Influence of different substituents linked on fluorene spacer in organic sensitizers on photovoltaic properties

Article abstract of DOI:10.1016/j.dyepig.2013.12.020

New metal-free organic sensitizers containing a fluorene unit as a π-conjugated system, a diphenylamine as an electron donor, and a cyanoacrylic acid moiety as an electron acceptor were synthesized and used for dye-sensitized solar cells. The photophysical and electrochemical properties of these dyes were investigated, and their performances as sensitizers in solar cells were measured. One solar cell containing the n-hexyl group in the fluorene unit produced an η of 5.15% (J_sc = 9.69 mA cm^-2, V_oc = 0.77 V, and ff = 0.70) under 100 mW cm^-2 simulated AM 1.5 G solar irradiation (100 mW cm^-2).

Full text of DOI:10.1016/j.dyepig.2013.12.020

Dyes and Pigments 104 (2014) 8e14
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Influence of different substituents linked on fluorene spacer in organic
sensitizers on photovoltaic properties
*
*
Zhenhua Ci, Xiaoqiang Yu , Chaolei Wang, Tingli Ma, Ming Bao
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116023, China
a r t i c l e i n f o
a b s t r a c t
Article history:
New metal-free organic sensitizers containing a fluorene unit as a p-conjugated system, a diphenylamine
Received 19 November 2013
Received in revised form
17 December 2013
Accepted 20 December 2013
Available online 31 December 2013
as an electron donor, and a cyanoacrylic acid moiety as an electron acceptor were synthesized and used
for dye-sensitized solar cells. The photophysical and electrochemical properties of these dyes were
investigated, and their performances as sensitizers in solar cells were measured. One solar cell containing
the n-hexyl group in the fluorene unit produced an
h
of 5.15% (Jsc ¼ 9.69 mA cmꢀ2, Voc ¼ 0.77 V, and
ff ¼ 0.70) under 100 mW cmꢀ2 simulated AM 1.5 G solar irradiation (100 mW cmꢀ2).
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Dye-sensitized solar cells
Organic dye
n-Hexyl group
Fluorene
p-Conjugated system
Photon-to-current conversion efficiency
1. Introduction
phene [28,29], benzo[b]thiophene [30,31], benzothiadiazole [32e
35], or dibenzosilole [36] have been designed and synthesized as
efficient sensitizers to achieve high conversion efficiency in DSSC
Dye-sensitized solar cells (DSSCs) have attracted significant
attention in recent years because of their high efficiency, low cost,
and facile fabrication [1e3]. DSSCs based on Ru(II)-polypyridyl
complex photosensitizers, such as N3, N719, and the black dye,
have achieved remarkable conversion efficiency of up to 11% under
AM 1.5 irradiation conditions [4e6]. However, the main drawbacks
of the Ru(II) complex sensitizers include the cost of ruthenium
metal, the requirement for cautious synthesis, and the tedious
purification process. Metal-free organic dyes have gained
increasing attention because of their unique advantages, such as
high molar absorption coefficient, ease of structure modification,
and relatively low material cost [7e10].
devices. The
photoelectronic properties of De
structures should be introduced to the
inhibit unfavorable aggregation [37]. However, the influence
of different substituted groups on the -conjugated bridge to avoid
aggregation has not been systematically studied.
This study presents three new organic dyes (F1, F2, and F3) with
different group-substituted fluorene moieties as -conjugated
p
-conjugated bridge has a great influence on the
eA dyes. Generally, some steric
-conjugated bridge to
p
p
pep
p
pep
p
bridge, a diphenylamine moiety as electron donor, and a cyano-
acetic acid as electron acceptor (Fig. 1). The study also investigates
the photophysical, electrochemical, and photovoltaic properties.
A common organic dye for DSSCs contains a structure of elec-
tron donor/acceptor (DeA) that is linked through a
p-conjugated
2. Experimental section
bridge; this structure is called the De eA molecular structure [11].
p
In this structure, triphenylamine derivatives have been widely used
as the electron donor (D), whereas a cyanoacrylic acid moiety acts
2.1. General analytical measurements
as the electron acceptor (A). De
peA dyes based on triphenylamine
All chemicals were used as received from commercial sources
without purification. Solvents for chemical synthesis, such as
dichloromethane, dimethylformamide, toluene, and tetrahydro-
furan, were purified by distillation. 2,7-Dibromo-9,9-diphenyl-9H-
fluorene [38] and 2,7-dibromo-9,9-dihexyl-9H-fluorene [39] were
synthesized according to the reported literature. 1H and 13C NMR
spectra were recorded on either Varian Inova-400 spectrometer
moieties with various -conjugated bridges, such as benzene [12e
p
18], thiophene [19e22], thienothiophene [23e27], dithienothio
* Corresponding authors. Tel.: þ86 411 84986181; fax: þ86 411 84986180.
E-mail address: yuxiaoqiang@dlut.edu.cn (X. Yu).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.12.020

Z. Ci et al. / Dyes and Pigments 104 (2014) 8e14
9
Fig. 1. Molecular structures of dyes F1, F2, and F3.
