New efficient dyes containing tert-butyl in donor for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2012.03.027

A series of D-π-A organic sensitizers that contains two 4-tert-butylbenzene moieties in the donor part of triphenylamine group are designed and characterized. All these dyes comprise the same donor and acceptor units while the different aromatic units are introduced as linkers between the donor and acceptor units. It is found that the tuning of the HOMO and LUMO energy level can be conveniently achieved by alternating the conjugate bridge. The DSSCs based on LI-17 show the best light to electricity conversion efficiency of 5.35% (J_sc = 12.65 mA cm^-2, V_oc = 675 mV, ff = 0.63) under standard global AM 1.5 solar light conditions (100 mW cm^-2), indicating that the introduction of tert-butyl groups may be able to play an anti-aggregation effect.

Full text of DOI:10.1016/j.dyepig.2012.03.027

Dyes and Pigments 95 (2012) 244e251
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New efficient dyes containing tert-butyl in donor for dye-sensitized solar cells
Jie Shi a, Zhaofei Chai a, Cheng Zhong a, Wenjun Wu b, Jianli Hua b, Yongqiang Dong c, Jingui Qin a,
**
,
a,
*
Qianqian Li a , Zhen Li
a Department of Chemistry, Hubei Key Lab on Organic and Polymeric Opto-Electronic Materials, Wuhan University, Wuhan 430072, China
b Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, China
c Department of Chemistry, Beijing Normal University, Beijing 100875, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of DepeA organic sensitizers that contains two 4-tert-butylbenzene moieties in the donor part
Received 15 January 2012
Received in revised form
23 March 2012
Accepted 26 March 2012
Available online 20 April 2012
of triphenylamine group are designed and characterized. All these dyes comprise the same donor and
acceptor units while the different aromatic units are introduced as linkers between the donor and
acceptor units. It is found that the tuning of the HOMO and LUMO energy level can be conveniently
achieved by alternating the conjugate bridge. The DSSCs based on LI-17 show the best light to electricity
conversion efficiency of 5.35% (Jsc ¼ 12.65 mA cmꢀ2, Voc ¼ 675 mV, ff ¼ 0.63) under standard global AM
1.5 solar light conditions (100 mW cmꢀ2), indicating that the introduction of tert-butyl groups may be
able to play an anti-aggregation effect.
Keywords:
Tert-butyl
Triphenylamine
Anti-aggregation
Dye-sensitized solar cells
Sensitizers
Ó 2012 Elsevier Ltd. All rights reserved.
Power conversion effciency
1. Introduction
the HOMO level of the dye and the redox potential of the electrolyte
[33e36]. From the viewpoint of chemists, it is necessary to remodel
With the consumption of fossil energy and the increasing
intensity of global warming, solar energy will play an important
role in the future world for its feature of renewable and clean [1]. In
1991, dye-sensitized solar cells (DSSCs) was first fabricated with the
Ru-complexes dye, and now some Ru sensitizers could achieve the
power conversion efficiencies in excess of 11% subsequently [2e11].
However, due to its rarity of this precious metal, large amounts of
researches have been carried out on metal-free organic dyes for its
advantages of low cost, easy synthesis, and high molar coefficient.
In order to improve photovoltaic efficiencies of DSSCs, many metal-
the molecular design of dyes. And an effective method to improve
photovoltaic performance of DSSCs is to inhibit intermolecular
aggregation. The phenomenon of -stacked aggregation on the
TiO2 electrodes always shortens the lifetime of excited electrons
which results in decreased photocurrent or leads to molecules
residing in the system that are not functionally attached to the
surface of TiO2 and thus act as filters, directly decreasing the elec-
tron injection greatly [37,38]. To weaken or avoid the aggregation
pep
p
from a strong intermolecular pep interaction, some co-adsorbents
such as deoxycholic acid (DCA) or chenodeoxycholic acid (CDCA)
have often been utilized with the aim to enhance its performance in
devices [39,40]. In addition, possible aggregation could be avoided
through appropriate structural modification such as introducing
alkyl to aniline and thiophene units [41e46]. Aromatic rings have
also been introduced to pyrrole via NeH bond in order to restrain
the aggregation of the dyes and achieve good performance in our
previous research [47e49]. Thiophene derivatives have been widely
used as conjugated bridge for sensitizers because of their high
polarizability, tunable spectroscopic and electrochemical proper-
ties. A lot of studies are concerning dyes containing thiophene or
furan as conjugated bridge, which reached high photovoltaic effi-
ciencies. However, it is relatively difficult to introduce anti-
free dyes have been synthesized and shown impressive
h values in
the range of 5e10.3% [12e32]. Four critical processes determine the
photovoltaic performance of the DSSCs: electron injection from the
dye to the conduction band of TiO2; recombination of the injected
electrons on TiO2 with the oxidized dye; dark current generated
due to the reduction of Iꢀ3 by the injected electrons; and the
regeneration of the dye by the electrolyte which is dependent on
* Corresponding author. Tel.: þ86 27 62254108; fax: þ86 27 68756757.
