, 146.9, 137.2, 131.4, 130.5, 129.6, 129.5, 124.9,
5 ml), Pd(PPh3)4 (20 mg, 0.018 mM) and compound 3 (240 mg, 123.9, 123.3, 123.1, 122.4, 121.5, 117.8, 117.2, 116.9, 114.5, 74.0,
0.4 mM) in 20 ml of THF were heated and reuxed for 15 min 73.7, 31.2, 31.1, 29.6, 29.4, 29.0, 28.8, 28.6, 28.5, 25.4, 25.3, 22.0,
under argon before the solution (4-formylphenyl)boronic acid 13.9. HRMS (ESI): m/z calculated for C48H52N4O4S2: 811.3352;
(75 mg, 0.5 mM) in THF was added. Aer reuxing for 20 h, the found 811.3349.
4070 | J. Mater. Chem. C, 2014, 2, 4063–4072
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
(Z)-2-Cyano-3-(5-(7-(5-(diphenylamino)thiophen-2-yl)-5,6-
Journal of Materials Chemistry C
3941;
J.
H.
Yum,
E.
Baranoff,
S.
Wenger,
¨
M. K. Nazeeruddin and M. Gratzel, Energy Environ. Sci.,
2011, 4, 842; L. Y. Han, A. Islam, H. Chen, C. Malapaka,
B. Chiranjeevi, S. F. Zhang, X. D. Yang and M. Yanagida,
Energy Environ. Sci., 2012, 5, 6057.
bis(octyloxy)benzo[c][1,2,5]thiadiazol-4-yl)furan-2-yl)acrylic acid
(DOBT-V). DOBT-V was synthesized in a similar way to DOBT-IV
by compound 4 with compound 5. The residue was puried by
chromatography with CH2Cl2–CH3CH2OH (100 : 6, v/v) as the
eluent to get 175 mg deep, dark red solid. The yield was 78%. 1H
NMR (400 MHz, DMSO-d6): d 8.44 (d, J ¼ 4.4 Hz, 1H), 7.98 (s,
1H), 7.61 (d, J ¼ 3.6 Hz, 1H), 7.50 (d, J ¼ 3.6 Hz, 1H), 7.38 (q, J ¼
7.2 Hz, 4H), 7.21 (d, J ¼ 7.2 Hz, 4H), 7.15 (q, J ¼ 7.2 Hz, 2H), 6.69
(d, J ¼ 4 Hz, 1H), 3.98–4.04 (m, J ¼ 7.2 Hz, 2H), 1.68–1.75 (m,
4H), 1.22–1.36 (m, 20H), 0.81–0.87 (m, 6H). 13C NMR (100 MHz,
DMSO-d6), 175.7, 175.1, 163.8, 160.1, 159.6, 156.0, 153.7, 152.4,
150.7, 148.7, 148.0, 146.5, 132.4, 132.2, 132.1, 131.1, 130.4,
130.0, 129.7, 125.1, 124.0, 120.1, 114.3, 114.1, 67.5, 31.2, 29.0,
28.7, 28.6, 25.5, 22.1, 13.9. HRMS (ESI): m/z calcd for
5 S. M. Feldt, E. A. Gibson, E. Gabrielsson, L. Sun, G. Boschloo
and A. Hagfeldt, J. Am. Chem. Soc., 2010, 132, 16714;
J. H. Yum, E. Baranoff, F. Kessler, T. Moehl, S. Ahmad,
T. Bessho, A. Marchioro, E. Ghadiri, J. E. Moser, C. Y. Yi,
¨
M. K. Nazeeruddin and M. Gratzel, Nat. Commun., 2012, 3,
2041; P. Salvatori, G. Marotta, A. Cinti, E. Mosconi,
M. Panigrahi, L. Giribabu, M. K. Nazeeruddin and F. De
Angelis, Inorg. Chim. Acta, 2013, 406, 106; K. B. Aribia,
¨
T. Moehl, S. M. Zakeeruddinand and M. Gratzel, Chem.
Sci., 2013, 4, 454.
6 M. Liberatore, A. Petrocco, F. Caprioli, C. La Mesa, F. Decker
and C. A. Bignozzi, Electrochim. Acta, 2010, 55, 4025;
X. P. Zong, M. Liang, C. R. Fan, K. Tang, G. Li, Z. Sun and
S. Xue, J. Phys. Chem. C, 2012, 116, 11241; C. J. Qin,
W. Q. Peng, K. Zhang, A. Islam and L. Y. Han, Org. Lett.,
2012, 14, 2532; P. Gao, Y. J. Kim, J. H. Yum,
C
46H50N4O5S2: 801.3144; found 801.3146.
