Effect of different acceptors in di-anchoring triphenylamine dyes on the performance of dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2014.01.012

Three di-anchoring triphenylamine dyes, which were coded as TPAC1, TPAR2 and TPACR2, were designed and synthesized for dye-sensitized solar cell application. The structural modification effect of different anchoring groups on photophysical, electrochemica

Full text of DOI:10.1016/j.dyepig.2014.01.012

Dyes and Pigments 105 (2014) 1e6
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Effect of different acceptors in di-anchoring triphenylamine dyes on
the performance of dye-sensitized solar cells
,
b
c
a
Guohua Wu a, Fantai Kong a , Yaohong Zhang , Xianxi Zhang , Jingzhe Li ,
*
Wangchao Chen a, Changneng Zhang a, Songyuan Dai d
,a,
*
a Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, PR China
b Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou 350002, PR China
c Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering,
Liaocheng University, Liaocheng 252059, PR China
d School of Renewable Energy, North China Electric Power University, Beijing 102206, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three di-anchoring triphenylamine dyes, which were coded as TPAC1, TPAR2 and TPACR2, were
designed and synthesized for dye-sensitized solar cell application. The structural modification effect of
different anchoring groups on photophysical, electrochemical and photovoltaic properties of the related
DSSCs was extensively investigated. With the variation from cyanoacetic acid via rhodanine-3-acetic acid
to co-rhodanine units, the molar extinction coefficients of the maximum absorption wavelength for the
Received 6 December 2013
Received in revised form
6 January 2014
Accepted 10 January 2014
Available online 22 January 2014
three dyes gradually increase due to the extension of
p system. In comparison with that for TPAC1 and
TPAR2, DSSC based on dye TPACR2 with double co-rhodanine groups shows the best overall conversion
efficiency of 4.64% with simultaneous enhancement of photocurrent and photovoltage, which is attrib-
uted to the higher molar extinction coefficient (6.5 ꢀ 104 Mꢁ1 cmꢁ1), broader absorption spectra, broader
IPCE spectra and longer electron lifetime.
Keywords:
Di-anchoring triphenylamine dye
Dye-sensitized solar cells
Organic sensitizer
Ó 2014 Elsevier Ltd. All rights reserved.
Photovoltaic performance
Electrochemical impedance spectroscopy
Electron acceptor
1. Introduction
By far, most typical metal-free organic dye sensitizers contain a
structure of “Donor (D)econjugated bridge (p)eAcceptor (A)”. In
As a crucial component in dye-sensitized solar cells (DSSCs), the
metal-free organic dyes have always attracted increased attention
in the past decades by virtue of their ease of inexpensive synthesis,
generally high molar extinction coefficients and tunable absorption
spectral response. In the last decade, numerous investigations
organic dyes, such as indoline [1e3], coumarin [4,5], cyanine [6e8],
perylene [9,10], triphenylamine [11e14], carbazole [15e17], tetra-
hydroquinoline [18,19], phenothiazine [20e23], phenoxazine
[24,25] and fluorene dyes [26,27], have been investigated. So far,
organic sensitizers have gained promising overall conversion effi-
order to utilize the sun light as much as possible, two common
structural strategies, incorporating more donor segments or
enlarging
p
-conjugated linker into the De
peA configuration to
form the DeDe
p
eA or De eA structure, have been considered
pep
[28e32]. However, the complexity in the process of synthesis is
the major problem. Different from donor and conjugated bridge
segments, the electron acceptor (A) carries a polar carboxylic acid
as anchoring group to TiO2 surface. Varying the numbers of anchor
groups via protonation state to tune the interfacial electron
transfer or photovoltaic properties is very important for Ru-
sensitizers. Especially, di-anchoring N719 dye gives higher cell
efficiency than the protonated N3 dye with quadri-anchoring
groups, which is attributed to the effect of the bound dye on the
energy of the TiO2 conducting band [33]. Similar to Ru-sensitizers,
the notion of incorporating double electron acceptor units into the
organic donor framework to form di-anchoring dye has been
proposed to further enhance the binding strength of dyes on the
TiO2. Several di-anchoring organic dyes, which have been
ciencies [1,11]
complexes.