(400 MHz for 1H; 100 MHz for 13C) or Bruker Avance II-400 spec-
trometer (400 MHz for 1H; 100 MHz for 13C); CDCl3 and TMS were
used as solvent and internal standard, respectively. High-resolution
mass spectra were recorded on either Q-TOF or GC-TOF mass
spectrometer.
nitrogen atmosphere for 24 h. After cooling to room temperature,
water was added to quench the reaction. The product was extracted
with dichloromethane (20 mL ꢁ 3). The combined organic layers
were dried over anhydrous MgSO4, and the solvent was removed
under vacuum. The crude product was purified by column chro-
matography on silica (petroleum ether/ethyl acetate ¼ 100/1, v/v)
to give a colorless oil 4 (1.2212 g, 2.33 mmol, 47%). 1H NMR
2.2. Theoretical calculations
(400 MHz, CDCl3)
d
7.58e7.53 (m, 2H), 7.49 (d, J ¼ 8.0 Hz, 1H), 7.43
(d, J ¼ 7.8 Hz,1H), 7.23 (s, 1H), 7.19 (s, 1H), 7.07e7.00 (m, 8H), 3.74 (s,
Gaussian 03 package was used for density functional theory
calculation [40]. The geometries and energies of F1, F2, and F3 were
determined using the B3LYP method with the 6-31G (d) basis set.
2H), 2.56 (t, J ¼ 7.6 Hz, 4H), 1.61e1.56 (m, 4H), 1.40e1.34 (m, 4H),
0.94 (t, J ¼ 7.2 Hz, 6H). 13C NMR (100 MHz, CDCl3)
d 147.9, 145.6,
145.0, 144.2, 140.7, 137.5, 134.7, 129.8, 129.2, 128.1, 124.4, 122.4,
120.4, 119.7, 119.3, 36.7, 35.1, 33.8, 22.5, 14.1. HRMS (EI): calcd. for
2.3. Synthesis
C
33H34NBr, 523.1875 [M]þ; found, 523.1870.
The synthetic routes to the F1, F2, and F3 dyes are shown in
Scheme 1.
2.3.2. Synthesis of 7-bromo-N,N-bis(4-butylphenyl)-9,9-diphenyl-
9H-fluoren-2-amine (5)
2.3.1. Synthesis of 7-bromo-N,N-bis(4-butylphenyl)-9H-fluoren-2-
amine (4)
The synthetic route of 5 is similar with 4 to give a white solid in
63% yield. mp 168e169 ꢂC. 1H NMR (400 MHz, d6-DMSO)
d 7.77 (dd,
A mixture of bis(4-butylphenyl)amine (1.407 g, 5 mmol), 2,7-
dibromo-9H-fluorene 1 (3.2401 g, 10 mmol), sodium tert-but-
oxide (0.961 g, 10 mmol), dppf (0.0641 g, 0.12 mmol), Pd(dba)2
(0.0575 g, 0.1 mmol), and toluene (80 ml) was refluxed under
J ¼ 8.0 Hz, 7.7 Hz, 2H), 7.55 (dd, J ¼ 8.1 Hz, 3.4 Hz, 2H), 7.29e7.22 (m,
6H), 7.08 (d, J ¼ 8.4 Hz, 4H), 7.02e6.84 (m, 10H), 2.56 (t, J ¼ 7.1 Hz,
4H),1.54e1.49 (m, 4H),1.32e1.23 (m, 4H), 0.87 (t, J ¼ 7.3 Hz, 6H). 13
C
NMR (100 MHz, CDCl3)
d 152.7, 152.2, 148.4, 145.3, 145.2, 139.3,
Scheme 1. The synthetic routes for F1, F2 and F3 dyes: i) bis(4-butylphenyl)amine, Pd(dba)2, dppf, tBuONa, toluene, 110 ꢂC; ii) (4-formylphenyl)boronic acid, Pd(PPh3)4, K2CO3, THF,
refluxed; iii) cyanoacetic acid, NH4OAc, glacial CH3COOH, refluxed.

10
Z. Ci et al. / Dyes and Pigments 104 (2014) 8e14
137.7, 132.5, 130.7, 129.4, 129, 128.3, 128.2, 126.8, 124.4, 122.1, 120.7,
120.3, 120.0, 65.4, 35.1, 33.7, 22.5, 14.1. HRMS (EI): calcd. for
C
2.3.7. Synthesis of 3-(4-(7-(bis(4-butylphenyl)amino)-9H-fluoren-
2-yl)phenyl)-2-cyanoacrylic acid (F1)
45H42NBr, 675.2501 [M]þ; found, 675.2494.
A mixture of 7 (0.5497 g, 1 mmol), cyanoacetic acid (1.1 mmol,
0.094 g), ammonium acetate (0.25 mmol, 0.02 g), and acetic acid
(5 mL) was refluxed under nitrogen atmosphere for 5 h. After
cooling to room temperature, water was added to quench the re-
action. The product was extracted with dichloromethane
(10 mL ꢁ 3). The combined organic layers were dried over anhy-
drous MgSO4, and the solvent was removed under vacuum. The
crude product was purified by column chromatography on silica
(dichloromethane/ethanol ¼ 20/1, v/v) to give a red solid F1
(0.4694 g, 0.761 mmol, 76%). mp 154e155 ꢂC. 1H NMR (400 MHz,
2.3.3. Synthesis of 7-bromo-N,N-bis(4-butylphenyl)-9,9-dihexyl-
9H-fluoren-2-amine (6)
The synthetic route of 6 is similar with 4 to give a colorless oil in
63% yield. 1H NMR (400 MHz, CDCl3)
d
7.47 (d, J ¼ 8.2 Hz, 1H), 7.44e
7.38 (m, 3H), 7.06e6.96 (m, 10 H), 2.57 (t, J ¼ 7.8 Hz, 4H), 1.81 (t,
J ¼ 8.0 Hz, 4H), 1.64e1.56 (m, 4H), 1.42e1.33 (m, 4H), 1.16e1.05 (m,
12 H), 0.96e0.91 (m, 6H), 0.81 (t, J ¼ 7.2 Hz, 6H), 0.64e0.63 (m, 4H).
13C NMR (100 MHz, CDCl3)
d 152.9, 151.6, 148.0, 145.6, 140.1, 137.2,
d6-DMSO)
d
8.06 (s, 1H), 7.99 (d, J ¼ 8.4 Hz), 7.88 (d, J ¼ 4.2 Hz, 2H),
136.8, 134.3, 129.8, 129.1, 129.0, 125.9, 123.9, 123.8, 122.7, 120.3,
119.9, 118.4, 55.3, 40.2, 35.1, 33.7, 31.5, 29.6, 23.7, 22.6, 22.5, 14.0.
