Effects of aromatic π-conjugated bridges on optical and photovoltaic properties of N,N-diphenylhydrazone-based metal-free organic dyes

Article abstract of DOI:10.1016/j.orgel.2011.08.010

A series of novel D-π-A hydrazone dyes (HB, HP, HF and HT) containing an N,N-diphenylhydrazone donor and 2-cyanoacetic acid acceptor linked by a different aromatic bridge (benzene, pyrrole, furan, and thiophene) have been designed and synthesized to evalu

Full text of DOI:10.1016/j.orgel.2011.08.010

Organic Electronics 12 (2011) 1992–2002
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Effects of aromatic
p-conjugated bridges on optical and photovoltaic
properties of N,N-diphenylhydrazone-based metal-free organic dyes
Ping Shen a,b, , Xinping Liu a, Shenghui Jiang a, Yuanshuai Huang a, Ling Yi a, Bin Zhao a,b
,
⇑
Songting Tan a,b
a College of Chemistry and Key Laboratory of Advanced Functional Polymeric Materials, College of Hunan Province, Xiangtan University, Xiangtan 411105, PR China
b Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 May 2011
Received in revised form 9 July 2011
Accepted 7 August 2011
Available online 3 September 2011
A series of novel D–p–A hydrazone dyes (HB, HP, HF and HT) containing an N,N-diph-
enylhydrazone donor and 2-cyanoacetic acid acceptor linked by a different aromatic bridge
(benzene, pyrrole, furan, and thiophene) have been designed and synthesized to evaluate the
aromatic bridge effects on the photophysical, electrochemical and the photovoltaic proper-
ties of the hydrazone-sensitized TiO2 solar cells. Each of the hydrazone-based dyes exhibited
different adsorption behavior, frontier molecular orbitals, and photovoltaic performance
Keywords:
Dye-sensitized solar cells
depending on the identity of aromatic
p-conjugated bridges. Specifically, HP-sensitized
TiO2 solar cell showed an obviously higher photocurrent, photovoltage, and power conver-
sion efficiency than other three dyes. We interpret that these results were stemmed from
light-harvesting abilities, the charge recombination rate and electron lifetime of different
aromatic bridges. Optical spectroscopy, cyclic voltammetry, density functional theory calcu-
lations, electrochemical impedance spectra, and photovoltaic measurements were
employed to support our proposal. With the addition of 1 mM chenodeoxycholic acid (CDCA)
as the coadsorbent, a maximum power conversion efficiency of a DSSC based on HP was
7.74% (Jsc = 16.17 mA/cm2, Voc = 0.69 V, FF = 0.694) under simulated AM 1.5 G solar irradia-
tion (100 mW/cm2). This work suggests that N,N-diphenylhydrazone and N-(2-ethylhexyl)
Aromatic p-conjugated bridges
N,N-Diphenylhydrazone
Photovoltaic performance
substituted pyrrole moieties can be used as an alternative and effective donor and
gated bridge, respectively, in the construction of efficient D– –A organic dyes.
p-conju-
p
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
include a transition metal in the structure and metal-free
organic dyes. At present, the most efficient DSSCs are
based on TiO2/Ru-bipyridyl dye systems, with the highest
power conversion efficiency (PCE) up to 11.7% being
achieved with a ruthenium dye (C106) [2]. Compared
with the traditional ruthenium dyes, metal-free organic
dyes have many advantages such as lower cost, easier
structural modification, and higher molar extinction coef-
ficient. A common strategy in the design of highly effi-
cient metal-free dyes for DSSCs is the linking of
The increasing energy demand and the concerns
about climate changes have led to a great focus on envi-
ronment friendly energy sources during the last years.
Dye-sensitized solar cells (DSSCs) are a potentially low-
cost alternative to conventional silicon photovoltaic de-
vices [1]. The photoactive dyes play a crucial role for
highly efficient DSSCs and they can be broadly classified
into two separate classes: organometallic dyes which
electron donor/acceptor (D–A) systems through
gated bridges, which is called the D– –A molecular
structure. This kind of D– –A compounds has been
p-conju-
p
⇑
Corresponding author at: College of Chemistry and Key Laboratory of
Advanced Functional Polymeric Materials, College of Hunan Province,
Xiangtan University, Xiangtan 411105, PR China.
p
found to possess photoinduced intramolecular charge
transfer (PICT) properties [3], which makes them as ideal
E-mail address: shenping802002@163.com (P. Shen).
1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.08.010

P. Shen et al. / Organic Electronics 12 (2011) 1992–2002
1993
dyes for DSSCs application. Using this strategy, many no-
vel D– –A metal-free dyes with various donor moieties,
tal-free organic dyes used in DSSCs. Once again, our re-
sults indicate that -conjugated bridge would have a
p
p
such as coumarin [4], indoline [5], triarylamine [6],
[bis(dimethylfluorenyl)amino]phenyl [7], phenothiazine
[8], tetrahydroquinoline [9], carbazole [10], porphyrin
[11], dithieno[3,2-b:20,30-d]thiophene [12], and pyrrole
[13], have been designed and synthesized as efficient
sensitizers for DSSCs in recent years. PCEs up to ꢀ9%
have been achieved for DSSCs using these metal-free
dyes [4]. Moreover, highly impressive PCEs (ꢀ10%) and
excellent stability were also reported in very recent pub-
lications [14]. Developing novel and highly efficient me-
tal-free organic dyes applied in DSSCs is thus an area of
strong current research activity [3].
large impact on photoelectronic properties of the me-
tal-free dyes sensitized TiO2 cells.
2. Experimental
2.1. Materials and reagents
1,1-Diphenylhydrazine hydrochloride, terephthalalde-
hyde, 2-furaldehyde, 2-thienaldehyde, thiophene, pyrrole,
and 2-ethylhexyl bromide were purchased from commer-
cial suppliers (Pacific ChemSource and Alfa Aesar) in ana-
lytical grade. 2-Carbaldehyde-N-ethylhexyl-pyrrole was
synthesized according to the previous published procedure
[25]. Toluene was dried and distilled over sodium/benzo-
phenone. DMF, CHCl3, and CH3CN were dried over by
accustomed methods and distilled before using. All other
solvents and chemicals used in this work were analytical
grade and used without further purification. All chromato-
graphic separations were carried out on silica gel
(200–300 mesh).
