Investigation of the effect of different anchoring groups in phenothiazine-triphenylamine-based di-anchoring dyes for dye-sensitized solar cells

Article abstract of DOI:10.1166/jnn.2017.14076

In dye-sensitized solar cells (DSSCs), as the excited electrons from dye molecules are injected into the conduction band of semiconductor films through the acceptor/anchoring moieties, the acceptor/ anchoring groups have significant influences on the phot

Full text of DOI:10.1166/jnn.2017.14076

Article
Journal of
Nanoscience and Nanotechnology
Vol. 17, 3181–3187, 2017
Copyright © 2017 American Scientific Publishers
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Printed in the United States of America
Investigation of the Effect of Different Anchoring Groups
in Phenothiazine-Triphenylamine-Based Di-Anchoring
Dyes for Dye-Sensitized Solar Cells
Nguyen Thi Hai, Le Quoc Bao, Suresh Thogiti∗, Rajesh Cheruku, Kwang-Soon Ahn, and Jae Hong Kim∗
Department of Chemical Engineering, Yeungnam University, 214-1, Dae-hakro 280, Gyeongsan,
Gyeongbuk 712-749, Republic of Korea
In dye-sensitized solar cells (DSSCs), as the excited electrons from dye molecules are injected into
the conduction band of semiconductor films through the acceptor/anchoring moieties, the accep-
tor/anchoring groups have significant influences on the photovoltaic properties of the dye sensitiz-
ers. This study examined the impacts of different acceptor/anchoring groups (cyanoacetic acid and
rhodanine-3-acetic) in phenothiazine-triphenylamine dye sensitizers on the optical, electrochemical
photovoltaic performance of DSSCs. The overall conversion efficiency was improved significantly by
replacing rhodanine-3-acetic acid (TPAPR) with cyanoacetic acid (TPAPC), which can be attributed
to the higher JSC and VOC of the TPAPC device. The lower efficiency of the device based on TPAPR
is due mainly to the broken conjugation between the 4-oxo-2-thioxothiazolidine ring and the acetic
Delivered by Ingenta to: State University of New York at Binghamton
acid, which affects electron injection from the excited state to the conduction band of TiO2. The
IP: 188.72.127.123 On: Mon, 06 Mar 2017 08:55:58
results are in good agreement with the photovoltaic properties of DSSCs.
Copyright: American Scientific Publishers
Keywords: Dye Sensitized Solar Cells, Electron Acceptor/Anchoring, Electrochemical
Properties, Photovoltaic Performance.
1. INTRODUCTION
high molar extinction coefficients, tunable molecular
energy levels, and cost-effectiveness.4 Some sensitizers,
such as coumarin-indoline-, and triphenylamine-based sen-
sitizers have achieved up to 10% power conversion effi-
ciencies using an iodine-based liquid electrolyte.23 The
donor-(ꢀ-bridged)-acceptor (D-ꢀ-A) structure is consid-
ered to be one of the most promising types of organic
sensitizers.9ꢁ10 When a D-ꢀ-A dye molecule is excited,
intramolecular charge transfer occurs by the flow of elec-
trons from the donor to the acceptor through a ꢀ-bridge,
and acceptor carrying functional groups, such as COOH,
may strongly bind to the TiO2 surface. The acceptor also
plays an important role that not only transfers excited elec-
trons to the conduction band of the semiconductor, but also
adsorbs the sensitizer on the semiconductor. Therefore, the
acceptor moiety of the D-ꢀ-A molecules should be inves-
tigated to achieve better electron transfer and improve the
efficiency.14–19
Dye sensitized solar cells (DSSCs) are promising alter-
natives to conventional p–n junction solar cells due to
low cost and flexible fabrication. The standard structure
of a DSSC consists of an electrochemical cell composed
of a sensitizer-adsorbed wide band gap semiconductor
electrode, an electrolyte containing a red-ox mediator as
hole-transporting material, and a Pt-coated counter elec-
trode. The mechanism of a DSSC is based on the injec-
tion of electrons from a photosensitizer to the conduction
band of a semiconductor. The oxidized photosensitizers are
reduced by electron injection from the electrolyte. There-
fore, the photosensitizer plays an important role in cap-
turing photons and generating electron/hole pairs, as well
as transferring them to the interface of a semiconductor
and electrolyte, respectively.1–9 The power conversion effi-
ciency is up to 10% using metal-complex sensitizers.10–13
Recently, metal-free organic sensitizers have attracted con-
siderable attention because of their great advantages com-
pared to metal sensitizers, such as environment friendly,
In this study, two new di-anchoring organic dyes were
designed and synthesized and the effects of different
acceptor groups (cyanoacetic acid and rhodanine-3-acetic)
on the photophysical, electrochemical and photovoltaic
∗Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 5
1533-4880/2017/17/3181/007
doi:10.1166/jnn.2017.14076
3181

Effect of Different Anchoring Groups in Phenothiazine-Triphenylamine-Based Di-Anchoring Dyes for DSSCs
Hai et al.