HRMS (EI): calcd. for C45H58NBr, 691.3753 [M]þ; found, 691.3755.
7.84 (d, J ¼ 7.6 Hz, 2H), 7.55 (dd, J ¼ 8.3 Hz, 8.0 Hz, 2H), 7.11 (d,
J ¼ 8.3 Hz, 5H), 6.94 (d, J ¼ 8.3 Hz, 5H), 3.86 (s, 2H), 3.01 (t, J ¼ 7.1 Hz,
4H),1.58e1.50 (m, 4H),1.36e1.29 (m, 4H), 0.90 (t, J ¼ 7.3 Hz, 6H). 13
C
NMR (100 MHz, d6-DMSO)
d 164.2, 148.2, 147.6, 145.6, 145.4, 144.1,
2.3.4. Synthesis of 4-(7-(bis(4-butylphenyl)amino)-9H-fluoren-2-
yl)benzaldehyde (7)
143.2,141.6,137.5,136.9,135.2,132.1,130.7,129.8,127.4,126.2,124.6,
123.7, 122.1, 121.5, 120.2, 119.6, 119.5, 45.7, 34.7, 33.6, 22.3, 14.3, 9.2.
MS (LD, m/z): calcd. for C43H40N2O2, 616.3090 [M]þ; found,
616.2932.
A mixture of compound 5 (1.049 g, 2 mmol), (4-formylphenyl)
boronic acid (0.4498 g, 3 mmol), Pd(PPh3)4 (0.116 g, 0.1 mmol),
Na2CO3 (1.106 g, 8 mmol) and THF-H2O (10 mLe2 mL) was refluxed
under nitrogen atmosphere for 24 h. After cooling to room tem-
perature, water was added to quench the reaction. The product was
extracted with dichloromethane (20 mL ꢁ 3). The combined
organic layers were dried over anhydrous MgSO4, and the solvent
was removed under vacuum. The crude product was purified by
column chromatography on silica (petroleum ether/ethyl
acetate ¼ 10/1, v/v) to give a yellow solid 7 (0.9796 g, 1.782 mmol,
2.3.8. Synthesis of 3-(4-(7-(bis(4-butylphenyl)amino)-9,9-
diphenyl-9H-fluoren-2-yl)phenyl)-2- cyanoacrylic acid (F2)
The synthetic route of F2 is similar with F1 to give a red solid in
78% yield. mp 208e209 ꢂC. 1H NMR (400 MHz, d6-DMSO)
d 8.05 (s,
1H), 7.97 (d, J ¼ 8.4 Hz, 2H), 7.89 (d, J ¼ 8.0 Hz, 1H), 7.81 (d,
J ¼ 8.4 Hz, 1H), 7.79e7.76 (m, 4H), 7.26e7.21 (m, 6H), 7.09e7.07 (m,
8H), 6.90 (d, J ¼ 8.5 Hz, 6H), 2.52 (t, J ¼ 7.1 Hz, 4H), 1.57e1.49 (m,
4H), 1.34e1.25 (m, 4H), 0.89 (t, J ¼ 7.3 Hz, 6H). 13C NMR (100 MHz,
89%). mp 90e91 ꢂC. 1H NMR (400 MHz, CDCl3)
d 10.00 (s, 1H), 7.93
(d, J ¼ 8.2 Hz, 2H), 7.80 (d, J ¼ 8.2 Hz, 2H), 7.75 (d, J ¼ 8.5 Hz, 2H),
7.64e7.62 (m, 2H), 7.10e7.02 (m, 10 H), 3.85 (s, 2H), 2.58 (t,
J ¼ 7.7 Hz, 4H), 1.63e1.57 (m, 4H), 1.41e1.34 (m, 4H), 0.95 (t,
d6-DMSO) d 152.8,151.3,148.2,145.7,145.3,144.6,140.3,137.5,133.1,
131.2, 129.1, 128.2128.1, 126.6, 124.4, 122.0, 120.9, 120.6, 119.8, 65.5,
35.1, 33.7, 22.5, 14.1, 1.1. HRMS (EI): calcd. for C55H48N2O2, 768.3716
[M]þ; found, 768.3725.
J ¼ 7.3 Hz, 6H). 13C NMR (100 MHz, CDCl3)
d 145.6, 145.0, 143.9,
142.3, 139.7, 137.6, 137.0, 135.2, 135.0, 134.9, 130.3, 130.2, 129.2,
129.0, 128.5, 127.7, 127.5, 127.4, 126.3, 124.5, 123.8, 122.3, 120.6,
119.7, 119.6, 37.0, 35.1, 33.7, 22.5, 14.0. HRMS (EI): calcd. for
2.3.9. Synthesis of 3-(4-(7-(bis(4-butylphenyl)amino)-9,9-dihexyl-
9H-fluoren-2-yl)phenyl)-2- cyanoacrylic acid (F3)
C
40H39NO, 549.3032 [M]þ; found, 549.3039.