The
p-conjugated bridge connecting the donor and
acceptor in the dye influences not only the region of light
absorbed by the DSSCs but also the degree of electron
injection from the dye’s excited state to the TiO2 surface.
Aromatic moiety is usually used as efficient bridge in the
design of dye molecules. Thiophene-based aromatic
compounds have been demonstrated as the effective
p-conjugated bridges by several research groups [15],
including ours [6g]. Imahori et al. [16] investigated the
effects of 5-membered heteroaromatic bridges on struc-
tures of porphyrin films and photovoltaic properties of
porphyrin-based DSSCs. Recently, Wang and co-workers
[17] also studied the influence of different heteroaromatic
bridges on photovoltaic properties of triphenylamine-
based organic dyes.
2.2. Characterization
1H NMR spectra was recorded on a Bruker Avance 400
instrument. Ultraviolet–visible (UV–vis) spectra of the
dyes were measured on a Perkin-Elmer Lamada 25 spec-
trometer. The PL spectra were obtained using Perkin-Elmer
LS-50 luminescence spectrometer. FT-IR spectra were
obtained on Perkin-Elmer Spectra One spectrophotometer.
The elemental analysis of all compounds were performed
with a Perkin-Elmer 2400 analyzer for C, H, N, and S deter-
mination. Electrochemical redox potentials were obtained
by cyclic voltammetry (CV) using a three-electrode cell
and an electrochemistry workstation (CHI830B, Chenhua
Shanghai). The working electrode was a Pt ring electrode;
the auxiliary electrode was a Pt wire, and saturated calo-
mel electrode (SCE) was used as reference electrode. Tetra-
butylammonium hexafluorophosphate (Bu4NPF6) 0.1 M
was used as supporting electrolyte in dry acetonitrile. Fer-
rocene was added to each sample solution at the end of the
experiments, and the ferrocenium/ferrocene (Fc+/Fc) redox
couple was used as an internal potential reference. The
potentials of dyes versus NHE were calibrated by addition
of 0.63 V to the potentials versus Fc+/Fc [6a]. The solutions
were purged with argon and stirred for 15 min before the
measurements. Electrochemical impedance experiments
for DSSCs were carried out in the dark with an electro-
chemical workstation (ZAHNER ZENNIUM) at room tem-
perature. Electrochemical impedance spectra (EIS) were
Aromatic hydrazone compounds are widely studied
and used as organic nonlinear optical materials [18]
and organic hole-transport materials for electrophoto-
graphic photoreceptors [19]. The main advantages of
such compounds are the simple synthesis and high en-
ough charge mobilities [19b]. Many triphenylamine-
[20], carbazole- [21] and thiophene-based hydrazone
[22] compounds were synthesized and used as glass-
forming hole-transport materials. Squaraine-based com-
pounds containing N,N-diphenylhydrazone moiety were
synthesized and applied as efficient small molecule do-
nors in bulk-heterojunction solar cells [23]. In addition,
Tran-Van et al. recently reported on the synthesis and
application of biscarbazole and terthiophene derivatives
containing hydrazone functional groups as hole-transport
materials in solid-state DSSCs [24]. However, aromatic
hydrazones compounds until now are few studied as
photoactive dyes for DSSCs. Therefore, we became inter-
ested in constructing metal-free organic dyes with
hydrazone moiety to make use of its excellent
hole-transport property for the electron donor.
Expanding on the foregoing points and as another try
of our efforts to explore new metal-free dyes, we herein
report on the design and synthesis of four D–p–A dyes
(HB, HP, HF, and HT, as shown in Fig. 1) consisting of
an N,N-diphenylhydrazone donor, a 2-cyanoacetic acid
acceptor, and a different aromatic bridge, i.e., benzene,
scanned in a
frequency range of 10ꢁ1–100 kHz with
applied potential set at open-circuit. The alternate current
(AC) amplitude was set at 10 mV. Geometry optimization
and electronic structure calculations of the hydrazone-
based dyes were performed using B3LYP functional and
3-21G⁄ basis set implemented in the Gaussian 03 program
package. Molecular orbitals were visualized by GaussView
3.0 software.
pyrrole, furan, and thiophene. The effects of the
p-conju-
gated bridges on the photophysical, electrochemical and
photovoltaic properties of these dyes were investigated
in detail. To the best of our knowledge, this is the first
time to develop hydrazones-based compounds as me-

1994
P. Shen et al. / Organic Electronics 12 (2011) 1992–2002
Fig. 1. Molecular structures of the dyes HB, HP, HF, and HT.
2.3. General procedure for fabrication and characterization of
DSSCs
2.4.1. 4-Carbaldehyde-benzene-N,N-diphenylhydrazone (1)
Under nitrogen atmosphere, a solution of N,N-diphenyl-
hydrazine hydrochloride (0.88 g, 4 mmol) in 30 mL of THF
was added dropwise into the mixture of terephthalalde-
hyde (0.94 g, 7 mmol), sodium acetate (0.82 g, 10 mmol)
and anhydrous MeOH (20 mL) at the room temperature.
The mixture was refluxed for 24 h and the color changed
gradually from yellow to dark brown. After cooling, the
resulting mixture was quenched with 40 mL distilled
water and extracted with chloroform. The combined
organic layer was washed with distilled water and brine,
dried over anhydrous MgSO4. The solvent was removed
by rotary evaporation. The crude product was purified by
silica gel column chromatography with petroleum ether/
dichloromethane (5/1, v/v) as eluent to yield 1 as a yellow
Fluorine-doped SnO2 conducting glass (FTO) were
cleaned and immersed in aqueous 40 mM TiCl4 solution
at 70 °C for 30 min, then washed with water and ethanol,
sintered at 450 °C for 30 min. The TiO2 colloid was pre-
pared from 12 g P25 (Degussa AG, Germany) following
the literature procedure [4]. First the 20–30 nm particles
sized TiO2 colloid was coated onto the above FTO glass
by sliding glass rod method to obtain a TiO2 film of
14 lm thickness after drying. Subsequently, the 200 nm
particles sized TiO2 colloid was coated on the electrode
by the same method, resulting in a TiO2 light-scattering
layer of 5 lm thickness. The double-layer TiO2-coated
FTO glass were sintered at 450 °C for 30 min, then treated
with TiCl4 solution and calcined at 450 °C for 30 min again.