1
I
(1.59 g, 53%). H MNR (300 MHz, DMSO-d6ꢂ ꢃ 9.89
(s, 2H), 7.86 (d, J = 8.7 Hz, 4H), 7.47 (d, J = 8.1 Hz,
2H), 7.33 (t, J = 7.5 Hz, 1H), 7.23 (d, J = 7.2 Hz, 2H),
7.18 (d, J = 8.7 Hz, 4H).
N
N
N
OHC
CHO
OHC
CHO
(2)
(1)
S
N
2.4. 4,4ꢀ-(4-iodophenylazanediyl)Dibenzaldehyde (2)
4,4ꢀ-(Phenylazanediyl)dibenzaldehyde (1) (3.01 g,
10 mmol), KI (0.83 g, 5 mmol), and KIO3 (1.07 g,
5 mmol) dissolved in acetic acid with a small amount
of water was placed into a 3 neck 250 ml round bot-
tom flask. The mixture was then heated under reflux
for 4 h. The reaction mixture was cooled to room
temperature, and water was then added to receive the pre-
cipitate. The mixture was then filtered and the precipitate
was washed with water to obtain 4,4ꢀ-(4-iodophenyl-
azanediyl)dibenzaldehyde (4.06 g, 95%) as a light yellow
S
N
CN
CN
N
COOH
HOOC
TPAPC
N
CHO
OHC
S
N
(3)
S
S
N
S
N
S
N
COOH
HOOC
O
O
1
solid. H MNR (300 MHz, DMSO-d6ꢂ ꢃ 9.872 (s, 2H),
TPAPR
7.83 (d, J = 8.4 Hz, 4H), 7.76 (d, J = 8.4 Hz, 2H), 7.17
Figure 1. Synthesis procedure for TPAPC and TPAPR.
(d, J = 8.4 Hz, 4H), 6.98 (d, J = 8.4 Hz, 2H).
2.5. 4,4ꢀ-(4-(10H-phenothiazin-10-yl)
performance of DSSCs were investigated. Phenothiazine
and triphenylamine units were chosen as the donor candi-
dates, as shown in Figure 1.
phenylazanediyl)Dibenzaldehyde (3)
Compound (2) (4.27 g, 10 mmol), phenothiazine (5.98 g,
30 mmol), bronze copper (0.38 g, 6 mmol), 18-crown-6
(1 g, 3 mmol) and K2CO3 (3.3 g, 25 mmol) were
heated under reflux in 1,2-dichlorobenzen for 12 h.
Subsequently, the solution was cooled to room tem-
2. EXPERIMENTAL DETAILS
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2.1. Materials and Instrumentation
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All starting materials and solvents were purchased from
Aldrich-Sigma and used without further purification.
perature, washed with water, and dried with MgSO .