The synthetic route of F3 is similar with F1 to give a red solid in
82% yield. mp 191e192 ꢂC. 1H NMR (400 MHz, d6-DMSO)
d 8.32 (s,
2.3.5. Synthesis of 4-(7-(bis(4-butylphenyl)amino)-9,9-diphenyl-
9H-fluoren-2-yl)benzaldehyde (8)
1H), 8.08 (s, 2H), 7.80 (s, 2H), 7.69e7.48 (m, 4H), 6.98e6.83 (m,
10H), 2.55 (t, J ¼ 7.6 Hz, 4H), 1.87e1.81 (m, 4H), 1.69e1.53 (m, 4H),
1.41e1.37 (m, 4H),1.18e1.10 (m,12 H), 0.95 (t, J ¼ 7.3 Hz, 6H), 0.75 (t,
J ¼ 6.9 Hz, 6H), 0.64e0.62 (m, 4H). 13C NMR (100 MHz, d6-DMSO)
The synthetic route of 8 is similar with 7 to give a yellow solid in
91% yield. mp 144e145 ꢂC. 1H NMR (400 MHz, d6-DMSO)
d 10.02 (s,
1H), 7.94 (dd, J ¼ 8.1 Hz, 10.3 Hz, 3H), 7.86 (d, J ¼ 8.1 Hz, 2H), 7.82
(dd, J ¼ 7.8 Hz, 7.4 Hz, 2H), 7.77 (s, 1H), 7.27e7.21 (m, 6H), 7.10e7.08
(m, 8H), 6.92e6.90 (m, 6H), 1.57e1.49 (m, 4H), 1.32e1.24 (m, 6H),
d
163.8, 154.0, 152.3, 151.4, 147.8, 145.5, 145.2, 141.5, 136.9, 134.8,
131.8,130.5,129.3,127.4,126.3,123.9,122.6,121.3,120.0,118.3,116.7,
103.2, 55.0, 34.7, 33.5, 31.3, 29.4, 23.8, 22.4, 22.2, 14.1. HRMS (EI):
calcd. for C55H64N2O2, 784.4968[M]þ; found, 784.4977.
0.91e0.84 (m, 8H). 13C NMR (100 MHz, CDCl3)
d 191.9, 153.0, 151.7,
148.6, 147.4, 145.8, 145.2, 140.9, 137.8, 137.7, 135.0, 132.8, 130.3,
129.2,128.4,128.3,127.6,127.1,126.8,125.1,124.5, 122.1,121.0,120.4,
120.0, 65.6, 35.2, 33.8, 31.7, 22.8, 22.5, 14.3, 14.2. HRMS (EI): calcd.
for C52H47NO, 701.3658 [M]þ; found, 701.3666.
2.4. Fabrication of DSSCs
The screen-printable TiO2 pastes were prepared according to the
procedure developed by Ma’s group [41]. A screen-printing tech-
nique was used to fabricate TiO2 films. First, the paste was depos-
ited on a fluorine-doped tin oxide (FTO) conductive glass (Asahi
Glass Co., Ltd.; sheet resistance, 10 U/sq). Second, the film was
sintered at 450 ꢂC for 30 min in atmospheric air, immersed in
40 mM TiCl4 solution for 30 min at 70 ꢂC, rinsed with water and
ethanol, and sintered at 500 ꢂC for 30 min. The film was dipped into
F1, F2, and F3 dye solutions (0.4 mM in CH2Cl2) for 18 h after
cooling to 80 ꢂC. Finally, dye-sensitized TiO2 photoelectrodes
2.3.6. Synthesis of 4-(7-(bis(4-butylphenyl)amino)-9,9-dihexyl-9H-
fluoren-2-yl)benzaldehyde (9)
The synthetic route of 9 is similar with 7 to give a yellow oil in
82% yield. 1H NMR (400 MHz, CDCl3)
d 10.05 (s, 1H), 7.96 (d,
J ¼ 8.3 Hz, 2H), 7.82 (d, J ¼ 8.2 Hz, 2H), 7.66 (d, J ¼ 7.8 Hz, 1H), 7.60e
7.54 (m, 3H), 7.12e6.99 (m, 10H), 2.58 (t, J ¼ 7.6 Hz, 4H), 1.94e1.87
(m, 4H), 1.63e1.59 (m, 4H), 1.41e1.37 (m, 4H), 1.16e1.07 (m, 12 H),
0.94 (t, J ¼ 7.3 Hz, 6H), 0.801 (t, J ¼ 6.9 Hz, 6H), 0.71e0.70 (m, 4H).
13C NMR (100 MHz, CDCl3)
d
191.9, 152.4, 151.6, 148.1, 147.8, 145.7,
(thickness, 12 mm) were obtained. The organic electrolyte was
141.8, 137.3,134.9,134.6,130.3,129.1,127.6,126.4,124.0,122.7,121.4,
120.6, 119.4, 118.4, 55.2, 40.3, 35.1, 33.7, 31.6, 29.7, 23.9, 22.6, 22.5,
14.1. HRMS (EI): calcd. for C52H63NO, 717.4910 [M]þ; found,
717.4918.
composed of 0.06 M LiI, 0.03 M I2, 0.1 M guanidinium thiocyanate,
0.6 M 1-propyl-3-methylimidazolium iodide (PMII), and 0.5 M tert-
butyl-pyridine in acetonitrile. The active area of DSSCs was
0.16 cm2. DSSC devices were assembled with counter electrodes

Z. Ci et al. / Dyes and Pigments 104 (2014) 8e14
11
Table 1
(thermally platinized FTO) using a thermoplastic frame (Surlyn,
60 m thick).
