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Multiple-Anchoring Triphenylamine Dyes for Dye-Sensitized Solar Cell Application Guohua Wu, Fantai Kong, Yaohong Zhang, Xianxi Zhang, Jingzhe Li, Wangchao Chen, Weiqing Liu, Yong Ding, Changneng Zhang, Bing Zhang, Jianxi Yao, and Songyuan Dai J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 11 Apr 2014 Downloaded from http://pubs.acs.org on April 12, 2014
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The Journal of Physical Chemistry
Multiple-Anchoring
Triphenylamine
Dyes
for
Dye-sensitized Solar Cell Application Guohua Wu, a Fantai Kong, *, a Yaohong Zhang, b Xianxi Zhang, *, c Jingzhe Li, a Wangchao Chen, a Weiqing Liu, a Yong Ding, a Changneng Zhang, a Bing Zhang, d Jianxi Yao d and Songyuan Dai *, a, d a
Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of
Sciences, Hefei, AnHui 230031, P. R. 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, P. R. China c
Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology,
School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, 252059, P. R. China d
School of Renewable Energy, North China Electric Power University, Beijing, 102206, P. R. China
ABSTRACT: A series of triphenylamine-based dyes (TPACR1, TPACR2 and TPACR3) with multiple co-rhodanine derivatives as acceptors were prepared and examined as sensitizers for dye-sensitized solar cells. The overall conversion efficiencies of DSSCs based on these dyes were in the range of 2.63 to 5.31%, in which TPACR2-based DSSC showed the best photovoltaic performance: a short-circuit current (Jsc) of 15.03 mA·cm–2, an open-circuit voltage (Voc) of 552 mV, and a fill factor (FF) of 0.64, corresponding to an overall efficiency of 5.31% under simulated AM 1.5 solar irradiation (100 mW·cm–2). Compared with mono-anchoring TPACR1 dye and tri-anchoring
TPACR3
dye,
di-anchoring
TPACR2
dye
had
relatively
slower
charge
recombination rate between injected electrons and triphenylamine dye cations, which was indicated by transient absorption spectra. Furthermore, the suppression of the charge recombination between injected electron and the I3– in the electrolyte lead to the longer electron lifetime observed with the DSSC based on TPACR2 dye. In comparison with TPACR1 and TPACR3 dye, TPACR2 dye showed the higher overall conversion efficiency with simultaneous enhancement of photocurrent and photovoltage.
Keywords: organic sensitizer; transient
recombination
absorption spectrum; multiple acceptors;
INTRODUCTION
dye-sensitized solar cell; charge
Metal-free organic dyes as sensitizers have 1
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drawn widespread academic and commercial
HOMO
attention because of their high molar extinction
Furthermore, a suitable number of acceptor
coefficient and easily adjustable spectral response.
groups are very critical to the balance between
Recently, a number of novel metal-free organic
photocurrent, photovoltage, spectral response
1-5
sensitizers, such as triphenylamine, coumarin, 18-19
9-10
cyanine,
11-13
tetrahydroquinoline,
phenoxazine been
26-28
perylene, 20-22
and
applied
levels
of
sensitizers.
and driving forces for highly efficient dyes. As
carbazole,
acceptor parts, cyanoacetic acid and rhodanine
23-25
3-acetic acid were always introduced into the dye
phenothiazine,
and fluorene dyes,
investigated
LUMO
6-8
indoline,
14-17
and
Page 2 of 20
29-31
in
have
sensitizers which was discussed in previous work. 42-44
DSSCs
successfully.
Especially, Abbotto et al.
45
and Jiang et al.
46
introduced an additional cyanoacetic acid or
Most traditional organic dye sensitizers contain
rhodanine 3-acetic acid anchor group to increase
the structure of “Donor (D) – conjugated bridge
the extinction coefficient of the sensitizer.
(π) – Acceptor (A)”. Generally, organic dyes used
However, the introducing of co-rhodanine units
for efficient solar cells are required to possess
into triphenylamine dyes as electron acceptor has
broad and intense absorption in the visible
not been reported in the literature.
spectral region. One strategy is to form D–π–π–A
In this study, we reported on the synthesis and
3, 32-35
characterization of triphenylamine-based dyes
structure by enlarging π–conjugated linker.
