Anti-recombination organic dyes containing dendritic triphenylamine moieties for high open-circuit voltage of DSSCs

Article abstract of DOI:10.1016/j.dyepig.2013.04.023

Three novel sensitizers with dendritic triphenylamine moieties were synthesized and used for dye-sensitized solar cells (DSSCs). Their absorption spectra, electrochemical and photovoltaic properties were extensively investigated. All three DSSCs exhibit high open-circuit voltage over 0.8 V. The photovoltaic results indicate that the dendritic triphenylamine units improve the open-circuit voltage, which is attributed to the retardation of charge recombination, demonstrating that non-planar and larger molecules exert better blocking function. Under standard global AM 1.5 solar conditions, the best performance of the DSSCs exhibits a short-circuit current density of 10.35 mA cm^-2, an open-circuit voltage of 0.836 V and a fill factor of 0.68, corresponding to an overall conversion efficiency of 5.87%.

Full text of DOI:10.1016/j.dyepig.2013.04.023

Dyes and Pigments 99 (2013) 74e81
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Anti-recombination organic dyes containing dendritic triphenylamine
moieties for high open-circuit voltage of DSSCs
a
b
a
b
,
, Lingyun Wang , Derong Cao a,*,
a
**
Hai Zhang , Jie Fan , Zafar Iqbal , Dai-Bin Kuang
c
Herbert Meier
a
School of Chemistry and Chemical Engineering, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology,
Guangzhou 510641, China
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of Chemistry and Chemical
b
Engineering, Sun Yat-sen University, Guangzhou 510275, China
c
Institute of Organic Chemistry, University of Mainz, Mainz 55099, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 January 2013
Received in revised form
Three novel sensitizers with dendritic triphenylamine moieties were synthesized and used for dye-
sensitized solar cells (DSSCs). Their absorption spectra, electrochemical and photovoltaic properties
were extensively investigated. All three DSSCs exhibit high open-circuit voltage over 0.8 V. The photo-
voltaic results indicate that the dendritic triphenylamine units improve the open-circuit voltage, which is
attributed to the retardation of charge recombination, demonstrating that non-planar and larger mol-
ecules exert better blocking function. Under standard global AM 1.5 solar conditions, the best perfor-
9
April 2013
Accepted 18 April 2013
Available online 28 April 2013
ꢀ
2
mance of the DSSCs exhibits a short-circuit current density of 10.35 mA cm , an open-circuit voltage of
.836 V and a fill factor of 0.68, corresponding to an overall conversion efficiency of 5.87%.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Dye-sensitized solar cell
Non-planar
0
Sensitizer
Triphenylamine
Recombination
Open-circuit voltage
1
. Introduction
triphenylamine dyes, coumarin dyes, carbazole dyes, indoline dyes
and fluorine dyes [11,23e35], which is mainly due to the excessive
loss of voltage during the dye-regeneration reaction. The usage of
iodide/triiodide electrolyte as a redox shuttle limits the attainable
open-circuit voltage (Voc), to 0.7e0.8 V and is thus a major draw-
back [20]. Therefore, in order to further improve the efficiency of
DSSCs, it is urgent to reduce the charge recombination and increase
the electron injection efficiency. Additionally, charge recombina-
Since the pioneering work of O’Regan and Grätzel in 1991 [1],
dye-sensitized solar cells (DSSCs) have received considerable
attention due to their low-cost fabrication and high power con-
version efficiency. Tremendous efforts have been made to improve
the photovoltaic performance of DSSCs [2e19]. Recently, over 12%,
11% and 10% [18,20e22] conversion efficiencies have been achieved
by employing zinc, ruthenium complex dyes and metal free organic
dyes under air mass 1.5 global (AM 1.5 G) irradiation, respectively.
Although organic dyes have advantages like high molar extinction
co-efficients, easy to fabrication over Ru complex dyes, but organic
dyes show lower efficiency than N719. For instance, compared with
the open-circuit voltage (Voc) of more than 0.8 V for N719, organic
dyes show relatively lower Voc ranging from 0.55 V to 0.75 V for
tion and electron injection efficiency are closely correlated with Voc,
which means Voc becomes one of the principle factors accounting
for the further improvement of the performance of DSSCs [36e40].
Photovoltage is determined by the difference between conduction-
band edge of TiO
energy can be modified by interfacial environment. Thus, photo-
voltage can be tuned by different dye attached on TiO surface
2
and redox potential. The conduction-band
2
because of the interfacial modification [23]. In recent years, various
efforts such as sensitizer modification, usage of additives, co-
adsorbents, and novel redox couples haves been made to improve
*
Corresponding author. Tel./fax: þ86 20 87110245.
Voc [41e44].
*
* Corresponding author. Tel.: þ86 20 84113015.
