Rational design of triphenylamine dyes for highly efficient p-type dye sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2014.01.024

Two new sensitizers based on triphenylamine-dicyanovinylene have been synthesized and used for p-type dye-sensitized solar cells. The best performance amongst solar cells was achieved by the dye with a ter-thiophene bridge ligand between carboxylic acid group and the triphenylamine part (with power conversion efficiency of 0.19%, short circuit current of 4.01 mA cm^-2, open circuit voltage at 144 mV, and fill factor of 0.33). Results indicate that the ter-thiophene groups in the dyes strongly affects both charge recombination and hole injection in the photoelectrode. In addition, the hexyl chains on the bridged thiophene rings also help to avoid dye aggregation on the nickel oxide film and block I^- in electrolyte from approaching the surface of nickel oxide, which leads to a reduction in the charge recombination between nickel oxide semiconductor and electrolyte. This study suggested that modification of the bridge moiety between triphenylamine and the carboxylic group by increasing thiophene units is a promising way for preventing charge recombination and increasing the power conversion efficiency.

Full text of DOI:10.1016/j.dyepig.2014.01.024

Dyes and Pigments 105 (2014) 97e104
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Rational design of triphenylamine dyes for highly efficient p-type dye
sensitized solar cells
,
,
,
*
Linna Zhu a 1, Hong Bin Yang b 1, Cheng Zhong c, Chang Ming Li a
a Institute for Clean Energy & Advanced Materials, Southwest University, Chongqing 400715, PR China
b School of Chemical and Biomedical, Nanyang Technological University, 70 Nan-yang Drive, Singapore 637457, Singapore
c Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new sensitizers based on triphenylamine-dicyanovinylene have been synthesized and used for p-
type dye-sensitized solar cells. The best performance amongst solar cells was achieved by the dye with a
ter-thiophene bridge ligand between carboxylic acid group and the triphenylamine part (with power
conversion efficiency of 0.19%, short circuit current of 4.01 mA cmꢀ2, open circuit voltage at 144 mV, and
fill factor of 0.33). Results indicate that the ter-thiophene groups in the dyes strongly affects both charge
recombination and hole injection in the photoelectrode. In addition, the hexyl chains on the bridged
thiophene rings also help to avoid dye aggregation on the nickel oxide film and block Iꢀ in electrolyte
from approaching the surface of nickel oxide, which leads to a reduction in the charge recombination
between nickel oxide semiconductor and electrolyte. This study suggested that modification of the
bridge moiety between triphenylamine and the carboxylic group by increasing thiophene units is a
promising way for preventing charge recombination and increasing the power conversion efficiency.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 5 December 2013
Received in revised form
26 January 2014
Accepted 27 January 2014
Available online 12 February 2014
Keywords:
Triphenylamine
Dye-sensitized solar cell
NiO
Thiophene
Charge recombination
Power conversion efficiency
1. Introduction
valence band (VB) of NiO semiconductor. Fast charge recombina-
tion between the injected holes and the reduced sensitizer is a
Dye sensitized solar cells (DSSCs) as one of the most promising
photovoltaic devices have attracted much attention due to their low
material cost, device flexibility and simple manufacturing process
compared to the silicon-based photovoltaic devices [1,2]. DSSCs
using sensitized n-type semiconductors such as TiO2 as a photo-
electrode (n-DSSCs) are mostly studied and have achieved high
photon conversion efficiency (PCE) [3e6]. As their inverse model, p-
type DSSCs (p-DSSCs) injecting holes to the valence band of a p-type
semiconductor such as NiO from the excited state of a sensitizer are
rarely studied, and their overall efficiency is much lower than n-
DSSCs [7e11]. Fundamentally and practically, it is very necessary to
investigate p-DSSCs for inexpensive tandem solar cells, in which
both electrodes are photoactive to offer great potential for signifi-
cant improvement of the existing solar cell efficiency [12e14].