However, organic dyes with this structure may
with
facilitate the charge recombination with the
co-rhodanine acceptors and their applications as
triiodide
sensitizers
ions
in
the
electrolyte
and
the
multiple in
octyl
chain
DSSC. Multiple
substituted co-rhodanine
unfavorable aggregations between dye molecules.
acceptors were employed in order to further
The other strategy is to form D–D–π–A structure
increase
by introducing more donor segments. Especially,
sensitizer due to its strong electron withdrawing
modification of the organic dyes with additional
effect and the extension of the π-conjugation.
donors to form so-called starburst dyes has
The molecular structures of sensitizers TPACR1,
aroused
great
their
coefficient
of
the
better
TPACR2 and TPACR3 are shown in Figure 1. The three dyes were designed to create multiple
dyes with this structure is the complexity in the
channels of electron injection into TiO2 electrode
process of synthesis. Aside from donor (D) and
for enhancement in the efficiency of DSSC. The
conjugated bridge (π) segments, the electron
influence
acceptor (A) also plays a very important role for
photophysical, electrochemical and photovoltaic
the performance control of DSSCs. Recently, dye
properties of the DSSCs based on the three dyes
sensitizers with triphenylamine (TPA) and its
was involved. Additionally, two different charge
derivatives as donor part have shown promising
recombination channels, one main channel
36-41
for
extinction
The major problem of organic
performance.
interest
the
1
of
multiple
acceptors
on
the
applications in photovoltaic devices. Due to the
between injected electrons and the I3– in the
special
electrolyte, and a second channel between
triphenylamine
structure,
multiple
electron acceptors can be introduced into the
injected
organic donor triphenylamine framework to form
cations, were extensively studied.
starburst
molecules.
The
multiple
acceptor
electrons
and
triphenylamine
dye
EXPERIMENTAL DETAILS
groups can affect not only the absorption spectra
Equipments.
or molar extinction coefficients but also the 2
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Absorption
spectra
were
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The Journal of Physical Chemistry
UV–Vis
under an illumination of a LED light source (λ=
spectrophotometer (Hitachi, Japan). Emission
610 nm) with different light intensities. The
spectra
frequency range was 0.3 Hz to 3 kHz.
recorded
on were
a
U–3900H
obtained
from
the
F–7000
spectrofluorimeter (Hitachi, Japan). Transient
Fabrication and characterization of DSSCs.
absorption measurements were carried out on a
The dye-sensitized TiO2 electrodes were prepared
LP920
by following the procedure reported in the
laser
flash
spectrometer
(Edinburgh
Instruments Ltd., Scotland) in conjunction with a
literature.
nanosecond
tunable
laser
particles (~10 μm) was screen-printed on the fluorine tin oxide (FTO) coated glass (12–14 Ω per
observed at 580 nm for TiO2 films sensitized with
square, TEC 15, USA). After that, the TiO2
dyes TPACR1, TPACR2 and TPACR3. The
thin-film electrodes were sintered at 450°C for 30
oxidation potentials of the three dyes adsorbed
min and used as the photoelectrode. After
on TiO2 films were measured in a three-electrode
cooling to room temperature, the TiO2 thin-film
electrochemical
CHI–660d
electrodes were immersed in a acetonitrile/
electrochemical analyzer (CH Instruments, Inc.,
tert-butanol (1:1, V/V) mixed solvent containing
China). TiO2 films stained with the sensitizers
3×10-4 mol·L-1 dye sensitizers for at least 12 h, then
were used as working electrodes. The supporting
rinsed with anhydrous acetonitrile and dried. To
electrolyte
cell
0.1
with
M
355II
Briefly, a double layer of TiO2
(Opotek, Inc., USA). Transient absorption signals
was
OPOLette
47
a
tetrabutylammonium
prepare the counter electrode, Pt catalyst was
perchlorate (TBAP) with dimethylformamide
deposited on FTO glass by spraying H2PtCl6
(DMF) as the solvent. The scan rate was 100
solution and pyrolysis at 410 °C for 20 min. The
mV·s–1. The photocurrent density-photovoltage
DSSCs used for photovoltaic measurements
(J–V) curves of the DSSCs were obtained using a
consists
3A grade solar simulator (Newport, USA, 94043A)
electrode, a 45 μm thermal adhesive film (Surlyn®,
–2
of
a
dye-adsorbed
TiO2
working
under AM 1.5 (100 mW·cm ) illumination. The
USA), an organic electrolyte and a counter
incident monochromatic photon to current
electrode. The organic electrolyte solution was a
conversion
mixture
efficiency
(IPCE)
spectra
were
of
0.6
M
1,
measured as a function of wavelength from 300 to
2-Dimethyl-3-propylimidazolium iodide (DMPII),
800 nm, which was recorded on QE/IPCE
0.1 M LiI and 0.1 M I2 in acetonitrile or 0.6 M
measurement
USA).
