Incorporating a stable fluorenone unit into D-A-π-A organic dyes for dye-sensitized solar cells

Article abstract of DOI:10.1039/c2jm33105e

Two new organic dyes based on a donor-acceptor-π-bridge-acceptor/anchor (D-A-π-A) configuration, HIQF1 and HIQF2, were synthesized, characterized, and successfully employed in dye-sensitized solar cells. The incorporation of the weakly electron-withdrawing fluorenone group as an additional acceptor enhanced the dyes' properties for solar energy conversion in several ways. First, the absorption spectra of the dyes were broadened, covering a large range of sunlight. Second, the absorption spectra observed upon dye adsorption on TiO₂ were only slightly blue-shifted as compared to the spectra of the dyes in solution. Third, the orbital energy levels and electron distributions of the D-A-π-A backbone were ideal for highly efficient electron transport and injection. The incorporation of long electron-donating alkoxy groups in the donor unit of HIQF1 further increased the dye's short-circuit photocurrent density (J_sc), open-circuit voltage (V_oc), and overall conversion efficiency (η), but had little negative effect on the fill factor (FF). Solar cells sensitized with HIQF1 showed excellent current-voltage characteristics; the J_sc, V _oc, and FF values were 12.26 mA cm^-2, 0.70 V, and 0.73, corresponding to a η value of 6.26%. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2jm33105e

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PAPER
Incorporating a stable fluorenone unit into D–A–p–A organic dyes for
dye-sensitized solar cells
*
Chuanjiang Qin, Ashraful Islam and Liyuan Han
Received 16th May 2012, Accepted 4th August 2012
DOI: 10.1039/c2jm33105e
Two new organic dyes based on a donor–acceptor–p–bridge–acceptor/anchor (D–A–p–A)
configuration, HIQF1 and HIQF2, were synthesized, characterized, and successfully employed in dye-
sensitized solar cells. The incorporation of the weakly electron-withdrawing fluorenone group as an
additional acceptor enhanced the dyes’ properties for solar energy conversion in several ways. First, the
absorption spectra of the dyes were broadened, covering a large range of sunlight. Second, the
absorption spectra observed upon dye adsorption on TiO2 were only slightly blue-shifted as compared
to the spectra of the dyes in solution. Third, the orbital energy levels and electron distributions of the
D–A–p–A backbone were ideal for highly efficient electron transport and injection. The incorporation
of long electron-donating alkoxy groups in the donor unit of HIQF1 further increased the dye’s short-
circuit photocurrent density (Jsc), open-circuit voltage (Voc), and overall conversion efficiency (h), but
had little negative effect on the fill factor (FF). Solar cells sensitized with HIQF1 showed excellent
current–voltage characteristics; the Jsc, Voc, and FF values were 12.26 mA cmꢀ2, 0.70 V, and 0.73,
corresponding to a h value of 6.26%.
to continually explore new functional groups as additional
Introduction
acceptors for highly efficient and low-cost DSCs.
Metal-free organic dyes have sparked a flurry of research activity
in the scientific community owing to their potential application
as highly efficient photosensitizers in dye-sensitized solar cells
(DSCs).1 Metal-free organic dyes, which are normally con-
structed with a donor–p–bridge–acceptor (D–p–A) configura-
tion, hold great promise owing to their high molar extinction
coefficients, facile structure modification, and no concern of the
rare metal resource.2 However, the overall conversion efficiencies
of DSCs based on metal-free organic dyes are lower than those of
DSCs based on ruthenium dyes, owing to the metal-free organic
dyes’ narrow coverage of the solar spectrum. Thus, there is
growing interest in developing new sensitizers with broader
absorption for DSCs applications. Recent research endeavours
toward this goal have involved the use of additional acceptor
chromophores between the donor and the p-bridge of the dye
molecule, leading to a D–A–p–A architecture, which facilitates
intramolecular charge transfer and shifts the bandgap energy to
permit harvesting of sunlight from a larger range of wavelengths.
For example, benzothiadiazole, quinoxaline, and benzotriazole
have been incorporated in D–A–p–A configurations as addi-
tional acceptors, and the resulting molecules exhibit good cell
performance and stability.3 Such favourable results encourage us
Fluorene derivatives have been widely applied in polymer solar
cells, polymer light-emitting diodes, and multiphoton-absorbing
materials owing to their unique electrochemical and optical
properties and their excellent hole-transporting abilities.4
Recently, Zhu et al. introduced fluorene into D–p–A organic
dyes as a conjunction bridge and used the resulting dyes in TiO2
DSCs.5 However, the conversion efficiency of even the best DSCs
constructed from these dyes is low (4.26%), owing to the narrow
absorption spectrum of the dye that results from its absorption
on TiO2. Incorporating electron-withdrawing groups, such as
ketones, at the C-9 position of fluorene stabilizes its LUMO, thus
remarkably enhancing the electrophilicity of the molecule. Thus,
fluorenone derivatives have been used as photosensitizers for
carbazole-containing conductive polymers.6 At the same time,
fluorenone derivatives modified with electron-donating groups
demonstrate a broadened absorption range due to charge
transfer from donor units to the carbonyl group.7 It is also worth
mentioning that fluorenone should also more benefit for stability
of DSCs due to its stable chemical property compared to fluorene
that had been proved in the yield of above mentioned optical
materials. Fluorenone is also attractive because it is commer-
cially available and inexpensive. For these reasons, we sought to
exploit the properties of fluorenone and D–A–p–A architectures
to develop a new sensitizer for DSCs. In this paper, we report for
the first time two novel fluorenone-based sensitizer dyes, HIQF1
and HIQF2 (Scheme 1), in which fluorenone is used as an
additional acceptor, substituted triphenylamines are used as
Photovoltaic Materials Unit and NIMS Saint-Gobain Center of Excellence
for Advanced Materials, National Institute for Materials Science, Sengen
1-2-1, Tsukuba, Ibaraki 305-0047, Japan. E-mail: Han.Liyuan@nims.go.