Typically, triphenylamine unit is often used as an electron
E-mail addresses: kuangdb@mail.sysu.edu.cn (D.-B. Kuang), drcao@scut.edu.cn
(
D. Cao).
donor for the photosensitizers because of its aggregation resistant
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.04.023

H. Zhang et al. / Dyes and Pigments 99 (2013) 74e81
75
non-planar molecular configuration as well as excellent electron
donating capability [45,46]. Despite of a variety of molecular en-
gineering of the TPA moiety, Voc of TPA based dye has not been
dramatically improved as expected over those of other organic
dyes. Tian et al. reported that an extra arylamine group can sta-
bilize the charge separated state and therefore enhance the value
of Voc [47,48]. Keeping in mind the above findings, we designed
and synthesized three new organic dyes with dendritic triphe-
nylamine moieties (T1e3) to enhance the open-circuit voltage of
DSSCs. Structural modifications were made to induce the bent
conformation as well as steric hindrance (Scheme 1). Their ab-
sorption spectra, electrochemical, photovoltaic properties have
been extensively investigated.
2. Experimental
All reagents were purchased from commercial sources and used
without further purification unless otherwise noted. Solvents were
Scheme 1. Synthesis of the dyes T1, T2 and T3.

76
H. Zhang et al. / Dyes and Pigments 99 (2013) 74e81
ꢁ
1
freshly distilled from appropriate drying agents prior to use. Con-
ventional Schlenk techniques were used, and reactions were car-
ried out under argon or nitrogen. Reactions were monitored by
using TLC and chromatographic separations were carried out on
silica gel (300e400 mesh).
Yield: 281 mg, 39.5%. mp 144e146 C. H NMR (CDCl
ppm 9.85 (s, 1H), 7.70 (d, J ¼ 4 Hz, 1H), 7.56e7.54 (m, 2H), 7.51e
7.49 (m, 4H), 7.47e7.45 (m, 4H), 7.31 (d, J ¼ 4 Hz, 1H), 7.28e7.24 (m,
3
, 400 MHz):
d
1
3
7H), 7.21e7.19 (m, 4H), 7.16e7.12 (m, 14H), 7.04e7.01 (m, 5H);
NMR (100 MHz, CDCl ): ppm 182.3, 165.5, 154.5, 148.9,147.1, 145.7,
44.3, 141.4, 137.7, 136.1, 134.4, 129.3, 127.6, 127.4, 127.3, 126.5, 125.3,
C
3
d
1
2
.1. Synthesis
124.4, 124.0, 123.0, 122.8. HRMS-ESI: calculated for C59
H
43
N
3
OS
þ
(MþH) /z 841.3121, found: 841.3128.
N,N-diphenyl-4-bromoaniline (2) [49], 5-[4-(diphenylamino)
0
0
phenyl]thiophene-2-carbaldehyde (4) [49], 5-{4-[bis(4-bromo
phenyl)amino]phenyl}thiophene-2-carbaldehyde (5) [49], 4-
2.1.4. (E)-3-{5-[4-(Bis(4 -(diphenylamino)-[1,1 -biphenyl]-4-yl)
amino)phenyl]thiophen-2-yl}-2-cyanoacrylic acid (T2)
[
(
N,N-di(4-bromophenyl)amino]benzaldehyde (8) [50], 4-{bis[4-
diphenylamino)biphenyl-4-yl]amino}benzaldehyde (9) [50],
A solution of compound 10 (163 mg, 0.194 mmol) in chloroform
(30 mL) and cyanoacetic acid (33 mg, 0.387 mmol) was refluxed in
the presence of piperidine (0.03 mL) for 20 h under Argon. After
cooling, the solution was poured into water (30 mL) and the
mixture was extracted with DCM. The combined organic phases
were synthesized according to the methods reported in the
literature with some modifications.
2
2
.1.1. 5-{4-[Bis(4-bromophenyl)amino]phenyl}-4-bromothiophene-
-carbaldehyde (6)
were dried over anhydrous MgSO
pressure. The pure product was obtained by silica gel chromatog-
raphy (CH Cl
:MeOH ¼ 20:1 as eluent) as a red powder in 90.9%
(160 mg) yield, mp 208e209 C. H NMR (DMSO-d
ppm 8.28 (s, 1H), 7.82 (s, 1H), 7.64e7.62 (m, 2H), 7.56e7.50 (m,
4
, and evaporated under reduced
Compound 4 (766 mg, 2.16 mmol) was dissolved in chloroform
2
2
ꢁ
ꢁ
1
(
50 mL) and the solution was cooled to 0 C. N-bromosuccinimide
6
, 400 MHz):
(
NBS) (2.3 g, 12.9 mmol) was added slowly and the mixture was
d
ꢁ
13
stirred for 1 h at 0 C. The mixture was allowed to warm to ambient
temperature and stirring was continued for 4 h. Then the mixture
was refluxed for 12 h. After being cooled to room temperature, the
reaction mixture was extracted with dichloromethane (DCM). The
combined organic layers were washed with water and brine, then
8H), 7.30e7.27 (m, 8H), 7.10e6.97 (m, 23H); C NMR (100 MHz,
CDCl ): ppm 147.6, 146.9, 145.4, 135.8, 134.2, 134.1, 129.2, 129.0,
3
d
128.2, 127.5, 127.3, 125.5, 125.2, 124.4, 124.2, 123.9, 123.8, 122.9,
þ
122.8. HRMS-ESI: calculated for C62
found: 908.2934.