In p-DSSCs, the dye molecules absorbs photons and generates
holes and electrons, followed by hole-transport from the acceptor
to the donor, then injecting through the carboxylic group into the
major challenge to greatly limit the power conversion efficiencies
[15e17]. Rational design of the dye molecules could help to over-
come this limitation. The organic sensitizer has advantages such as
high molar absorption coefficients, facile molecular design and
cost-effectiveness [18]. Since Lindquist and co-workers’ first
demonstration for generation of cathodic photocurrent with an
erythrosine-sensitized NiO cathode [19], numerous organic sensi-
tizers such as coumarin, perylene monoimide and porphyrin de-
rivatives have been explored [20e22]. “Pushepull” dyes with an
anchoring group on the triphenylamine donor were first developed
by Sun [7]. The p-DSSCs based on these dyes exhibit high incident
photon-to-current conversion efficiencies (IPCEs), among which 4-
(bis-{4-[5-(2,2-dicyanovinyl)-thiophene-2-yl]-phenyl}amino)ben-
zoic acid (P1) shows the highest efficiency (h) (w0.15%) [7,11].
Inspired from this work, various “pushepull” dyes have been
developed. Diketopyrrolopyrrole (DPP) based novel dyad gives
promising efficiency of 0.07% with Iꢀ/I3- electrolyte, and a PCE of
0.18% is obtained with cobalt complex as a redox shuttle [9]. Yen
et al. increased the amount of anchoring groups on triphenylamine
donor to facilitate hole injection, resulting in h of 0.08%e0.09% [23].
Organic dyads comprising a perylene monoimide (PMI) dye con-
nected to a naphthalene diimide (NDI) or a fullerene (C60) with a
* Corresponding author.
E-mail addresses: ecmLi@swu.edu.cn, ecmli@ntu.edu.sg (C.M. Li).
1
The two authors contribute equally to this article.
http://dx.doi.org/10.1016/j.dyepig.2014.01.024
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

98
L. Zhu et al. / Dyes and Pigments 105 (2014) 97e104
cobalt electrolyte in p-DSSCs has accomplished a PCE of 0.14% [24].
The performance has been further increased to 1.3% when Tris(1,2-
diaminoethane)Cobalt(II)/(III) based electrolytes were introduced
[25]. More efficient sensitizers by varying the length of the oligo-
thiophene bridge ligands between triphenylamine donor and PMI
acceptor unveil long-lived charge separated excited states to result
workstation (model 660A). The potentials are quoted against the
ferrocene internal standard.
2.1. Synthesis
The synthetic procedures for intermediates of T3 and T4 are
shown in the electronic Supplementary information. T3 and T4
were synthesized by Suzuki Coupling reaction between brominated
triphenylamine and thiophene boronic acid before finally under-
going a Knoevenagel reaction with malononitrile as reported for
dye T1 (Synthetic routes shown in Scheme 2) [28]. The final
products were fully characterized by 1H and 13C NMR spectroscopy,
MALDI-TOF spectrometry and elemental analysis as well (detailed
synthetic routes, see ESI).
in the most efficient p-DSSCs up to date (
most of the reported arylamine dyes show PCE still lower than 0.1%
[26,27].
h
¼ 0.43%) [9]. However,
Quite recently we have synthesized a novel dye T1 based on a
triphenylamine donor and dicyanovinyl acceptor (Scheme 1) [28].
Compared to P1 reported by Sun et al. [7], a thiophene ring is
inserted between triphenylamine and carboxylic acid group. Results
show that T1 outperforms P1 under the same test condition. In light
of this work, herein we design and synthesize two new dyes T3 and
T4, in which the conjugation length between triphenylamine and
carboxylic group further increased by inserting ter-thiophene and
fluorene units respectively (Scheme 1). Hexyl chains on the bridged
thiophene rings in T3 and T4 are introduced to maintain solubility,
which are also expected to play an important role in preventing dye
aggregation and blocking of Iꢀ in electrolyte approaching the NiO
surface for charge recombination. The influences of different link-
ages toward the photovoltaic properties of these organic dyes were
investigated in detail to provide valuable knowledge for rational
design of high performance dyes for p-DSSCs.