After cooling to 100 °C, the TiO2 electrodes were soaked in
a CH3CN solution with 0.5 mM dyes (and 1 mM chenode-
oxycholic acid as a coadsorbent) and kept at room temper-
ature under dark for 24 h.
solid. Yield: 67% (0.85 g). FT-IR (KBr, cmꢁ1): 1667 (
mC@O).
(ppm) 9.97 (s, 1H),
1H NMR (400 MHz, CDCl3):
d
7.83–7.85 (d, 2H, J = 8.0 Hz), 7.73–7.75 (d, 2H, J = 8.0 Hz),
7.48–7.44 (m, 4H), 7.25–7.23 (m, 6H), 7.15 (s, 1H). Elem.
Anal. Calcd. for C20H16N2O: C, 79.98%; H, 5.37%; N, 9.33%.
Found: C, 79.70%; H, 5.85%; N, 9.56%.
The dye-adsorbed TiO2 electrode washed with ethanol
and dried. A drop of electrolyte solution was deposited
onto the surface of the electrode and a Pt foil counter elec-
trode was clipped onto the top of the TiO2 electrode to
assemble a dye sensitized solar cells for photovoltaic per-
formance measurements. The electrolyte consisted of
0.5 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine (TBP)
in 3-methoxypropionitrile and the efficient irradiated area
of cell was 0.196 cm2. The photocurrent–voltage (J–V)
characteristics of solar cells were measured using a Keith-
ley 2602 Source meter under the 100 Mw/cm2 irradiation
of a 500 W Xe lamp with a global AM 1.5 filter for solar
spectrum simulation. Meanwhile, the photoelectric con-
version efficiency was calculated. The measurement of
monochromatic incident photon-to-current conversion
efficiencies (IPCE) was performed by a Zolix DCS300PA
Data acquisition system and other optical system.
2.4.2. The general synthetic procedure of (2)
In a 100 mL three-necked flask, a solution of N-ethyl-
hexylpyyrole-2-carbaldehyde (or 2-furaldehyde or 2-thien
aldehyde, 5 mmol) in 30 mL of THF was added dropwise into
the mixture of N,N-diphenylhydrazine hydrochloride
(1.54 g, 7 mmol), sodium acetate (0.82 g, 10 mmol) and
anhydrous MeOH (20 mL) under nitrogen atmosphere. The
mixture was refluxed for 24 h and the color turned gradually
from yellow to dark brown. After cooling, the resulting mix-
ture was quenched with 40 mL distilled water and extracted
with chloroform. The combined organic layer was washed
with distilled water and brine, dried over anhydrous MgSO4.
The solvent was removed by rotary evaporation. The crude
product was purified by silica gel column chromatography
with petroleum ether/dichloromethane (4/1, v/v) as eluent.
2.4.2.1. N-Ethylhexylpyrrole-2-carbaldehyde-N,N-diphenylhyd-
razone (2a). Yield: 76% (1.42 g). 1H NMR (400 MHz, CDCl3):
d (ppm) 7.46–7.42 (m, 5H), 7.24–7.17 (m, 5H), 7.10 (s, 1H),
6.88 (s, 1H), 6.67–6.65 (m, 2H), 4.34 (d, 2H, J = 5.6 Hz),
1.31–1.01 (m, 9H), 0.85 (t, 3H, J = 7.3 Hz), 0.72 (t, 3H,
J = 7.2 Hz). Elem. Anal. Calcd. for C25H31N3: C, 80.39%; H,
8.37%; N, 11.25%. Found: C, 80.40%; H, 8.47%; N, 11.66%.
2.4. Synthesis
The synthetic routes to the four dyes were shown in
Scheme 1. The detailed synthetic procedures were as
follows.

P. Shen et al. / Organic Electronics 12 (2011) 1992–2002
1995
Scheme 1. Synthesis of the HB, HP, HF, and HT dyes.
2.4.2.2. Furan-2-carbaldehyde-N,N-diphenylhydrazone
(2b). Yield: 70% (0.92 g). 1H NMR (400 MHz, CDCl3): d
(ppm) 7.43–7.39 (m, 6H), 7.20–7.27 (m, 5H), 7.08 (s, 1H),
6.46–6.42 (m, 2H). Elem. Anal. Calcd. for C17H14N2O: C,
77.84%; H, 5.38%; N, 10.68%. Found: C, 77.52%; H, 5.47%;
N, 10.80%.
C
26H31N3O: C, 77.77%; H, 7.78%; N, 10.46%. Found: C,
77.40%; H, 7.97%; N, 10.66%.
2.4.3.2.
5-Formyl-furan-2-carbaldehyde-N,N-diphenylhyd-
razone(3b). Yield: 81% (0.39 g). FT-IR (KBr, cmꢁ1): 1665
(m
C@O). 1H NMR (400 MHz, CDCl3): d (ppm) 9.56 (s, 1H),
7.46–7.43 (m, 6H), 7.26–7.23 (m, 4H), 7.19 (d, 1H,
J = 3.5 Hz), 7.07(s, 1H), 6.81 (d, 1H, J = 3.2 Hz). Elem. Anal.
Calcd. for C18H14N2O2: C, 74.47%; H, 4.86%; N, 9.65%. Found:
C, 74.32%; H, 4.77%; N, 9.80%.