jp; Fax: +81 29-859-2304; Tel: +81 29-859-2305
19236 | J. Mater. Chem., 2012, 22, 19236–19243
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prepared via two sequential asymmetrical Suzuki coupling
reactions of 2,7-dibromofluorenone, first with a substituted tri-
phenylamino borate and then with 5-formylthiophen-2-boronic
acid. The resulting monoaldehyde-substituted precursors were
subjected to Knoevenagel condensations with cyanoacetic acid
to afford HIQF1 and HIQF2. Purification of the mixtures by
silica chromatography furnished HIQF1 and HIQF2 as air-
stable black powders with high purity. The dyes were fully
characterized by 1H and 13C NMR spectroscopy as well as mass
spectrometry. The obtained dyes were deep red solids and
were soluble in common organic solvents, such as chloroform,
dimethylformamide (DMF), and acetonitrile.
Photophysical characterization
Fig. 1 depicts the ultraviolet-visible (UV-vis) spectra of HIQF1
and HIQF2 measured in DMF. All spectroscopic data for
HIQF1 and HIQF2 are listed in Table 1. The intense absorption
peaks at 396 nm (3 ¼ 5.32 ꢁ 104 Mꢀ1 cmꢀ1) for HIQF1 and
393 nm (3 ¼ 4.98 ꢁ 104 Mꢀ1 cmꢀ1) for HIQF2 were attributed to
the p–p* transitions of the conjugated aromatic moieties. The
weak and low-energy bands observed at around 500 nm for
HIQF1 and HIQF2 corresponded to intramolecular charge
transfer absorption. Estrada et al. also reported similar weak
absorption features with absorption band at around 440 nm for
fluorenone-based donor–acceptor–donor type molecules without
cyano unit.8
Scheme 1 Chemical structures of HIQF1 and HIQF2.
donors, thiophene is a p-spacer, and 2-cyano-2-propenoic acid is
an acceptor/anchor. We studied the influence of long alkyl chains
on photovoltaic performance using controlled intensity-modu-
lated photovoltage spectroscopy and achieved excellent current–
voltage characteristics (Jsc, Voc, and FF) of 12.26 mA cmꢀ2
,
0.70 V, and 0.73, corresponding to an high overall conversion
efficiency (h) of 6.26% for HIQF1.
Results and discussion
Both HIQF1 and HIQF2 with donor–p–bridge–acceptor
configuration showed a broad and red shifted absorption band
due to introduction of the thiophene group, and the strongly
electron-withdrawing character of the cyano unit, which
enhanced interaction between the donor and acceptor. When
adsorbed on a transparent TiO2 electrode (4 mm thickness), both
dyes showed similar absorption spectra, and the absorption
peaks were slightly blue-shifted compared with the correspond-
ing peaks in the solution absorption spectra, due to deprotona-
tion of the anchoring group and differences in the polarity of
the media (Fig. 2).9 Both in solution and on TiO2, the
absorption peaks of HIQF1 were located at longer wavelengths
(labs. in DMF ¼ 396, 482 nm; labs. on TiO2 ¼ 392 nm) than those of
HIQF2 (labs. in DMF ¼ 393, 477 nm; labs. on TiO2 ¼ 385 nm).
The introduction of the electron-donating alkoxy group into the
triphenylamino moiety of the HIQF1 donor should have
Synthesis and characterization
Scheme 2 shows the synthetic protocol for the new organic dyes,
HIQF1 and HIQF2. Each of the two dye precursors was easily
Scheme 2 Synthetic routes of HIQF1 and HIQF2: (a) 2,7-dibromo-
fluorenone, toluene,
2 M K2CO3, Pd(PPh3)4, reflux; (b) 5-for-
mylthiophen-2-boronic acid, toluene, methanol, K2CO3, Pd(dppf)Cl2,
microwave, reflux; (c) CNCH2COOH, piperidine, chloroform, reflux.
Fig. 1 UV-vis absorption spectra of HIQF1 and HIQF2 in DMF.
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Table 1 Absorption and electrochemical properties of HIQF1 and HIQF2
Absorptiona [nm] (3b [103 Mꢀ1 cmꢀ1])
lmax on TiO2 [nm]c
E0–0 [V]d
Eox [V]e
Eox [V]f
*
Dye
HIQF1
HIQF2
396 (53.2), 484 (8.2)
393 (49.8), 477 (7.7)
392
385
2.10
2.16
1.02
1.15
ꢀ1.08
ꢀ1.01
a
Absorption peaks of dyes in DMF. b Molar absorption coefficient determined from the absorbance for a molar concentration (2 ꢁ 10ꢀ6 M) of the dyes
with a path length of 1 cm at corresponding wavelength. c Absorption maximum on TiO2. d Optical bandgap E0–0 ¼ hc/lonset ¼ 1238.83/lonset
Eox ¼ |
abs
abs e
.