2.1.1. Synthesis of dye T3 (500-(4-(bis(4-(5-(2,2-dicyanovinyl)
thiophen-2-yl)phenyl)amino)phenyl)-30,300-dihexyl-[2,20:50,200-
terthiophene]-5-carboxylic acid)
To a solution of compound 7 (200 mg, 0.22 mmol) in degassed
toluene (120 mL) were added malononitrile (44 mg, 0.66 mmol)
and 0.1 mL Et3N. The mixture was stirred at 90 ꢁC overnight
under an Ar atmosphere. After cooled to room temperature,
water (100 mL) was added, the solution was extracted with
CHCl3, and dried over anhydrous sodium sulfate. The solvent was
evaporated, the residue was purified by column chromatography
over silica gel using EtOH/CHCl3 (1:7, v/v) as the eluent to give
the final product T3 (170 mg, 77%) as a dark red solid. 1H NMR
2. Experimental details
The 1H NMR and 13C NMR spectra were recorded on a BRUKER
AVANCE 300 MHz NMR Instrument in CDCl3, CD3COCD3 and
DMSO-d6, using tetramethylsilane as an internal reference. GC-Ms
was recorded on a GCMS-QP2010 Plus Spectrometer. MALDI-TOF
was performed on a Bruker Autoflex instrument, using 1,8,9-
trihydroxyanthracene as a matrix. Elemental analyses of carbon,
hydrogen, and nitrogen were performed on a Carlorerba-1106
microanalyzer. UVeVis absorption spectra were recorded on Shi-
madzu 160A spectrophotometer. Electrochemical experiments
(300 MHz, CD3OCD3): d (ppm): 8.35 (brs, 1H), 7.93 (brs, 1H),
7.76e7.65 (m, 8H), 7.48e7.17 (m, 12H), 2.82 (brm, 4H), 1.71 (brm,
4H), 1.32 (brm, 12 H), 0.89 (brm, 6H). 13C NMR (75 MHz,
CD3OCD3):
d (ppm): 180.17, 168.43, 158.27, 155.29, 151.55, 148.33,
141.81, 141.38, 133.90, 132.51, 130.01, 127.17, 126.68, 126.18,
125.30, 124.47, 124.10, 114.41, 113.75, 75.09, 35.28, 31.50, 30.25,
30.17, 29.71, 29.46, 22.44, 13.50. MALDI-TOF-MS: m/z 1019.55
(Mþ). Anal. calcd for C59H49N5O2S5 (%): C, 69.45; H, 4.84; N, 6.86.
Found: C, 69.80; H, 4.76; N, 6.39. IR,
n
(cmꢀ1): 2225 (C^N), 1699
were performed using
a
CH Instruments electrochemical
(C]O), 1600, 1560 (aromatic rings).
Scheme 1. Molecular structures of T1, T2, T3 and T4.

L. Zhu et al. / Dyes and Pigments 105 (2014) 97e104
99
Scheme 2. Synthetic routes for dye molecules T3 and T4, respectively.
2.1.2. Synthesis of dye T4 (7-(4-(bis(4-(5-(2,2-dicyanovinyl)
thiophen-2-yl)phenyl)amino)phenyl)-9,9-dihexyl-9H-fluorene-2-
carboxylic acid)
2H), 7.29e7.21 (m, 6H), 2.06 (m, 4H), 1.14e1.06 (m, 12 H), 0.78e0.73
(m, 6H), 0.66 (m, 4H). 13C NMR (75 MHz, CD3OCD3):
(ppm): 167.9,
156.3, 153.5, 152.5, 151.9, 149.5, 146.5, 146.4, 142.7, 141.0, 140.3,
138.4, 133.5, 129.3, 129.2, 128.8, 128.7, 127.9, 127.2, 125.4, 123.6,
122.1, 120.5, 115.3, 114.7, 76.0, 56.3, 40.8, 24.6, 23.2, 14.2. MALDI-
TOF-MS: m/z 937.68 (Mþ). Anal. calcd for C60H51N5O2S2 (%): C,
d
The synthetic procedure for dye T4 is just the same as that for
dye T3 described above. 1H NMR (300 MHz, CDCl3):
d
(ppm): 8.18e
8.12 (m, 2H), 7.86e7.77 (m, 4H), 7.72e7.62 (m, 10H), 7.43e7.41 (m,

100
L. Zhu et al. / Dyes and Pigments 105 (2014) 97e104
76.81; H, 5.48; N, 7.46. Found: C, 76.93; H, 5.21; N, 7.16. IR, n
(cmꢀ1):
2222 (C^N), 1704 (C]O), 1597, 1573 (aromatic rings).