2.4.2.3. Thiophene-2-carbaldehyde-N,N-diphenylhydrazone
(2c). Yield: 72% (1.0 g). 1H NMR (400 MHz, CDCl3): d
(ppm) 7.44–7.42 (m, 4H), 7.31 (s, 1H), 7.27 (s, 1H), 7.22–
7.20 (m, 6H), 7.97–7.93 (m, 2H). Elem. Anal. Calcd. for
C
17H14N2S: C, 73.35%; H, 5.07%; N, 10.06%; S, 11.52%.
2.4.3.3. 5-Formyl-thiophene-2-carbaldehyde-N,N-diphenylhyd-
razone (3c). Yield: 85% (0.52 g). FT-IR (KBr, cmꢁ1): 1668
Found: C, 73.50%; H, 5.07%; N, 10.50%; S, 11.58%.
(m
C@O). 1H NMR (400 MHz, CDCl3): d (ppm) 9.86 (s, 1H),
7.63 (d, 1H, J = 3.5 Hz), 7.62 (s, 1H), 7.48 (d, 1H,
J = 3.5 Hz), 7.47–7.45 (m, 4H), 7.27–7.21 (m, 2H), 6.99–
2.4.3. The general synthetic procedure of (3)
In a 100 mL three-necked flask, compound 2 (2 mmol)
and dried DMF (0.23 mL, 3 mmol) were dissolved into
1,2-dichloroethane (20 mL). After the solution was cooled
to 0 °C, POCl3 (0.27 mL, 3 mmol) was added dropwise.
The mixture was stirred at room temperature for 0.5 h
and then heated up to reflux for another 12 h. After cooling
to room temperature, saturated aqueous solution of NaOAc
(20 mL) was added into the reaction mixture and stirred
violently for 1 h. Then the resulting mixture was extracted
with chloroform. The combined organic layer was washed
with distilled water and brine, dried over anhydrous
MgSO4. Solvent was removed by rotary evaporation, and
the crude product was purified by silica gel column chro-
matography with petroleum ether/dichloromethane (5/1,
v/v) as eluent.
6.98 (m, 4H). Elem. Anal. Calcd. for
70.56%; H, 4.61%; N, 9.14%; S, 10.47%. Found: C, 70.50%;
H, 4.67%; N, 9.16%; S, 10.53%.
C18H14N2OS: C,
2.4.4. The general synthetic procedure of sensitizers
Under nitrogen atmosphere, a mixture of compound 1
(or 3, 1.2 mmol), 2-cyanoacetic acid (0.21 g, 2.4 mmol),
piperidine (0.5 mL), and CH3CN (30 mL) was refluxed at
80 °C for 24 h. After cooling, most of solvent was removed
under vacuum and the residue was dropped into a mixture
of petroleum ether and HCl (0.1 M) to form a precipitation.
The crude product was further purified by silica gel column
chromatograph eluted with dichloromethane/methanol
(15/1, v/v) to obtain dyes.
2.4.3.1. 5-Formyl-N-ethylhexypyrrolel-2-carbaldehyde-N,N-
2.4.4.1. 5-(2-Cyanoacrylicacid-3-yl)-benzene-2-carbalde-hyde-N,
diphenylhydrazone (3a). Yield: 83% (0.67 g). FT-IR (KBr,
N-diphenylhydrazone (HB). Yield: 72% (0.32 g). FT-IR (KBr,
cmꢁ1): 1666 (
m
C@O). 1H NMR (400 MHz, CDCl3): d (ppm)
cmꢁ1): 2227 ( C@O). 1H NMR (400 MHz, CDCl3):
mC„N), 1693 (m
9.45 (s, 1H), 7.46–7.42 (m, 5H), 7.24–7.17 (m, 5H), 7.09
(s, 1H), 6.90 (d, 1H, J = 4.1 Hz), 6.67 (d, 1H, J = 4.2 Hz),
4.33 (d, 2H, J = 5.8 Hz), 1.26–1.03 (m, 9H), 0.83 (t, 3H,
J = 7.1 Hz), 0.70 (t, 3H, J = 7.4 Hz). Elem. Anal. Calcd. for
d (ppm) 8.27 (s, 1H), 7.85 (s, 1H), 7.76–7.55 (m, 4H), 7.30–7.26
(m, 6H), 7.24–7.19 (m, 4H). Elem. Anal. Calcd. for C23H17N3O2:
C, 75.19%; H, 4.66%; N, 11.44%. Found: C, 75.30%; H, 4.97%; N,
11.60%.

1996
P. Shen et al. / Organic Electronics 12 (2011) 1992–2002
2.4.4.2. 5-(2-Cyanoacrylicacid-3-yl)-N-ethylhexylpyrrole-2-
overlapped with the solar emission will affect the device
photocurrent to a large extent; thus, we measured the ultra-
violet–visible (UV–vis) absorptions of the dyes HB, HP, HF,
and HT both in THF solutions and on TiO2 films (Fig. 2 and
Fig. 3, respectively). The characteristic data are collected in
Table 1.
carbaldehyde-N,N-diphen-ylhydrazone (HP). Yield: 74%
(0.42 g). FT-IR (KBr, cmꢁ1): 2210 ( C@O). 1H
mC„N), 1670 (m
NMR (400 MHz, CDCl3): d (ppm) 8.04 (s, 1H), 7.84 (d, 1H,
J = 4.5 Hz), 7.08 (s, 1H), 7.52–7.44 (m, 4H), 7.21–7.17 (m,
6H), 6.82 (d, 1H, J = 4.6 Hz), 4.05 (d, 2H, J = 7.4 Hz),
1.55–1.11 (m, 9H), 0.84 (t, 3H, J = 7.0 Hz), 0.75 (t, 3H,
J = 7.3 Hz). Elem. Anal. Calcd. for C29H32N4O2: C, 74.33%;
H, 4.88%; N, 11.96%. Found: C, 74.10%; H, 4.97%; N, 11.60%.