DMPII, 0.1 M LiI, 0.1 M I2 and 0.5 M TBP in
Electrochemical impedance spectroscopy (EIS)
acetonitrile. The area of the TiO2 film electrodes
measurements of the DSSCs were performed
was 0.25 cm2.
kit
(Newport,
using an AUTOLAB PGSTAT 302N analyzer
Synthesis. The synthetic route of TPACR1,
(Metrohm, Switzerland) in the frequency region
TPACR2 and TPACR3 dyes is shown in Figure 2.
from 50 mHz to 1000 kHz. The applied voltage bias
was
photocurrent
–0.55
V.
FTPA and DFTPA were synthesized according to
Intensity-modulated
spectroscopy
(IMPS)
the corresponding literature methods.
and
48-49
The
final step was a Knoevenagel condensation
intensity-modulated photovoltage spectroscopy
between
(IMVS) were performed on IM6ex (Zahner
the
carbaldehyde
and
different
equivalent
Company, Germany) and the light emitting
(Z)-2-(2-(3-octyl-4-oxo-2-thioxothiazolidin-5-ylid
diodes (Expot, Zahner, Germany) supplied the
ene)-4-oxothiazolidin-3-yl) acetic acid in the
modulated light. IMPS and IMVS were measured
presence of ammonium acetate in acetic acid. 3
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Page 4 of 20
Detailed synthesis procedures for the three dyes
three dyes were similar to those in acetonitrile/
are shown in supporting information.
tert-butanol (1:1, V/V)
mixed
solvent. The
wavelength peaks of absorption spectra on TiO2
RESULTS AND DISCUSSION
are listed in Table 1. Compared to the spectra in
Absorption spectra. The UV–Vis absorption
solution, the absorption peaks for TPACR1,
spectra of the three organic dyes in acetonitrile/
TPACR2 and TPACR3 on TiO2 films appeared
tert-butanol (1:1, V/V) solution are displayed in
almost no change. Generally, the absorption
Figure 3 and the parameters are listed in Table 1.
maxima of organic dyes on TiO2 films would
Each of these compounds exhibited a minor one
change due to the different effect of the
at 340–420 nm and one major absorption band at
deprotonation in adsorption process and the
420–600 nm. The former was assigned to the
aggregation state of dyes on TiO2 films. In our
localized aromatic π–π∗ transitions and the latter
case, no shift between organic dyes on TiO2 films
was
(CT)
and in solution was observed which might be
transition between the triphenylamine donating
ascribed to the canceling effect of J-type
unit and the co-rhodanine anchoring moiety.
aggregation 51-52 and deprotonation. In addition, it
Especially, the above two absorption bands of
was noteworthy that the absorption spectra of
TPACR2 dye were red-shifted compared with
the three dyes anchored onto the TiO2 film
those of TPACR1 and TPACR3 dyes. With
showed a slightly broad profile compared to
increasing
those in acetonitrile/ tert-butanol (1:1, V/V)
attributed
to
anchoring
a
charge-transfer
group
number,
the
maximum absorption peak showed red-shift first
mixed
then blue-shift, which could be ascribed to the
light-harvesting.
conjugated
system
increasing
first
then
major
absorption
band
and
–1
bands of the C=O stretching mode were at 1711 cm−1 for TPACR1, 1715 cm−1 for TPACR2 and 1720
4
L·mol ·cm ) for TPACR2 and 500 nm (2.1×10 L·mol ·cm )
for
TPACR3,
respectively.