2.2. Photophysical measurements
UVevis absorption of samples was measured using a spectro-
photometer (UV-2450, Shimadzu). For transient absorption spec-
troscopy and time-resolved emission experiments, an LP920-KS
instrument from Edinburgh Instruments, equipped with an iCCD
camera from Andor and a photomultiplier tube.
2.3. Fabrication and characterization of p-DSSCs
Before fabricating NiO nanoparticle films onto FTO glass, a
compact p-type photocathode blocking layer (compact NiO layer)
was deposited by spin-coating nickel acetate (þ98%, aldrich) in
ethanol (ꢂ99.7%, Merck) solution (0.2 M) at 2000 rpm on FTO glass
and sintered at 150 ꢁC for 15 min. The nickel acetate layer was
thermally decomposed by the following reaction during the high
temperature sintering process: Ni(CH3COO)2 þ 4O2 / NiO þ
4CO2 þ 3H2O, and formed a compact NiO layer to improve the
contact between FTO and the NiO nanoparticle film. The p-type NiO
nanoparticle film was prepared by two cycle “doctor-blading” the
precursor solution onto the substrate, and after each “doctor-blad-
ing” process the filmwas sintered at 450 ꢁC for 30 min. NiO precursor
solution was prepared by dissolving anhydrous NiCl2 (1 g) and F108
(1 g) into a mixture of Mill-Q water (3 mL) and ethanol (6 mL). The
Fig. 1. UVevis absorption of dyes T1, T3 and T4 measured in CH2Cl2 solution
(0.03 mM).
transition in T4 as a result of the shorter conjugation length of
fluorene group in comparison to ter-thiophene groups in T3.
Generally, the molar extinction coefficient of T3 and T4 is higher
than T1, indicating more photons harvested (Table 1). Cyclic vol-
tammograms (CVs) were performed in CH2Cl2 solution, using 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) as the sup-
porting electrolyte. The HOMO orbitals of all the dyes studied are
below the top of NiO valence band (ꢀ5.04 eV), and the LUMO or-
bitals are above that of the redox mediator (ꢀ4.15 eV), suggesting
sufficient driving force for hole injection and dye regeneration of
these dyes. The optical band gap of T1, T3 and T4 were determined
from the intersection of absorption and emission spectra in CH2Cl2
film thickness measured by FE-SEM was about 1.6
mm. The NiO
electrode with a 0.25 cm2 geometric area was immersed in a 0.3 mM
dye solution at room temperature for 16 h, followed by rinsing with
dichloromethane and drying in air.
The solar cells were assembled with a platinized FTO counter
electrode using a thermoplastic frame (Surlyn, SX1170-25
mm
(E0-0 ¼ 1240/
l). Detailed optical parameters and electrochemical
properties obtained from the measured CVs are summarized in
Table 1.
thick). The redox electrolyte (containing 0.8 M LiI and 0.15 M I2 in
acetonitrile) was introduced through a pre-drilled hole at the
counter electrode, which was sealed afterwards. The current den-
sity (J)-voltage (V) characteristics were measured using a Keithley
2420 m in dark and under illumination of a sun 2000 solar simu-
lator (Abet) with 100 mW cmꢀ2 AM 1.5G spectrum. The intensity of
the solar simulator was calibrated by standard Si photovoltaic cell.