Optical and electrochemical properties of F1, F2, and F3 dyes.
m
Dye Absorptiona
Emissiona Oxidation potential
Eox[V]b
E0e0 [V]c ELUMO[V]
2.5. Measurements
lmax ε at lmax
lmax
[nm] [Mꢀ1 cmꢀ1
]
[nm]
(versus NHE) (Abs/Em) (versus NHE)
Absorption and emission spectra were recorded with HP8453
(USA) and PTI700 (USA), respectively. Electrochemical measure-
ment was carried out on a BAS100W (USA) electrochemistry
workstation. The irradiation source for the photocurrentevoltage
measurement was an AM 1.5 solar simulator (16S-002, Solar Light
Co. Ltd., USA). The incident light intensity was 100 mW cmꢀ2
calibrated with a standard silicon solar cell. The currentevoltage
curve was obtained by linear sweep voltammetry method using an
electrochemical workstation (LK9805, Lanlike Co. Ltd., China). The
incident photon-to-current conversion efficiency (IPCE) measure-
ment was performed by a Hypermonolight (SM-25, Jasco Co. Ltd.,
Japan).
F1
F2
F3
405
418
441
17,732
23,695
26,016
645
636
623
0.95
0.97
0.93
2.46
2.38
2.41
ꢀ1.51
ꢀ1.41
ꢀ1.48
a
Absorption and emission spectra were measured in CH2Cl2, with a concentra-
tion of 1.0 ꢁ 10ꢀ5 M at room temperature.
b
The oxidation potential of the dyes was measured under the following condi-
tions: working electrode, Pt; electrolyte, 0.1
M tetrabutylammonium hexa-
fluorophosphate, n-Bu4NPF6 in THF; scan rate, 0.1 V/s. Potentials measured vs Feþ/Fe
were converted to NHE by addition of þ0.63 V.
c
The E0-0 energies were estimated from the intercept of the normalized ab-
sorption and emission spectra.
Comparedꢀw1 ith F1 (405 nm, 17,732 Mꢀ1 cmꢀ1) and F2 (418 nm,
23,695 M
cmꢀ1), the maximum absorption of F3 (441 nm,
26,016 Mꢀ1 cmꢀ1) was slightly distant. Noticeably, the molar
extinction coefficients of the three dyes were higher than that of
the N719 dye (14.0 ꢁ 103 Mꢀ1 cmꢀ1) [5], thus indicating good light
harvesting ability.
3. Results and discussion
3.1. Synthesis
The organic dyes F1, F2, and F3 were synthesized following the
steps depicted in Scheme 1. Compounds of 4e6 were prepared via
the BuchwaldeHartwig cross-coupling reaction of bis(4-
butylphenyl)amine with the corresponding dibromo compounds.
Coupling reactions of 4e6 with 4-formyl phenyl boronic acid under
the Suzuki reaction led to their corresponding aldehydes 7e9.
Finally, the aldehydes were condensed with cyanoacetic acid to
obtain the target compounds F1, F2, and F3 via Knöevenagel re-
action in the presence of ammonium acetate.
3.3. Molecular orbital calculations
The molecular geometries and electron distributions of F1, F2,
and F3 were obtained using the density function theory calcula-
tions with Gaussian 03 program. The calculations were performed
with the B3LYP exchange correlation that is functional under 6-31G
(d) basis. Results are shown in Fig. 3. The electron distribution
before light irradiation locates mainly on the donor units while
moving to the acceptor units close to the anchoring groups after
light irradiation. This distribution favors the electron injection from
the dye molecules to the conduction band edge of TiO2.
3.2. Absorption properties in solution
The optical and electrochemical properties of the three dyes are
summarized in Fig. 2 and Table 1. Fig. 2 shows that all dyes in CH2Cl2
solutions gave two distinct absorption bands: one relatively weak
band in the near-ultraviolet region (300 nme320 nm) that corre-
3.4. Electrochemical properties
The redox behavior of the F1, F2, and F3 dyes was studied by
cyclic voltammetry (Fig. 4) to investigate the ability of electron
transfer from the excited dye molecules to the conductive band of
TiO2. The cyclic voltammograms of these dyes were measured in a
solution of 0.1 M n-Bu4NPF6 in CH2Cl2. A three-electrode cell con-
taining a Pt-coil working electrode, a Pt wire counter electrode, and
a Ag/AgCl reference electrode was employed. The ferrocene/ferri-
cenium redox couple was used as an internal reference. The
examined highest occupied molecular orbital (HOMO) levels and
the lowest unoccupied molecular orbital (LUMO) levels were
collected, as shown in Table 1. HOMO values (0.93 Ve0.97 V vs.
NHE) were more positive than the Iꢀ/I3ꢀ redox couple (0.4 V vs.
NHE), thus suggesting that the oxidized dyes can thermodynami-
cally accept electrons from Iꢀ ion in iodide/triiodide electrolyte for
regeneration. Electron injection from the excited sensitizers to the
conduction band of TiO2 should be energetically favorable because
of the more negative LUMO values (ꢀ1.41 V to ꢀ1.51 V vs. NHE)
compared with the conduction band edge energy level of the TiO2
electrode (at approximately ꢀ0.5 V vs. NHE) [42]. Table 1 shows
that the introduction of the phenyl and n-hexyl groups to fluorine
can change the HOMOeLUMO energy gaps of the dyes narrowly.