** Corresponding author. Tel.: þ86 27 62306885; fax: þ86 27 68756757.
E-mail addresses: qianqian-alinda@163.com (Q. Li), lizhen@whu.edu.cn (Z. Li).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.03.027

J. Shi et al. / Dyes and Pigments 95 (2012) 244e251
245
aggregation group to thiophene and furan units [50e53]. Therefore,
we try to synthesize the molecules which contain anti-aggregation
groups in donor, and set thiophene derivatives and furan as
conjugated bridge.
Tert-butyl was widely used in many opto-electronic materials
such as organic light-emitting diodes (OLED), for its superior
Ag/AgCl electrode were used as the working electrode and quasi-
reference electrode, respectively. The ferrocene/ferrocenium redox
couple was used for potential calibration. Elemental analyses were
performed by a Carlo Erba 1106 microelemental analyzer. HR-ESI-
TOF mass spectra were recorded on a Waters Micromass LCT
Premier XE.
performance in preventing
coplanar structure also introduced tert-butyl to inhibit aggregation
[54e56]. Interestingly, a few research groups reported some De eA
pep stacking. Some dyes possess
2.2.1. Synthesis of 4-(bis-(4-bromo-phenyl)-amino)-benzaldehyde
To a solution of 4-(diphenylamino) benzaldehyde (30 mmol,
8.0 g) in dichloromethane (150 mL) was added bromine
(60.0 mmol, 9.6 g) dropwise within 1.5 h at 0 ꢁC. The resultant
mixture was stirred at room temperature for 6 h and then aqueous
KOH was added. The mixture was extracted with dichloromethane.
The organic layer was collected, washed with water, dried over
anhydrous sodium sulfate. Filtered and the solvent was removed by
rotator evaporation. The residue was purified by column chroma-
tography over silica gel to give the product (9.20 g, 71%) as green
p
dyes bearing tert-butyl moiety and its functions in molecule design
was to improve the solubility [57]. Herein, two 4-tert-butylbenzene
moieties were chosen as additional groups of triphenylamine. First,
the solubility of the dye will be significantly improved; secondly, the
introduction of the benzene ring could expand the molecular
conjugated system thus enhancing their ability of light harvesting,
and thirdly, there are two tert-butyl in the donor side just like two
umbrellas, which keep a certain distance between molecules,
reflecting their function to prevent aggregation. Thiophene, 2,20-
bithiophene, 3,4-ethylenedioxythiophene (EDOT) and furan group
were selected to serve as conjugation bridge for their excellent
performance in electronic transmission (Scheme 1).
yellow solid. 1H NMR (CDCl3, 300 MHz)
d (ppm): 9.84 (s, 1H, eCHO),
7.71 (d, J ¼ 8.7 Hz, 2H, ArH), 7.48 (d, J ¼ 8.7 Hz, 4H, ArH), 7.06e7.01
(m, 6H, ArH).
2.2.2. Synthesis of 1
A mixture of 4-(bis-(4-bromo-phenyl)-amino)-benzaldehyde
(11.6 mmol, 5.0 g), 4-tert-butylphenylboronic acid (25.5 mmol,
4.5 g), sodium carbonate (10.0 or 20.0 equiv), THF (monomer
concentration about 0.025 M)/water (2:1 in volume), and tetra-
kis(triphenylphosphine)palladium Pd(PPh3)4 (3e5 mol%) was
carefully degassed and charged with nitrogen. The reaction was
stirred for 32 h at 80 ꢁC. After cooled to room temperature, the
organic layer was separated, dried over sodium sulfate, and evap-
orated to dryness. The crude product was purified by column
chromatography over silica gel to give the product (4.84 g, 78%) as
2. Experimental
2.1. Materials
Tetrahydrofuran (THF) was dried over and distilled from KeNa
alloy under an atmosphere of dry nitrogen. N,N-Dimethylforma-
mide (DMF) was dried over and distilled from CaH2 under an
atmosphere of dry nitrogen. 1,2-Dichloromethane was dried over
and distilled from phosphorus pentoxide. Phosphorus oxychloride
was freshly distilled before use. All reagents were purchased from
Alfa Aesar and used as received. Compound 2- and 4-(diphenyla-
mino)benzaldehyde was prepared following the procedure repor-
ted in the literature [58].
a yellow solid. Yield: 75%. 1H NMR (CDCl3, 300 MHz)
d (ppm): 9.76
(s, 1H, eCHO), 7.64 (d, J ¼ 8.1 Hz, 1H, ArH), 7.55e7.41 (m, 8H, ArH),
7.26e7.16 (m, 5H, ArH), 7.06 (d, J ¼ 4.8 Hz, 2H, ArH), 6.98 (d,
J ¼ 8.1 Hz, 2H, ArH), 6.91e6.83 (m, 2H, ArH), 1.36 (s, 18H, eCH3).