Incident photon-to-electron conversion efficiency (IPCE) mea-
surements were performed without bias illumination with respect
to a calibrated silicon diode. The monochromic light was supplied
3.2. Theoretical calculations
Density functional theory (DFT) calculations were performed in
Gaussian 09 at B3LYP/6-31þG(d) level to analyze the electron
density distribution of the frontier orbitals of the dye molecules
[29]. The molecular-orbital energy diagram of T1, T3 and T4 were
shown in Fig. 2. The HOMO orbitals are mainly located on the tri-
phenylamine part, and the LUMO orbitals are mainly distributed on
the dicyanovinylene groups and the neighboring bridge ligands.
Notably, the calculated results reveal that the HOMO orbital dis-
tribution in dye T3 is more evenly and efficiently to ensure fast
intramolecular hole transfer from the acceptor to the donor as
observed in dye T1. Contrarily, in T4, the HOMO orbital is located on
most of the conjugation moieties but only slightly distributed on
the fluorene part, leading spatially a little far distance from NiO
semiconductor for more difficult hole injection from HOMO orbital
of T4 to NiO. Therefore, upon photoexcitation, T3 could offer better
charge injection at the metal oxide/dye interface.
by xenon light illuminating through
a Cornerstone mono-
chromator. A chopper was placed after the monochromator and the
signal was collected by Merlin lock-in radiometry after amplifica-
tion by the current preamplifier. Electrochemical impedance
spectroscopy (EIS) was carried out with Solartron 1260 þ 1294
impedance analyzer under illumination at open-circuit potential in
dark with AC amplitude of 10 mV over a range from 20000 to
0.02 Hz.
3. Results and discussion
3.1. Photophysical properties of T3 and T4
The UVevis absorption of T1, T3 and T4 in dichloromethane
3.3. Photovoltaic performances
(CH2Cl2) are shown in Fig 1, and exhibiting two bands, the one at
around 380 nm corresponding to the
p
e
p
* transition of the whole
Typical currentevoltage characteristics of p-DSSCs sensitized
with T3 and T4 on double-layered NiO film with Iꢀ/I3- electrolyte
measured under standard AM 1.5 G conditions (100 mW cmꢀ2) are
shown in Fig 3. Dye T1 was also evaluated under the same condition
for comparison. It turned out that dyes dissolved in the DMF-CH2Cl2
mixture (V:V ¼ 1:5) showed the best performance with Iꢀ/I3ꢀ
electrolyte. Dye T3 sensitized cells exhibits a power conversion
conjugation system, and the one located at longer wavelength
(w500 nm) ascribed to the charge transfer process. The absorbance
of T3 and T4 in the visible region is at longer wavelength than that
of T1, due to their increased conjugation length. However, the
absorbance of T4 in the region around 410 nm is particularly lower
than that of T3, which probably comes from the blue shift of pep*

L. Zhu et al. / Dyes and Pigments 105 (2014) 97e104
101
Table 1
Optical and electrochemical properties of dyes T3 and T4 (dye T1 was also evaluated for comparison).
Dyes
labs
(
3
(104Mꢀ1 cmꢀ1))a
lem (nm)
E0-0 (eV)b
Excited state lifetime (ns)c
Eon set.ox (V vs FCþ/FC)d
EHOMO (ev)e
ELUMO (eV)f
T1
T3
T4
374 (3.50)
387 (4.78)
363 (5.24)
506 (4.01)
504 (4.65)
514 (5.23)
644
587
664
2.25
2.24
2.15
3.14
3.17
2.91
0.42
0.42
0.38
ꢀ5.53
ꢀ5.53
ꢀ5.54
ꢀ3.28
ꢀ3.29
ꢀ3.39
a
Absorption spectra were measured in CH2Cl2 solution.
b
c
d
e
f
E0-0 was determined from the intersection of absorption and emission spectra in dichloromethane.
Excited state lifetime was determined from the transient emission spectra.
Oxidation potentials of dyes were measured in CH2Cl2 with 0.1 M TBAPF6 with a scan rate of 50 mV sꢀ1
EHOMO was calculated by (5.1 þ E(onset.ox vs FCþ/FC)).