All the dyes exhibit two distinct absorption bands
(Fig. 2): one absorption band is in the UV region
(280–350 nm) corresponding to the
p–p
⁄ electron transi-
tions of the conjugated molecules; and the other is in the
visible region (380–550 nm) that can be assigned to an
intramolecular charge transfer (ICT) between the N,N-
diphenylhydrazone donating unit and the cyanoacrylic
acid acceptor moiety [26]. It can be seen that the maxi-
mum absorption (kmax) of the visible region red-shifts from
425 nm to 452 nm, 465 nm, and 469 nm for HB, HF, HT and
HP, respectively. This red-shifted phenomenon may be
ascribed to the different aromatic ability of the three
five-membered heteroaromatic bridges. The aromatic abil-
ity of furan is larger than that of thiophene and pyrrole due
to the stronger electronegative of oxygen element than
that of nitrogen and sulphur elements. We can see that
the kmax values in the visible region shift to a lower energy
with the decreased electronegativity of heteroatoms,
which is in accordance with results of Wang et al. [17].
2.4.4.3. 5-(2-Cyanoacrylicacid-3-yl)-furan-2-carbaldehyde-N,
N-diphenylhydrazone (HF). Yield: 72% (0.31 g). FT-IR (KBr,
cmꢁ1): 2221 ( C@O). 1H NMR (400 MHz,
mC„N), 1685 (m
CDCl3): d (ppm) 7.96 (s, 1H), 7.48–7.44 (m, 6H), 7.28–
7.26 (m, 4H), 7.22 (d, 1H, J = 3.5 Hz), 7.03 (s, 1H), 6.82 (d,
1H, J = 3.5 Hz). Elem. Anal. Calcd. for
C21H15N3O3: C,
70.58%; H, 4.43%; N, 11.76%. Found: C, 70.30%; H, 4.57%;
N, 11.80%.
2.4.4.4. 5-(2-Cyanoacrylicacid-3-yl)-thiophene-2-carbalde-hy
de-N,N-diphenylhydrazone (HT). Yield: 74% (0.33 g). FT-IR
(KBr, cmꢁ1): 2213
(mC„N), 1673
(m
C@O). 1H NMR
(400 MHz, DMSO-d6): d (ppm) 8.36 (s, 1H), 7.82 (d, 1H,
J = 3.4 Hz), 7.52 (s, 1H), 7.50 (d, 1H, J = 3.5 Hz), 7.37–7.26
(m, 6H), 7.19–7.17 (m, 4H). Elem. Anal. Calcd. for
As depicted in Table 1, the molar extinction coefficient (
e
)
)
C21H15N3O2S: C, 67.54%; H, 4.05%; N, 11.25%; S, 8.59%.
of the ICT bands of the four dyes (15.9–24.6 ꢂ 103 M–1 cm–1
Found: C, 67.58%; H, 4.07%; N, 11.60%; S, 8.63%.
are higher than that of the standard N719 sensitizer
(14.2 ꢂ 103 M–1 cm–1) [27], indicating a better ability of
light harvesting of these new N,N-diphenylhydrazone-
based metal-free organic dyes than that of classical organo-
3. Results and discussion
3.1. Synthesis and characterization of sensitizers
metallic dyes. Moreover, the e of the ICT bands increases in
the order of HB < HT < HF < HP, which suggests that HP
should hold the best ability of light harvesting. The higher
The synthesis of the four hydrazone-based organic
dyes HB, HP, HF, and HT is outlined in Scheme 1. The three
dyes HP, HF, and HT were synthesized by the similar
stepwise synthetic protocol. Firstly, the five-membered
heterocyclic aldehyde (2-furaldehyde, 2-thienaldehyde
or 2-carbaldehyde-N-ethylhexyl-pyrrole) reacted with
N,N-diphenylhydrazine hydrochloride to afford five-
membered heterocycle-based hydrazone 2a, 2b or 2c
according to the previous reports [22,23] in high yields.
Then Vilsmeier formylation of 2a, 2b and 2c afforded the
key intermediates 3a, 3b and 3c with good yields
(81–85%), respectively. Finally, Knoevenagel condensation
reactions of aldehyde derivatives 3a, 3b or 3c with twofold
excess of cyanoacetic acid afforded the target dyes HP, HF,
or HT in acetonitrile using piperidine as catalyst. Compared
to the former three dyes, the dye HT with the phenyl unit
as the conjugated bridge was synthesized by a relatively
simple procedure. N,N-Diphenylhydrazine hydrochloride
was reacted with excess terephthalaldehyde to afford com-
pound 1. Then 1 reacted with cyanoacetic acid by Knoeve-
nagel condensation to give the dye HT. The structures of all
dye molecules were characterized unambiguously with
FT-IR, 1H NMR spectroscopy and elemental analyze.
e
of the organic dyes allows a correspondingly thinner nano-
crystalline film so as to avoid the decrease of the film
mechanical strength. This also benefits the electrolyte diffu-
sion in the film and reduces the recombination possibility of
the light-induced charges during transportation [28].
When the dyes were attached on TiO2 films, the absorp-
tion spectra may shift more or less as compared with that
3.2. Optical properties
Fig. 2. Absorption spectra of the four dyes in THF solutions (10ꢁ5 M) at
25 °C.
In DSSCs, sensitizer is a key and unique component with
function of light harvesting. Its spectral response
a

P. Shen et al. / Organic Electronics 12 (2011) 1992–2002
1997
the interface between a donor component and an acceptor
component (dye and titania in DSSC), featuring a favorite
energy alignment, although a hot electron injection is also
possible [30]. Here we measured the oxidation potential of
the four dyes by means of cyclic voltammetry (CV) to eval-
uate the possibility of electron transfer from the excited
dye molecule to the conductive band of TiO2. CV was per-
formed in CH3CN with 0.1 M Bu4NPF6 as a supporting elec-
trolyte (Fig. 4). Table 2 summarizes all the electrochemical
properties of the four dyes.