44
H N
4
O
2
S (M) /z 908.3179,
4
dried over anhydrous MgSO . The solvent was removed via rotary
0
0
evaporator, the residue was purified by column chromatography on
silica gel (PE:DCM:EA ¼ 30:15:1 as eluent) to give the product as a
2.1.5. 5-{4-[Bis(4 -(diphenylamino)-[1,1 -biphenyl]-4-yl)amino]
phenyl}-4-(4-(diphenylamino)phen-yl)thiophene-2-carbaldehyde (11)
A mixture of 6 (633 mg, 1.07 mmol), 7 (1.24 g, 4.29 mmol),
tetrakis(triphenylphosphine)palladium(0) (148 mg, 0.128 mmol),
and potassium carbonate (6 mL of 2 M aqueous solution,
12.8 mmol) in THF (50 mL) was stirred and heated to reflux under
argon for 20 h. The reaction mixture was poured into water
(50 mL) and extracted with DCM. The combined extract was
ꢁ
1
yellow solid in 51.4% (653 mg) yield, mp 170e172 C. H NMR
(
(
d
CDCl
m, 4H), 7.07 (m, 2H), 7.01 (m, 4H); C NMR (100 MHz, CDCl
ppm 181.5, 148.3, 147.8, 145.7, 140.6, 140.2, 132.7, 130.0, 126.6,
25.7, 122.3, 116.9, 108.1. APCI-MS: calculated for C23 NOS
3
, 400 MHz): d ppm 9.83 (s, 1H), 7.70 (s, 1H), 7.59 (m, 2H), 7.41
13
3
):
1
(
H15Br
3
þ
MþH) /z 589.8419, found: 589.8282.
4
washed with brine and dried over anhydrous MgSO . The solvent
was removed by rotary evaporation, and the crude product was
0
0
2
.1.2. (E)-3-{4-[Bis(4 -(diphenylamino)-[1,1 -biphenyl]-4-yl)amino]
phenyl}-2-cyanoacrylic acid (T1)
purified by column chromatography on silica gel to give the
ꢁ
Compound 9 (300 mg, 0.395 mmol) was dissolved in 30 mL of
chloroform, and cyanoacetic acid (67 mg, 0.788 mmol) was refluxed
in the presence of piperidine (0.1 mL) for 20 h under argon. After
cooling the reaction mixture was poured into water (30 mL) and
extracted with DCM. The combined organic phases were dried over
product as a yellow solid in 27.3% (317 mg) yield, mp 155e157 C.
1
3
H NMR (CDCl , 400 MHz): d ppm 9.88 (s, 1H), 7.75 (s, 1H), 7.50e
13
7.45 (m, 8H), 7.28e7.10 (m, 35H), 7.05e7.01 (m, 11H); C NMR
(100 MHz, CDCl ): ppm 182.7, 148.5, 148.2, 147.7, 147.3, 147.0,
3
d
145.8, 140.4, 139.4, 138.6, 136.0, 134.4, 129.9, 129.7, 129.6, 129.3,
anhydrous MgSO
pressure. The pure product was obtained by column chromatog-
raphy using silica gel (CH Cl
:MeOH ¼ 20:1 as eluent) as a red
powder in 55.2% (180 mg) yield, mp 184e186 C. H NMR (DMSO-
, 400 MHz): ppm 7.81 (s, 1H), 7.94 (m, 2H), 7.68 (m, 4H), 7.60 (m,
H), 7.35e7.31 (m, 8H), 7.26 (m, 4H), 7.09e6.99 (m, 18H); C NMR
100 MHz, DMSO-d ): ppm 164.5, 151.7, 147.5, 147.2, 144.6, 137.0,
33.6, 133.2, 130.1, 128.1, 127.9, 126.8, 124.7, 123.8, 123.6, 119.7, 117.8,
4
, and the solvent was evaporated under reduced
129.0, 127.6, 127.4, 125.2, 124.7, 124.4, 124.0, 123.2, 122.9, 122.1.
þ
HRMS-ESI: calculated for C77
1084.4014.