2.2. Instrumentation
2.2.3. General synthesis of 3aed
To a solution of the (4-(bis(40-tert-butylbiphenyl-4-yl)amino)
phenyl)methanol (5.4 g, 10 mmol) in 25 mL triethylphosphite was
added iodine (2.53 g, 10 mmol) at 0 ꢁC slowly. The resulting mixture
was stirred for 5 min at 0 ꢁC, then warmed to room temperature
and stirred for 2.5 h. After evaporation of triethylphosphite, column
chromatography and removal of trace triethyl phosphate under
vacuum at 50 ꢁC afforded compound 2 as light green solid (5.33 g,
85%). Compound 2 (3 mmol) was suspended in 20 mL of anhydrous
1H and 13C NMR spectroscopy study was conducted with a Varian
Mercury 300 spectrometer using tetramethylsilane (TMS;
d
¼ 0 ppm)
as internal standard. UVevisible spectra were obtained using a Shi-
madzu UV-2550 spectrometer. Differential pulse voltammetric
(DPV) was carried out on a CHI 660 voltammetric analyzer at room
temperature in nitrogen-purged anhydrous acetonitrile with tetra-
butylammonium hexafluorophosphate (TBAPF6) as the supporting
electrolyte at a scanning rate of 100 mV/s. A platinum disk and an
S
LI-17
S
S
S
LI-18
LI-19
N
spacer
=
spacer
CN
COOH
O
O
O
LI-20
Scheme 1. The structures of LI-17, LI-18, LI-19 and LI-20.

246
J. Shi et al. / Dyes and Pigments 95 (2012) 244e251
tetrahydrofuran under an atmosphere of dry nitrogen. t-BuOK
(0.56 g, 5 mmol) was added directly as a solid and the resultant
mixture was stirred at room temperature for 10 min. After the
addition of the solution of thiophene-2-carbaldehyde, 2,20-bithio-
8H, ArH), 6.91 (s, 1H, eCH]CHe), 6.80 (s, 1H, eCH]CHe), 4.37 (t,
J ¼ 27.3 Hz, 4H, eCH2e), 1.36 (s, 18H, eCH3). C47H45NO3S: C, 80.19;
H, 6.44; N, 1.99; Found: C, 80.29; H, 6.49; N, 1.52.
phene-5-carbaldehyde,
3,4-ethylene-dioxothio-phene-2-
2.2.4.4. 4d. Red solid. Yield: 72%. 1H NMR (CDCl3, 300 MHz)
carbaldehyde or furan-2-carbaldehyde (2.5 mmol) in anhydrous
tetrahydrofuran (20 mL) dropwise, the reaction mixture was stirred
at room temperature overnight, then poured into 100 mL of water.
The organic product was extracted with chloroform, and dried over
anhydrous Na2SO4. After removing the solvent, the crude product
was purified through a silica gel chromatography column.
d (ppm): 9.57 (s, 1H, eCHO), 7.52e7.39 (m, 11H, ArH), 7.33 (s, 1H,
ArH), 7.22e7.12 (m, 8H, ArH), 6.82 (d, J ¼ 16.5 Hz, 2H, eCH]CHe),
6.49 (s, 2H, ArH), 1.36 (s, 18H, eCH3). C45H43NO2: C, 85.81; H, 6.88;
N, 2.22; Found: C, 85.46; H, 6.62; N, 2.08.
2.2.5. General synthesis of LI-17eLI-20
A mixture of 4aed (1.00 equiv) and cyanoacetic acid (1.20 equiv)
were vacuum-dried, then MeCN (30 mL), THF (30 mL) and piperi-
2.2.3.1. 3a. Light yellow solid. Yield: 74%. 1H NMR (CDCl3,
300 MHz)
d
(ppm): 7.52e7.40 (m, 12H, ArH), 7.31 (d, J ¼ 8.1 Hz, 4H,
dine (10 mL) were added. The solution was refluxed for 3e6 h. After
ArH), 7.21e7.09 (m, 7H, ArH), 6.99 (s, 1H, eCH]CHe), 6.77 (s, 1H,
eCH]CHe), 1.37 (s, 18H, eCH3). Anal. Calcd for: C44H43NS: C,
85.53; H, 7.01; N, 2.27; Found: C, 85.71; H, 6.94; N, 2.15.
the solution was cooled, the organic layer was removed under
vacuum. The pure product was purified by recrystallization or
column chromatography.
2.2.3.2. 3b. Pale yellow solid. Yield: 81%. 1H NMR (CDCl3, 300 MHz)
2.2.5.1. LI-17. Dark red solid. Yield: 67%. 1H NMR (CDCl3, 300 MHz)
d
(ppm): 7.52e7.44 (m, 13H, ArH), 7.37 (d, J ¼ 8.1 Hz, 2H, ArH),
d (ppm): 8.41 (s, 1H, eCH]), 7.89 (s, 1H, ArH), 7.58e7.38 (m, 15H,
7.21e7.06 (m, 9H, ArH), 6.94 (s, 1H, ArH), 6.85 (d, J ¼ 16 Hz, 2H,
eCH]CHe), 1.37 (s, 18H, eCH3). Anal. Calcd for: C48H45NS2: C,
82.36; H, 6.48; N, 2.00; Found: C, 82.63; H, 6.42; N, 2.23.