The first oxidation potential vs. NHE (Eox), which corre-
sponds to the highest occupied molecular orbital (HOMO
vs. NHE), was calibrated by addition of 0.63 V to the poten-
tial (vs. SCE) vs. Fc/Fc+ by CV (Table 2). The first oxidation
potentials of the four dyes HB, HP, HF, and HT were mea-
sured as 1.23, 1.15, 1.28, and 1.29 V, respectively. It is easy
to see that the negative shifts of the Eox (0.08, 0.13, and
0.14 V) were observed for HB, HF, and HT vs. HP, respec-
tively, which means that HP has the lowest HOMO level
among the four dyes. The decrease of HOMO levels of the
dyes could show a positive effect on the DSSCs perfor-
mance due to the narrower gap between the HOMO level
and the redox potential of iodine/iodide leading to less
waste of energy [17]. We also can see that all the HOMO
levels of four dyes are more positive than the iodide/triio-
dide redox couple (ꢀ0.42 V vs. NHE), indicating that the
oxidized dyes formed after electron injection into the con-
duction band of TiO2 could accept electrons from Iꢁ ions in
the electrolyte thermodynamically.
Fig. 3. Absorption spectra of the four dyes adsorbed on TiO2 surface.
Table 1
Maximum absorption and emission data of the four dyes.
a
Dye
kmax
(
ce/Mꢁ1 cmꢁ1
)
kmaxb/nm
HB
HP
HF
HT
425(15,900)
469(27,000)
452(24,600)
465(18,300)
295(9300)
310(5300)
304(15,800)
295(8500)
418
457
437
469
a
b
c
Maximum absorption in THF solutions (10ꢁ5 M).
Maximum absorption on TiO2 films.
e
is the molar extinction coefficient at kmax of maximum absorption.
By neglecting any entropy change during light absorp-
tion, the reduction potential vs. NHE (Ered), which corre-
sponds to the lowest unoccupied molecular orbital
(LUMO vs. NHE), can be obtained from Eox and E0–0 value
determined from the intersection of absorption and emis-
sion spectra, namely, EoxꢁE0–0. The calculated LUMO levels
of HB, HP, HF, and HT were ꢁ1.33, ꢁ1.36, ꢁ1.21, and
ꢁ1.09 V vs. NHE (Table 2), respectively, which all are suffi-
ciently higher than that of the conduction band (CB) level
in solutions because of strong interactions between the
dyes and the semiconductor surface, which can lead to
form aggregates of the dyes on semiconductor surface,
such as H-aggregation for the blue shift or J-aggregation
for the red shift. From Figs. 2 and 3 and Table 1, it can be
seen that absorption spectra of the dyes HB, HP, and HF
on TiO2 films displayed broader and a blue shift of 7 nm,
12 nm, and 15 nm, respectively, compared to those in
THF solutions due to H-aggregation. However, the absorp-
tion spectrum of HT on TiO2 film red shifted (4 nm) slightly
in comparison with that in solution, indicating the dye pos-
sesses the J-aggregation on semiconductor surface.
Another noteworthy feature in the spectra of dyes
adsorbed on TiO2 surfaces is an apparent tailing toward
longer wavelength region (Fig. 3). In solutions the low
energy edge of absorption bands does not exceed 550 nm
(Fig. 1), yet on TiO2 the absorption threshold extends to
750 nm. The long wavelength absorption should be result
from the formation of a certain degree of aggregates on
the surface of TiO2. The absorption of aggregates is respon-
sible for the induction of photocurrent in the regions of
P600 nm (Fig. 5).
of TiO2 (ꢁ0.5 V vs. NHE). Provided that energy gap (Egap
)
between LUMO levels and CB level of TiO2 of 0.2 eV is nec-
essary for efficient electron injection [4a], these driving
3.3. Electrochemical properties
While a good light absorption is an essential require-
ment for photovoltaic cells, the following charge genera-
tion from the closely bound electron–hole pair (Frenkel
exciton) confined in a single organic molecule [29] also
determines device operation. This process is triggered at
Fig. 4. Cyclic voltammograms of the three dyes: working electrode, Pt
ring; auxiliary electrode, Pt wire; reference electrode, Hg/Hg2Cl2; scan-
ning rate is 100 mV/s.

1998
P. Shen et al. / Organic Electronics 12 (2011) 1992–2002
Table 2
3-methoxy-propionitrile. Fig. 5 shows incident photon-
a
Electrochemical data of the four dyes
.
to-electron conversion efficiency (IPCE) as a function of
the wavelength for the sandwiched DSSCs based on the
four dyes as sensitizers. As shown in Fig. 5, the IPCE of
HP sensitizer exceeds 60% in the spectral region from
400 to 520 nm, reaching its maximum over 80% at
425 nm, while other three dyes produce maximum IPCE
about 60%. The relatively higher maximum IPCE for HP
compared with other three dyes is well in agreement with
the tendency of UV–vis absorption properties. In compari-
son with HB and HF, HT exhibits a broader IPCE spectrum.
These results imply that HP and HT would show a rela-
tively large photocurrent in DSSCs. Additionally, it should
be noted that the four dyes still show weak IPCE response
at the range of long wavelength region (650–750 nm), as
shown in Fig. 5. We believe that this feature should be as-
cribed two reasons: one is the weak absorption band of the
dyes on TiO2 films at the range of 650–750 nm (see Fig. 3),
which is consistent with our and the other reports [32]; the
other is the effect of light scattering by TiO2 nanoparticles,
which increases the photocurrents for the weak absorption
in that region.
Fig. 6 showscurrent density–voltage (J–V) characteristics
of devices based on the four dyes. The detailed photovoltaic
parameters of the short-circuit photocurrent density (Jsc),
open-circuit photovoltage (Voc), fill factor (FF), and power
conversion efficiency (PCE) are listed in Table 4. According
to Fig. 6, it is clear that the photovoltaic performances of
the DSSCs can be evidently affected by the aromatic bridges
in the dye molecules. The PCE increases in the order of HB
(4.39%) < HF (4.66%) < HT (5.29%) < HP (7.18%). HP exhibits
the best PCE due to both the highest Jsc (14.76 mA/cm2) and
Voc (0.69 V) values. Obviously, the tendency of a gradually
increased PCEs for the four dyes with various aromatic
bridges shows be along with the increase of Jsc. Consistent
with the increased tendency of IPCE, the measured Jsc in-
creases in the order of HB (9.64 mA/cm2) < HF (10.62 mA/
cm2) < HT (11.04 mA/cm2) < HP (14.76 mA/cm2). In view
of the similar configuration and anchoring mode of the four
sensitizers,wecomparetheirratiosof molarextinctioncoef-
ficients and the absorptivities of dye coated titania films,
deducing that HB has a lower surface coverage than those
of other three dyes. In addition, the remarkably increased
Jsc value of HP compared with other three dyes can be as-
cribed to its relatively better light harvesting ability, reflect-
ing in its better IPCE spectrum.