These results clearly demonstrate that these dyes are potentially
efficient dyes for DSSCs.
sponds to the
pep* electron transition and another with a strong
absorption in the visible region (420 nme460 nm). The bands can
be assigned to an intramolecular charge transfer between the tri-
arylamine donating unit and the cyanoacrylic acid anchoring
moiety, thereby producing an efficient charge-separated state.
40
F1
F2
F3
30
20
10
0
3.5. Photovoltaic performance
300
400
500
600
Wavelength (nm)
The action spectrum, or the IPCE as a function of wavelength,
was measured to evaluate the photoresponse of the photoelectrode
Fig. 2. Absorption spectra of F1, F2, and F3 in CH2Cl2 at ambient temperature.

12
Z. Ci et al. / Dyes and Pigments 104 (2014) 8e14
Fig. 3. Frontier HOMO and LUMO orbitals of F1, F2, and F3 dyes.
in the whole spectral region. F1, F2, and F3 sensitizers were used to
manufacture solar cell devices. Fig. 5 shows the IPCE obtained with
0.06 M LiI, 0.03 M I2, 0.1 M guanidinium thiocyanate, 0.6 M PMII,
and 0.5 M tert-butyl-pyridine in acetonitrile as redox electrolyte.
The three dyes can efficiently convert visible light into photocur-
rent in the region of 300 nme600 nm. A solar cell based on F3
showed the highest IPCE value of 75% at 490 nm. In addition, the
cell exhibited a broad IPCE spectrum with IPCE values (>70%)
ranging from 390 nm to 520 nm. However, the IPCE spectra of F2
and F1 were slightly low, with a maximum IPCE of 68% at 445 nm
and 69% at 450 nm, respectively.
Fig. 6 shows the currentevoltage curves of the DSSCs based on
the F1, F2, and F3 dyes under standard global AM 1.5 G solar irra-
diation, and the results are shown in Table 2. The DSSCs based on
the F3 dye showed the best performance, with a short-circuit
photocurrent density of 9.69 mA cmꢀ2, an open-circuit voltage of
0.77 V, and a fill factor of 0.70, thereby yielding an overall con-
version efficiency (
h
) of 5.15%. For a fair comparison, the N719-
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
20
10
F1
F2
F3
0
F1
F2
F3
-10
300
350
400
450
500
550
600
650
700
0.5
1.0
1.5
Wavelength (nm)
Fig. 4. Cyclic voltammogram of F1, F2, and F3 in CH2Cl2.
Fig. 5. IPCE spectra for DSSCs based on the as-synthesized dyes.

Z. Ci et al. / Dyes and Pigments 104 (2014) 8e14
13
16
14
12
10
8
circuit current density. The use of n-hexyl-substituted fluorene
moiety can avoid the aggregation of the dyes and conse-
quently achieve a high conversion efficiency (5.15%). Our results
pep
F1
F2
F3
N719
indicate that the alkyl group is better than the aryl group in
avoiding the
pep aggregation. The detailed investigation into the
effect of the substitution on
ongoing in our laboratory.
p-spacer with other bulky groups is
Acknowledgments
We are grateful to the National Natural Science Foundation of
China (No. 21002010 and 21273026) for their financial support. This
work was also supported by the Fundamental Research Funds for
the Central Universities (DUT12LK44).
6
4
2
References
0
[1] O’Regan B, Grätzel M. A low-cost, high-efficiency solar cell based on dye-
sensitized colloidal TiO2 films. Nature 1991;353:737e9.
[2] Grätzel M. Dye-sensitized solar cells. J Photochem Photobiol C 2003;4:
145e53.
0.0
0.2
0.4
0.6
0.8
Voltage (V)
Fig. 6. Current - potential (JeV) curve for the DSSCs based on F1, F2 and F3.
[3] Grätzel M. Conversion of sunlight to electric power by nanocrystalline dye-
sensitized solar cells. J Photochem Photobiol A 2004;164:3e14.
[4] Nazeeruddin MK, Kay A, Rodicio I, Humphry-Baker R, Müller E, Liska P, et al.
Conversion of light to electricity by cis-X2bis(2,2’-bipyridyl-4,4’-dicarboxylate)
ruthenium(II) charge-transfer sensitizers (X ¼ Clꢀ, Brꢀ, Iꢀ, CNꢀ and SCNꢀ) on
nanocrystalline TiO2 electrodes. J Am Chem Soc 1993;115:6382e90.
[5] Nazeeruddin MK, Zakeeruddin SM, Humphry-Baker R, Jirousek M, Liska P,
Vlachopoulos N, et al. Acid-base equilibria of (2,2’-bipyridyl-4,4’-dicar-
boxylic acid) ruthenium(II) complexes and the effect of protonation on
charge-transfer sensitization of nanocrystalline titania. Inorg Chem
1999;38:6298e305.
sensitized TiO2 solar cell showed an efficiency of 7.46%, with a JSC of
15.04 mA cmꢀ2, a VOC of 0.72 V, and a fill factor of 0.69. The con-
version efficiency of F3 reached 70% of the N719 cell efficiency.
Compared with the F1-based cell, the F2-based cell showed a lower
JSC value. This result indicates that the phenyl substituent can serve
as a
p to aggregate with the p-spacer of the F2 dye. The F3-based
[6] Nazeeruddin MK, Péchy P, Renouard T, Zakeeruddin SM, Humphry-Baker R,
Comte P, et al. Engineering of efficient panchromatic sensitizers for nano-
crystalline TiO2-based solar cells. J Am Chem Soc 2001;123:1613e24.
[7] Ning ZJ, Zhang Q, Wu WJ, Pei HC, Liu B, Tian H. Starburst triarylamine based
dyes for efficient dye-sensitized solar cells. J Org Chem 2008;73:3791e7.