Epc + Epa|/2 with the cathodic and anodic peak potentials Epa, Epc from the oxidation wave. f Eox ¼ Eox ꢀ E0–0
.
*
increased the donor–acceptor character of the dye, thus
explaining the observed red-shift in the absorption spectra of
HIQF1 compared with HIQF2. These results indicate that
introducing fluorenone into the D–A–p–A architecture can
effectively narrow the bandgap and thus permit the harvesting of
more sunlight, both for the dye in solution and adsorbed on
TiO2.
and 2.16 eV, respectively. The calculated Eox* values for HIQF1
and HIQF2 were ꢀ1.08 and ꢀ1.01 V (vs. NHE), respectively, and
were much more negative than the conduction-band level of TiO2
(approximately ꢀ0.5 V vs. NHE).12 These results indicate a
sufficient driving force for electron injection from the excited
state of the sensitizer dyes to the TiO2 conduction band.
Theoretical approach
Electrochemical characterization
It is important for an efficient sensitizer dye to possess a well-
delocalized p-framework and directional electron distributions.
To gain better insight into the electron distributions and
molecular geometries of the dyes, we performed time-dependent
density functional theory calculations, and the optimized
molecular structures and frontier molecular orbital profiles are
shown in Fig. 4. Molecular-orbital calculations showed that the
HOMO was mainly delocalized on the triphenylamino unit for
both HIQF1 and HIQF2. These results suggest that delocaliza-
tion of the HOMO on the donor moiety may facilitate reduction
of the oxidized dye by reaction with Iꢀ, making the dye suitable
for highly efficient solar cells. The LUMOs of HIQF1 and
HIQF2 were located predominantly on the cyanoacetic acid units
and extended to the fluorenone moieties. Such electron distri-
butions facilitate electron injection from the photoexcited
sensitizer dye to TiO2. Thus, HOMO–LUMO excitation would
move the photoexcited electrons on the p-framework from the
donor unit to the cyanoacrylic acid moiety via the additional
acceptor-fluorenone group, which also is beneficial for efficient
charge separation and electron injection. Moreover, since it is
more weakly electrophilic than the cyano group, the electron-
withdrawing ketone unit of the fluorenone does not limit electron
injection. In our previous study, 4H-cyclopenta[2,1-b:3,4-b’]
The electrochemical properties of HIQF1 and HIQF2 were
measured by cyclic voltammetry in a typical three-electrode
electrochemical cell, with the dye-sensitized TiO2 films serving
as the working electrodes and 0.1 M tetra-n-butylammonium
hexafluorophosphate as the supporting electrolyte (Table 1). The
reference electrode was Ag/AgCl calibrated with Fc+/Fc as an
internal reference. HIQF1 exhibited a reversible oxidative wave
with a oxidation potential, Eox, at 1.02 V (vs. NHE); whereas a
reversible oxidative wave with a Eox at 1.15 V was observed for
HIQF2 (Fig. 3). These values were low enough for efficient
regeneration of the oxidized dyes via electron donation from
iodide (ꢂ0.4 V vs. NHE).10 HIQF1, which has an alkoxy-
substituted triphenylamino donor unit, had an oxidation
potential ca. 100 mV more negative than that of HIQF2, which
has a methyl-substituted triphenylamino donor unit. This result
indicates that the introduction of electron-donating alkoxy
groups to the triphenylamino moiety caused a negative shift in
the anodic Eox. The excited-state oxidation potential values,
Eox*, of the two dyes were calculated from their oxidation
potentials (Eox) and the optical energy gaps, E0–0, determined
from the onset value of the absorption spectra on transparent
TiO2 films.11 The E0–0 values for HIQF1 and HIQF2 were 2.10
Fig. 2 UV-vis absorption spectra of HIQF1 and HIQF2 on TiO2
Fig. 3 Cyclic voltammetry of HIQF1 and HIQF2 adsorbed on TiO2 film
(4 mm thickness) deposited on TCO glass.
nanoparticles.
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of the alkoxy groups in its donor moiety. The IPCEs of the two
dyes maintained a high plateau of about 80% throughout much
of the visible region. Taking into account the reflection and
absorption losses by the conducting glass of the DSCs, the effi-
ciency of electric current generation in this range reached about
90% over a broad wavelength range extending from 350 to
600 nm. Thus, with further modification to increase the red-shift
in absorbance, this type of novel fluorenone-based sensitizer dye
could be very promising for high-efficiency DSCs.