The product was isolated on a silica gel column
Copyright: American Scientific Publishers
4
1
The H-nuclear magnetic resonance (NMR) spectra were
with a mixture of 60% hexane:35% CH2Cl2:5% ethyl
acetate as the eluent to give 4,4ꢀ-(4-(10H-phenothiazin-10-
yl)phenylazanediyl)dibenzaldehyde (3) as a yellow solid
recorded on a Bruker Advance NMR 300 MHz spectrom-
eter. The UV-Vis spectra were recorded by Cary 5000 UV-
Vis-NIR Spectrophotometer. The cyclic voltammograms
were estimated with each dye (0.3 mM) and 0.1 mM tetra-
butyl amoniumtetrafluoroborate in a DMF solution, scan
rate of 50 mV/s, and three electrodes, Pt, Ag/AgCl, and
Pt wire as the working, reference, and counter electrode,
respectively.
1
(3.14 g, 63%). H MNR (300 MHz, DMSO-d6ꢂ ꢃ 9.902
(s, 2H), 7.88 (d, J = 8.7 Hz, 4H), 7.39 (d, J = 2.4 Hz,
4H), 7.29 (d, J = 9.6 Hz, 4H), 7.11 (d, J = 7.5 Hz, 2H),
7.02 (t, J = 8.5 Hz, 2H), 6.9 (t, J = 7.5 Hz, 2H), 6.42
(d, J = 8.1 Hz, 2H).
2.6. 3,3ꢀ-(4,4ꢀ-(4-(10H-phenothiazin-10-yl)
2.2. Synthesis Proceduce
Figure 1 presents the synthetic scheme for the three new
sensitizers.
phenylazanediyl)Bis(4,1-phenylene))
Bis(2-cyanoacrylic acid) (TPAPC)
An acetic acid solution of compound 3 (2.45 g, 5 mmol),
2-cyanoacetic acid (1.04 g, 12.5 mmol), and ammonium
acetate (0.95 g, 12.5 mmol) was heated under reflux for
10 h. After cooling the mixture to room temperature, the
solution was poured into the ice water to allow precipita-
tion. The mixture was filtered and washed with water and
then purified on a silica gel column using 95% CHCl3:4%
methanol:1% acetic acid as the eluent to obtain TPAPC as
a yellow solid (2.69 g, 85%). 1H MNR (300 MHz, DMSO-
d6ꢂ ꢃ 7.91 (s, 2H), 7.87 (d, J = 8.7 Hz, 4H), 7.35 (d, J =
2.4 Hz, 4H), 7.21 (d, J = 8.7 Hz, 4H), 7.1 (d, J = 7.5 Hz,
2H), 6.99 (t, J = 7.5 Hz, 2H), 6.9 (t, J = 7.5 Hz, 2H), 6.4
(d, J = 8.1 Hz, 2H).
2.3. 4,4ꢀ-(phenylazanediyl)Dibenzaldehyde (1)
Triphenylamine (2.45 g, 10 mmol) was dissolved in 1,2-
dichloroethane, and DMF (1.8 g, 25 mmol) was then
ꢁ
added. The solution was cooled to 0 C and POCl3 (3.8 g,
35 mmol) was added drop wise. After color changed to
red, the solution was heated under reflux in a nitrogen
atmosphere for 6 h. The mixture was diluted with CH2Cl2,
rinsed with water and dried with MgSO4 before being
filtered through a silica gel column with a mixture of
70% hexane: 30% CH2Cl2 as the eluent to give 4,4ꢀ-
(phenylazanediyl)dibenzaldehyde as a light yellow solid
3182
J. Nanosci. Nanotechnol. 17, 3181–3187, 2017

Hai et al.
