Novel organic dyes incorporating a carbazole or dendritic 3,6-diiodocarbazole unit for efficient dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2013.09.025

Four organic photosensitizers incorporating a carbazole or 3,6-diiodocarbazole unit as the electron donor, a benzene/thiophene or oligothiophene moiety as the conjugated spacer, and 2-cyanoacrylic acid as the electron acceptor have been synthesized. The p

Full text of DOI:10.1016/j.dyepig.2013.09.025

Dyes and Pigments 100 (2014) 269e277
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel organic dyes incorporating a carbazole or dendritic
3,6-diiodocarbazole unit for efficient dye-sensitized solar cells
,
b
a
Li-Lin Tan a, Li-Jun Xie a, Yong Shen a, Jun-Min Liu a , Li-Min Xiao , Dai-Bin Kuang ,
*
,
Cheng-Yong Su a
*
a MOE Laboratory of Bioinorganic and Synthetic Chemistry/KLGHEI of Environment and Energy Chemistry,
State Key Laboratory of Optoelectronic Materials and Technologies, Lehn Institute of Functional Materials,
School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China
b School of Computer Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four organic photosensitizers incorporating a carbazole or 3,6-diiodocarbazole unit as the electron
donor, a benzene/thiophene or oligothiophene moiety as the conjugated spacer, and 2-cyanoacrylic acid
as the electron acceptor have been synthesized. The photovoltaic performance data are quite sensitive to
the structural modification of sensitizer. The introduction of the benzene/thiophene linker benefits from
lower tendency to aggregate, but disfavors the electron transport between donor and acceptor. The
addition of a thiophene unit in the bridge efficiently red-shifts the absorption response, however, organic
Received 24 June 2013
Received in revised form
13 September 2013
Accepted 15 September 2013
Available online 22 September 2013
dyes have some pep aggregation. Incorporation of 3,6-diiodocarbazole as a dendritic donor unit not only
enhances the molar extinction coefficients of the absorption but also suppresses the charge recombi-
nation with electrolyte.
Keywords:
Organic dye
Carbazole
Dendritic 3,6-diiodocarbazole
Thiophene
Ó 2013 Elsevier Ltd. All rights reserved.
Dye-sensitized solar cells
Photovoltaic performance
1. Introduction
indoline [5], and triphenylamine [6] derivatives have been devel-
oped as promising candidates with the power conversion efficiency
Dye-sensitized solar cells (DSSCs) based on nanocrystalline TiO2
have attracted significant attention as low-cost photovoltaic de-
vices due to their high conversion efficiencies over 11% in standard
air mass 1.5 and good stability [1]. In these cells as one of the key
components ruthenium dyes are used as light absorbers. But the
high cost of ruthenium, the necessity of purification treatments,
and the low molar extinction coefficients make the research on
alternative, metal-free organic dyes appealing for their application
in large DSSC modules [2]. Organic photosensitizers have the
advantage of high extinction coefficients in the visible region [3].
Also, the position and intensity of the charge transfer transition in
organic dyes can be tuned by simple structural modifications such
as variation of donor strength and nature of the conjugation
pathway [3b]. Therefore, enormous progress has been made in this
field, and several organic photosensitizers based on coumarin [4],
closing to ruthenium dyes [3b,7]. Although organic photosensi-
tizers exhibit excellent spectral properties, they tend to form ag-
gregates on the semiconductor surface, resulting in self-quenching
of the dye excited stated. Another disadvantages of organic pho-
tosensitizers are low long-term stability and easy interfacial
recombination dynamics, thus leading to low open-circuit photo-
voltage. Many efforts have been made to design efficient organic
photosensitizers through structural modifications in order to pre-
vent the aggregation of dyes and to diminish the charge recombi-
nation between the electrons on TiO2 film and acceptors [3b,8]. A
successful approach was achieved by introducing more donor
segments to the primary donor, thereby forming donor-donor-
bridge-acceptor (D-D- -A) structures. Compared with the D- -A
structure constructed by extending -conjugated bonding bridges,
D-D- -A dyes benefit from lower tendency to aggregate and better
p
p
p
p
p
thermo-stability, and meanwhile their absorption regions can be
extended and molar extinction coefficients can be enhanced. Tian
et al. has reported a series of D-D-p-A structural organic dyes
incorporating several donor groups into the triphenylamine
framework with starburst configuration, resulting in the red-shift
* Corresponding authors. Tel./fax: þ86 20 84115178.