[8] Wiberg J, Marinado T, Hagberg DP, Sun LC, Hagfeldt A, Albinsson B. Dis-
tance and driving force dependencies of electron injection and recombi-
cell gives a higher JSC value than the F1-based cell, thereby
reflecting its better ability to avoid
the n-hexyl as a substituent [43].
pep aggregation as a result of
Chenodeoxycholic acid (CDCA) is used as coadsorbent to
dissociate the -stacked dye aggregates and to improve the elec-
p
tron injection yield, thus affording a high JSC value (Table 2). For the
DSSCs sensitized by the dyes, the JSC values of the F1 and F2 dyes
with CDCA increased by 0.38 and 0.11 mA cmꢀ2, respectively.
However, the JSC values were reduced from 0.52 mA cmꢀ2 to
9.17 mA cmꢀ2 for the F3 dye because of the decrease in the loading
dyes. The results confirm again that the use of the n-hexyl chain can
nation dynamics in organic dye-sensitized solar cells.
J Phys Chem B
2010;114:14358e63.
[9] Chang DW, Lee HJ, Kim JH, Park SY, Park SM, Dai LM, et al. Novel quinoxaline-
based organic sensitizers for dye-sensitized solar cells. Org Lett 2011;13:
3880e3.
[10] Gao FF, Wang Y, Shi D, Zhang J, Wang MK, Jing XY, et al. Enhance the optical
absorptivity of nanocrystalline TiO2 film with high molar extinction coeffi-
cient ruthenium sensitizers for high performance dye-sensitized solar cells.
J Am Chem Soc 2008;130:10720e8.
avoid the pep aggregation of the dyes and consequently achieve a
[11] Späth M, Sommeling PM, van Roosmalen JAM, Smit HJP, van der Burg NPG,
Mahieu DR, et al. Reproducible manufacturing of dye-sensitized solar cells on
a semi-automated baseline. Prog Photovolt 2003;11:207e20.
[12] Hwang S, Lee JH, Park C, Lee H, Kim C, Park C, et al. A highly efficient organic
sensitizer for dye-sensitized solar cells. Chem Commun 2007:4887e9.
[13] Kitamura T, Ikeda M, Shigaki K, Inoue T, Anderson NA, Ai X, et al. Phenyl-
conjugated oligoene sensitizers for TiO2 solar cells. Chem Mater 2004;16:
1806e12.
[14] Chen KF, Hsu YC, Wu QY, Yeh MCP, Sun SS. Structurally simple dipolar organic
dyes featuring 1,3-cyclohexadiene conjugated unit for dye-sensitized solar
cells. Org Lett 2009;11:377e80.
[15] Yang CH, Chen HL, Chuang YY, Wu CG, Chen CP, Liao SH, et al. Characteristics
of triphenylamine-based dyes with multiple acceptors in application of dye-
sensitized solar cells. J Power Sources 2009;188:627e34.
high short-circuit current density.
4. Conclusions
In summary, three new organic sensitizers (F1, F2, and F3)
containing different groups (hydrogen, phenyl, and n-hexyl)
substituted on fluorene moieties were designed and synthesized
for DSSCs. The introduction of phenyl-substituted fluorene moiety
(F2) increased the
pep aggregation, thus leading to low short-
[16] Liang M, Xu W, Cai FS, Chen PQ, Peng B, Chen J, et al. New triphenylamine-
based organic dyes for efficient dye-sensitized solar cells. J Phys Chem C
2007;111:4465e72.
[17] Kim C, Choi H, Kim S, Baik C, Song K, Kang MS, et al. Molecular engineering of
organic sensitizers containing p-phenylene vinylene unit for dye-sensitized
solar cells. J Org Chem 2008;73:7072e9.
[18] Xu W, Peng B, Chen J, Liang M, Cai FS. New triphenylamine-based dyes for
dye-sensitized solar cells. J Phys Chem C 2008;112:874e80.
[19] Liu WH, Wu IC, Lai CH, Lai CH, Chou PT, Li YT, et al. Simple organic molecules
bearing a 3,4-ethylenedioxythiophene linker for efficient dye-sensitized solar
cells. Chem Commun 2008:5152e4.
[20] Hagberg DP, Marinado T, Karlsson KM, Nonomura K, Qin P, Boschloo G, et al.
Tuning the HOMO and LUMO energy levels of organic chromophores for dye
sensitized solar cells. J Org Chem 2007;72:9550e6.
[21] Shi D, Cao YM, Pootrakulchote N, Yi ZH, Xu MF, Zakeeruddin SM, et al. New
organic sensitizer for stable dye-sensitized solar cells with solvent-free ionic
liquid electrolytes. J Phys Chem C 2008;112:17478e85.
Table 2
Photovoltaic performance of DSSCs sensitized with the as-synthesized dyes
compared with the N719 dye.a
Dye
CDCA
JSC/mA cmꢀ2
Voc/mV
ff
h/%
F1
0 mM
0.8 mM
0 mM
0.8 mM
0 mM
0.8 mM
0 mM
9.40
9.78
8.51
8.62
9.69
9.17
15.04
0.73
0.75
0.73
0.74
0.77
0.72
0.72
0.67
0.69
0.69
0.71
0.70
0.66
0.69
4.62
5.11
4.26
4.55
5.15
4.39
7.46
F2
F3
N719
a
The DSSCs had an active area of w0.16 cm2 and used an electrolyte composed of
0.06
M LiI, 0.03 M I2, 0.1 M guanidinium thiocyanate, 0.6 M 1-propyl-3-
methylimidazolium iodide (PMII), and 0.5 M tert-butyl-pyridine in acetonitrile.