ArH), 7.24 (s, 1H, ArH), 7.19 (s, 1H, ArH), 7.09 (d, J ¼ 6.3 Hz, 4H, ArH),
6.97 (d, J ¼ 6.6 Hz, 2H, eCH]CHe), 1.27 (s, 18H, eCH3). 13C NMR
(DMSO-d6, 75 MHz) d (ppm): 164.4, 150.2, 148.0, 146.2, 137.2, 135.8,
134.6, 132.5, 130.7, 129.1, 128.3, 127.5, 126.6, 126.3, 125.4, 123.2,
2.2.3.3. 3c. Yellow solid. Yield: 82%. 1H NMR (CDCl3, 300 MHz)
117.6, 97.8, 34.8, 31.7. HRMS (HR-ESI-TOF): m/z calcd for
d
(ppm): 7.47e7.37 (m, 10H, ArH), 7.28 (d, J ¼ 8.1 Hz, 2H, ArH), 7.11
C
48H44N2O2S [M þ H]þ 713.3202, found 713.3206.
(d, J ¼ 8.1 Hz, 4H, ArH), 7.05e7.01 (t, J ¼ 14.4 Hz, 3H, ArH), 6.95 (s,
1H, ArH), 6.78 (s, 1H, eCH]CHe), 6.72 (s, 1H, eCH]CHe), 6.13 (s,
1H, ArH), 4.16 (t, J ¼ 21.3 Hz, 4H, eCH2e), 1.29 (s, 18H, eCH3). Anal.
Calcd for: C46H45NO2S: C, 81.74; H, 6.71; N, 2.07; Found: C, 81.61; H,
6.63; N, 1.92.
2.2.5.2. LI-18. Dark red solid. Yield: 71%. 1H NMR (CDCl3, 300 MHz)
(ppm): 8.42 (s, 1H, eCH]), 7.93 (s, 1H, ArH), 7.59e7.37 (m, 17H,
d
ArH), 7.31 (s, 1H, ArH), 7.20 (s, 1H, ArH), 7.10 (d, J ¼ 7.2 Hz, 4H, ArH),
6.99 (d, J ¼ 8.1 Hz, 2H, eCH]CHe), 1.28 (s, 18H, ArH). 13C NMR
(DMSO-d6, 75 MHz)
d (ppm): 164.3, 150.1, 147.4, 146.4, 145.6, 141.5,
2.2.3.4. 3d. Yellow solid. Yield: 65%. 1H NMR (CDCl3, 300 MHz)
137.2, 135.6, 134.7, 133.8, 131.3, 129.9, 128.5, 128.2, 126.6, 126.3,
d
(ppm): 7.50e7.38 (m,10H, ArH), 7.29 (d, J ¼ 8.1 Hz, 2H, ArH), 7.19 (d,
125.1, 123.7, 117.6, 99.8, 34.8, 31.7. HRMS (HR-ESI-TOF): m/z calcd for
C
J ¼ 7.5 Hz, 2H, ArH), 7.15e7.04 (m, 9H, ArH), 6.97 (s,1H, eCH]CHe),
52H47N2O2S2 [M þ H]þ 795.3079, found 795.3096.
6.81 (s, 1H, eCH]CHe), 1.36 (s, 18H, eCH3). Anal. Calcd for:
C
44H43NO: C, 87.81; H, 7.20; N, 2.33; Found: C, 87.56; H, 6.98; N, 2.51.
2.2.5.3. LI-19. Dark red solid. Yield: 65%. 1H NMR (CDCl3, 300 MHz)
(ppm): 8.16 (s, 1H, eCH]), 7.61e7.44 (m, 10H, ArH), 7.43 (d,
d
2.2.4. General synthesis of 4aed
J ¼ 8.1 Hz, 4H, ArH), 7.18e7.10 (m, 6H, ArH), 6.96 (d, J ¼ 8.1 Hz, 2H,
DMF (2.4 mmol) was added to freshly distilled POCl3 (1.8 mmol)
under an atmosphere of dry nitrogen at 0 ꢁC, and the resultant
solution was stirred until its complete conversion into a glassy
solid. After the addition of compound 3aed (1.2 mmol) in 1,2-
dichloromethane (20 mL) dropwise, the mixture was stirred at
room temperature overnight, then poured into an aqueous solution
of sodium acetate (1 M, 300 mL), and stirred for another 2 h. The
mixture was extracted with chloroform for several times, the
organic fractions were combined and dried over anhydrous Na2SO4.