H
56
N
4
OS (M) /z 1084.4169, found:
2
2
ꢁ
1
0
0
d
4
(
1
6
d
2.1.6. (E)-3-{5-[4-(Bis(4 -(diphenylamino)-[1,1 -biphenyl]-4-yl)
amino)phenyl]-4-(4-(diphenylamino)ph-enyl)thiophen-2-yl}-2-
cyanoacrylic acid (T3)
13
6
d
A solution of compound 11 (300 mg, 0.276 mmol), cyanoacetic
acid (47 mg, 0.553 mmol) in chloroform (30 mL) was refluxed in the
presence of piperidine (0.04 mL) for 20 h under Argon. After
cooling, the reaction mixture was poured into water (30 mL) and
extracted with DCM. The combined organic phase was dried over
þ
1
00.0. HRMS-ESI: calculated for C58
42 4 2
H N O (M) /z 826.3302,
found: 826.3311.
0 0
.1.3. 5-{4-[Bis(4 -(diphenylamino)-[1,1 -biphenyl]-4-yl)amino]
2
phenyl}thiophene-2-carbaldehyde (10)
anhydrous MgSO
pure product was obtained by silica gel column chromatography
(CH Cl
:MeOH ¼ 20:1 as eluent) as a red powder in 73.9% (235 mg)
yield, mp 193e195 C. H NMR (DMSO-d
1H), 7.90 (s, 1H), 7.54e7.50 (m, 8H), 7.31e7.15 (m, 16H), 7.09e6.88
4
, and evaporated under reduced pressure. The
A mixture of 5 (434 mg, 0.85 mmol), 7 (734 mg, 2.54 mmol),
tetrakis(triphenylphosphine)palladium(0) (97 mg, 0.084 mmol),
and potassium carbonate (3.3 mL of 2 M aqueous solution,
2
2
ꢁ
1
6
, 400 MHz): d ppm 8.32 (s,
6
.77 mmol) in THF (25 mL) was stirred and heated to reflux under
13
argon for 20 h. The reaction mixture was poured into water and
extracted with DCM, and the combined extracts were washed with
3
(m, 30H); C NMR (100 MHz, CDCl ): d ppm 147.7, 147.4, 147.3,
147.0, 145.6, 136.1, 134.3, 130.0, 129.9, 129.7, 129.3, 127.6, 127.4,
brine, dried over anhydrous MgSO
rotary evaporation, and the crude product was purified by column
chromatography on silica gel to give the product as a yellow solid.
4
. The solvent was removed by
125.4, 125.3, 124.7, 124.6, 124.4, 124.0, 123.2, 122.9, 122.8, 121.9.
þ
HRMS-ESI: calculated for C80
1151.4065.
H
57
N
5
O
2
S (M) /z 1151.4227, found:

H. Zhang et al. / Dyes and Pigments 99 (2013) 74e81
77
2
.2. Measurement
(0.2 mL), terpineol (3.0 g) and ethyl cellulose (0.5 g) to form ho-
mogeneous slurry. Finally, the slurry was sonicated for 15 min in an
1H and 13C NMR spectra were recorded on a Bruker 400 MHz
ultrasonic bath to form a viscous white TiO
photoanodes (about 16 m in thickness) were prepared via screen-
printing process onto FTO glass. The prepared films were annealed
2 2
paste. Then the TiO
spectrometer in CDCl
reference. The melting point was taken on Tektronix X4 microscopic
melting point apparatus and uncorrected. The absorption and
emission spectra of the dyes in chloroform solution (1 ꢂ 10 M)
were measured at room temperature by Shimadzu UV-2450 UVeVis
spectrophotometer and Fluorolog III photoluminescence spec-
3
or DMSO-d
6
with tetramethylsilane as inner
m
ꢁ
ꢁ
ꢁ
through a procedure (325 C for 5 min, 375 C for 5 min, 450 C for
ꢀ
5
ꢁ
15 min, and 500 C for 15 min) to remove the organic substances.