cm−1 for TPACR3, respectively. When the three
In
dyes were adsorbed on TiO2, the absorption
comparison with the conventional ruthenium dye N719
intensity of the C=O stretching mode was greatly
[tetrabutylammonium
reduced. Moreover, the antisymmetric stretching
cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicar
bands of carboxylate were appeared obviously
boxylato)ruthenium(II)] (1.47×104 L·mol−1·cm−1 at 535 nm),
50
for
Figure 5. The characteristic FT-IR absorption
L·mol–1·cm–1) for TPACR1, 518 nm (5.7×104 –1
beneficial
by the three corresponding dyes are listed in
of the three dyes were 496 nm (5.5×104 –1
was
cm-1 region) of the three dyes and TiO2 adsorbed
the
corresponding molar extinction coefficient values
–1
which
FT-IR spectra. The FT-IR spectra (1800-900
decreasing. The maximum absorption peaks in the
solvent,
(1623 cm−1 for TPACR1, 1626 cm−1 for TPACR2 and
the three dye molecules showed
1627 cm−1 for TPACR3). This indicated that the
obviously higher absorption coefficients, which
surface complexes of the three dyes were mostly
could be beneficial for light harvesting.
attached
The normalized absorption spectra of the three
through
a
bidentate
coordination
instead of a monodentate one.
dyes (TPACR1, TPACR2 and TPACR3) adsorbed
Electrochemical properties. To obtain and
onto TiO2 films (~2.5μm) after 12 h adsorption are
understand the molecular orbital energy levels of
shown in Figure 4. When the dyes adsorbed on
the three sensitizers TPACR1, TPACR2 and
the TiO2 surface, the absorption spectra of the
TPACR3, cyclic voltammetry was performed in 4
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The Journal of Physical Chemistry
DMF and the results were summarized in Table 1.
TPACR3, the HOMO and LUMO electron density
Figure 6 shows the cyclic voltammetry curves of
geometry distributions are all over the whole
TPACR1, TPACR2 and TPACR3 adsorbed on
molecular structures. Especially for the LUMO of
TiO2 films in DMF. The SCE reference electrode
TPACR3, it was obvious that only one of the
was calibrated using a ferrocene/ferrocenium
co-rhodanine units has the larger electron
+
(Fc/Fc ) redox couple as an external standard.
density geometry distributions, which directly
The oxidation potentials, corresponding to the
determines the main electron moving direction.
highest occupied molecular orbital (HOMO)
The HOMO–LUMO excitation induced by light
level
a
irradiation for TPACR1, TPACR2 and TPACR3
quasi-reversible couple at 1.34 V, 1.37 V and 1.46 V
could move the electron distribution from the
versus
electrode).
triphenylamine moiety to the co-rhodanine
Furthermore, it was noteworthy that the lowest
framework, which was favorable to an efficient
unoccupied molecular orbital (LUMO) level of
photoinduced electron transfer from the dye to
TPACR1 (–0.91 V) < TPACR2 (–0.77 V) < TPACR3
the TiO2 electrode.
of
the
NHE
three (normal
sensitizers, hydrogen
show
(–0.71 V) show that the LUMO energy level of the
Transient absorption measurements. The
dye was increased with an increased numbers of
transient absorption technique was applied to the
acceptors leading to the smaller driving force.
dyes TPACR1, TPACR2 and TPACR3 to evaluate
Suitable LUMO and HOMO levels of the three
the rate of the recombination process of injected
dyes can match the conduction band (–0.5 V vs.
electrons with the oxidized dye and the dye
NHE)
53
of the TiO2 electrode and the redox
potential (0.4 V vs. NHE)
54
cation dynamics of interception by iodide.
56-57
of the iodine/iodide
The recombination and regeneration, two charge
electrolyte. Therefore it was feasible for the
transfer processes, follow the ultrafast electron
electron inject from the excited state of dyes into
injection from the dye excited state into TiO2 and
the conduction band of TiO2 and the oxidized
compete kinetically. Figure 8 shows transient
dyes form after electron injection into the
absorption kinetics of TPACR2 adsorbed on TiO2
conduction band of TiO2 can accept electrons
in presence of pure acetonitrile or redox
from I– thermodynamically, which demonstrates
electrolyte (EL) under similar conditions ( The
that the three dyes can work as the sensitizers for
transient absorption spectra of TPACR1 and
DSSCs.
TPACR3 adsorbed on TiO2 are shown in
Theoretical calculations. Theoretical studies
supporting information). In the electrochemically
based on DFT calculations were performed for
inert pure solvent (ACN) without the redox
the three dyes to gain insight into the geometric
couple, the decay of the absorption signal reflects
electronic structures by using the Gaussian 09
the dynamics of the recombination of injected
program package
55
at the B3LYP/6-31G(d) level.
electrons
with
the
oxidized
dye.