E-mail addresses: liujunm@mail.sysu.edu.cn (J.-M. Liu), cesscy@mail.sysu.edu.cn
(C.-Y. Su).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.09.025

270
L.-L. Tan et al. / Dyes and Pigments 100 (2014) 269e277
in absorption and the suppression of charge recombination with
electrolyte [9].
2. Experimental section
In this work, we report a novel class of organic photosensitizers,
containing carbazole or carbazole dendrimer donors and cyanoa-
2.1. General methods
crylic acid acceptors (DX1eDX4). They are linked via a
p
-bridge
All reagents were obtained from commercial sources and used as
received. Tetrahydrofuran (THF), methylbenzene (MB) and chloro-
form (CHCl3) was purified using MBRAUN MB SPS-800 system.
Other solvents were dried over sodium or calcium hydrides and
distilled before used. 1H NMR and 13C NMR spectra were recorded
on a Mercury-Plus 300FT-NMR spectrometer in DMSO-d6 or CDCl3,
respectively. IR spectra were obtained on a Thermo Scientific Nicolet
330 infrared spectrophotometer. MS data were obtained using an
LCQ DECA XP liquid chromatographymass spectrometry. UVevis
absorption spectra were measured using a Shimadzu UV-2450
spectrometer. Emission spectra were measured using Shimadzu
RF-5301PC spectrometer. Cyclic voltammograms (CV) were recor-
ded using a CHI 832 electrochemical analyzer with FTO/TiO2/Dye as
working electrode, Ag/AgCl as reference electrode, Pt wire as
counter electrode. CVs were measured with 0.1 M tetrabuty-
lammonium hexafluorophosphate (Bu4NPF6) as a supporting elec-
trolyte in CH2Cl2. Scan rate was kept as 50 mV sꢀ1 for all compounds.
The highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) energies of triarylamine organic
dyes obtained from the theoretical calculations were coupled with
the redox potential obtained from the CV measurements as shown
in main text. 4a [14] and 4c [15] were synthesized according to
constituted by benzene/thiophene or oligothiophene. Carbazole is a
common heterocyclic compound with interesting photo- and
electro-chemistries [10], and it is known as an OPC [11] (organic
photo conductor) or a hole-transporting material [12]. The incor-
poration of hole-conductors of carbazole moieties into organic dyes
as electron donors have been proved to exhibit supersensitized
effects by retarding interfacial charge-recombination dynamics and
thus achieving long-lived photoinduced charge separation [13].
Moreover, the dendritic carbazole, which has a rigid and highly
twisted starburst structure, shows unique functions in several de-
vices or systems that involve electron transfer [14]. Use of
the dendritic carbazole donor could be not only beneficial to high
molar extinction coefficients of the absorption but also helps to
suppress dark current upon appropriate incorporation of D-D-p-A
structure. We therefore set out to synthesize a novel dye (DX4)
consisting of a 3,6-Diiodocarbazole donor, a 2-cyanoacrylic acid
acceptor, and an oligothiophene spacer at the first time. With these
dyes as the photosensitizers, we fabricated corresponding DSSCs,
and found that the electro-optical properties of DX4 relative with
DX1e3 show significant improvements which manifests in the
overall efficiency for the DSSCs.