Fig. 6 shows photocurrent–voltage (J–V) curves of DSCs
sensitized with HIQF1 and HIQF2 under standard AM 1.5 solar
light conditions. The short-circuit photocurrent density (Jsc),
open-circuit voltage (Voc), fill factor (FF) and overall conversion
efficiency (h) for each dye-TiO2 electrode are summarized in
Table 2. The DSCs sensitized with HIQF1 showed a Jsc of 12.26
mA cmꢀ2, a Voc of 0.70 V, and a FF of 0.73, corresponding to an
h value of 6.26%. Under the same device operating conditions,
the current–voltage characteristics (Jsc, Voc, FF, and h) of
HIQF2 were 11.46 mA cmꢀ2, 0.65 V, 0.76, and 5.66%, respec-
tively. These results indicate that the attachment of long electron-
donating alkoxy groups onto the donor unit of HIQF1 slightly
increased Jsc, Voc, and h. These increases were attributed to the
following reasons: (1) the electron-donating alkoxy groups –
elevated the HOMO energy level and narrowed the bandgap of
HIQF1, causing the dye to absorb longer-wavelength light and
thus increasing Jsc; and (2) the introduction of the bulky alkoxy
groups suppressed charge recombination between injected elec-
trons and iodine, resulting in a higher Voc. Thus, this class of
alkoxy-substituted compounds serves as a basis for further
design of new potential sensitizers fabricated by introducing
suitable substituents on phenyl rings to enhance the light-har-
vesting efficiency of the sensitizer dye and, furthermore, to
prevent surface aggregation of the sensitizer.14
Fig. 4 Frontier molecular orbital profiles of HIQF1 and HIQF2 at the
B3LYP/6-31G* level.
dithiophen-4-one was used as an additional acceptor in a D–A–A
configuration for DSCs.13 Because it was strongly electron
withdrawing, the LUMO of this sensitizer dye was located
predominantly on the 4H-cyclopenta[2,1-b:3,4-b’]dithiophen-4-
one moiety and then extended to the cyanoacetic acid units,
resulting in low electron injection efficiency. Thus, the present
results again demonstrate that fluorenone is a good candidate as
an additional acceptor that not only effectively decreases the
bandgap but also facilitates electron transport.
Device performances
The photovoltaic performance of DSCs sensitized by HIQF1 and
HIQF2 was investigated under standard AM 1.5 irradiation
(100 mW cmꢀ2) using an electrolyte composed of 0.6 M dime-
thylpropyl-imidazolium iodide, 0.05 M I2, 0.1 M LiI, and 0.5 M
tert-butyl pyridine in acetonitrile. Fig. 5 shows the photocurrent
action spectra for DSCs sensitized with HIQF1 and HIQF2,
where the incident photon-to-current conversion efficiency
(IPCE) values are plotted as a function of wavelength. Both dyes
efficiently sensitized nanocrystalline TiO2 over the whole visible
range from 300 to 700 nm. HIQF1 exhibited a slightly broader
IPCE action spectrum than did HIQF2, and this result was
attributed to the lower bandgap of HIQF1 owing to the presence
CEM and IMVS characterization
To further understand the improvement in Voc observed for
DSCs sensitized with HIQF1 and HIQF2, we first measured the
relative conduction band position of TiO2 by means of a charge
extraction method developed by Frank et al.15 As shown in
Fig. 7.
DSCs sensitized with both HIQF1 and HIQF2 exhibited a
linear increase in Voc as a function of the logarithm of electron
Fig. 5 IPCE spectra of DSCs based on HIQF1 and HIQF2.
This journal is ª The Royal Society of Chemistry 2012
Fig. 6 J–V curves of DSCs sensitized with HIQF1 and HIQF2.
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Table 2 Current–voltage characteristics of DSCs sensitized with HIQF1
and HIQF2
Jsc (mA
cmꢀ2
Dye
)
Voc [V]
FF
h (%)
HIQF1
HIQF2
12.26
11.46
0.70
0.65
0.73
0.76
6.26
5.66
density. Notably, the plots for the two dyes overlapped, indi-
cating that DSCs sensitized with HIQF1 or HIQF2 had the same
conduction band position regardless of the substituents on the
triphenylamine donor unit. Since the conduction band edge was
not influenced by these two dyes, the increase in Voc for HIQF1
compared to HIQF2 should be ascribed to the suppression of
charge recombination that is related to the electron lifetime (s).
Intensity-modulated photovoltage spectroscopy was used to
measure s, and Fig. 8 shows the relationship between s and
electron density for DSCs sensitized with HIQF1 and HIQF2.
HIQF1 exhibited a much longer s than HIQF2 at the same
electron density, implying that recombination between electrons
Fig. 8 Electron lifetime (s) as a function of Voc for DSCs sensitized with
HIQF1 and HIQF2. Electron lifetime was measured by means of inten-
sity-modulated photovoltage spectroscopy.
HIQF1. The calculated electron lifetime increases from 7.8 ms
(HIQF2) to 10.3 ms of HIQF1, which is derived from eqn (1):
ꢀ
s ¼ Cm ꢁ RCT
(1)
on the TiO2 surface and I3 ions in the electrolyte was consid-
erably suppressed by HIQF1 compared to HIQF2. Thus, the
improvement in Voc was attributed to suppression of charge
recombination by the longer alkyl chains of HIQF1. Introduc-
tion of a stable fluorenone structure into the dye configuration
can also be expected to improve DSC durability, and the stability
of DSCs sensitized with these dyes is currently under
investigation.
where Cm is interfacial chemical capacitance. This strongly
supports the speculation that the bulky alkoxy groups of HIQF1
can act as an effective blocker between the TiO2 layer and the
electrolyte, leading to more effective suppression of the back
ꢀ
reaction of the injected electron with the I3 in the electrolyte,
which is also reflected in the improved Voc, yielding substantially
enhanced DSCs efficiency.