.
ELUMO was calculated by EHOMO þ E0-0
.
efficiency (
h
) of 0.19% with a high short circuit current density (Jsc)
Apparently, the absorbance of the dyes sensitized photoelectrode
descends in the order of T3>T1>T4, especially at the region around
400 nm, and the LHE spectrum follows the same trends (Fig. 4(b)).
It is noted that the absorption of these dyes on NiO films is quite
different from that measured in solution (the absorption of the dyes
follows T4>T3>T1 in solution), which probably due to the much
lower dye loading amount of T4 on NiO film, as compared to dye T1
and T3 (Table 2). To determine the dye loading amount, the dye
molecules were desorbed in tetrabutylammonium hydroxide
(Bu4NOH)/DMF (0.05 M) solution for 12 h. The calculated dye
of 4.01 mA cmꢀ2 and an open circuit potential (Voc) of 0.144 V; T1
under the same condition shows
and Voc of 0.125 V; while T4 delivers the lowest
h
of 0.11% with Jsc of 2.82 mA cmꢀ2
of 0.06% with Jsc of
h
only 1.69 mA cmꢀ2 and Voc of 0.123 V. Detailed data of these p-
DSSCs in DMF-CH2Cl2 mixture are summarized in Table 2,
demonstrating that T3 outperforms the other two dyes in regard to
open-circuit photovoltage and short-circuit current, which are in
good agreement with their IPCE results (Fig 3(b)). Meanwhile, dyes
dissolved in single solvent DMF or CH2Cl2 were also investigated,
but only poor performances achieved (Fig. S2).
loading amount of T1, T3 and T4 are 30.6, 27.1 and 24.4 nmol cmꢀ2
,
As shown in JeV curves and IPCE spectra, by inserting the ter-
thiophene bridge ligand between triphenylamine and the carbox-
ylic group, both Jsc and Voc of the photoelectrode are significantly
enhanced, while the inserted fluorene group leads to a prominent
decrease of Jsc. This is in accordance with the calculated orbital
distributions and the excited state lifetime discussed above. In
general, the IPCE (also photocurrent) of a photoelectrode is deter-
respectively. Obviously, for the T4 sensitized cell, the photo-
electrode performance is limited by dramatically reduced dye
loading amount on NiO film. According to the definition of IPCE, the
differences in IPCE cannot be completely ascribed to the differences
in LHE. The calculated Binj
ꢃ
hc value is about 0.66 and 0.74 for T1
and T3 dyes sensitized photoelectrodes in the visible region [31].
Considering the same charge transfer resistance of NiO electrodes,
we argue that dye T3 sensitized photoelectrode has higher hole
injection efficiency at the NiO/dyeeelectrolyte interface and less
charge recombination loss for transporting to external circuit. On
the other hand, for dye T4, lower dye loading amount as well as the
lowest excited state lifetime restricts the overall performance by
refining the hole injection efficiency.
mined by three parameters in terms of IPCE(
l) ¼ LHE(
l
) ꢃ Binj
ꢃ
hc,
which are the light harvesting efficiency (LHE), the charge injection
efficiency (Binj) from the excited dye to the photoelectrode and the
charge collection efficiency (hc) related to charge loss by recombi-
nation during charge transport. The light harvesting efficiency of
different dyes sensitized NiO photoelectrodes were studied by UVe
vis absorption spectra. The absorption of the dyes on NiO films
together with the light harvesting efficiency (LHE) spectra (LHE is
defined as 1e10ꢀA, where A is the absorbance of the photo-
electrode) of the sensitized electrodes were shown in Fig. 4 [30].
To confirm the higher hole injection efficiency and the less
charge recombination loss in T3 sensitized photoelectrode,
impedance spectra (IS) of devices sensitized by T1 and T3 at
different bias voltage in dark and at open-circuit voltage bias under
Fig. 2. Molecular orbital energy diagram of the dyes T1, T3 and T4.