(h), which is comparable to ruthenium-based
* Corresponding authors. Key Laboratory of Novel Thin Film Solar Cells, Institute
of Plasma Physics, Chinese Academy of Sciences, 350 Shushanhu Road, P.O. Box:
1126, Hefei 230031, Anhui, PR China. Tel.: þ86 551 65593222.
E-mail addresses: kongfantai@163.com (F. Kong), sydai@ipp.ac.cn (S. Dai).
0143-7208/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2014.01.012

2
G. Wu et al. / Dyes and Pigments 105 (2014) 1e6
designed and synthesized for use in DSSCs, have demonstrated
better cell performance than mono-anchoring De eA sensitizers
with an improved photocurrent due to the extension of the
company. All the other solvents and the chemicals are puriss grade
and used without further purification.
p
p
-
conjugated system and the enhanced molar extinction coefficient
[34e40]. Because the electrons from the photoexcitation of the
dye molecules are injected to conduction band of the semi-
conductor through the electron acceptor parts, changes in the
electron acceptor of the dye sensitizers can result in a significant
variation of electronic and photovoltaic properties. As regards
acceptor parts, cyanoacetic acid and rhodanine-3-acetic acid
generally carried anchoring group to TiO2 surface in the single De
2.2. Synthesis
In order to investigate the structural modification of different
electron acceptor groups upon the photophysical, electrochemical
and photocurrent densityevoltage characteristics of the DSSCs,
double electron acceptor groups were applied into the three dyes
with triphenylamine electron donor. The synthetic route of TPAC1,
TPAR2 and TPACR2 dyes is shown in Fig. 1. DFTPA and TPAR2 were
synthesized according to the corresponding literature methods
[44]. The final step was a Knoevenagel reaction between the car-
baldehyde and two equivalent of different electron acceptors
(cyanoacetic acid, rhodanine-3-acetic acid or (Z)-2-(2-(3-octyl-4-
oxo-2-thioxothiazolidin-5-ylidene)-4-oxothiazolidin-3-yl) acetic
acid) in the presence of ammonium acetate in acetic acid.
p
eA sensitizers which were discussed in previous work [41e43].
In addition, due to the strong electron withdrawing ability and the
extension of the -conjugation framework, co-rhodanine unit
p
have also been successfully utilized for the application of DSSCs
[1,3]. However, the contributions of different electron acceptors in
di-anchoring dyes on the electronic and photovoltaic properties
have not been well explored. Recently, we present three di-
anchoring dyes comprised a triphenylamine group as an electron
donor, which are coded as TPAC1, TPAR2 and TPACR2. The three
di-anchoring dyes were designed to have double electron accep-
tors (cyanoacetic acid, rhodanine-3-acetic acid, co-rhodanine
unit), which are shown in Fig. 1. With the variation from cyano-
2.2.1. Synthesis of TPAC1
A 25 mL acetic acid solution of DFTPA (151 mg, 0.5 mmol),
cyanoacetic acid (85 mg, 1.0 mmol) and ammonium acetate (20 mg,
0.26 mmol) was refluxed for 6 h under argon atmosphere. After
cooling to room temperature, the precipitate was filtered and
washed by distilled water. The crude product was purified by col-
umn chromatography (methylene chloride/methanol ¼ 10/1) to
obtain TPAC1 (200 mg, 92%) as a red solid. 1H NMR (500 MHz,
acetic acid via rhodanine-3-acetic acid to co-rhodanine units, a p-
conjugated extension of electron acceptor in di-anchoring organic
sensitizers was observed for expanding the possibility of
enhancing the optical and photovoltaic properties of the sensi-
tizers. In our study, in comparison with cyanoacetic acid or rho-
danine-3-acetic acid, the sensitizer with double co-rhodanine
units as the anchor group exhibited the best overall conversion
efficiency with simultaneous enhancement of photocurrent and
photovoltage.