In the Bode phase plot (as shown in Fig. 10), the middle-
frequency peak (in the 1–100 Hz range) is indicative of the
electron recombination between electrolyte and TiO2. This
parameter is related to the electron lifetime and is ultimately
correlated with Voc. The middle-frequency peaks of the DSCs
based on HIQF1 shift to lower frequency relative to that of
HIQF2, indicating a longer recombination lifetime for the
former case. This result is agreement with the fact that HIQF1
possess higher Voc values (0.70 V) than HIQF2 (0.65 V). Thus,
the presence of long alkyl chains in HIQF1 increases not only
charge generation and injection but also the electron lifetime,
which leads to the both improvement of Voc and Jsc, and ulti-
mately the conversion efficiency.
EIS characterization
We performed electrochemical impedance spectra to further
study charge recombination kinetics at the TiO2/dye/electrolyte
interface under open circuit and illuminated conditions. As
shown in Fig. 9, the Nyquist plots exhibited three semicircles, the
largest semicircle (intermediate frequencies, 1–100 Hz) represents
the electron transfer resistance (RCT) at the TiO2/dye/electrolyte
interface, which showed RCT in DSC based on HIQF1 (5.44 U) is
larger than that of HIQF2 based cell (5.19 U), indicating that the
electron recombination resistance increased from HIQF2 to
Fig. 7 Voc as a function of electron density for DSCs sensitized with
HIQF1 and HIQF2. Electron density was measured by means of a charge
extraction method.
Fig. 9 EIS Nyquist plots for DSCs sensitized with HIQF1 and HIQF2.
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without further purification. THF and toluene were dried
over molecular sieves and distilled under argon from sodium
benzophenone ketyl immediately prior to use. 4-(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)-N,N-bis(4-ethyl-hexyloxyphenyl)-
aniline and 4-(4,4,5,5-tetramethyl-1,3,2-dioxa-borolan-2-yl)-
N,N-bis(4-methylphenyl)aniline were synthesized according the
reference.16 Column chromatography was performed using
Wakogel-C300 as a stationary phase.
Dye synthesis and characterization
4-(7-Bromofluorenone-2-yl)-N,N-bis(4-ethylhexyloxyphenyl)-
aniline. A mixture of 2,7-dibromofluorenone (0.34 g, 1 mmol),
4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-N,N-bis(4-ethyl-
hexyloxyphenyl)aniline and Pd (PPh3)4 (50 mg) were suspended
in toluene (50 ml) and 2 M K2CO3 (10 ml) was refluxed for 24 h
under argon. The crude product was extracted into dichloro-
methane, and the organic layer was washed with water and dried
over anhydrous magnesium sulfate. After removal of the solvent
under reduced pressure, the residue was purified by column
chromatography on silica gel (hexane/dichloromethane, 1/2, v/v)
to yield a yellow solid (0.68 g, 89.6%). 1H NMR (600 MHz,
CDCl3, d): 7.85 (s, 1H, Fn-H), 7.76 (d, J ¼ 1.8 Hz, 1H, Fn-H),
7.68 (d, J ¼ 7.2 Hz, 1H, Fn-H), 7.61 (dd, J ¼ 7.8 Hz, J ¼ 1.8 Hz,
1H, Fn-H), 7.52 (d, J ¼ 7.8 Hz, 1H, Fn-H), 7.41 (m, 2H, Th-H),
7.39 (d, J ¼ 7.8 Hz, 1H, Fn-H), 7.08 (s, 4H, Ph-H), 6.97 (s, 2H,
Ph-H), 6.85 (d, J ¼ 8.4 Hz, 4H, Ph-H), 3.82 (s, 4H, –OCH2–),
1.72 (m, 2H, CH), 1.49 (m, 8H, –CHCH2CH2–), 1.32 (m, 8H,
–CH2–), 0.92 (m, 12H, –CH3). 13C NMR (150 MHz, CDCl3, d):
192.62, 156.03, 149.01, 143.17, 142.42, 141.42, 140.23, 137.18,
136.12, 138.48, 132.50, 130.72, 127.59, 127.16, 126.91, 122.53,
122.44, 121.60, 120.81, 119.99, 115.35, 70.74, 39.48, 30.56, 29.12,
23.88, 23.07, 14.11, 11.16. MS (MOLDI-TOF, m/z): [M]+ calcd
for C47H32BrNO3. 757.3; found, 757.3.
Fig. 10 EIS Bode plots for DSCs sensitized with HIQF1 and HIQF2.
Conclusions
In summary, we have demonstrated for the first time the
synthesis of two stable fluorenone-based sensitizer dyes, HIQF1
and HIQF2, in which a fluorenone moiety served as an addi-
tional acceptor in a D–A–p–A framework. These dyes exhibited
strong charge transfer absorption around 500 nm both in solu-
tion and adsorbed on TiO2. Both dyes efficiently sensitized
nanocrystalline TiO2 over the whole visible range from 300 to
700 nm and maintained a high IPCE plateau of about 80% over
most of the visible region. Addition of long alkoxy substituents
in HIQF1 not only slightly red-shifted the absorption spectrum
but also suppressed charge recombination. As a result, solar cells
sensitized with HIQF1 showed a higher h (6.26%) compared to
cells sensitized with HIQF2 (5.66%). The results reported here
provide preliminary evidence that fluorenone, which is both
stable and inexpensive, is an effective building unit for high-
efficiency DSC sensitizers. Further studies are underway to
develop novel fluorenone-based sensitizer dyes for further
broadening the absorption spectra to enhance the dyes’ light-
harvesting ability in the infrared wavelength region.