102
L. Zhu et al. / Dyes and Pigments 105 (2014) 97e104
Fig. 3. JeV curves (a) and IPCE spectra (b) of T1, T3 and T4 sensitized photoelectrodes.
(Both spectra were obtained by dissolving the dyes in DMF:DCM 1:5 (V:V) mixed
solution).
Fig. 4. (a) UVevis absorption of dye molecules on NiO films, and (b) LHE spectra of NiO
photoelectrode sensitized with T1, T3 and T4 (dyes dissolved in DMF-CH2Cl2mixed
solvents). The absorbance of NiO film was subtracted from the absorbance of dyes on
NiO films.
illumination were measured (Fig. 5). In dark with external bias,
holes are injected from the external, followed by recombination at
the metal oxide/electrolyte interface via oxidation of Iꢀ ions in the
electrolyte. Thus recombination resistance (Rrec) at the metal oxide/
electrolyte interface and the free charge lifetime could be obtained
by analyzing the IS with equivalent circuit (as shown in Figure S3)
[32,33]. Nyquist plots of NiO electrodes sensitized with T1 and T3
with ꢀ0.1 V bias are shown in Fig. 5(a), in which the semicircle
diameter for the measured devices is T3>T1 in dark, implying a
larger resistance of T3 against interfacial hole transfer from the NiO
valence band to Iꢀ than T1 based devices. The Rrec values derived
from the impedance data with transmission line model are plotted
in Fig. 5(b) as a function of the bias voltage. The Rrec of T1 and T3
decreases with increased bias voltage due to the increased hole
concentration in the valence band. The fact that the Rrec of T3 is
larger than that of T1 at the same external bias in dark suggests a
higher energy barrier for recombination of hole in valence band
with Iꢀ in T3 sensitized cell at the same hole concentration.
Considering the dye loading amount of these dyes (T1>T3), it could
be concluded that dye T3 could more efficiently block Iꢀ
approaching the NiO surface for charge recombination, due to the
alkyl chains on thiophene rings in T3. The hole lifetime is plotted in
Fig. 5(c) as a function of bias voltage, which is acquired by the
inversion of frequency of the maximum imaginary impedance
component at the medium frequency (1/2p max) of the semicircle. It
f
has been reported that the charge carrier lifetime is inversely
proportional to fmax, and thus a lower fmax indicates a longer
effective charge lifetime to indicate a lower charge recombination
[34]. Obviously, T3 sensitized electrode has the longest hole life-
time, which is consistent with the highest recombination resistance
discussed above, and the synergistic effects result in the high Voc of
T3 sensitized p-DSSCs.
Under illumination at open-circuit voltage bias, there is no net
current flow through the cell. All the injected holes from the excited
dyes are recaptured by Iꢀ before being extracted to the external
circuit. Meanwhile, the reduced dye is regenerated by Iꢀ3 . Thus, the
semicircle of the IS in the intermediate frequency range (Fig. 5(a))
also represents the transfer resistance at the NiO/dyeeelectrolyte
interface, which is the apparent combination effect of hole injec-
tion, charge recombination and the reduced dye regeneration
processes [28]. The charge transfer resistance at the NiO/dyee
electrolyte interface of the devices obtained from the semicircle of
Table 2
Performances of p-type DSSCs based on T1, T3 and T4 under 1 Sun, AM 1.5G illu-
mination, and dye loading amount of three kinds of dyes on photocathodes.
Dyes Jsc(mA/cm2) Voc(mV) FF (%) PCE (%) Dye Loading Amount (nmol/cm2)a
T1 2.82
T3 4.01
T4 1.69
125
144
123
31
33
29
0.11
0.19
0.06
30.6
27.1
24.4
a
To determine the dye loading amount, the dye-loaded NiO photocathodes with
an area of 1.0 cm2 were immersed into 3 mL 0.05 M Bu4NOH/DMF) for 12 h under
dark condition to desorb the dye molecules. By comparing the UVeVis spectra of the
desorbed dye solution with corresponding dye (0.003 mM) dissolved in 0.05 M
Bu4NOH/DMF, loading amounts of the three dyes on NiO were calculated.