CDCl3):
d/ppm: 7.86 (s, 1H, CH), 7.84 (s, 1H, CH), 7.36 (t, 2H,
J ¼ 7.5 Hz, ArH), 7.23 (t, 1H, J ¼ 8.5 Hz, ArH), 7.28 (d, 2H, J ¼ 8.5 Hz,
ArH), 7.15 (d, 4H, J ¼ 8.0 Hz, ArH), 7.00 (d, 2H, J ¼ 8.0 Hz, ArH). 13
C
NMR (125 MHz, CDCl3): d/ppm: 151.5, 145.7, 144.9, 133.8, 132.7,
129.5, 127.7, 126.2, 125.8, 125.4, 119.4, 116.5. MALDI-TOF-MS (m/z):
calcd for (Mꢁ2H)ꢁ C26H15O4N3: 433.1063, found: 433.1093.
2. Experimental section
2.2.2. Synthesis of TPACR2
The same procedure as for TPAC1 but with (Z)-2-(2-(3-octyl-4-
oxo-2-thioxothiazolidin-5-ylidene)-4-oxothiazolidin-3-yl) acetic
acid (160 mg, 0.4 mmol) were used. The crude product was purified
by column chromatography (methylene chloride/methanol ¼ 10/1)
to obtain TPACR2 (150 mg, 70%) as a red solid. 1H NMR (500 MHz,
2.1. Materials
Tetrabutylammonium perchlorate (TBAP), 4-tert-butylpyridine
(TBP), lithium iodide (LiI) and iodine (I2) were purchased from
Aldrich and used as received. The starting materials triphenylamine
CDCl3):
d/ppm: 7.78 (s, 1H, CH), 7.76 (s, 1H, CH), 7.54 (d, 2H,
and
(Z)-2-(2-(3-octyl-4-oxo-2-thioxothiazolidin-5-ylidene)-4-
J ¼ 8.5 Hz, ArH), 7.40 (t, 2H, J ¼ 7.7 Hz, ArH), 7.26 (t, 1H, J ¼ 7.5 Hz,
oxothiazolidin-3-yl) acetic acid was purchased from chemsolarism
ArH), 7.18 (d, 8H, J ¼ 8.0 Hz, ArH), 4.76 (s, 4H, CH2), 4.06 (t, 4H,
Fig. 1. The synthetic route of the three dyes (TPAC1, TPAR2 and TPACR2). (a) POCl3, DMF, reflux, 5 h; (b) cyanoacetic acid, rhodanine-3-acetic acid or (Z)-2-(2-(3-octyl-4-oxo-2-
thioxothiazolidin-5-ylidene)-4-oxothiazolidin-3-yl) acetic acid, ammonium acetate, acetic acid, reflux, 6 h.

G. Wu et al. / Dyes and Pigments 105 (2014) 1e6
3
J ¼ 8.0 Hz, CH2), 1.67 (m, 4H, CH2), 1.36e1.24 (m, 20H, CH2), 0.87(t,
6H, J ¼ 2.5 Hz, CH3). 13C NMR (125 MHz, CDCl3):
d/ppm: 190.5,172.2,
169.0, 166.8, 149.2, 148.0, 144.9, 143.7, 133.8, 132.0, 131.0, 129.7,
126.5, 123.3, 121.6, 117.1, 94.6, 44.7, 44.4, 31.3, 29.2, 28.7, 26.4, 22.2,
13.6. MALDI-TOF-MS (m/z): calcd for (M)ꢁ C52H55O8N5S6:
1069.2375, found: 1069.1925.
2.3. Fabrication and characterization of DSSCs
The dye-sensitized TiO2 electrodes were prepared by following
the procedure reported in the literature [45]. Briefly, a double layer
of TiO2 particles (w10
mm) was screen-printed on the fluorine tin
oxide (FTO) coated glass (12e14
U
per square, TEC 15, USA). After
that, the TiO2 thin-film electrodes were sintered at 450 ꢂC for
30 min and used as the photoelectrode. After cooling to room
temperature, the TiO2 thin-film electrodes were immersed in a
CHCl3 solvent containing 3 ꢀ 10ꢁ4 mol Lꢁ1 dye sensitizers for at
least 15 h, then rinsed with anhydrous CHCl3 and dried. To prepare
the counter electrode, Pt catalyst was deposited on FTO glass by
spraying H2PtCl6 solution and pyrolysis at 410 ꢂC for 20 min. The
DSSCs used for photovoltaic measurements consist of a dye-
Fig. 2. The absorption spectra of dyes TPAC1, TPAR2 and TPACR2 in CHCl3 solutions
(3 ꢀ 10ꢁ5 M).