Dye kint/nm E0–0/eV Eox/V vs. NHE Ered/V vs. NHE Egap/V
HB
HP
HF
HT
485
495
498
520
2.56
2.51
2.49
2.38
1.23
1.15
1.28
1.29
ꢁ1.33
ꢁ1.36
ꢁ1.21
ꢁ1.09
0.83
0.86
0.71
0.59
a
E0–0 values were calculated from intersection of the normalized
absorption and the emission spectra (kint): E0–0 = 1240/kint. The first oxi-
dation potential (vs. NHE), Eox was measured in acetonitrile and cali-
brated by addition of 0.63 V to the potential versus Fc/Fc+. The reduction
potential, Ered
,
was calculated from EoxꢁE0–0
. Egap is the energy gap
between the Ered of dye and the CB level of TiO2 (ꢁ0.5 V vs. NHE).
forces are sufficiently large for effective electron injection.
Therefore, the four dyes used as sensitizers will have suffi-
cient driving force for electron transfer from the excited
dye molecules to the CB of TiO2 electrode. Noticeably, the
relatively large Egap of the dyes allow for the addition of
the 4-tertbutylpyridine (TBP) into the electrolyte, which
can shift the conduction band of TiO2 negatively about
0.3 V and consequently improve the open-circuit voltage
(Voc) and total power energy conversion efficiency (PCE)
[31]. The Egap of the four dyes ranges from 0.59 (HT) to
0.86 eV (HP) (Table 2). Accordingly, an increased Egap
may yield higher Voc and enhance the PCE considerably,
which predicts the dye HP may give a higher Voc and PCE.
These above-discussed results indicate that the intro-
duction of aromatic cycles (benzene, pyrrole, furan and thi-
ophene) in
p-conjugated bridges could easily tune the
absorption spectra and HOMO and LUMO energy levels of
the dyes.
3.4. Molecular orbital calculations
In order to gain an insight into the molecular structure
and electron distribution, density functional theory (DFT)
calculations were performed on the four dyes using the
Gaussian 03 program package. In particular we used
B3LYP as exchange-correlation functional and 3-21G⁄ as
basis set. The electron distribution of the HOMO and LUMO
of the dyes is shown in Table 3. It is obvious that the
electron density of HOMO is localized mainly on the
N,N-diphenylhydrazone moiety and is extended along the
p
-conjugated bridge to the central region of the molecule.
On the other hand, the LUMO is localized over the cyanoac-
rylic unit through aromatic bridges. We notice that the
HOMO–LUMO excitation induced by light irradiation could
move the electron distribution from the N,N-diphenylhyd-
razone unit to the cyanoacrylic acid moiety via aromatic
-conjugated bridges and the photo-induced electron
transfer from the dyes to the TiO2 electrode can occur effi-
ciently by the HOMO–LUMO transition.
Generally, Voc becomes low along with the decrease of
aromatic ability of aromatic p-conjugated bridges. The im-
proved Voc value (0.69 V) of HP compared to other three
dyes (0.61–0.67 V) could be attributed to the following
two aspect reasons. One is that the additional hydrophobic
alkyl chain of pyrrole bridge in HP can effectively retard
the charge recombination between injected electron and
I3ꢁ ions in the electrolyte, which is favorable for increasing
the electron lifetimes and hence to improve Voc
[33a,6i,6h,11b]. We will discuss it in detail as the following
electrochemical impedance spectroscopy studies. The
other reason can be attributed to the comparatively low-
er-lying HOMO of the HP (estimated from the CV), which
offers enough larger driving force for the reduction of the
oxidized dye. This, in turn, will lead to a slower back
p
3.5. Photovoltaic performance of DSSCs
DSSCs were fabricated using these dyes as the
sensitizers, with an effective area of 0.196 cm2, TiO2
particles on FTO, and the electrolyte composed of 0.5 M
LiI, 0.05 M I2, and 0.5 M 4-tert-butyl-pyridine (TBP) in

P. Shen et al. / Organic Electronics 12 (2011) 1992–2002
1999
Table 3
Frontier molecular orbitals of the HOMO and LUMO calculated with DFT on a B3LYP/3-21G⁄ level of the four dyes.
electron transfer from TiO2 to the dye and result in a lager
Voc value [33b].
which results from the significant improvement of Jsc val-
ues. The obviously improved PCEs of HB (5.11%) and HF
(5.54%) indicate that CDCA plays a important role in sup-
pressing the aggregation conformation of dyes on TiO2 sur-
face. In the case of HP (7.74%), the relatively little increase
implies that the CDCA has a slight impact on the aggrega-
tion the dye, which can be ascribe to effectively inhibiting
aggregation of alkyl group to pyrrole unit [6g,10a,33a]. The
above photovoltaic results indicate strongly that coadsorp-
tion of CDCA is effective to improve solar cell performance
by suppressing the aggregation conformation of dyes on
TiO2 surface.
It has been reported that chenodeoxycholic acid (CDCA)
can enhance the performance of the DSSCs because CDCA
can reduce the aggregation of the dyes on the TiO2 surface
[6g,8b,9a,34]. To investigate the influence of aggregation of
dyes on the photovoltaic performance, CDCA was added
into the dye solution as a coadsorbent and the relative pho-
tovoltaic data are also collected in Table 4. As shown in Ta-
ble 4, after the addition of CDCA (1 mM) to the dye bath,
PCEs of DSSCs based on the four dyes are increased by
16%, 8%, 19%, and 10% for HB, HP, HF, and HT respectively,

2000
P. Shen et al. / Organic Electronics 12 (2011) 1992–2002
Fig. 7. EIS Nyquist plots for DSSCs based on the four dyes measured in the
dark under ꢁ0.65 V bias.