COOH
S
CHO
S
ii
iii
N
I
N
N
N
N
CN
1a
1b
DX1
NC
i
S
S
S
iii
COOH
CHO
N
N
S
S
S
i
2a
DX2
NH
S
S
S
i
iii
CHO
COOH
S
NC
N
DX3
3a
iv
v
N
N
N
I
I
N
N
N
vi
vii
N
S
i
iii
N
Boc
NH
S
S
S
S
S
OHC
4b
CN
4a
DX4
HOOC
4c
(i) Cu, K2CO3, 18-Crown-6, 1,2-dichlorobenzene; (ii) Pd(PPh3)4, NaCO3, THF+H2O;
(iii) piperidine, CHCl 3+CH3CN; (iv) KI , KIO3 , HAc; (v) DMAP, acetone (vi) CuI,
K3PO4, dioxane; (vii) TFA, water, anisole and toluene.
Scheme 1. Synthesis of carbazole -based dyes DX1, DX2, DX3, and DX4.

L.-L. Tan et al. / Dyes and Pigments 100 (2014) 269e277
271
50000
reported literature. 1-3a, 1b, 4b, and DX1e4 were prepared ac-
DX1
DX2
DX3
DX4
a
cording to the procedures listed in the Supporting Information.
40000
30000
20000
10000
0
2.2. Preparation of TiO2 electrode
The TiO2 electrode was prepared according to the reported
literature procedure [16].
2.3. Fabrication of DSSCs
TiO2 films with 15 mm in thickness were prepared by following
the literature procedure [17] and soaked in 40 mM TiCl4 aqueous
solution at 70 ꢁC, which improved the photocurrent and photo-
voltaic performance of DSSCs. After 30 min, the TiO2 films were
washed with water and ethanol and then sintered at 520 ꢁC for
30 min. After cooling to 80 ꢁC, the TiCl4 treated TiO2 electrodes
were immersed into 0.3 mM CH2Cl2 solution of the organic dye and
kept at room temperature for 16 h. The active area of the dye-
coated TiO2 film was 0.16 cm2, which was measured by pro-
filometer (AMBIOS, XP-1). The dye-sensitized TiO2/FTO films were
sandwiched together with Pt coated FTO glass which was used as a
counter electrode. Platinized counter electrodes were fabricated by
thermal deposition of H2PtCl6 solution (5 mM in isopropanol) onto
FTO glass at 400 ꢁC for 15 min. The electrolyte was injected into the
space between the sandwiched cells.
300
400
500
600
700
800
Wavelength (nm)
DX1
DX2
DX3
DX4
b
1.2
0.8
0.4
2.4. Dye loading measurements
The dye loading measurements on TiO2 films were carried out by
desorbing the dye into 0.8 mol/L triethylamine solution in CH2Cl2
and then measuring the ultravioletevisible absorption spectra of
the resultant solution with the same dilution. The adsorbed density
of each dye was calculated from the difference concentration of
each solution before and after TiO2 film immersion.
400
500
600
Wavelength (nm)
700
Fig. 1. (a) The absorption spectra of DX1, DX2, DX3, and DX4 in CH2Cl2 solution. (b)
The Absorption spectrum of photosensitizers DX1, DX2, DX3, and DX4 on TiO2 nano-
crystalline films.
2.5. Electrochemical impedance spectroscopy
carbazole and excess amount of 1,4-diiodobenzene in the presence
of Cu powder as a catalyst and K2CO3. Then, 1b was synthesized by
using the palladium-catalyzed Suzuki coupling reaction. The final
step to obtain DX1 was a Knoevenagel condensation with cyano-
acetic acid to convert carbaldehydes to cyanoacrylic acids. The dyes
DX2e3 were conveniently obtained from carbazole in three steps
involving a suitable phase transfer catalyzed N-alkylation, Suzuki
reaction to liberate the aldehyde, and Knoevenagel condensation
with cyanoacetic acid. The 3,6-Diiodocarbazole based organic dye
DX4 was synthesized in six steps. First, 9-Boc-3,6-diiodocarbazole
(I2BocCz) was prepared by the iodination of the carbazole fol-
lowed by Boc protection of the amine group. Next, I2BocCz and the
carbazole were reacted by the N-arylation reaction, and then the
mixture was deprotected with a mixture of toluene, trifluoroacetic
acid (TFA), water, and anisole to produce 4a. Then, 4b was syn-
thesized using Suzuki coupling reaction, and finally DX4 was ob-
tained by Knoevenagel condensation.