(Z)-2-Cyano-3-(5-(7-(5-(diphenylamino)thiophen-2-yl)-5,6-
bis(octyloxy)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)acrylic
acid (DOBT-VI). DOBT-VI was synthesized in a similar way to
DOBT-IV by compound 4 with compound 6. The residue was
puried by chromatography with CH2Cl2–CH3OH (100 : 8, v/v)
as the eluent to obtain 71.5 mg of a deep, dark red solid. The
¨
T. W. Holcombe, M. K. Nazeeruddin and M. Gratzel,
1
J. Mater. Chem. A, 2013, 1, 5535; A. N. Cai, Y. L. Wang,
M. F. Xu, Y. Fan, R. Z. Li, M. Zhang and P. Wang, Adv.
Funct. Mater., 2013, 23, 1846.
yield was 72%. H-NMR (400 MHz, DMSO-d6): d 10.75 (s, 1H),
8.60 (s, 1H), 8.46 (s, 1H), 7.60 (s, 1H), 7.40 (s, 2H), 7.11–7.16
(m, 4H), 7.00 (d, J ¼ 7.6 Hz, 4H), 6.90–6.93 (m, 4H), 4.07 (s, 2H),
3.66 (s, 2H), 1.98 (s, 2H), 1.14–1.27 (m, 22H), 0.74 (m, 6H). 13C-
NMR (100 MHz, DMSO-d6), 152.7, 152.6, 150.1, 147.6, 137.7,
131.9, 129.2, 126.9, 124.6, 123.1, 121.9, 116.2, 74.2, 74.1, 31.9,
30.1, 30.0, 29.7, 29.5, 29.4, 29.3, 26.1, 26.0, 22.6, 13.6, 13.5.
HRMS (ESI): m/z calculated for C46H50N4O4S3: 817.2916; found
817.2914.
7 J. Liu, X. D. Yang, A. Islam, Y. Numata, S. F. Zhang,
N. T. Salim, J. Chen and L. Y. Han, J. Mater. Chem. A, 2013,
1, 10889; X. D. Yang, S. F. Zhang, K. Zhang, J. Liu,
C. J. Qin, H. Chen, A. Islam and L. Y. Han, Energy Environ.
Sci., 2013, 6, 3637; J. Liu, Y. Numata, C. J. Qin, A. Islam,
X. D. Yang and L. Y. Han, Chem. Commun., 2013, 49, 7587;
Y. Bai, J. Zhang, D. F. Zhou, Y. H. Wang, M. Zhang and
P. Wang, J. Am. Chem. Soc., 2011, 133, 11442; X. F. Lu,
Q. Y. Feng, T. Lan, G. Zhou and Z. S. Wang, Chem. Mater.,
2012, 24, 3179.
8 Y. P. Hong, J. Y. Liao, D. R. Cao, X. F. Zang, D. B. Kuang,
L. Y. Wang, H. Meier and C. Y. Su, J. Org. Chem., 2011, 76,
8015; Y. Hao, X. C. Yang, M. Z. Zhou, J. Y. Cong,
X. N. Wang, A. Hagfeldt and L. C. Sun, ChemSusChem,
2011, 4, 1601; Y. Ooyama, Y. Shimada, S. Inoue, T. Nagano,
Y. Fujikawa, K. Komaguchi, I. Imae and Y. Harima, New J.
Chem., 2011, 35, 111; S. Haid, M. Marszalek, A. Mishra,
M. Wielopolski, J. Teuscher, J.-E. Moser, R. Humphry-
Acknowledgements
This work was supported by NSFC/China (2116110444,
21172073, 91233207 and 21372082) and the National Basic
Research 973 Program (2013CB733700).
Notes and references
¨
1 B. O'Reganand and M. Gratzel, Nature, 1991, 343, 737.
2 J. Burschka, N. Pellet, S. J. Moon, R. Humphry-Baker, P. Gao,
¨
M. K. Nazeeruddin and M. Gratzel, Nature, 2013, 499, 316;
¨
¨
M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501,
395.