After removing the solvent under vacuum, the crude product was
purified through a silica gel chromatography column.
eCH]CHe), 4.43 (t, J ¼ 24.8 Hz, 4H, eCH2e), 1.29 (s, 18H, eCH3).
13C NMR (DMSO-d6, 75 MHz)
d (ppm): 163.7, 148.9, 148.2, 146.7,
144.9, 138.0, 135.9, 134.6, 131.2, 130.9, 129.3, 127.7, 127.0, 125.3,
125.1, 124.2, 121.8, 116.7, 92.9, 64.1, 33.6, 30.5. HRMS (HR-ESI-TOF):
m/z calcd for C50H47N2O4S [M þ H]þ 771.3257, found 771.3234.
2.2.5.4. LI-20. Dark red solid. Yield: 80%. 1H NMR (CDCl3, 300 MHz)
d
(ppm): 7.99 (s, 1H, eCH]), 7.63e7.42 (m, 14H, ArH), 7.34 (s, 1H,
ArH), 7.29 (s, 1H, ArH), 7.13 (d, 2H, ArH), 7.03 (d, 4H, ArH), 6.84 (s,
2H, eCH ¼ CHe), 1.30 (s, 18H, eCH3). 13C NMR (DMSO-d6, 75 MHz)
d
(ppm): 164.9, 159.7, 150.4, 148.4, 146.4, 137.8, 137.4, 136.1, 133.4,
130.6, 129.4, 128.5, 126.8, 126.5, 125.7, 123.5, 117.7, 98.1, 35.1, 31.9.
HRMS (HR-ESI-TOF): m/z calcd for C48H44N2O3 [M þ H]þ 697.3430,
found 697.3425.
2.2.4.1. 4a. Red solid. Yield: 80%. 1H NMR (CDCl3, 300 MHz)
d
(ppm): 9.85 (s, 1H, eCHO), 7.67 (s, 1H, ArH), 7.53e7.39 (m, 15H,
ArH), 7.22e7.11 (m, 8H, ArH, eCH]CHe), 1.37 (s, 18H, eCH3). Anal.
Calcd for: C45H45NOS: C, 83.68; H, 6.71; N, 2.17; Found: C, 83.79; H,
6.76; N, 2.01.
2.3. Device fabrication
A layer of ca. 5
FTO conducting glass by screen printing and then dried for
6 min at 125 ꢁC. This procedure was repeated 2 times (ca. 10
m)
and finally coated by a layer (ca. 4 m) of TiO2 paste (Ti-nano-
xide 300) as the scattering layer. The tri-layer TiO2 electrodes
were gradually heated under an air flow at 275 ꢁC for 5 min,
325 ꢁC for 5 min, 375 ꢁC for 5 min, 450 ꢁC for 15 min, and 500 ꢁC
for 15 min. The sintered film was further treated with 0.2 M
TiCl4 aqueous solution at room temperature for 12 h, then
mm TiO2 (13 nm paste, T/SP) was coated on the
2.2.4.2. 4b. Red solid. Yield: 61%. 1H NMR (CDCl3, 300 MHz)
d
(ppm): 9.77 (s, 1H, eCHO), 7.59 (s, 1H, ArH), 7.44e7.37 (m, 12H,
m
ArH), 7.30 (d, J ¼ 7.5 Hz, 2H, ArH), 7.18e7.04 (m, 8H, ArH), 6.98e6.82
(m, 3H, ArH, eCH]CHe),1.29 (s, 18H, eCH3). C49H45NOS2: C, 80.84;
H, 6.23; N, 1.92; Found: C, 80.50; H, 6.54; N, 1.74.
m
2.2.4.3. 4c. Red solid. Yield: 66%. 1H NMR (CDCl3, 300 MHz)
d
(ppm): 9.88 (s, 1H, eCHO), 7.51e7.38 (m, 12H, ArH), 7.21e7.01 (m,

J. Shi et al. / Dyes and Pigments 95 (2012) 244e251
247
Br
OH
B
spacer
CHO
NaBH4
OH
CHO
N
O
N
N
CHO
P
Triethyl phosphite
Pd(PPh3)4, K2CO3
EtO OEt
Br
S
S
S
a
S
b
c
1
2
spacer
O
O
O
d
cyanoacetic acid,
piperidine, CH3CN
POCl3
N
N
N
spacer
spacer
CHO
spacer
DMF
CN
COOH
LI-17: spacer= thiophene
LI-18: spacer= 2,2'-bithiophene
LI-19: spacer= 3,4-ethylenedioxothiophene
Li-20: spacer= furan
3a-d
4a-d
Scheme 2. Synthetic route of LI-17, LI-18, LI-19 and LI-20.