The electrochemical impedance spectroscopy (EIS) measurements
were performed with a Zennium electrochemical workstation (ZAH-
NER) with the frequency range from 10 mHz to 1000 kHz. The
magnitude of the alternative signal was 10 mV. The impedance mea-
surements were carried out under forward bias of ꢀ0.65 V in the dark.
2.6. Characterization of DSSCs
The currentevoltage characteristics were measured by using a
Keithley 2400 source meter under simulated AM 1.5 G one sun
(100 mW cmꢀ2) illumination provided by solar simulator (91192,
1 kW Xe lamp with optical filter, Oriel). The electrolyte solution is
composed of 0.6 M PMII, 0.03 M I2, 0.05 M LiI, 0.1 M guanidinium
thiocyanate (GuSCN), and 0.5 M 4-tertbutylpyridine (TBP) in
acetonitrile and valeronitrile (85:15 v/v).
2.7. Computational methods
3.2. Photophysical properties
The geometry structures of four dyes are optimized by means of
TD-DFT methods with MO62X functional at 6-311þg(2d,2p) basis
set level.
The absorption spectra of the dyes in CH2Cl2 solution are dis-
played in Fig. 1(a), and the corresponding data are presented in
Table 1. All the dyes possess two major prominent peaks at around
290 and >400 nm. The band in the ultraviolet region is probably
originating from the electronic transitions localized within the
carbazole or starburst carbazole segment. The absorption occurring
in the visible region is of charge transfer character, which is sen-
sitive to the nature of the conjugation pathway and red-shifts on
progressive addition of thiophene units. The extinction coefficients
3. Results and discussion
3.1. Synthesis and structural characterization
The synthetic approach of DX1e4 is outlined in Scheme 1. As for
DX1, 1a was synthesized by an Ullman-type condensation of

272
L.-L. Tan et al. / Dyes and Pigments 100 (2014) 269e277
Table 1
Absorption and electrochemical data of the dyes.
Dye
/104 Mꢀ1 cmꢀ1
Eox/V
(vs Ag/AgCl)
Eox/V
E0e0/eVb
EHOMO/eV
ELUMO/eV
Amounts/10ꢀ6
mol cmꢀ2
(
(vs Fc/Fcþ)a
(vs vacuum)c
(vs vacuum)d
DX1
DX2
DX3
DX4
0.87 (416)
0.87 (440)
1.73 (467)
3.46 (465)
1.37
1.43
1.28
1.30
0.74
0.80
0.65
0.67
2.56
2.38
2.29
2.30
ꢀ5.54
ꢀ5.60
ꢀ5.45
ꢀ5.47
ꢀ2.98
ꢀ3.22
ꢀ3.16
ꢀ3.17
1.43
1.14
1.10
0.53
a
Oxidation potential of dyes was measured with a scan rate of 50 mV sꢀ1 (vs NHE).
b
c
E0e0 was determined from the intersection of absorption and emission spectra in CH2Cl2.
EHOMO [eV] ¼ ꢀ(Eox (vs Fc/Fcþ) þ 4.8).
d
ELUMO [eV] ¼ EHOMO þ E0e0
.
of all the absorption band of DX4 with starburst carbazole donor is
largest among the four dyes, which is the result of the increase of
the number of carbazole units and the conjugation length. The
greater maximum absorption coefficients of the organic dyes allow
a correspondingly thinner nanocrystalline film so as to avoid the
decrease of the film mechanical strength. This also benefits the
electrolyte diffusion in the film and reduces the recombination
possibility of the light-induced charges during transportation. In
addition, the dye DX3 bearing terthiophene as linker shows a sig-
nificant red-shift and enhancement in extinction coefficient in the
longer-wavelength band when compared with that of the dyes DX1
and DX2. Also, the peak position of the charge transfer transition
for DX1 is much shorter when compared with the other three dyes,
probably due to the electron-deficiency of the benzene linker
relative to the thiophene unit [18].
that for DX1e3, which may be caused by its cone-shaped molecular
configuration. For DX4, the starburst carbazole donors can expand
the molecular size and lead to the smallest amount of adsorbed
dyes on the photoelectrode.