Effect of Different Anchoring Groups in Phenothiazine-Triphenylamine-Based Di-Anchoring Dyes for DSSCs
2.7. 2,2ꢀ-(5Z,5ꢀZ)-5,5ꢀ-(4,4ꢀ-(4-(10H-phenothiazin-10-yl)
phenylazanediyl)Bis(4,1-phenylene))Bis(methan-
1-yl-1-ylidene)Bis(4-oxo-2-thioxothiazolidin-3-yl-
5-ylidene) Diacetic Acid (TPAPR)
spectroscopy (EIS) was performed using a computer-
controlled potentiostat (IVIUMSTAT, IVIUM) at the open
circuit voltage with a 10 mV amplitude and an AC fre-
quency range between 100 kHz and 0.1 Hz (PEC-L11,
Peccell Technologies, Inc.). The electron diffusion coeffi-
cient and electron lifetime of the DSSCs were measured
by the stepped light-induced transient measurements of
the photocurrent and voltage (SLIM-PCV) using a diode
laser (Helium-Neon laser power supply, Thor Lab, ꢆ =
632.8 nm). The current transient was monitored using a
digital phosphor oscilloscope (Tektronic DPO 4102B-L)
through a current amplifier.
An acetic acid solution of compound 3 (2.45 g, 5 mmol),
rhodanine-3-acetic acid (2.35 g, 12.5 mmol), and ammo-
nium acetate (0.95 g, 12.5 mmol) was heated under reflux
for 10 h. After cooling the mixture to room temperature,
the solution was poured into the ice water to form the pre-
cipitate. The mixture was filtered and washed with water,
and then purified on a silica gel column using a mixture of
95% CHCl3:4% methanol:1% acetic acid as the eluent to
obtain TPAPR as an orange solid (3.46 g, 82%). 1H MNR
(300 MHz, DMSO-d6ꢂ ꢃ 7.84 (s, 2H), 7.68 (d, J = 8.7 Hz,
4H), 7.41 (d, J = 2.4 Hz, 4H), 7.30 (d, J = 8.7 Hz, 4H),
7.11 (d, J = 7.5 Hz, 2H), 7.03 (t, J = 7.5 Hz, 2H), 6.91
(t, J = 7.5 Hz, 2H), 6.41 (d, J = 8.1 Hz, 2H).
3. RESULTS AND DISCUSSION
3.1. Optical and Electrochemical Properties
Figures 2(a and b) shows the UV-Vis absorption spectra
of TPAPC and TPAPR in a DMF solution and on TiO2
films, respectively. TPAPC in a DMF solution exhibits an
absorption band in the range, 300–500 nm, with a molar
2.8. Solar Cell Fabrication
Fluorine-doped tin oxide (FTO) transparent conducting
glass substrates (Pilkington, 15 ꢄ/cm2ꢂ were cleaned with
methanol, D.I water, and acetone. Transparent TiO2 paste
(20–30 nm in diameter, Dyesol Ltd.) was coated on the
cleaned FTO glasses (Pilkington, 15 ꢄ/cm2ꢂ using the
extinction coefficient (ꢇꢂ of 29,800 M−1 cm−1 at ꢆmax
=
405 nm (Table I). TPAPR exhibits two major promi-
nent bands at 300–390 nm and 390–550 nm with molar
maximum extinction coefficients of 50, 400 M−1cm−1 at
ꢆmax = 405 nm. The absorption peak at approximately
300–390 nm of the TPAPR solution can be assigned to
a ꢀ–ꢀ∗ transition and the absorption peak around 390–
550 nm is assigned to intramolecular charge transfer (ICT)
ꢁ
doctor blade technique, followed by sintering at 450 C
for 30 min. A TiO2 particle scattering layer (200 nm in
diameter, Dyesol Ltd.) was deposited on the transparent
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ꢁ
nano-porous TiO2 films, followed by sintering at 450 C
IP: 188.72.127.123 On: Mon, 06 Mar 2017 08:55:58
for 30 min. Two layers of TiO2 films were treated with
Copyright: American Scientific Publishers
ꢁ
a 40 mM TiCl4 aqueous solution at 70 C for 30 min
ꢁ
1.0
0.8
0.6
0.4
0.2
0.0
(A)
and then sintered at 450 C for 30 min. After cooling
to 100 ꢁC, the TiO2 films were immersed in 0.3 mM
dye solutions at 25 ꢁC for 24 h in the dark and the
residual dye was rinsed off with acetonitrile to give the
working electrode. A platinum paste were deposited on
the FTO glasses using the doctor blade technique, fol-
lowed by sintering at 450 ꢁC for 30 min to give the
counter electrodes. The working electrode and Pt counter
electrodes were assembled into a sealed sandwich cell
with a 60 ꢅm thick Surlyn film (Dupont), and the cell
was filled with an electrolyte solution through pre-drilled
holes on the Pt counter electrode. The electrolyte solution
consists of 1-butyl-3-methylimidazolium iodide (0.7 M),
lithium iodide (LiI, 0.2 M), iodine (I2, 0.05 M), and
t-butylpyridine (TBP, 0.5 M) in acetonitrile/valeronitrile
(85:15, v/v).