14
Z. Ci et al. / Dyes and Pigments 104 (2014) 8e14
[22] Yum JH, Hagberg DP, Moon SJ, Karlsson KM, Marinado T, Sun LC, et al. A light-
resistant organic sensitizer for solar-cell applications. Angew Chem Int Ed
2009;48:1576e80.
[23] Li G, Jiang KJ, Li YF, Li SL, Yang LM. Efficient structural modification of
triphenylamine-based organic dyes for dye-sensitized solar cells. J Phys Chem
C 2008;112:11591e9.
[33] Lee DH, Lee MJ, Song HM, Song BJ, Seo KD, Pastore M, et al. Organic dyes
incorporating low-band-gap chromophores based on -extended benzothia-
diazole for dye-sensitized solar cells. Dyes Pigments 2011;91:192e8.
[34] Seo KD, Choi IT, Park YG, Kang S, Lee JY, Kim HK. Novel D-A- -A coumarin
p
p
dyes containing low band-gap chromophores for dye-sensitised solar cells.
Dyes Pigments 2012;94:469e74.
[24] Xu MF, Li RZ, Pootrakulchote N, Shi D, Guo J, Yi ZH, et al. Energy-level and
[35] Haid S, Marszalek M, Mishra A, Wielopolski M, Teuscher J, Moser J, et al.
Significant improvement of dye-sensitized solar cell performance by small
molecular engineering of organic D-
p-A sensitizers in dye-sensitized solar
cells. J Phys Chem C 2008;112:19770e6.
structural modification in
Mater 2012;22:1291e302.
[36] Akhtaruzzaman Md, Seya Y, Asao N, Islam A, Kwon E, EI-Shafei A, et al. Donor-
acceptor dyes incorporating a stable dibenzosilole -conjugated spacer for
p-conjugated donoreacceptor dyes. Adv Funct
[25] Wang MK, Xu MF, Shi D, Li RZ, Gao FF, Zhang GL, et al. High-performance
liquid and solid dye-sensitized solar cells based on a novel metal-free organic
sensitizer. Adv Mater 2008;20:4460e3.
p
[26] Zhang GL, Bai Y, Li RZ, Shi D, Wenger S, Zakeeruddin SM, et al. Employ a
bisthienothiophene linker to construct an organic chromophore for efficient
and stable dye-sensitized solar cells. Energy Environ Sci 2009;2:92e5.
[27] Zhang GL, Bala H, Cheng YM, Shi D, Lv XJ, Yu QJ, et al. High efficiency and
stable dye-sensitized solar cells with an organic chromophore featuring a
dye-sensitized solar cells. J Mater Chem 2012;22:10771e8.
[37] Wu YZ, Zhang X, Li WQ, Wang ZS, Tian H, Zhu WH. Hexylthiophene-featured
DeAepeA structural indoline chromophores for coadsorbent-free and
panchromatic dye-sensitized solar sells. Adv Energy Mater 2012;2:149e56.
[38] Huang WY, Huang SY. Sterically encumbered fluorene-based poly(arylene
ether)s containing spiro-annulated substituents on the main chain. Macro-
molecules 2010;43:10355e65.
[39] Saikia G, Iyer PK. Facile C-H alkylation in water: enabling defect-free materials
for optoelectronic devices. J Org Chem 2010;75:2714e7.
binary
p-conjugated spacer. Chem Commun 2009;2:198e200.
[28] Qin H, Wenger S, Xu M, Gao F, Jing X, Wang P, et al. An organic sensitizer with
a fused dithienothiophene unit for efficient and stable dye-sensitized solar
cells. J Am Chem Soc 2008;130:9202e3.
[29] Tian HN, Yang XC, Chen RK, Zhang R, Hagfeldt A, Sun LC. Effect of different dye
baths and dye-structures on the performance of dye-sensitized solar cells
based on triphenylamine dyes. J Phys Chem C 2008;112:11023e33.
[30] Choi H, Lee JK, Song K, Kang SO, Ko J. Novel organic dyes containing bis-
dimethylfluorenyl amino benzo[b]thiophene for highly efficient dye-
sensitized solar cell. Tetrahedron 2007;63:3115e21.
[40] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al.
Gaussian 03, revision C.02. Wallingford, CT: Gaussian Inc.; 2004.
[41] Guo W, Shen YH, Wu LQ, Gao YR, Ma TL. Effect of N dopant amount on the
performance of dye-sensitized solar cells based on N-doped TiO2 electrodes.
J Phys Chem C 2011;115:21494e9.
[42] Hao Y, Yang XC, Cong JY, Tian HN, Hagfeldt A, Sun LC. Efficient near infrared
[31] Choi H, Baik C, Kang SO, Ko J, Kang MS, Nazeeruddin MK, et al. Highly efficient
and thermally stable organic sensitizers for solvent-free dye-sensitized solar
cells. Angew Chem Int Ed 2008;47:327e30.
[32] Velusamy M, Thomas KRJ, Lin JT, Hsu YC, Ho KC. Organic dyes incorporating
low-band-gap chromophores for dye-sensitized solar cells. Org Lett 2005;7:
1899e902.
DepeA sensitizers with lateral anchoring group for dye-sensitized solar cells.
Chem Commun 2009:4031e3.
[43] Koumura N, Wang ZS, Mori S, Miyashita M, Suzuli E, Hara K. Alkyl-function-
alized organic dyes for efficient molecular photovoltaics. J Am Chem Soc
2006;128:14256e7.