3.3. Electrochemical properties
To evaluate the possibility of electron transfer from the excited
dye molecule to the conduction band of TiO2, the cyclic voltam-
mograms (CV) of three photosensitizers were measured (Fig. S1)
in CH2Cl2 solution, using 0.1
M tetrabutylammonium hexa-
fluorophosphate as a supporting electrolyte. All the dyes demon-
strate quasi-reversible redox waves at a moderately high oxidation
potential. As seen from Table 1, the first oxidation potentials of all
four dyes are more positive than the Iꢀ/I3ꢀ redox couple, providing a
thermodynamic driving force for efficient dye regeneration. The
energy levels of the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) derived from the
oxidation potentials and absorption/emission data (Fig. S2) of four
organic dyes are summarized in Table 1 [20]. Judging from the
LUMO values, the excited-state energy levels of three dyes are
All the dyes when adsorbed on TiO2 exhibits red-shifted and
broadened absorption profile (Fig. 1(b)) in comparison to that
measured in solution, which could be attributed to the aggregation
or electronic coupling of the dyes on the TiO2 surface [9f,19]. In
addition, the amount of the dyes adsorbed on the TiO2 surface is
shown in Table 1. The dye loading amounts of for DX4 is lower than
Fig. 2. Computed frontier molecular orbitals and electronic distribution in the dyes.

L.-L. Tan et al. / Dyes and Pigments 100 (2014) 269e277
273
100
80
60
40
20
0
group through the
p
bridge constituted by the benzene and/or
DX1
thiophene moieties. Thus, the HOMOeLUMO excitation induced by
light irradiation could move the electron distribution from the
carbazole segment to the anchoring unit through the conjugation
pathway. Moreover, as depicted in Table S2, for the four dyes, the S1
state mainly consists of mixed transitions, in which the delocal-
ization of electron density from carbazole to carboxylic groups is
apparent. This, in combination with the near unity oscillator
DX2
DX3
DX4
strength (f), confirms the charge transfer (pep*) type of transition
in the first excited state, supporting the aforementioned experi-
mental results. In addition, the computational UVeVis spectra of
them are consistent with their absorption spectra in CH2Cl2 solu-
tion (Fig. S4).
3.5. Photovoltaic performance of the DSSCs
400
500
600
700
800
Wavelength/nm
The incident monochromatic photon-to-current conversion ef-
ficiency (IPCE) with a sandwich cell based on DX1e4 using 0.6 M 1-
methyl-3-propyl imidazolium iodide (PMII), 0.1 M guanidinium
thiocyanate (GuNCS), 0.05 M LiI, 0.03 M I2, 0.5 M tert-butylpyridine
in a mixture of acetonitrile and valeronitrile(85: 15) as redox
electrolyte is shown in Fig. 3. The onset of IPCE for the device of DX4
was ca. 700 nm. IPCE values higher than 70% were observed in the
range of 400e550 nm with a maximum value of 80% at 490 nm for
the device based on DX4. The action spectrum of a DSSC with DX3
exceeds 60% in the visible spectral region from 400 to 540 nm,
reaching its maximum of 75% at 490 nm. The action spectrum of
DX3 is red-shifted by 50 nm and 100 nm relative to the DX2 and
DX1, respectively.
Fig. 3. The IPCE spectra for DSSCs based on four dyes.
much higher than the bottom of the conduction band of TiO2,
indicating that the electron injection process from the excited dye
molecule to TiO2 conduction band is energetically favorable.
