Multiple-Anchoring Triphenylamine Dyes for Dye-Sensitized Solar Cell

Apr 11, 2014 - Caroline R. English and Robert J. Hamers. The Journal of Physical Chemistry C 2015 119 (50), 27972-27981. Abstract | Full Text HTML | P...
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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