The
The molecular orbital distributions of TPACR1,
recombination lifetimes obtained by the well
TPACR2 and TPACR3 are shown in Figure 7. It
fitted decays with a single exponential were 4.7 μs,
can be seen that the electron density of the
11.8 μs and 1.8 μs for TPACR1, TPACR2 and
HOMO for TPACR1 was almost localized on all
TPACR3, respectively. The corresponding rate
over the whole molecular structure. The electron
constants of back-electron transfer were 2.1×105 s–1,
density of the LUMO was mostly localized on the
8.5×104 s–1 and 5.6×105 s–1. The recombination rate
co-rhodanine framework. But for TPACR2 and
of TPACR2 was significantly slower than those of 5
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TPACR1 and TPACR3 dyes. In the presence of
attributed to the decrease of light harvesting for
the redox couple, a much faster decay of the
the three dyes. IPCE is represented by the
oxidized dye signal was observed. The fit to first
following equation: IPCE= LHE × ηc × ηinj , where
exponential function gives a rate constant of
LHE is the light harvesting efficiency, ηc is the
6
–1
6
3.5×10 s , 5.3×10 s
–1
6
–1
and 1.3×10 s , giving a
electron collecting efficiency and ηinj is the
typical lifetime of 284 ns, 187 ns and 779 ns for
electron injection efficiency. From the LUMO
dye regeneration on TPACR1, TPACR2 and
energy level of the three dyes discussed above
TPACR3, respectively. The yield of interception
(–0.91 V for TPACR1, –0.77 V for TPACR2, –0.71 V
by iodide can be evaluated to be 94%, 98% and
for TPACR3), it can be obtained that the driving
57%
TPACR3,
force between the dyes and TiO2 is reduced
respectively. Furthermore, according to the two
accordingly which is disadvantageous for the
charge transfer processes, the rate constants of
electron injection for TPACR2 dye. So the
for
TPACR1,
TPACR2
and
6
–1
dye regeneration were obtained as 3.29×10 s for 6
–1
5
broader and higher IPCE of DSSC based on
–1
TPACR2 compared with that for TPACR1 and
for TPACR3. Thus, the dye regeneration process
TPACR3 leading to better photoresponse in the
of the TPACR2 dye with the double co-rhodanine
visible light can be explained by two factors. On
units was faster than those of TPACR1 and
one hand, the LHE for TPACR2 dye is better than
TPACR1, 5.22×10 s for TPACR2 and 7.40×10 s
TPACR3, which indicated that there was certain
that for TPACR1 and TPACR3 dyes due to the
relationship between conjugation length and the
higher molar extinction coefficient and broader
dye regeneration rate.58 These results indicate
absorption band. On the other hand, although
that the as synthesized sensitizers TPACR1,
there is a little difference in the electron
TPACR2 and TPACR3 can be regenerated and
transport
time
the dye cations can be intercepted by the redox
TPACR2,
the
mediator.
TPACR2 dye is larger than that for TPACR1 and
DEVICE PERFORMANCE
TPACR3 dyes, which leads to higher ηc (the
photon-to-current
efficiency
between
recombination
TPACR1 time
(τn)
and for
equation ηc = 1 – (τd / τn) as discussed below) for
Photovoltaic performances of DSSCs. The incident
(τd)
TPACR2 dye.
(IPCE)
spectra as a function of the wavelength for DSSCs
The photocurrent density-photovoltage (J–V)
based on the TPACR1, TPACR2 and TPACR3
curves of the DSSCs based on TPACR1, TPACR2
dyes are shown in Figure 9. It was clear that the
and TPACR3 under simulated AM 1.5 solar
IPCE of TPACR2-sensitized DSSC has a higher
irradiation (100 mW·cm–2) are shown in Figure 10.