washed with water and ethanol, and annealed at 450 ꢁC for
30 min. After the film was cooled to 50 ꢁC, it was immersed into
a 3 ꢂ 10ꢀ4 M dye bath in CH2Cl2 solution and maintained in the
dark for 24 h at room temperature. The electrode was then
rinsed with CH2Cl2 and dried. The size of the TiO2 electrodes
used was 0.28 cm2. To prepare the counter electrode, the Pt
catalyst was deposited on cleaned FTO glass by coating with
a drop of H2PtCl6 solution (0.02 M 2-propanol solution) with
heat treatment at 400 ꢁC for 15 min. A hole (0.8 mm diameter)
was drilled on the counter electrode using a drill-press. The
perforated sheet was cleaned with ultrasound in an ethanol bath
for 10 min. For the assembly of DSSCs, the dye-covered TiO2
electrode and Pt-counter electrode were assembled into
a sandwich type cell and sealed with a hot-melt gasket of
25 mm thickness made of the ionomer Surlyn 1702
(DuPont). The electrolyte consisting of 0.6 M 1,2-dimethyl-3-
propylimidazolium iodide (DMPII), 0.1 M LiI, 0.05 M I2, and
0.5 M 4-tert-butylpyridine (TBP) in a mixture of acetonitrile and
methoxypropionitrile (volume ratio, 7:3) was introduced into
the cell via vacuum backfilling from the hole in the back of the
counter electrode. Finally, the hole was sealed using a UV-melt
gum and a cover glass.
0.196 cm2. The photocurrent action spectra were measured with
an IPCE test system consisting of a Model SR830 DSP Lock-In
Amplifier and
a Model SR540 Optical Chopper (Stanford
Research Corporation, USA), a 7IL/PX150 xenon lamp and power
supply, and a 7ISW301 Spectrometer.
3. Results and discussion
3.1. Synthesis of the dyes
The whole synthetic route is shown in Scheme 2. Under the
normal Suzuki coupling reaction, 1 was obtained from the bromine
atom exposed intermediate and 4-tert-butylphenylboronic acid
with good yield. Then, the followed Wittig reaction between 2 and
aromatic aldehyde gave the corresponding compounds 3ae3d,
which underwent another Vilsmeier reaction to yield the alde-
hyde 4ae4d. Finally, the four dyes were produced from 4ae4d
upon the Knoevenagel condensation reaction with cyanoacetic
acid in the presence of piperidine. All the compounds and four
new organic sensitizers (LI-17eLI-20) were confirmed by 1H and
13C NMR, and HRMS.
3.2. Absorption spectra
2.4. Photovoltaic properties measurements
The four dyes were soluble in common organic solvents, such as
dichloromethane, THF, DMF, and DMSO. As shown in their UVevis
spectra tested in ethanol (Fig. 1, Table 1), two distinct absorptions
were observed around 340 and 450 nm. The absorption band
Photovoltaic measurements employed an AM 1.5 solar
simulator equipped with a 300 W xenon lamp (Model No. 91160,
Oriel). The power of the simulated light was calibrated to
100 mW cmꢀ2 using a Newport Oriel PV reference cell system
(Model 91150V). JeV curves were obtained by applying an
external bias to the cell and measuring the generated photo-
current with a Keithley model 2400 digital source meter. The
voltage step and delay time of photocurrent were 10 mV and
40 ms, respectively. Cell active area was tested with a mask of
around 340 nm was assigned to a
pep* transition, while the
absorption band with lmax around 450 nm corresponding to an
intramolecular charge transfer (ICT) between the TPA donor part
and the acceptor end group. The absorption maximum of LI-20 at
456 nm was red shift by 6 nm in comparison with that of LI-17, as
LI-20 containing a furan unit instead of thiophene. And LI-19

248
J. Shi et al. / Dyes and Pigments 95 (2012) 244e251
1.5
1.2
0.9
0.6
0.3
0.0
0.8
0.6
0.4
0.2
0.0
LI-17
LI-18
LI-19
LI-20
LI-17
LI-18
LI-19
LI-20
330 400 470 540 610 680
300 370 440 510 580 650
Wavelength(nm)
Wavelength(nm)
Fig. 2. UVevis spectra of the dyes on TiO2 films.
Fig. 1. UVevis spectra of dyes in ethanol. Concentration: 3.0 ꢂ 10ꢀ5 mol/L.
vs NHE (Ered), which corresponded to the lowest unoccupied
exhibits a red-shift peaks at 467 nm (
3
¼ 37,600 Mꢀ1 cmꢀ1) in
molecular orbital (LUMO), could be calculated from Eox to E0ꢀ0
.