Then TiO
2
films were soaked in 0.04 M TiCl
4
aqueous solution for
ꢁ
30 min at 70 C to improve the photocurrent and photovoltaic
trometer, respectively. The absorption spectra of the dyes on TiO
film were measured by UV-3010 UVeVis spectrophotometer. Elec-
trochemical redox potentials were measured by Cyclic Voltammetry
2
performance. Treated films were rinsed with deionized water and
ꢁ
ethanol and then sintered again at 520 C for 30 min. After cooling to
ꢁ
ꢀ4
80 C the films were immersed in a 5.0 ꢂ 10 M solution of T1e3
(
CV), using three electrode cells and an Ingsens 1030 electro-
dyes for 16 h (0.5 mM dye in dichloromethane). Afterward these
chemical workstation (Ingsens Instrument Guangzhou, Co. Ltd.,
China) in a one compartment at a scan rate of 50 mV/s. The Ag/AgCl
in KCl (3 M) solution and an auxiliary platinum wire were utilized as
films were rinsed with CH
organic dye molecules. To evaluate their photovoltaic performance,
the dye-sensitized TiO /FTO glass films were assembled into a
sandwiched type together with Pt/FTO counter electrode. Platinized
2 2
Cl in order to remove physical adsorbed
2
reference and counter electrodes while dye coated TiO
used as working electrode. Tetrabutylammonium perchlorate (n-
Bu NClO ) 0.1 M in THF was used as supporting electrolyte. All
2
films were
counter electrodes were fabricated by thermal depositing H
solution (5 mM in isopropanol) onto FTO glass at 400 C for 15 min.
The electrolyte (0.6 M 1-methyl-3-propylimidazoliumiodide (PMII),
2
PtCl
6
ꢁ
4
4
electrochemical measurements were calibrated by using ferrocene
as standard (0.63 V vs. NHE). The current voltage characteristics
were recorded by using a Keithley 2400 source meter under simu-
2
0.05 M LiI, 0.10 M guanidinium thiocyanate, 0.03 M I and 0.5 M tert-
butylpyridine) in acetonitrile/valeronitrile (85:15) was injected from
a hole made on the counter electrode into the space between the
2
lated AM 1.5 G (100 mW/cm ) illumination with a solar light
simulator (Oriel, Model: 91192). A 1000 W Xenon arc lamp (Oriel,
Model:6271) served as a light source and its incident light intensity
was calibrated with an NREL standard Si solar cell. The incident
monochromatic photon-to-current conversion efficiency (IPCE)
spectra were measured as a function of wavelength from 350 to
sandwiched cells. The active area of the dye coated TiO
0.16 cm .
2
film was
2
3. Result and discussion
8
00 nm on the basis of a Spectral Products DK240 monochromator.
3.1. Synthesis
The electrochemical impedance spectra (EIS) measurements were
conducted on the electrochemical workstation (Zahner Zennium) in
dark condition, with an applied bias potential of ꢀ0.84 V. A 10 mV
AC sinusoidal signal was employed over the constant bias with the
frequency ranging between 1 MHz and 10 mHz. The impedance
parameters were determined by fitting of the impedance spectra
using Z-view software. The amount of dye load was measured by
The synthetic routes of three dyes are shown in Scheme 1. In-
termediate 4 was prepared from TPA via bromination of compound
1 and Suzuki coupling reaction of compound 3. Compounds 5 and 6
were obtained by bromination reaction of 4 using N-bromosucci-
nimide followed by the MichaeliseArbuzov reaction. The Suzuki
coupling reaction was made with the respective bromides and (4-
(diphenylamino)phenyl)boronic acid (7) to yield the aldehydes 9, 10
and 11. Then, compound 9 was transformed into T1 via Knoevenagel
reaction in 55.2% yield. Similarly, compounds T2 and T3 were ob-
tained in 90.9% and 73.9% yields, respectively. All the intermediates
and target products were purified by column chromatography and
the new compounds were characterized with standard spectro-
desorbing the dye from the films with 0.1 M NaOH in THF/H
2
O (1:1)
and measuring UVeVis spectrum.
2.3. Fabrication of DSSCs
Fluorine-doped tin oxide (FTO) glasses were washed with
detergent, water, ethanol and acetone in an ultrasonic bath for
removing dirt and debris. Anatase TiO nano-particles (20 nm) were
prepared through a hydrothermal treatment with a precursor so-
lution containing Ti(OBu) (10 mL), ethanol (20 mL), acetic acid
18 mL) and deionized water (50 mL) according to the previously
reported paper [51,52]. The synthesized TiO powder (1.0 g) was
1
13
scopic techniques like H NMR, C NMR and HRMS.
2
3.2. Optical properties
4
(
Absorption and emission spectra of three dyes in dilute solu-
ꢀ5
2
tions of chloroform (1 ꢂ 10 M) are shown in Fig. 1a. The char-
ground for 40 min in the mixture of ethanol (8.0 mL), acetic acid
acteristic data are listed in Table 1. In the UVeVis spectra, the dyes
3 2
Fig. 1. Absorption and emission spectra of T1e3 in CHCl solutions (a) and on TiO films (16 mm) (b).