3 S. Mathew, A. Yella, P. Gao, R. Humphry-Baker,
B. F. E. Curchod, N. Ashari-Astani, I. Tavernelli,
Baker, S. M. Zakeeruddin, M. Gratzel and P. Bauerle, Adv.
Funct. Mater., 2012, 22, 1291; N. Cai, Y. L. Wang, M. F. Xu,
Y. Fan, R. Z. Li, M. Zhang and P. Wang, Adv. Funct. Mater.,
2013, 23, 1846.
¨
U. Rothlisberger, M. K. Nazeeruddin and M. Gratzel, Nat.
Chem., 2014, 6, 242.
9 Y. Z. Wu and W. H. Zhu, Chem. Soc. Rev., 2013, 42, 2039.
10 M. Velusamy, K. R. Justin Thomas, J. T. Lin, Y. C. Hsu and
K. C. Ho, Org. Lett., 2005, 7, 1899; W. H. Zhu, Y. Z. Wu,
S. T. Wang, W. Q. Li, X. Li, J. Chen, Z. S. Wang and
H. Tian, Adv. Funct. Mater., 2011, 21, 756; Y. Z. Wu,
X. Zhang, W. Q. Li, Z. S. Wang, H. Tian and W. H. Zhu,
Adv. Energy Mater., 2012, 2, 149; M. Zhang, Y. L. Wang,
M. F. Xu, W. T. Ma, R. Z. Li and P. Wang, Energy Environ.
Sci., 2013, 6, 2944.
4 A. Yella, H. W. Lee, H. N. Tsao, C. Y. Yi, A. K. Chandiran,
M. K. Nazeeruddin, E. W. G. Diau, C. Y. Yeh,
S. M. Zakeeruddin and M. Gratzel, Science, 2011, 334, 629;
C. M. Lan, H. P. Wu, T. Y. Pan, C. W. Chang, W. S. Chao,
C. T. Chen, C. L. Wang, C. Y. Lin and E. W. G. Diau, Energy
Environ. Sci., 2012, 5, 6460; A. K. Chandiran, N. Tetreault,
R. Humphry-Baker, F. Kessler, E. Baranoff, C. Y. Yi,
¨
¨
M. K. Nazeeruddin and M. Gratzel, Nano Lett., 2012, 12,
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 4063–4072 | 4071

View Article Online
Journal of Materials Chemistry C
Paper
11 Y. Cui, Y. Z. Wu, X. F. Lu, X. Zhang, G. Zhou, F. B. Miapeh, 18 L. Chen, X. Li, W. J. Ying, X. Y. Zhang, F. L. Guo, J. Li and
W. H. Zhu and Z. S. Wang, Chem. Mater., 2011, 23, 4394; J. L. Hua, Eur. J. Org. Chem., 2013, 9, 1770.
J. Y. Mao, F. L. Guo, W. J. Ying, W. J. Wu, J. Li and 19 L. Y. Lin, C. H. Tsai, F. Lin, T. W. Huang, S. H. Chou,
J. L. Hua, Chem.–Asian J., 2012, 7, 982.
C. C. Wu and K. T. Wong, Tetrahedron, 2012, 68, 7509;
Y. C. Chen, H. H. Chou, M. C. Tsai, S. Y. Chen, J. T. Lin,
C. F. Yao and K. Chen, Chem.–Eur. J., 2012, 18, 5430;
L. Y. Lin, C. H. Tsai, K. T. Wong, T. W. Huang, C. C. Wu,
S. H. Chou, F. Lin, S. H. Chen and A. I. Tsai, J. Mater.
Chem., 2011, 21, 5950; S. W. Chiu, L. Y. Lin, H. W. Lin,
Y. H. Chen, Z. Y. Huang, Y. T. Lin, F. Lin, Y. H. Liu and
K. T. Wong, Chem. Commun., 2012, 48, 1857.
12 D. W. Chang, H. J. Lee, J. H. Kim, S. Y. Park, S. M. Park, L. Dai
and J. B. Baek, Org. Lett., 2011, 13, 3880; K. Pei, Y. Z. Wu,
W. J. Wu, Q. Zhang, B. Q. Chen, H. Tian and W. H. Zhu,
Chem.–Eur. J., 2012, 18, 8190; X. F. Lu, Q. Y. Feng, T. Lan,
G. Zhou and Z. S. Wang, Chem. Mater., 2012, 24, 3179;
K. Pei, Y. Z. Wu, A. Islam, Q. Zhang, L. Y. Han, H. Tian
and W. H. Zhu, ACS Appl. Mater. Interfaces, 2013, 5, 4986.