intramolecular charge transfer (ICT) between the triphenylamine
donor and the electron acceptor. The absorption peak values are in
the order of TPACR2 (535 nm) > TPAR2 (497 nm) > TPAC1
(426 nm). Note that the above two absorption bands are also red-
shifted with the variation from cyanoacetic acid via rhodanine-3-
acetic acid to co-rhodanine units. The bathochromic shift which is
desirable for harvesting light from the solar spectrum should be
adsorbed TiO2 working electrode, a 45 mm thermal adhesive film
(SurlynÒ, USA), an organic electrolyte and a counter electrode. The
organic electrolyte solution was a mixture of 0.6 M 1, 2-Dimethyl-3-
propylimidazolium iodide (DMPII), 0.1 M LiI and 0.1 M I2 in aceto-
nitrile or 0.6 M DMPII, 0.1 M LiI, 0.1 M I2 and 0.5 M TBP in aceto-
nitrile. The area of the TiO2 film electrodes was 0.25 cm2.
assigned to the extension of
p system. All the molar extinction
coefficients of the maximum absorption wavelength for the three
dyes obviously increase with the variation from cyanoacetic acid
via rhodanine-3-acetic acid to co-rhodanine units. The higher
2.4. Equipments
molar extinction coefficient for TPACR2 (6.5 ꢀ 104 Mꢁ1 cmꢁ1
)
Absorption spectra were performed on a U-3900H UVeVis
spectrophotometer (Hitachi, Japan). Emission spectra were ob-
tained from the F-7000 spectrofluorimeter (Hitachi, Japan). The
oxidation potentials of the three dyes adsorbed on TiO2 films were
measured in a three-electrode electrochemical cell with a CHI-
660d electrochemical analyzer (CH Instruments, Inc., China). TiO2
films stained with the sensitizers were used as working electrodes.
Pt wire was used as the auxiliary electrode and Saturated Calomel
Electrode (SCE) was used as reference electrode. The supporting
electrolyte was 0.1 M tetrabutylammonium perchlorate (TBAP)
with dimethylformamide (DMF) as the solvent. The scan rate was
100 mV sꢁ1. Electrochemical impedance spectroscopy (EIS) mea-
surements of the DSSCs were performed using an AUTOLAB
PGSTAT 302N analyzer (Metrohm, Switzerland) in the frequency
region from 50 mHz to 1000 kHz. The applied voltage bias
is ꢁ0.55 V. The photocurrent densityephotovoltage (JeV) curves of
the DSSCs were obtained using a 3A grade solar simulator (New-
port, USA, 94043A) under AM 1.5 (100 mW cmꢁ2) illumination. The
incident monochromatic photon-to-current conversion efficiency
(IPCE) spectra were measured as a function of wavelength from
300 to 900 nm, which was recorded on QE/IPCE measurement kit
(Newport, USA).
compared with that for TPAC1 and TPAR2 indicates a good ability
for light harvesting.
Fig. 3 shows the normalized absorption spectra of the three dyes
on 2.5 mm thick TiO2 films after 12 h adsorption. Compared with the
spectra in CHCl3 solution, a blue-shift of the absorption spectra was
observed in the two dyes TPAC1 (20 nm) and TPAR2 (6 nm) on TiO2
surface, which can be attributed to the strong interactions between
the two dyes and the semiconductor surface especially the for-
mation of H-type aggregation. Furthermore, TPAC1 dye has larger
blue-shifted value as compared to TPAR2 dye, indicating that
TPAC1 dye has a more tendency to aggregate on TiO2. However, the
absorption spectrum of TPACR2 on TiO2 film shows no difference in
comparison with that in solution indicating the dye has no ten-
dency to aggregate, which is attributed to the presence of the octyl
substituted rhodanine ring.