Noticeably, the HOMOeLUMO gap
DE decreases with the incre-
ment of the number of carbazole electro-donors and thiophene
groups in the molecule, in qualitative agreement with theoretical
computation results, which may favor light harvesting and hence
photocurrent generation in DSSCs.
3.4. Molecular orbital calculations
The photoelectrochemical properties of dyes sensitized TiO2
electrodes under irradiation of Xe lamp (100 mW cmꢀ2) are listed
in Table 2, and the corresponding photocurrentevoltage curves are
To scrutinize the geometrical and photophysical properties,
molecular orbital calculations of DX1e4, were carried out using the
TD-DFT and B3LYP/3-21G* program. The thienyl moiety in DX2e4 is
also coplanar with the 2-cyano-acrylic acid acceptor, due to the
shown in Fig. 4. The short-circuit current (Jsc) and overall yield (h)
for the four dyes lie in the order DX4 > DX3 > DX2 > DX1, which is
in accordance with their IPCE results. The DX4-sensitized device
generates the highest conversion efficiency among four photosen-
sitizers, which may be due to its broad and intense photocurrent
action spectrum. Of particular importance is a great (the 57 mV)
increase in the open-circuit voltage (Voc) of the DX4-based cell
relative to the DX3-based cell. The improved Voc value is because
the starburst carbazole structure might be beneficial for retarding
the electron transfer from TiO2 to the oxidized dye or electrolyte,
which would increase the electron lifetime and enhance the open-
circuit voltage. In addition, although DX1-based cell gave the
extended p-bond conjugation (Fig. S3). Such a group leads to a large
electronic interaction that may facilitate the charge separation.
Different from DX2, DX1 possesses phenyl and thienyl groups as
linkers and the angle between both planes is 28.6ꢁ, which does not
favor the electron transport between donor and acceptor but
benefit from lower tendency to aggregate. Interestingly, as for DX4,
all dihedral angles between the phenyl planes in carbazole based
donors are larger than 64.0ꢁ (Table S1) and they are all noncoplanar
with each other, which can help to inhibit the close
gation effectively between the starburst structures.
pep aggre-
The electronic structures of dyes in CH2Cl2 solution were also
mimicked, which is the solvent used to record the experimental
spectra. The electron distribution of the HOMOs and LUMOs of DX1,
DX2, DX3, and DX4 are shown in Fig. 2. Clearly, the HOMOs of these
12
DX1
DX2
10
DX3
compounds are delocalized over the carbazole
p system with the
DX4
highest electron density located at the nitrogen atoms of the
carbazole moiety. It can be seen that the HOMOs in DX2 are more
delocalized than those in DX1, due to the better coplanarity in
conjugated pathway. The LUMOs are located in the anchoring
8
6
4
2
0
Table 2
The performance parameters of the dye-sensitized solar cells.
Dye
Jsc/mA cmꢀ2
Voc/mV
FF
h(%)
DX1
DX2
DX3
DX4
N719a
6.17
6.70
9.98
10.65
14.94
601
589
586
643
765
0.74
0.74
0.74
0.71
0.69
2.74
2.94
4.30
4.86
7.93
0.0
0.2
0.4
0.6
Photovoltage(V)
a
N719 was used as the reference and measured under the same experimental
Fig. 4. Photocurrent density vs voltage for DSSCs based on dyes under AM 1.5 G
conditions.
simulated solar light (100 mW cmꢀ2).

274
L.-L. Tan et al. / Dyes and Pigments 100 (2014) 269e277
10
DX1
a
100
80
60
40
20
0
a
DX1
DX2
DX3
DX4
DX2
DX3
DX4
1
0
100
200
300
0.1
30
60
90
120
150
R/Ohm
50
40
30
20
10
0
Light intensity/mW cm-2
DX1
DX2
b
b
DX1
DX2
DX3
DX4
DX3
100
DX4
10
10
Frequence/Hz
10
10
10
10
10
10
10
10
Fig. 5. Electrochemical impedance spectra (a-Nyquist plot; b-Bode plot) of DSSC for
20
40
60
80
100 120 140 160
four dyes.