5-{7-{4-[N,N-Bis(4-ethylhexyloxyphenyl)amino]phenyl}-fluo-
renone-2-yl}thiophenyl-2-carbaldehyde. The 5-formyl-2-thio-
pheneboronic acid (0.156 g, 1 mmol), 4-(7-bromofluorenone-2-
yl)-N,N-bis(4-ethylhexyloxyphenyl)aniline (0.38 g, 0.5 mmol),
Pd(PPh3)4 (10 mg) and K2CO3 (1 g), were dissolved in toluene
(10 ml) and methanol (10 ml) and the solution was heated by
microwave to reflux for 2 hours under the argon atmosphere. The
crude product was extracted into dichloromethane, and the
organic layer was washed with water and dried over anhydrous
magnesium sulfate. After removal of the solvent under reduced
pressure, the residue was purified by column chromatography on
silica gel (hexane/dichloromethane, 1/2, v/v) to yield a red solid
(0.3 g, 75%). 1H NMR (600 MHz, CDCl3, d): 9.89 (s, 1H, CHO),
7.92 (s, 1H, Th-H), 7.86 (s, 1H, Th-H), 7.75 (m, 2H, Fn-H), 7.67
(m, 1H, Fn-H), 7.54 (d, J ¼ 7.8 Hz, 2H, Fn-H), 7.45-7.41 (m, 3H,
Ph-H + Fn-H), 7.09 (d, J ¼ 8.4 Hz, 4H, Ph-H), 6.98 (d, J ¼ 8.4
Hz, 2H, Ph-H), 6.86 (d, J ¼ 8.4 Hz, 4H, Ph-H), 3.83 (m, 4H,
–OCH2–), 1.73 (m, 2H, –CH–), 1.44 (m, 8H, –CHCH2CH2–),
1.32 (m, 8H, –CH2–), 0.94 (m, 12H, –CH3). 13C NMR (150 MHz,
CDCl3, d): 193.18, 182.67, 156.06, 152.76, 149.06, 145.02, 142.68,
142.57, 141.45, 140.20, 137.34, 135.35, 135.11, 133.58, 132.47,
130.62, 127.19, 126.94, 124.47, 122.37, 121.85, 121.10, 120.88,
119.94, 115.37, 70.73, 39.48, 30.56, 29.13, 23.89, 23.08, 14.12,
Experimental
Materials and instruments
Absorption spectra were measured in N,N-dimethylformamide
(DMF) solution or on a TiO2 film (thickness ¼ 4 mm) with a
UV-vis-NIR spectrophotometer (UV-3600, Shimadzu). 1H
(600 MHz) and 13C NMR (150 MHz) spectra were measured
with a DRX-600 spectrometer (Bruker BioSpin). Mass spectra
were measured on a Shimadzu Biotech matrix-assisted laser
desorption ionization (MALDI) mass spectrometer. Cyclic vol-
tammetry was performed on a CH Instruments 624D potentio-
stat/galvanostat system.
A
conventional three-electrode
configuration consisting of a platinum working electrode, a
Pt-wire counter electrode, and an Ag/AgCl reference electrode
was used. The solvent in all measurements was acetone nitrile,
and the supporting electrolyte was 0.1 M [Bu4N]PF6. Ferrocene
was added as a calibrant after each set of measurements, and all
potentials reported were quoted with reference to the ferrocene-
ferrocenium (Fc/Fc+) couple at a scan rate of 50 mV sꢀ1. All
chemicals and reagents were used as received from suppliers
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 19236–19243 | 19241

View Article Online
11.17. MS (MOLDI-TOF, m/z): [M+] calcd for C52H55NO4S,
acid (0.156 g, 1 mmol), 4-(7-bromofluorenone-2-yl)-N,N-bis(4-
methylphenyl)aniline (0.27 g, 0.5 mmol) and Pd(PPh3)4 (10 mg)
were dissolved in toluene (30 ml) and 2 M K2CO3 (5 ml), and the
solution was heated to reflux for 24 hours under the argon
atmosphere. To the reaction solution was added water (50 ml),
and this was extracted with dichloromethane, dried with
magnesium sulfate, and concentrated under reduced pressure.