ethanol compared to the others, the appreciable red shift is
attributed to the presence of EDOT moieties, which enhanced the
extent of electron delocalization over the whole molecule. The
molar extinction coefficient of LI-17, 24,500 Mꢀ1 cmꢀ1 (450 nm), is
much higher than that of N719, indicating that these dyes have
good light-harvesting ability. Both blue and red shift of the
absorption band were reported for dipolar organic dyes upon
adsorption on the TiO2 surface, due to different interaction between
the dyes and TiO2, or between the dyes. In this study, the absorption
spectra of the dyes became broadened after adsorption on the TiO2
surface (Fig. 2), which should favor the light harvesting of the solar
cells and thus increase the photocurrent response region, leading to
the increase of Jsc. The maximum absorption peaks for LI-17eLI-20
on the TiO2 films are red shift by 4, 27, 6 and 19 nm respectively, in
comparison with their spectra in solutions. The red shifts of the
absorption spectra on TiO2 of LI-17eLI-20 are due to the J-aggre-
gation on the TiO2 surface, in which J-aggregation can also present
readily because of the carboxyl unit of the dyes. Such broaden and
red-shift absorption has been turned up in other organic dyes, and
this phenomenon proves that the introduction of tert-butyl may
Table 1 summarized the electrochemical properties of the four
dyes. The potential energy of ionization decreased slightly in the
order of LI-17 > LI-20 > LI-19 > LI-18. As shown in Fig. 4, the
HOMO levels thus obtained within the range of 0.94e1.01 eV,
which were more positive than the iodine/iodide redox potential
value (0.4 eV), indicating that the oxidized dyes formed after
electron injection into the conduction band of TiO2 could thermo-
dynamically accept electrons from Iꢀ ions. The LUMO levels of the
dyes were estimated by the values of Eox and the E0e0 band gaps,
and these were sufficiently more negative than the conduction-
band-edge energy level (Ecb) of the TiO2 electrode (ꢀ0.5 V vs.
NHE), which implies that electron injection from the excited dye
into the conduction band of TiO2 is energetically permitted.
3.4. Theoretical approach
The structures of the dyes were analyzed using a the B3LYP
exchange-correlation functional and a 6-31g* basis set; salvation
effects were included by means of the polarizable continuum
model to gain further insight into the correlation between structure
and the physical properties as well as the device performance. To
model the electronic state of the dyes adsorbed on the TiO2 surface,
we employed the carboxylate of the dyes because they should be
bonded to the TiO2 surface in these form. The electron distribution
of the HOMO and LUMO of LI-17eLI-20 was shown in Fig. 5. The
indeed inhibit the pep stacking and H-aggregation.
3.3. Electrochemical properties
The oxidation potential of the two dyes was determined from
the peak potentials (Ep) by DPV (Fig. 3). The oxidation potential of
the dyes was calculated from the peak potentials (Ep) by DPV; the
oxidation potential vs NHE (Eox) corresponded to the highest
occupied molecular orbital (HOMO), while the reduction potential
Table 1
LI-17
LI-18
Absorbance and electrochemical properties of dyes.
a
a
b
c
d
Dye
lmax
3
lmax E0e0 Ep
(nm) (eV) (V)
Eoxe (V) vs Eredf (V) vs
(nm) (Mꢀ1 cmꢀ1
)
NHE
NHE
0.5
0.7
0.9
1.1 0.5
0.7
0.9
1.1
potential vs Ag/AgCl (V)
potential vs Ag/AgCl (V)
LI-17 450
LI-18 441
LI-19 467
LI-20 456
24,500
32,400
23,700
33,200
454
468
473
475
2.29 0.81, 0.93 1.01
2.21 0.74, 0.95 0.94
ꢀ1.28
ꢀ1.27
ꢀ1.21
ꢀ1.19
2.20 0.79
2.19 0.80
0.99
1.00
a
Absorption spectra of dyes measured in ethanol with the concentration of
3 ꢂ 10ꢀ5 mol/L.
b
Absorption spectra of dyes adsorbed on the surface of TiO2.
The band gap, E0e0 was derived from the observed optical edge.
c
LI-20
LI-19
d
Ep is the peak of DPV (the DPV of the dyes were measured in CH2Cl2 with 0.1 M
(n-C4H9)4NPF6 as electrolyte (scanning rate, 100 mV/s; working electrode and
counter electrode, Pt wires; reference electrode, Ag/AgCl)).
0.5 0.6 0.7 0.8 0.9 0.6 0.7 0.8 0.9 1.0
e
The oxidation potential (Eox) referenced to calibrated Ag/AgCl was converted to
potential vs Ag/AgCl (V)
potential vs Ag/AgCl (V)
the NHE reference scale: Eox ¼ Ep þ 0.197 V.
f
Ered was calculated from Eox to E0e0
.
Fig. 3. DPV of dyes.

J. Shi et al. / Dyes and Pigments 95 (2012) 244e251
249
80
60
40
20
0
E NHE
-2.0
LI-17
LI-18
LI-19
LI-20
-1.5
-1.0
-0.5
0.0
LI-17 LI-18
LI-19
LI-20
TiO
-
I /I
-
0.5
380
460
540
620
700
Wavelength (nm)
1.0
Fig. 6. Spectra of monochromatic incident photon-to-current conversion efficiency
(IPCE) for DSSCs based on LI-17eLI-20.
1.5
Fig. 4. Schematic representation of the band positions in DSSCs based on LI-17eLI-20.