Table 1
UVeVis, emission and electrochemical data.
d
e
f
Dye
Abs lamax/nm
(εb/Mꢁ1 cmꢁ1
Em la
nm
/
lc
/
E0
V (vs
/
E0e0
/
E0
/
(Sþ/S*)
ex
max
(Sþ/S)
)
nm
V (Abs/ V (vs
on TiO2 NHE)
Em)
NHE)
TPAC1
TPAR2
426 (2.0 ꢀ 104) 568
497 (4.8 ꢀ 104) 577
406
491
535
1.34
1.25
1.37
2.58
2.17
2.14
ꢁ1.24
ꢁ0.92
ꢁ0.77
3. Results and discussion
TPACR2 535 (6.5 ꢀ 104) 612
3.1. Absorption spectra
a
Absorption and emission peaks were measured in CHCl3 solution
(3.0 ꢀ 10ꢁ5 mol Lꢁ1) at room temperature.
The absorption spectra of the three dyes TPAC1, TPAR2 and
TPACR2 in diluted solution of CHCl3 (3 ꢀ 10ꢁ5 M) are shown in
Fig. 2. The data are listed in Table 1. The absorption spectra of the
three dyes TPAC1, TPAR2 and TPACR2 in CHCl3 display two
distinct absorption bands at around 300e395 nm and 400e
600 nm, respectively. The weak absorption peaks in the UV band
b
The molar extinction coefficient at corresponding wavelength of the absorption
spectra.
c
Absorption maximum on TiO2.
Oxidation potentials of the three dyes adsorbed on TiO2 films were measured in
d
DMF containing 0.1 mol Lꢁ1 TBAP with a scan rate of 100 mV sꢁ1, NHE: standard
hydrogen electrode.
e
E0e0 transition energy, estimated from the intersection between the absorption
correspond to the
pep* electron transition and the strong ab-
and emission spectra in CHCl3 solution.
f
sorption peaks in the visible band can be assigned to an
The E0
of the three dyes were calculated from E0(Sþ/S) ꢁ E0e0
.
(Sþ/S*)

4
G. Wu et al. / Dyes and Pigments 105 (2014) 1e6
were performed with the B3LYP exchange correlation functional
under 6-31G (d) basis set. Fig. 5 shows the frontier molecular or-
bitals of the three dyes. It can be seen that for TPAC1, TPAR2 and
TPACR2, the HOMO electron density geometry distributions are all
over the whole molecular structures especially in the conjugated
systems. Neglecting the unsubstituted benzene ring, the LUMO
electron density geometry distributions for the three dyes are also
all over the whole molecular structures. Furthermore, the LUMO
electron density geometry distribution of TPAR2 and TPACR2 is
mainly concentrated on the rhodanine framework, especially on
the carbonyl and thiocarbonyl. In spite of this, the electrons which
are generated upon photoexcitation in the three organic dyes
anchored onto the TiO2 film surface, can be successively transferred
from triphenylamine to electron acceptor group (cyanoacetic acid
group, rhodanine-3-acetic acid group or co-rhodanine group) and
finally into the conduction band of TiO2.
Fig. 3. The normalized absorption spectra of these dyes on the TiO2 films.
3.2. Electrochemical properties
3.4. Photovoltaic performances of DSSCs
Electrochemical properties of TPAC1, TPAR2 and TPACR2 were
investigated by cyclic voltammetry in dimethylformamide (DMF)
solution containing 0.1 M tetrabutylammonium perchlorate (TBAP)
as supporting electrolyte. Fig. 4 shows the cyclic voltammetry
curves of TPAC1, TPAR2 and TPACR2 adsorbed on TiO2 films and the
results were summarized in Table 1. It is shown that the HOMO
(E0(Sþ/S)) levels of TPAC1, TPAR2 and TPACR2 were sufficiently more
positive than the iodine/triiodide redox potential value (0.4 V vs.