L. Zhu et al. / Dyes and Pigments 105 (2014) 97e104
103
excited dye molecules. Consequently, better hole injection of T3
compared to T1 results in higher Iꢀ concentration and hole con-
centration in VB of NiO, and further leads to lower charge transfer
resistance of T3 photoelectrode at the NiO/electrolyte interface
under illumination.
Moreover, we also tested the photovoltaic performances of T3 in
the presence of different concentration of deoxycholic acid (DCA)
(Figure S4). Result shows that the introduction of coadsorbent
gradually decreased Jsc and Voc with increased DCA concentration.
This is very likely that the amount of dye molecules adsorbed on
NiO surface was reduced by the competitive coadsorption of DCA,
thus leading to significant loss of light harvesting [36]. The result
here could also indicate that the design motif of the dye structures
are rationale and can efficiently avoid dye aggregation on NiO film.
4. Conclusions
In brief, two new dyes based on triphenylamine and dicyano-
vinylene as electron donors and acceptors were synthesized to
demonstrate rational design of dyes for favorable synergistic effects
in nDSSCs. Compared to T1 reported earlier, the bridge ligand be-
tween triphenylamine and the anchoring carboxylic group are
increased by incorporating ter-thiophene and fluorene units
respectively, to produce dye T3 and T4. The best performance
among solar cells with dyes based on an arylamine structure was
achieved by dye T3 (
h
¼ 0.19%, Jsc ¼ 4.01 mA cmꢀ2, Voc ¼ 144 mV,
and FF ¼ 0.33). Results indicate that the ter-thiophene groups in T3
strongly affects both charge recombination and hole injection in
the photoelectrode. In addition, the hexyl chains on the bridged
thiophene rings could also help to avoid dye aggregation on the NiO
film and block Iꢀ in electrolyte from approaching the surface of NiO,
which leads to reducing the charge recombination process between
NiO semiconductor and electrolyte. In contrast, dye T4 introducing
fluorene unit between triphenylamine and carboxylic acid shows
even poorer efficiency compared to T1, perhaps mainly because of
the relatively smaller dye loading amount and poorer hole injection
efficiency of T4. This study suggested that modification of the
bridging moiety between triphenylamine and the carboxylic group
by increasing thiophene units is a promising way for preventing
charge recombination and as a result increasing the power con-
version efficiency.
Acknowledgments
This work is partially financially Institute for Clean Energy &
Advanced Materials, Southwest University, Chongqing, P.R. China. L.
Zhu thanks the National Natural Science Foundation of China (No.
51203046). Hong Bin Yang is grateful for financial support from the
Agency of Science, Technology and Research (A*Star), Singapore
under SERC Grant No. 122 020 3053.
Fig. 5. (a) Electrochemical impedance spectra on NiO electrodes sensitized with the
P1, T1 and T3 dyes with ꢀ0.1 V under dark condition and under 1 sun at open-circuit
voltage, (b) dependence of Rrec and (c) hole lifetime on the applied bias voltage.
the IS measured at open-circuit voltage under illumination are
Appendix A. Supplementary data
198
U
(T1) and 105
U
(T3), which are smaller than that measured at
(T1) and 342 (T3))
open-circuit voltage bias in the dark (300
U
U
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2014.01.024
(Table S2). Similar to the n-DSSCs, the disparity of charge transfer
resistance at NiO/dyeeelectrolyte interface in the dark and under
illumination originates from the differentiation of the local Iꢀ
concentration. In dark, Iꢀ is generated at the counter electrode and
penetrates into the NiO film by a diffusion process [35]. Under
illumination, Iꢀ is formed “in situ” by dye regeneration at the NiO/
electrolyte interface. As discussed above, higher LUMO level of the
dyes (ꢀ3.28 eV (T1) and ꢀ3.09 eV (T3)) than the redox mediator
(ꢀ4.15 eV) suggests sufficient driving force for dye regeneration.
Thus the Iꢀ concentration could be determined by the reduced dye
concentration, which is relevant to the injection ability of the
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