value exceeding 60% in the range of 370–630 nm
The
as compared with that of TPACR3 and N719 dyes
dyes-based DSSCs are summarized in Table 2 in
and reaches its maximum of 78% at 590 nm. On
comparison with that of N719. Under AM1.5
the other hand, the onset of IPCE for TPACR2
irradiation, the TPACR2-based DSSC exhibited a
was extended to 700 nm, which corresponds to
short-circuit current (Jsc) of 15.03 mA·cm–2, an
red-shift of 50 nm compared with TPACR1 and
open-circuit voltage (Voc) of 0.552 V, and a fill
TPACR3. This was also consistent with the result
factor (FF) of 0.64, which corresponded to an
of the absorption spectra of the dyes in solution
overall efficiency of 5.31%. Under the same
and on TiO2 films. The decrease of the IPCE
conditions, photovoltaic parameters (Jsc, Voc, FF,
above 650 nm in the long-wavelength region was
and η) of the TPACR1 cell were 12.97 mA·cm-2,
photovoltaic
6
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properties
of
the
three
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The Journal of Physical Chemistry
0.526 V, 0.67, and 4.60%. As a comparison, the
and the corresponding τe values for the DSSCs
N719 dye gave a referenced η value of 6.21%. The
based on dyes TPACR1, TPACR2 and TPACR3
lower Voc of the DSSCs observed with the three
were 33 ms, 55 ms and 2 ms, respectively. It was
new dyes might be attributed to a faster
clear that the lifetime and Rct order were
recombination
injected
consistent with the Voc values of TPACR2 (552
electrons and I3 in electrolytes in comparison to
mV), TPACR1 (526 mV) and TPACR3 (414 mV).
that for N719 dye, which was similar to that for
The longer electron lifetime observed with the
rate
between
the
–
triphenylamine, coumarin, carbazole, indoline,
DSSC based on TPACR2 dye showed more
porphyrin and phthalocyanine dyes. 19, 42, 44, 59-62
effective suppression of the back reaction of the
Electrochemical impedance spectroscopy
injected electron with the I3– in the electrolyte,
analysis. To characterize the charge transfer
which was reflected in the improvements seen in
resistances
the photovoltage, yielding enhanced device
impedance
of
the
cells,
spectroscopy
Electrochemical
(EIS)
analysis
efficiency. Therefore, it
was
was reasonable to
performed in the dark under a bias of –0.55 V.
conclude that TPACR2 gives the highest values of
Figure 11 showed the EIS Nyquist plots for DSSCs
Voc compared with that of TPACR1 and TPACR3.
based on TPACR1, TPACR2 and TPACR3. A
To study the dynamics of electron transport
typical EIS spectrum of a DSSC exhibited three
and charge recombination of the DSSCs based on
semicircles in the Nyquist plots. In general, the
the three dyes, Intensity-modulated photocurrent
first semicircle represents the charge transfer
spectroscopy (IMPS) and intensity-modulated
resistances at the Pt/electrolyte interface. In
photovoltage spectroscopy (IMVS) were further
Figure 12, it was obvious that the series resistance
investigated.
(RS) and charge transfer resistance at the
recombination time (τn) can be calculated from
Pt/electrolyte interface (RCE) corresponding to
the expression τn = 1/2πfn. Similarly, the electron
the first semicircle were almost the same in
transport time (τd) can be estimated from the
TPACR1, TPACR2 and TPACR3 dyes-based
expression τd = 1/2πfd, where fn and fd is the
DSSCs due to using the same electrolyte and
characteristic frequency minima of the IMVS and
electrode
semicircle
IMPS imaginary components. Figure 12(a) and
corresponds to the charge transfer resistance at
Figure 12(b) showed recombination time (τn) and
the interface between TiO2/dye/electrolyte. The
the transport time (τd) curves as a function of
third semicircle in the low frequency range
light intensity. All DSSCs showed the decrease of
(1–0.01
impedance
τn and τd with the increase of light intensity. The
corresponding to the Warburg diffusion process
IMVS showed that the values of electron lifetime
-
material.
Hz)
The
represents
second
the
-
The
electron
lifetime
or
of I /I3 in the electrolyte. The charge transfer
or recombination time (τn) were in the order of
resistance at the TiO2/dye/electrolyte interface
TPACR3 < TPACR1 < TPACR2, in agreement
(Rct) corresponding to the second semicircle of
with the results of EIS measurement, which led to
the Nyquist plot (Figure 11.) was in the order of
the higher photovoltage of the DSSC based on
TPACR2 (78 Ω) > TPACR1 (58 Ω) > TPACR3 (6
TPACR2. Furthermore, the IMPS showed that the
Ω), indicating that the recombination rate is
transport time values of the DSSCs based on
increased in the order of TPACR2 < TPACR1