TPAPC
TPAPR
300
400
500
600
700
Wavelength (nm)
(B)
4
TPAPC
TPAPR
3
2
1
0
2.9. Photovoltaic Characterization
The photocurrent density–voltage (J–V ) characteristics
of the prepared DSSCs were measured under AM 1.5
irradiation with an incident power of 100 mW/cm2
(PEC-L11, Peccell Technologies, Inc.). The incident
monochromatic photon-to-current efficiencies (IPCEs)
were recorded as a function of the light wavelength
using an IPCE measurement instrument (PEC-S20,
Peccell Technologies, Inc.). Electrochemical impedance
400
450
500
550
600
650
700
Wavelength (nm)
Figure 2. Absorption spectra of TPAPC and TPAPR in a DMF solu-
tion (A) and on TiO2 film (B).
J. Nanosci. Nanotechnol. 17, 3181–3187, 2017
3183

Effect of Different Anchoring Groups in Phenothiazine-Triphenylamine-Based Di-Anchoring Dyes for DSSCs
Hai et al.
Table I. Optical and electrochemical properties of the TPAPC and
TPAPR sensitizers.
LUMO= HOMO−E0−0. As shown in Table I, the HOMO
level of TPAPC and TPAPR are −5.16 eV −5.15 eV,
respectively, which are more positive than the redox
potential of I−/I3− (−4.80 eV),14 making sufficient driv-
ing force for the dye regeneration reaction. The LUMO
level of TPAPC and TPAPR are −2.64 eV and −2.82 eV,
respectively which are more negative than the conduc-
tion band of TiO2 (−4.2 eV), ensuring that the two
dyes can inject electrons efficiently from the excited dye
molecules to the conduction band of TiO2. The LUMO
level of TPAPC is more negative than that of TPAPR,
indicating the easier electron transfer from the LUMO
level of TPAPC to the conduction band of TiO2 than
TPAPR.3
ꢇmax
(M−1cm−1
ꢆmax (nm)
solution (TiO2ꢂ E0−0 (eV)
HOMO LUMO
Dye
ꢂ
(eV)
(eV)
TPAPC
TPAPR
29800
50400
405 (428)
476 (478)
2.51
2.32
−5ꢈ16
−5ꢈ15
−2ꢈ64
−3ꢈ82
between the phenothiazine-triphenylamine donor group
and the rhodanine-3-acetic acid acceptor group.10 The
absorption band of TPAPR with rhodanine-3-acetic acid
as the acceptor in solution shows a red shift compared
to that of TPAPC with cyanoacetic acid as the acceptor,
which is attributed to the extended ꢀ-conjugation system
through the 4-oxo-2-thioxothiazolidine ring, leading to a
higher electron withdrawal strength of rhodanine-3-acetic
acid compared to cyanoacetic acid. In particular, the high
molar extinction coefficient of both dyes indicate the good
properties for light-harvesting in DSSCs. Based on the
absorption spectra of the TPAPC and TPAPR solution, the
zeroth–zeroth transition energies (E0−0ꢂ (band gap) were
estimated to be 2.51 eV and 2.32 eV, respectively.20ꢁ21
Compared to the absorption spectrum of dyes in solu-
tion, the adsorbed-TPAPC on the TiO2 film exhibited
large red shifts (ꢉꢆmax ≈ 23 nm), while TPAPR shows
a small red shift (ꢉꢆmax ≈ 2 nm), which indicates that
3.2. Photovoltaic Properties
The photovoltaic performance was investigated by the cur-
rent density–voltage (J–V ) characteristics of the DSSCs
employing TPAPC and TPAPR as the sensitizer under
standard global 1.5 AM solar light, 100 mW/cm2
(Fig. 4(a)); Table II lists the extracted parameters.