NHE), ensuring that there is enough driving force for the dye
regeneration reaction. On the other hand, the estimated excited
state potential (E0(Sþ/S*)) corresponding to the LUMO levels of
The incident photon-to-current conversion efficiency (IPCE)
spectra of the cells based on the three dyes and N719 are shown in
Fig. 6. The solar cell based on TPACR2 shows high IPCE above 60% in
the range of 390e630 nm and with the highest value of 74% at
570 nm. For the other two dyes sensitized solar cells, the IPCE
reaches maxima of 69% at 570 nm for TPAR2 and 77% at 450 nm for
TPAC1, respectively. On the other hand, the onset of IPCE for
TPACR2 and TPAR2 is extended to 690 nm and 640 nm, which
corresponds to red shifts of 100 nm and 50 nm respectively in
comparison with TPAC1. Consistent with their absorption spectra in
CHCl3 solvent and on the transparent TiO2 films, the broader IPCE
values for the TPACR2-based DSSC may be attributed to its broader
absorption, which leads to a higher short circuit photocurrent
density compared with that of TPAC1 and TPAR2. Furthermore, the
onsets of IPCE spectra for the three dyes are significantly broadened
compared to their absorption spectra in solution, which is observed
TPAC1, TPAR2 and TPACR2, calculated from E0
ꢁ E0e0
,
(Sþ/S)
are ꢁ1.24 V, ꢁ0.92 V and ꢁ0.77 V, respectively. It is obvious that the
LUMO level of the dye is increased with the increase of the unit of
rhodanine ring leading to the smaller driving force. More impor-
tantly, the LUMO levels of the three dyes are more negative than the
conduction band of TiO2 (ꢁ0.5 V), indicating that the electron in-
jection process from the excited dye molecule to the conduction
band of TiO2 is energetically permitted. From these values, we can
clearly conclude that these organic dyes could be used as sensi-
tizers in DSSCs.
3.3. Theoretical calculations
To investigate the molecular structure and electron distribution
of the three organic dyes, the three dyes have been optimized using
DFT calculations with Gaussian 09 program [46]. The calculations
Fig. 4. The cyclic voltammetry plots of TPAC1, TPAR2 and TPACR2 attached to nano-
crystalline TiO2 films deposited on conducting FTO glass.
Fig. 5. Molecular orbital distributions of TPAC1, TPAR2 and TPACR2.

G. Wu et al. / Dyes and Pigments 105 (2014) 1e6
5
Fig. 7. Current densityevoltage curves of the DSSCs sensitized with TPAC1, TPAR2 and
TPACR2 under light (100 mW cmꢁ2, AM 1.5 irradiation) and dark conditions.
Fig. 6. The IPCE spectra of the DSSCs sensitized with TPAC1, TPAR2 and TPACR2 dyes.
the cells. Fig. 8 showed the electrochemical impedance spectra for
the DSSCs based on the three sensitizers (TPAC1, TPAR2 and
TPACR2) under a forward bias of ꢁ0.55 V in the dark. Three
semicircles located in the high-, middle- and low-frequency regions
were observed in the Nyquist plots, which are attributed to the
charge transfer at the Pt/electrolyte interface, the electron trans-
port at the TiO2/dye/electrolyte interface and Warburg diffusion
process of Iꢁ/Iꢁ3 in the electrolyte, respectively. It is obvious that the
radius of the middle semicircle (Rct) is decreased in the order of
for other dyes [5,21,22,26,41]. The exact reason still needs further
studies. Here, red shift of the onsets in IPCE spectra for the DSSC
based on the three dyes may be attributed to the interaction of Liþ
ions adsorbing on the TiO2 film from electrolyte solution with the
carbonyl group in the dye [5].
Photovoltaic performances of the TPAC1, TPAR2 and TPACR2
sensitized TiO2 film electrodes with a liquid electrolyte are listed in
Table 2 under AM 1.5 solar simulator illumination (100 mW cmꢁ2),
and the corresponding photocurrent density (J)evoltage (V) curves
for the DSSCs based on the three dyes are shown in Fig. 7. The
TPACR2 (40
U) > TPAC1 (17 U) > TPAR2 (6 U) implying the ac-
celeration of the charge recombination between injected electrons
and Iꢁ3 in the electrolyte (TPACR2 < TPAC1 < TPAR2), with a
consequent increase of the dark currents for the three dyes
observed in JeV test. The electron lifetimes of TPAC1, TPAR2 and
TPACR2 are obtained with 8 ms, 4 ms and 31 ms, respectively.