The residue was purified by silica gel column chromatography
(hexane/dichloromethane ¼ 1/2) to obtain 5-{7-{4-[N,N-bis-
(4-methylphenyl)amino]phenyl}-fluore-none-2-yl}thiophenyl-2-
carbaldehyde (0.48 g, 85%). 1H NMR (600 MHz, CDCl3, d): 9.91
(s, 1H, CHO), 7.96 (d, J ¼ 1.8 Hz, 1H, Th-H), 7.91 (d, J ¼ 1.8 Hz,
1H, Th-H), 7.80 (dd, J ¼ 7.8 Hz, J ¼ 1.8 Hz, 1H, Fn-H), 7.77 (d,
J ¼ 3.6 Hz, 1H, Fn-H), 7.73 (dd, J ¼ 7.8 Hz, J ¼ 1.8 Hz, 1H, Fn-
H), 7.59 (d, J ¼ 7.8 Hz, J ¼ 1.8 Hz, 2H, Fn-H), 7.45 (m, 3H, Ph-
H), 7.09 (m, 5H, Ph-H), 7.07 (s, 1H, Ph-H), 7.05 (d, J ¼ 8.4 Hz,
4H, Ph-H), 2.33 (s, 6H). 13C NMR (150 MHz, DMSO-d6, d):
193.19, 182.73, 152.77, 148.45, 144.99, 142.72, 142.48, 141.70,
139.11, 137.39, 135.38, 135.13, 133.69, 133.10, 132.69, 132.54,
131.94, 130.03, 127.30, 125.03, 124.52, 122.55, 122.08, 121.93,
121.14, 120.96, 20.88. MS (MOLDI-TOF, m/z): [M+] calcd for
C38H27NO2S, 561.2; found, 561.4.
789.4; found, 789.5.
2-Cyano-3-{5-{7-{4-[N,N-bis(4-ethylhexyloxyphenyl)amino]-
phenyl}-fluorenone-2-yl}thiophenyl}acrylic acid (HIQF1). To a
stirred solution of 5-{7-{4-[N,N-bis(4-ethylhexyloxyphen-yl)
amino]phenyl}-fluorenone-2-yl}thiophenyl-2-carbaldehyde (0.2 g,
0.25 mmol) and cyanoacetic acid (68 mg, 0.8 mmol) in chloro-
form (5 ml) was added piperidine (140 mg, 1.6 mmol). The
reaction mixture was refluxed under argon for 12 h and then
acidified with 2 M aqueous hydrochloric acid (10 ml). The crude
product was extracted into chloroform, and the organic layer was
washed with water and dried over anhydrous magnesium sulfate.
After removal of the solvent under reduced pressure, the residue
was purified by flash chromatography with chloroform and
methanol/chloroform (1/10, v/v) in turn as eluents to yield a deep
1
red powder (0.13 g, 79.2% yield). H NMR (600 MHz, DMSO-
d6, d): 8.43 (s, 1H, C]CH), 8.01 (d, J ¼ 9 Hz, 1H, Th-H), 7.98
(d, J ¼ 9 Hz, 1H, Th-H), 7.95 (s, 1H, Fn-H), 7.92 (d, J ¼ 3.6 Hz,
1H, Fn-H), 7.91 (d, J ¼ 7.8 Hz, 1H, Fn-H), 7.86 (s, 2H, Fn-H),
7.82 (s, 1H, Fn-H), 7.61 (d, J ¼ 9 Hz, 2H, Ph-H), 7.06 (d, J ¼ 8.4
Hz, 4H, Ph-H), 6.94 (d, J ¼ 9 Hz, 4H, Ph-H), 6.82 (d, J ¼ 9 Hz,
2H, Ph-H), 3.84 (d, J ¼ 6 Hz, 4H, –OCH2–), 1.67 (m, 2H, –CH–),
1.43 (m, 8H, –CHCH2CH2–), 1.29 (m, 8H, –CH2–), 0.88 (m,
12H, –CH3). 13C NMR (150 MHz, DMSO-d6, d): 192.85, 163.76,
156.18, 149.06, 144.68, 141.86, 141.68, 140.85, 140.02, 135.78,
135.07, 134.95, 133.57, 133.22, 132.98, 130.03, 127.71, 127.58,
126.32, 122.68, 122.55, 121.55, 121.51, 119.28, 117.44, 116.03,
70.58, 39.20, 30.42, 28.93, 23.80, 23.00, 14.44, 11.42. HRMS
(ESI, m/z): [M + H]+ calcd for C55H56N2O5S, 856.3910; found,
856.3914. Anal. calcd for C55H56N2O5S: C 77.07, H 6.59, N 3.27;
found: C 77.12, H 6.50, N 3.06%.
2-Cyano-3-{5-{7-{4-[N,N-bis(4-methylphenyl)amino]phenyl}-
fluorenone-2yl}thiophenyl}-acrylic acid (HIQF2). HIQF2 was
synthesized according to the procedure described for HIQF1 and
was obtained as a deep red powder (0.15 g, 75% yield). 1H NMR
(600 MHz, DMSO-d6, d): 8.37 (s, 1H, COOH), 8.32 (s, 1H, C]
CH), 8.01 (d, J ¼ 7.8 Hz, 1H, Th-H), 7.95 (d, J ¼ 7.8 Hz, 1H, Th-
H), 7.93 (s, 1H, Fn-H), 7.89 (m, 4H, Fn-H), 7.85 (s, 1H, Fn-H),
7.66 (d, J ¼ 8.4 Hz, 2H, Ph-H), 7.16 (d, J ¼ 8.4 Hz, 4H, Ph-H),
6.98 (d, J ¼ 8.4 Hz, 4H, Ph-H), 6.95 (d, J ¼ 8.4 Hz, 2H, Ph-H),
2.26 (s, 6H, –CH3). 13C NMR (150 MHz, DMSO-d6, d): 192.84,
163.66, 148.31, 144.84, 144.50, 141.97, 141.66, 136.11, 135.09,
134.96, 133.75, 133.35, 133.19, 131.49, 130.66, 127.85, 126.26,
125.35, 122.70, 122.61, 121.72, 121.69, 121.48, 20.91. HRMS
(ESI, m/z): [M + H]+ calcd for C41H28N2O3S, 628.1821; found,
628.1823. Anal. calcd for C41H28N2O3S: C 78.32, H 4.49, N 4.46;
found: C 78.36, H 4.26, N 4.43%.