13 S. Y. Qu, W. J. Wu, J. L. Hua, C. Kong, Y. T. Long and H. Tian, 20 M. Liang and J. Chen, Chem. Soc. Rev., 2013, 42, 3453.
´
`
J. Phys. Chem. C, 2010, 114, 1343; S. Y. Qu, B. Wang, 21 J. Preat, C. Michaux, J. M. Andre and E. A. Perpete, Int.
F. L. Guo, J. Li, W. J. Wu, C. Kong, Y. T. Long and J. Quantum Chem., 2012, 112, 2072.
J. L. Hua, Dyes Pigm., 2012, 92, 1384; S. Y. Qu, C. J. Qin, 22 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
A. Islam, Y. Z. Wu, W. H. Zhu, J. L. Hua, H. Tian and
L. Y. Han, Chem. Commun., 2012, 48, 6972; J. H. Yum,
T. W. Holcombe, Y. J. Kim, K. Rakstys, T. Moehl,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, et al., Gaussian 09, Revision
A.2, Gaussian Inc., Wallingford CT, 2009.
J. Teuscher, J. H. Delcamp, M. K. Nazeeruddin and 23 A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
¨
M. Gratzel, Sci. Rep., 2013, 3, 2446.
24 W. J. Hehre, R. Ditcheld and J. A. Pople, J. Chem. Phys.,
1972, 56, 2257.
14 W. J. Ying, J. B. Yang, M. Wielopolski, T. Moehl, J. E. Moser,
P. Comte, J. L. Hua, S. M. Zakeeruddin, H. Tian and 25 A. D. Becke, J. Chem. Phys., 1996, 104, 1040.
¨
M. Gratzel, Chem. Sci., 2014, 5, 206.
26 J. Tomasi, B. Mennucci and R. Cammi, Chem. Rev., 2005,
105, 2999.
15 Q. Y. Feng, X. F. Lu, G. Zhou and Z. S. Wang, Phys. Chem.
Chem. Phys., 2012, 14, 7993; Q. Y. Feng, W. Y. Zhang, 27 J. X. He, F. L. Guo, X. Li, W. J. Wu, J. B. Yang and J. L. Hua,
G. Zhou and Z. S. Wang, Chem.–Asian J., 2013, 8, 168. Chem.–Eur. J., 2012, 18, 7903.
16 M. Velusamy, K. R. J. Thomas, J. T. Lin, Y. C. Hsu and 28 H. N. Tsao, P. Comte, C. Y. Yi and M. Gratzel,
K. C. Ho, Org. Lett., 2005, 7, 1899; J. J. Kim, H. Choi, ChemPhysChem, 2012, 13, 2976.
J. W. Lee, M. S. Kang, K. Song, S. O. Kang and J. Ko, J. 29 J. Bisquert, F. Fabregat-Santiago, I. Mora-Sero, G. Garcia-
¨
´
´
Mater. Chem., 2008, 18, 5223.
Belmonte and S. Gimenez, J. Phys. Chem. C, 2009, 113,
17 W. H. Zhu, Y. Z. Wu, S. T. Wang, W. Q. Li, X. Li, J. Chen,
17278.
¨
Z. S. Wang and H. Tian, Adv. Funct. Mater., 2011, 21, 756; 30 G. Schlichthorl, S. Y. Huang, J. Sprague and A. J. Frank, J.
¨
Phys. Chem. B, 1997, 101, 8141; J. Kruger, R. Plass,
Y. Z. Wu, M. Marszalek, S. M. Zakeeruddin, Q. Zhang,
¨
¨
H. Tian, M. Gratzel and W. H. Zhu, Energy Environ. Sci.,
M. Gratzel, P. J. Cameron and L. M. Peter, J. Phys. Chem. B,
2012, 5, 8261; Y. Wu, X. Zhang, W. Li, Z. S. Wang, H. Tian
and W. H. Zhu, Adv. Energy Mater., 2012, 2, 149.
2003, 107, 7536; J. van de Lagemaat, N. G. Park and
A. J. Frank, J. Phys. Chem. B, 2000, 104, 2044.
4072 | J. Mater. Chem. C, 2014, 2, 4063–4072
This journal is © The Royal Society of Chemistry 2014