Therefore, the larger charge recombination resistance and
enhanced electron lifetime may be the intrinsic reason for the
higher Voc value of the DSSC based on TPACR2 compared to that for
the TPAC1 and TPAR2.
TPACR2 sensitized cell gave an overall conversion efficiency (h) of
4.64% with
a short circuit photocurrent density (Jsc) of
13.16 mA cmꢁ2, an open circuit voltage (Voc) of 534 mV and a fill
factor (FF) of 0.66. Under the same conditions, the TPAC1 and
TPAR2 sensitized cells gave Jsc values of 8.56 and 11.21 mA cmꢁ2, Voc
of 467 and 411 mV and FF of 0.71 and 0.68, corresponding to
h
values of 2.86% and 3.15%, respectively. Evidently, TPACR2 shows
better solar cell performance than TPAC1 and TPAR2, especially in
Jsc and Voc. Consistent with the trend of the photocurrent integrated
from the IPCE spectra, the remarkably enhanced Jsc value of the
TPACR2-based DSSC may be attributed to its higher and broader
absorption in comparison with TPAC1- and TPAR2-based DSSCs.
The lower Voc of the DSSCs observed with the three new dyes may
be attributed to a faster recombination rate between the injected
electrons and Iꢁ3 in electrolytes in comparison to that for N719 dye,
which can be seen clearly from the dark currents in JeV test. This
result strongly suggests that co-rhodanine unit can be an excellent
electron acceptor system for organic dyes to improve the photo-
voltaic performance.
4. Conclusions
In summary, three di-anchoring dyes TPAC1, TPAR2 and
TPACR2, comprised the same donor unit and double different
electron acceptors, were synthesized for dye-sensitized solar cells.
The absorption spectra are red-shifted with the variation from
cyanoacetic acid via rhodanine-3-acetic acid to co-rhodanine units.
Among the three dyes, photovoltaic performance of the device with
TPACR2 dye showed a higher overall conversion efficiency of 4.64%
3.5. EIS analysis
Electrochemical impedance spectroscopy (EIS) measurements
were performed to characterize the charge transfer resistances of
Table 2
Photovoltaic performance of DSSCs with the three dyes.a
Dye
Jsc/mA cmꢁ2
Voc/mV
FF
h/%
TPAC1b
TPAR2b
TPACR2b
N719c
8.56
11.21
13.16
12.47
467
411
534
667
0.71
0.68
0.66
0.74
2.86
3.15
4.64
6.17
a
Irradiating light: simulated AM 1.5 irradiation (100 mW cmꢁ2); working area:
0.25 cm2.
b
The electrolyte was a solution of 0.6 M DMPII, 0.1 M LiI and 0.1 M I2 in
acetonitrile.
c
The electrolyte solution was a mixture of 0.6 M DMPII, 0.1 M LiI, 0.1 M I2 and
0.5 M TBP in acetonitrile.
Fig. 8. EIS Nyquist plots for DSSCs based on TPAC1, TPAR2 and TPACR2 under dark.
The inset shows the equivalent circuit.

6
G. Wu et al. / Dyes and Pigments 105 (2014) 1e6
under simulated AM 1.5 solar irradiation (100 mW cmꢁ2), which
were attributed to the higher molar extinction coefficient, broader
absorption spectra, broader IPCE spectra and longer electron life-
time. The result strongly suggests that co-rhodanine unit can be an
excellent electron acceptor system for organic dyes to improve the
photovoltaic performance. Further improvement of DSSCs with
organic sensitizers based on the structure of TPACR2 by molecular
engineering aiming for better photovoltaic performance is in
progress.
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Acknowledgments
This work was financially supported by the National Basic
Research Program of China under Grant No. 2011CBA00700, the
National Natural Science Foundation of China under Grant Nos.
(21003130, 21171084 and 21173227) and the External Cooperation
Program of the Chinese Academy of Sciences under Grant No.
GJHZ1220.
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