78
H. Zhang et al. / Dyes and Pigments 99 (2013) 74e81
Table 1
Band gap (calculated by DFT/B3LYP), absorption, and electrochemical parameters for organic dyes.
a
Band gapa
absb/nm)/(
ꢀ1
ꢀ1
HOMO/eVc
E
0e0/eVd
LUMO/eVe
Dye
(HOMO/LUMO) /eV
(
l
3
/M cm
)
labs on TiO
2
/nm
T1
T2
T3
ꢀ4.93/ꢀ2.25
ꢀ4.83/ꢀ2.53
ꢀ4.79/ꢀ2.47
2.68
2.3
2.32
356(35579), 458(25096)
359(18661), 502(8363)
354(68980), 493(16470)
421
488
498
ꢀ5.07
ꢀ5.12
ꢀ5.16
2.36
2.25
2.28
ꢀ2.71
ꢀ2.87
ꢀ2.88
a
DFT/B3LYP calculated values.
b
c
Absorptions of charge-transfer transition were measured in CHCl
3
.
HOMO (vs. NHE) of dyes measured by cyclic voltammetry in 0.1 M tetrabutylammonium perchlorate in THF solution as supporting electrolyte, Ag/AgCl as reference
electrode and Pt as counter electrode.
d
E
0e0: 0e0 transition energy measured at the onset of absorption spectra.
e
LUMO (vs. NHE) was calculated by HOMOeE0e0
.
exhibit two prominent bands, appearing at 300e400 nm and 400e
00 nm, respectively. The former is ascribed to a localized aromatic
* transition and the latter is of intramolecular charge-transfer
character [53]. The intensity of * transitions was higher than
that of the ICT (Fig. 1a). The absorption maxima for T1e3 in CHCl
3.3. Electrochemical properties
6
p
e
p
The oxidation potential (Eox), corresponding to the highest
occupied molecular orbital (HOMO) level of dyes, were measured
by cyclic voltammetry (CV) in THF solutions on a dye films as
working electrode (Fig. 3). As shown in Fig. 3 and Table 1, the
ionization energy decreased gradually in the order of T3 > T2 > T1.
It is noteworthy that inserting a thiophene unit close to the
acceptor group promotes the HOMO level. Suitable HOMO and
LUMO levels are necessary to match the conduction band (ꢀ0.5 V
pep
3
are 458 nm, 502 nm and 493 nm in the visible region, respectively.
Compounds T2 and T3 have thiophene group in conjugated chains,
which enhance the extent of electron delocalization over the whole
molecules, so their maximum absorption peaks are red shifted as
compared to T1. The threshold wavelengths of the absorption
spectra for T1, T2 and T3 are 551 nm, 583 nm, 595 nm, respectively,
indicating that the introduction of a thiophene unit into the mo-
lecular structure of the sensitizers expanded and broadened the
absorption range [53]. It is well known that a wider absorption
range is favorable for the enhancement of the DSSCs performance,
as more photons can be harvested. Under similar conditions the T3
sensitizer shows an absorption peak at 493 nm that is slightly blue-
shifted relative to that of T2 which can be readily interpreted by the
molecular modeling study of the two dyes. The ground state
2
vs. NHE) of the TiO electrode and the redox potential (0.4 V vs.
NHE) of the iodine/iodide electrolyte. The excitation transition
energies (E0e0) were estimated to be 2.36 eV, 2.25 eV and 2.28 eV,
respectively from the intersection of absorption and photo-
luminescence spectra. The HOMO and LUMO values vs. vacuum
were transformed via the equation EHOMO ¼ ꢀe (4.88 þ Vox) [18],
where Vox is the onset potential vs. ferrocene of reduction or oxi-
dization of sensitizers. The energy offsets of LUMOs (ꢀ2.71 eV for
T1, ꢀ2.87 eV for T2 and ꢀ2.88 eV for T3) of dye molecules with
respect to the titania conduction band edge (ꢀ4.00 eV) provide
thermodynamic driving forces for charge generation. The energy
offsets of HOMOs (ꢀ5.07 eV for T1, ꢀ5.12 eV for T2 and ꢀ5.16 eV for
T3) relative to that (ꢀ4.60 eV) of iodide supply negative Gibbs free
energy changes for dye regeneration reaction. Through a systematic
comparison on electrochemical data (Table 1) of all organic dyes, it
is clear that the T2 dye has a relative small HOMO/LUMO gap of
2.28 eV, generally consistent with its low-energy absorption peak.
ꢁ
structure of T3 possesses at 36.45 twist between B4 and S2 unit.
ꢁ
The dihedral angle of S2 and B5 unit in T3 is 47.57 (Fig. 2). For T2,
ꢁ
the dihedral angles of the corresponding units are 20.46 , giving a
more planar configuration. Accordingly, a red shift of T2 compared
to T3 derives from more delocalization over an entire conjugated
system in T2. Emission maxima of the dyes can be found at 500e
7
00 nm when the dyes are excited at their respective absorption
bands at 400e600 nm.