4-(7-Bromofluorenone-2-yl)-N,N-bis(4-methylphenyl)aniline. 4-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-N,N-bis(4-meth-
ylphenyl)aniline (0.4 g, 1 mmol), 2,7-dibromofluorenone (0.34 g,
1 mmol) and Pd (PPh3)4 (50 mg) were suspended in toluene
(50 ml) and 2 M K2CO3 (10 ml), and the resulting mixture was
heated to reflux for 16 hours under the argon atmosphere. After
completion of the reaction, water (120 ml) was added to the
reaction solution, and this was extracted with dichloromethane,
dried with magnesium sulfate, and concentrated under reduced
pressure. The residue was purified by silica gel column chroma-
tography (hexane/dichloromethane ¼ 1/2) to obtain 4-(7-bro-
Cell fabrication and characterization
The DSCs were fabricated as follows. A double-layer TiO2
photoelectrode (thickness 15 mm; area 0.25 cm2) was used as a
working electrode. A 10 mm main transparent layer with titania
particles (ꢂ20 nm) and a 5 mm scattering layer with titania
particles (ꢂ400 nm) were screen-printed on the fluorine-doped
tin oxide conducting glass substrate. A solution of HIQF1 and
HIQF2 (3 ꢁ 10ꢀ4 M) in acetonitrile/tert-butyl alcohol (1/1, v/v)
was used to coat the TiO2 film with the dye. Deoxycholic acid
(20 mM) was added to the dye solution as a co-adsorbent to
prevent aggregation of the dye molecules. The electrodes were
immersed in the dye solutions and then kept at 25 ꢃC for 24 h to
adsorb the dye onto the TiO2 surface. The dye-coated TiO2 film
was used as the working electrode, and platinum-coated con-
ducting glass was used as the counter-electrode. The two elec-
trodes were separated by a Surlyn spacer (50 mm thick) and
sealed up by heating the polymer frame. The electrolyte was
composed of 0.6 M dimethylpropylimidazolium iodide, 0.05 M
mofluorenone-2-yl)-N,N-bis(4-methylphenyl)aniline as
a red
solid (0.36 g, 68%). 1H NMR (600 MHz, CDCl3, d): 7.86 (d, J ¼
1.8 Hz, 1H, Fn-H), 7.77 (d, J ¼ 1.8 Hz, 1H, Fn-H), 7.69 (dd, J ¼
7.8 Hz, J ¼ 1.8 Hz, 1H, Fn-H), 7.61 (dd, J ¼ 7.8 Hz, J ¼ 1.8 Hz,
1H, Fn-H), 7.53 (d, J ¼ 7.8 Hz, 1H, Fn-H), 7.45 (d, J ¼ 7.2 Hz,
2H, Ph-H), 7.41 (d, J ¼ 7.8 Hz, 1H, Fn-H), 7.10 (d, J ¼ 8.4 Hz,
4H, Ph-H), 7.07 (s, 1H, Ph-H), 7.06 (s, 1H, Ph-H), 7.04 (d, J ¼ 8.4
Hz, 4H, Ph-H), 2.33 (s, 6H, –CH3). 13C NMR (150 MHz, CDCl3,
d): 192.56, 148.39, 144.94, 143.11, 142.29, 141.63, 137.20, 136.11,
134.48, 133.06, 132.66, 132.01, 130.02, 127.61, 127.27, 125.00,
122.61, 122.56, 122.11, 121.64, 120.83, 20.88. MS (MOLDI-
TOF, m/z): [M]+ calcd for C33H24BrNO. 529.1; found, 529.5.
5-{7-{4-[N,N-Bis(4-methylphenyl)amino]phenyl}-fluorenone-2-
yl}thiophenyl-2-carbaldehyde. The 5-formyl-2-thiopheneboronic
19242 | J. Mater. Chem., 2012, 22, 19236–19243
This journal is ª The Royal Society of Chemistry 2012

View Article Online
J. Chen, Z.-S. Wang and H. Tian, Adv. Funct. Mater., 2011, 21,
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current–voltage characteristics were measured using a black
metal mask and edge with an aperture area of 0.25 cm2 under
standard AM 1.5 sunlight (100 mW cmꢀ2, WXS-155S-10:
Wacom Denso Co. Japan).17 Monochromatic incident photon-
to-current conversion efficiency spectra were measured with
monochromatic incident light of 1 ꢁ 1016 photons cmꢀ2 under
100 mW cmꢀ2 in director current mode (CEP-2000BX, Bunko-
Keiki).
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Acknowledgements
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P. Scandiucci de Freitas, U. Scherf and E. J. W. List, J. Chem. Phys.,
This work was supported by NIMS Saint-Gobain Center of
Excellence for Advanced Materials and Core Research for
Evolutional Science and Technology (CREST) of the Japan
Science and Technology Agency.
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J. Mater. Chem., 2012, 22, 19236–19243 | 19243