Light intensity/mW cm-2
lowest Jsc and
h among the four cells, its Voc is higher than that of
1.00
0.98
0.96
DX2 and DX3-based cells. The enhancement in Voc could be
attributed to their aggregation-resistant nonplanar linkers con-
taining the phenyl and thienyl groups, which can block the
approach of the Iꢀ3 ion to a certain degree.
Electrochemical impedance spectroscopy (EIS) analyses were
also performed to elucidate above photovoltaic findings. In the
Nyquist plots (Fig. 5(a)) the radius of the middle semicircle reflects
c
the electron recombination resistance. The electron lifetime (s)
values derived from curve fitting are 93.8, 92.1, 87.8, and 94.9 ms for
DX1, DX2, DX3, and DX4, respectively. The results are consistent
with the Voc values of the devices. The electron lifetime is improved
DX1
DX2
DX3
DX4
upon incorporation of the starburst carbazole group, i.e., s (DX4) > s
(DX3). We thus speculate that the starburst hydrophobic group
could form a substantial compact sensitizer layer at the surface of
the TiO2 to prevent the approach of the redox couple.
20
40
60
80
100
120
140
160
The Bode phase plots shown in Fig. 5(b) likewise support the
differences in the electron lifetime for TiO2 films derivatized with
the four dyes. The middle-frequency peaks of the DSSCs based on
DX1 and DX4 shift to lower frequency relative to that of DX2 and
DX3, indicating a shorter recombination lifetime for the latter
species. The increase in the recombination lifetime in the TiO2 film
is associated with a pronounced rise in the charge transfer resis-
tance, which depends on the structure of the dyes. Judging from the
optimized structures of DX2 and DX3 bearing oligothiophene as
linkers, the thienyl moieties in the dyes are coplanar with each
other, which may facilitate the electron communication. However,
these coplanar molecules, especially for DX3, are susceptible to
-2
Light intensity/mW cm
Fig. 6. (a) Incident light intensity dependent recombination time constants, (b) inci-
dent light intensity dependent transport time constants, and (c) the charge collection
efficiency from IMPS and IMVS.
aggregation for the strong dipoleedipole interaction between the
extended delocalized
p-bonds. Close pep aggregation between
molecules can aggravate charge recombination for the triiodide and
can easily penetrate through the big interspaces between chro-
mophores, resulting in the decrease of Voc.

L.-L. Tan et al. / Dyes and Pigments 100 (2014) 269e277
275
Intensity modulated photocurrent spectroscopy (IMPS) and in-
tensity modulated photovoltage spectroscopy (IMVS) were carried
out to further investigate the different photovoltaic behaviors of
photovoltaic performance was shown to be quite sensitive to the
substituent on the photosensitizers. Some general performance
trends are clearly discerned as follows: i) The introduction of the
benzene/thiophene linker in DX1, to a degree, prevents the ag-
gregation of dyes and diminishes the charge recombination be-
tween the electrons on TiO2 film and acceptors, but it disfavors the
electron transport between donor and acceptor, which reduces the
four dyes. Fig. 6(a) shows recombination time (sr) of DSSCs based on
DX1, DX2, DX3 and DX4 dyes under various incident light in-
tensities. The results indicate that the recombination time of the
devices displays a systematic trend DX4 > DX1 > DX2 > DX3,
consistent with the EIS results described above. On the other hand,
spectral response, thus leading to a lowest Jsc and
h. ii) Increasing
the electron transport times (
increase in the order DX1 > DX2 > DX4 > DX3 as shown in Fig. 6(b).