2
The absorption spectra of T1e3 on 16 mm TiO films after 16 h
absorption were measured (Fig. 1b). Compared to the spectrum in
solution, the maximum absorption peaks of T1 and T2 on TiO films
3.4. Molecular orbital calculations
2
were blue-shifted by 37 nm and 14 nm, respectively, which is
possibly due to the deprotonation of carboxylic acid of the dye upon
To get further insight into the geometrical and electronic dis-
tribution of the dyes, density functional theory (DFT) calculations
were performed at B3LYP/6e31G* level with the Gaussian 03W
program package. Energies and the electronic distribution of the
frontier molecular orbitals (HOMOs and LUMOs) of the dyes T1e3
computed are presented in Fig. 4. In the optimized structures of
adsorption on the TiO
54,55]. However, the maximum absorption peak of T3 was red-
shifted by 5 nm (Table 1), which is possibly due to formation of J-
aggregates of T3 on the TiO film [56].
2
film or due to formation of H-aggregates
[
2
Fig. 2. The optimized geometries of the T1, T2 and T3 calculated with DFT on the B3LYP/6e31G* level.

H. Zhang et al. / Dyes and Pigments 99 (2013) 74e81
79
3.5. Photovoltaic performances of the DSSCs
The incident monochromatic photon-to-current conversion
efficiency as a function of incident wavelength for DSSCs based
on the dyes using liquid electrolyte are shown in Fig. 5a. It is
evident that all three dyes can efficiently convert visible light to
photocurrent in the region from 400 nm to 700 nm. The IPCE of
dye T3 exceeds 70% over the spectral region from 420 nm to
5
30 nm, reaching its maximum of 78.1% at 470 nm and tail off
toward 700 nm. JeV characteristics were further measured under
irradiation of AM 1.5 G full sunlight and presented in Fig. 5b. It is
noteworthy that all of the three dyes have high open-circuit
voltages (853, 822 and 836 mV), which is due to the introduction
of dendritic triarylamine antennas into the molecule to form the
DeDepeA structure. These structural modifications avoid the
2
Fig. 3. Cyclic voltammogram plots of T1e3 attached to nanocrystalline TiO film
deposited on conducting FTO glass.
charge recombination processes of injected electrons with triio-
dide in the electrolyte and lead to an increase in open-circuit
voltage [36,48]. T2 dye based DSSCs showed a lower Voc (822 mV)
than T1 and T3 dyes based DSSCs. One plausible explanation is
that the extended conjugation and good planarity can enhance
the charge recombination and result in a lower Voc [11,57]. As
shown in Table 2, the T3 dye-sensitized solar cell exhibits the
three dyes, the electron cloud of the highest occupied frontier
molecular orbitals were delocalized over the triphenylamine en-
tities, while the lowest unoccupied molecular orbitals were mainly
distributed over the cyanoacrylic acid groups. It suggests that
HOMO to LUMO excitation moves the electron density distribution
from triphenylamine donor moieties to cyanoacrylic acid acceptor
best overall light-to-electricity conversion efficiency (
h
) of 5.87%
with Jsc of 10.35 mA cm , Voc of 0.836 V, and a fill factor (ff) of
.68. Under the similar conditions, the T1 and T2 dye-sensitized
ꢀ
2
0
ꢀ2
moiety via different p-spacers. The effective intramolecular charge
solar cell gave a Jsc value of 8.71 and 10.74 mA cm , Voc of 0.853
and 0.822 V and ff of 0.67 and 0.63, corresponding to an value
of 4.97% and 5.56%. From these results (Table 2), it was observed
that the value of T2 based cell was higher than that of T1 based
transfer process indicates that the electron injection from the
excited dyes to titanium oxide CB is feasible. On the basis of these
results, introduction of triphenylamine group into dye molecules is
expected to give larger steric interactions, which would lead to less
h
h
cell due to a large Jsc. The poor efficiency of T1 dye-sensitized cell
was attributed to the lower light harvesting capability in visible
range, evidenced by the UVeVis absorption spectra (Fig. 1). A
reverse trend was observed in Jsc, i.e. T2 > T3, corresponding to
dye-aggregation on the TiO
device performances.
2
thin film, resulting in enhanced DSSCs
ꢀ
2
ꢀ2
1
0.74 mA cm , 10.35 mA cm , respectively, for which the main
reason may be too large a dihedral angle between the thiophene
moiety (S2) and the benzene ring (B4), resulting in less efficient
intramolecular charge transfer [58]. The adsorbed amounts
ꢀ
7
ꢀ7
on the TiO
and 1.59 ꢂ 10
Table 2).