The energy scale is indicated in electron volts using the normal hydrogen electrode
(NHE).
the values were all higher than 50% in the region of 380e510 except
LI-20. The value of LI-17 exceeds 60% over the spectral region from
400 to 530 nm, reaching its maximum of 68% at 460 nm. The IPCE
values of LI-18 reached its maximum of 58% at 450 nm. And the LI-
19 reached 62% at 450 nm. From the IPCE performance of dyes, we
can infer that, the DSSCs based on LI-17 would generate the highest
conversion yield in these dyes.
Fig. 7 shows the photocurrentevoltage (JeV) plots of DSSCs
fabricated with these dyes under AM 1.5G simulated sunlight at
100 mW cmꢀ2, and the pertinent performance data are described in
Table 2, the photovoltaic characteristic parameters, i.e., short circuit
current (Jsc), open-circuit photovoltage (Voc), fill factor (ff), and
HOMO consists of delocalized
p orbitals on spacer unit, with
greatest density in TPA, and the LUMO of dyes show a greater
contribution localized in cyanoacrylic acid part, giving the strongest
DeA strength. Thus, the HOMO to LUMO excitation induced by light
irradiation could move the electron distribution from the whole
molecules to the anchoring moieties. As depicted above, it also
revealed that the dyes were completely planar except for the 4-tert-
butylbenzene moieties linked to TPA group to avoid steric clashes,
and those bonded on the TPA ring had some electron transition to
the planar system. The two tert-butyl moieties at the head of the
molecule act just like two umbrellas, to keep a certain distance
between the dyes when absorbed on TiO2.
solar-to-electrical photocurrent density (h). The broadening of the
IPCE spectra is desired for a larger photocurrent which explains the
higher efficiency observed for the dyes. DSSC based on LI-17 show
the highest solar to electricity conversion efficiency of 5.35%
(Jsc ¼ 12.65 mA cmꢀ2, Voc ¼ 675 mV, ff ¼ 0.63) among these dyes,
which should be ascribed to its best performance in IPCE. From
Fig. 4, we observed that the LI-17 showed the deepest HOMO and
broadest band gap, but not the highest Voc, these may ascribe to the
charge recombination. The excited state oxidation potentials of the
dyes assume the following order: LI-20 > LI-19 > LI-18 > LI-17.
More negative excited state oxidation potential observed for LI-17
3.5. Photovoltaic performance of DSSCs
The incident photon-to-current conversion efficiencies (IPCEs)
of dyes as shown in Fig. 6, were obtained with a sandwich cell using
0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 0.1 M LiI, 0.05 M
I2 and 0.5 M 4-tert-butylpyridine (TBP) in dry acetonitrile (CH3CN).
The IPCE of the dyes illustrate that the visible light can be converted
to photocurrent efficiently in the region from 400 to 620 nm, and
Fig. 5. Frontier orbitals of the dyes LI-17eLI-20 optimized at the B3LYP/6-31þG (D) level.

250
J. Shi et al. / Dyes and Pigments 95 (2012) 244e251
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269e77.
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Voltage (V)
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Dye
Jsc (mA cmꢀ2
)
Voc (V)
ff
h (%)
LI-17
LI-18
LI-19
LI-20
12.65
11.8
10.31
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0.64
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a
Illumination: 100 mW cmꢀ2 simulated AM 1.5 G solar light; electrolyte con-
taining: 0.1 M LiI þ 0.05 M I2 þ 0.6 M DMPII þ 0.5 M TBP in the mixed solvent of
acetonitrile and 3-methoxypropionitrile (7:3, v/v).
among the series, gives larger driving force the electron injection
from this dye. However, this trend is not matching with the
photovoltaic efficiency trend, indicating that other factors also
influence the photovoltaic performance. Therefore we believe that
the tert-butyl in donor does have play an important role in anti-
aggregation, but for different structures of the dyes, the effect of
the anti-aggregation to the dyes was not similar, and the results
indicated that the function is effective to the dyes with thiophene
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4. Conclusions
In conclusion, a new series of organic dyes (LI-17eLI-20), which
contain TPA substituted by two 4-tert-butylbenzene moieties as the
donor, a cyanoacrylic acid as the acceptor and anchoring groups,
and thiophene, 2,20-bithiophene, 3,4-ethylenedio-xythiophene
(EDOT) and furan group as the conjugate bridge, were synthesized
and applied in DSSCs. The 4-tert-butylbenzene moieties were
employed as a part of anti-aggregation group. The absorption
spectra, electrochemical and photovoltaic performance of the
dyes were studied. Among these dyes, DSSCs based on LI-17 show
the best light to electricity conversion efficiency of 5.35%
(Jsc ¼ 12.65 mA cmꢀ2, Voc ¼ 675 mV, ff ¼ 0.63). The preliminary
results demonstrated that the introduction of tert-butyl groups
may be able to play the anti-aggregation effect, and more follow-up
work is being carried out in our laboratory.
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We are grateful to the National Science Foundation of China (no.
21002075), and the National Fundamental Key Research Program
(no. 2011CB932702) for financial support.
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