Compared with DX3, DX4 has a longer d. Considering the inter-
action with the dye and electrolyte, we deduced it could be the
reason that DX4 having starburst structure might limit the access to
the surface by Liþ ions which are known to assist the electron
diffusion along the TiO2 network, resulting in a slower photo-
injected electron transport to FTO glass. In addition, the charge
collection efficiencies (hcc) determined by IMPS/IMVS through the
s
d) of DSSCs based on the four dyes
the conjugation length of the -linkers from DX2 to DX3 can
p
extend the absorption cross section and conduct the bathochromic
shift, however, DX3 tends to form aggregates on the semi-
conductor. iii) Incorporation of dendritic carbazole as a donor unit
in DX4 not only enhances the molar extinction coefficients of the
absorption but also improves the electron lifetime by leading to an
effective spatial separation of the charges, which exhibits the
s
highest Jsc, Voc, and
h among the four dyes. Our studies open ave-
nues for the development of organic dyes featuring dendritic
carbazole as electron donors. By appropriate structural modifica-
tions, electron-rich dendritic carbazole can be developed, which
may serve as an efficient donor.
equation hcc ¼ 1ꢀsd
/sr are shown in Fig. 6(c). The results indicate
that the differences of charge collection efficiency among the dyes
are very small, being essentially in the range of 0.990e0.998,
implying the main factor affecting the IPCEs and overall conversion
efficiencies is the LHE and
F inj, in agreement with the above
Acknowledgment
discussion.
The long-term stability of DSSCs is an important requirement
for their practical application. Among many factors affecting the
stability of DSSCs, the lifetime is an especially critical factor.
Koatoh et al. have developed a simple and efficient method to
evaluate the stability of dyes by accelerating the dye aging process
upon light irradiation on dye-loaded TiO2 film without redox
electrolyte [21]. Grätzel and Zhu et al. have also tested the photo-
stability of their dyes by this method [9d]. Fig. S5 shows the
photographs of the samples of DX1, DX2, DX3, and DX4 adsorbed
on TiO2 surfaces before and after 60 min irradiation. No dramatic
change in color was observed for the all the samples. Fig. S6 shows
the absorption curves of dyes DX1, DX2, DX3, and DX4 with aging
upon light irradiation of AM 1.5 light (5, 10, 30, and 60 min). No
substantial change in absorbance was observed for DX3 and DX4,
indicating they are stable enough according to Koatoh’s experi-
ence. For DX1 and DX2, the absorbance at 400e500 nm decreased
slightly without no any distinct absorption peak shift as a result of
light irradiation. Compared with DX1 and DX2, the photo-stability
of DX3 and DX4 seems higher, suggesting that delocalization of
holes on the oligothiophene moieties of DX3 and DX4 may
enhance their intrinsic photo-stability [21,22]. Meanwhile, the
peak positions did not change, implying that no photochemical
reaction occurred.
According to the above analyses, the device efficiencies described
from IeV curves and IPCE values are in the order of
DX4 > DX3 > DX2 > DX1, and the electron lifetimes obtained in EIS
andIMVSmeasurementsshow theorderof DX4 > DX1 > DX2 > DX3.
The outcome of DX4 with the highest cell efficiency may be ratio-
nalized by the following reasons: (1) DX4 has a higher molar ab-
sorption coefficient, resulting in better light harvesting; (2) low
aggregation of DX4 improves its electron injection efficiency and Jsc;
(3) the longer electron lifetime of DX4 leads to a higher Voc. On the
other hand, the somewhat higher cell efficiencies of DX2 and DX3
than DX1 are mainly attributed to the better spectral properties and
electron communication, albeit with shorter electron lifetimes.
This work was supported by the 973 Program of China
(2012CB821701), NSF of China (61232009, 21272292, 21173272, and
21121061), Doctoral Fund of Ministry of Education of China
(20101102110018), and Science and Technology Planning Project of
Guangdong Province (2011J2200053), and High Performance Grid
Computing Platform of Sun Yat-sen University, Guangdong Prov-
ince Key Laboratory of Computational Science, Guangdong Province
Computational Science Innovative Research Team.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2013.09.025.
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