When we nearly finished the experimental works, a series of
2
films were measured as 1.28 ꢂ 10 , 1.51 ꢂ 10
,
ꢀ7
ꢀ2
mol cm
for T1, T2 and T3, respectively
(
similar dendritic triphenylamine dyes TT1e3 were reported by
Lu et al. [24]. The dendritic triphenylamine units were linked
with vinyl group in the dyes. The TT1e3 based DSSCs exhibited
good performance, but their Voc were not very high (0.65e
0
.71 V). A reasonable explanation is that the linker of vinyl
group can keep more planar structure, and therefore reduce the
bent conformation of the molecules. In our dyes the dendritic
triphenylamine units were linked each other directly, inducing
more bent conformation and leading to a higher Voc (0.82e
0
.85 V).
Electrochemical impedance spectroscopy (EIS) analysis was
performed to elucidate the photovoltaic findings (Fig. 6).
Impedance spectra for T1, T2 and T3 dyes sensitized solar cells
were measured in the dark condition under a forward bias
of ꢀ0.84 V with a frequency range of 0.1 Hze100 kHz. The
Nyquist plots (Fig. 6) showed two semicircles, the small and
large semicircles, respectively located in the high and middle
frequency regions, which were assigned to charge transfer at
Pt/electrolyte and TiO
2
/dye/electrolyte interface, respectively
[
11,59e62]. Small circles were almost similar in all the dyes
due to the use of same counter electrode and electrolyte. Fig. 6
showed that the radius of the large semicircle were increased in
the order of T2 < T3 < T1, indicating that the electron recom-
bination resistance augments from T2, T3 to T1. The electron
Fig. 4. The frontier orbitals of T1, T2 and T3 optimized at the B3LYP/6e31G* level.

80
H. Zhang et al. / Dyes and Pigments 99 (2013) 74e81
Fig. 5. (a) IPCE plots for the DSSCs. (b) JeV curves of DSSCs based on the dyes.
lifetime calculated by fitting the equation
s
r
¼ Rrec ꢂ CPE2
4. Conclusion
(
CPE2, chemical capacitance) [7,63,64] were 299 ms, 215 ms and
2
49 ms for T1, T2 and T3 dyes sensitized cells, respectively. The
In summary, three new dendritic triphenylamine-based DeDe
longer electron lifetime observed with T1 dye-sensitized cells
peA sensitizers (T1, T2 and T3) were synthesized for DSSCs. The
indicates more effective suppression of the recombination of the
overall light-to-electricity conversion efficiency of the DSSCs
sensitized with branched T1, T2, and T3 dyes were 4.97%, 5.56%, and
5.87% respectively. As a result, compared to the reported organic
dyes, high open-circuit voltages were obtained. Dye T1 sensitized
solar cell delivered the highest open-circuit photovoltage of
853 mV, which is related to introducing non-planar TPA moiety and
thereby increasing steric hindrance against electron back reactions.
ꢀ
ꢀ
injected electron with the I /I
3
in the electrolyte, leading to
improvement in the Voc. The result was also in agreement with
the observed shift in the Voc value under standard global AM 1.5
illumination. The dye T2 has low electron lifetime as compared
to the dye T3, indicating that the triphenylamine of the sub-
stituent at 4-position of thiophene may avoid the charge
recombination, which leads to an increase in open-circuit
voltage.
By introducing the thiophene unit as
p-bridge, Voc obviously
decreased. However, the open-circuit voltage increased signifi-
cantly from 822 to 836 mV with the introduction of a TPA side chain
at 4-position of thiophene. The results of this work indicate that the
extended conjugation and good planarity can enhance the charge
recombination and result in a lower Voc, introducing bulky groups
Table 2
a
Photovoltaic parameters for DSSCs of organic dyes.
sc/mA cmꢀ2
H (%)b
/sc
into side chain of p-bridge may be the effective method for
increasing the open-circuit voltage of DSSCs.
Dye
J
V
oc/mV
ff
s
Amount of dye
load (mol cm 2)
ꢀ
7
T1
T2
T3
8.71
10.74
10.35
853
822
836
0.669
0.630
0.678
4.97
5.56
5.87
0.2993
0.2150
0.2492
1.28 ꢂ 10ꢀ
ꢀ
7
7
1.51 ꢂ 10
1.59 ꢂ 10ꢀ
Acknowledgments
a
J
sc: short-circuit photocurrent density; Voc: open-circuit photovoltage; ff: fill
: total power conversion efficiency.
Performance of DSSCs measured in
We are grateful to the National Natural Science Foundation of
China (21272079, 20873183, 21072064), the Natural Science
Foundation of Guangdong Province, China (10351064101000000,
S2012010010634) and the Fund from Guangzhou Science and
Technology Project, China (2012J4100003) for the financial support.
factor;
h
b
0.16 cm2 working area on
a FTO
a
ꢀ
1
(
7
U
square ) substrate.
The electron lifetime calculated by fitting the equation
chemical capacitance).
c
s ¼ Rrec ꢂ CPE2 (CPE2,
r
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