Fig. 5. IPCE plots for the DSSCs based on the four dyes.
Fig. 8. EIS Bode plots for DSSCs based on the four dyes measured in the
dark under ꢁ0.65 V bias.
Fig. 6. The J–V characteristics for DSSCs based on the four dyes.
Fig. 7. Three semicircles are observed in the Nyquist plots.
The small and large semicircles respectively located in the
high- and middle-frequency regions, are assigned to the
charge transfer at Pt/electrolyte and TiO2/dye/electrolyte
interface, respectively [33]. Another small semicircle, which
should have appeared at the low-frequency region, is over-
lapped by the middle-frequency large semicircle. The
charge recombination resistance (Rrec) at the TiO2 surface
can be deduced by fitting curves from the range of the mid-
dle-frequency using a Z-view software. Rrec is related to the
charge recombination rate between injected electron and
electron acceptor (I3ꢁ) in the electrolyte, estimated by the
large semicircle width. A large Rrec means the small charge
recombination rate and vice versa. The Rrec values for HP
Table 4
Photovoltaic performance of DSSCs based on the four dyes.
Dye
Jsc (mA/cm2)
Voc (V)
FF
PCE (%)
HBa
HBb
HPa
HPb
HFa
HFb
HTa
HTb
9.64
11.35
14.76
16.17
10.62
12.78
11.04
12.69
0.67
0.66
0.69
0.69
0.61
0.62
0.64
0.65
0.680
0.682
0.705
0.694
0.719
0.699
0.749
0.707
4.39
5.11
7.18
7.74
4.66
5.54
5.29
5.83
a
Dye bath: CH3CN solution (0.5 mM).
Dye bath: CH3CN solution (0.5 mM) with addition of CDCA (1 mM).
m trans-
b
For all cells, the double-layer TiO2 films are composed of ꢀ14
parent layer and ꢀ5 m scatter layer.
l
and HB were estimated to be 186 and 97
In contrast, HF and HT were found to have relatively smaller
Rrec values at 84 and 57 , respectively. The result appears
X, respectively.
l
X
3.6. Electrochemical impedance spectroscopy studies
to be consistent with the larger Voc values for the dyes HP
(0.69 V) and HB (0.67 V). The significantly increased Rrec
values of HP and HB imply the retardation of the charge
Electrochemical impedance spectroscopy (EIS) is a pow-
erful technique of characterizing the important interfacial
charge transfer processes in a DSSC. The EIS was measured
in the dark to elucidate correlation of Voc with those dyes.
The Nyquist plots of DSSCs with four dyes are shown in
ꢁ
recombination between injected electron and I3 ions in
the electrolyte, with a consequent increase of Voc
.
At the same time, DSSCs based on HP and HB produced
higher Voc than HF and HT, which can be explained by the

P. Shen et al. / Organic Electronics 12 (2011) 1992–2002
2001
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electron lifetime. Fig. 8 shows the Bode plot for DSSCs
based on HB, HP, HF, and HT. Two peaks in Fig. 8 located
at the high-frequency (right) and middle-frequency (left)
respectively correspond to the small semicircle (left) and
large semicircle (right) in the Nyquist plots (Fig. 7). The re-
ciprocal of the peak frequency for the middle-frequency
peak is regarded as the electron lifetime since it represents
the charge transfer process at the TiO2/dye/electrolyte
interface. It is evident that the electron lifetime for the
DSSC based on HT and HF is smaller than those for DSSCs
based on HP and HB, thereby explaining the Voc increase
from HT and HF to HP and HB.
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In summary, we have designed and synthesized a series
of novel hydrazone-based D–
p–A dyes (HB, HP, HF, and
HT), by employing different aromatic
p-conjugated bridges
such as benzene, pyrrole, furan, and thiophene in combina-
tion with the hydrophobic N,N-diphenylhydrazone donor
and the hydrophilic cyanoacrylic acid acceptor. The effects
of the aromatic p-conjugated bridges on the photophysical,
electrochemical and photovoltaic properties of these dyes
were extensively investigated. It was found that the absorp-
tion spectra and HOMO and LUMO energy levels can be
conveniently tuned by alternating the aromatic bridges.
Molecular orbital calculations showed that the HOMO–
LUMO excitation could readily move the electron
distribution from the N,N-diphenylhydrazone donor to the
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bridges on photovoltages through the electrochemical
impedance experiments and achieved a preliminary conclu-
sion that the high Voc of HP based DSSC resulted from the rel-
atively small charge recombination rate and long electron
lifetime due to the additional hydrophobic alkyl chain of
pyrrole bridge. DSSCs based on these dyes have achieved
5.11–7.74% PCEs when CDCA (1 mM) was added into the
dye bath as a coadsorbent. For the first time we have proved
that N,N-diphenylhydrazone and N-(2-ethylhexyl)-substi-
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promising donor and
D– –A dyes, which expands the selection scope of building
blocks for further dye design. Further structural modifica-
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p
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Acknowledgements
(c) C.-P. Hsieh, H.-P. Lu, C.-L. Chiu, C.-W. Lee, S.-H. Chuang, C.-L. Mai,
W.-N. Yen, S.-J. Hsu, Eric W.-G. Diau, C.-Y. Yeh, J. Mater. Chem. 20
(2010) 1127;
This work was supported by the General Program of the
Education Department of Hunan Province (No. 09C946),
National Natural Science Foundation of China (No.
50973092, 21004050, 51003089), and National Natural
Science Foundation of Hunan Province (No. 10JJ4007).
(d) T. Bessho, S.M. Zakeeruddin, C.-Y. Yeh, Eric W.-G. Diau, M.
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