The overall conversion efficiency (ꢊꢂ of a DSSC is
determined by the short circuit current density (JSCꢂ,
open circuit voltage (VOCꢂ, fill factor (FF ) of the
cell, and intensity of incident light (Pin).1 The FF for
DSSC is defined as the ratio of the actual maximum
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TPAPC tends to J-aggregate on the electrode surface
IP: 188.72.127.123 On: Mon, 06 Mar 2017 08:55:58
more than TPAPR. Because of the large size, introduc-
ing the rhodanine-3-acetic acid unit as an acceptor in
place of cyanoacetic acid leads to a better anti-aggregation
effect.
(A)
Copyright: American Scientific Publishers
TPAPC
TPAPR
6
4
2
0
To investigate the molecular orbital energy levels of
TPAPC and TPAPR, cyclic voltammetry (CV) was con-
ducted in a dimethylformamide (DMF) solution using
0.1 M tetrabutyl amoniumtetrafluoroborate as the support-
ing electrolyte (Fig. 3). The highest occupied molecu-
lar orbital (HOMO) is estimated from its first oxidation
potential (Eoxꢂ and the lowest unoccupied molecular
orbital (LUMO) is calculated from the expression of
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Potential (V)
70
60
50
40
30
20
10
0
(B)
TPAPC
TPAPR
TPAPC
TPAPR
1.5
1.0
0.5
0.0
300
400
500
600
700
800
–0.5
–0.4 –0.2 0.0
0.2
0.4
0.6
0.8
1.0
Wavelength (nm)
Potential (V)
Figure 4. J–V characteristics (A) and IPCE (B) spectra of the DSSCs
containing TPAPC and TPAPR as a sensitizer under simulated standard
solar light illumination (AM 1.5, 100 mW/cm2ꢂ.
Figure 3. Cyclic voltammograms of TPAPC and TPAPR dissolved in
DMF.
3184
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Hai et al.
Effect of Different Anchoring Groups in Phenothiazine-Triphenylamine-Based Di-Anchoring Dyes for DSSCs
Table II. Photovoltaic parameters of the TPAPC and TPAPR sensitized
DSSCs.
(A) –100
–80
TPAPC
TPAPR
Dye
JSC (×10 A/m2ꢂ
VOC (V)
FF (%)
ꢊ (%)
–60
TPAPC
TPAPR
5.76
2.73
0.675
0.558
68.53
73.46
2.66
1.12
–40
obtainable power (JMP × VMPꢂ to the product of the VOC
and JSC.
–20
0
ꢊ = ꢋJSC ꢌmA cm−2ꢂ×VOC (V)×FF ꢌ%ꢂꢍ
0
10 20 30 40 50 60 70 80 90 100
Z' (Ohm)
/Pin (mW cm−2
ꢂ
(1a)
(1b)
(B) –100
–80
FF = JMP ×VMP/JSC ×VOC
TPAPC
TPAPR
When the TPAPR was used as sensitizer, the solar cell
produced a JSC of 2.73 mA/cm2, VOC of 0.558 V, and FF
of 73.46%, corresponding to a ꢊ of 1.12%. Under the
same conditions, the TPAPC sensitized solar cell showed
higher efficiency (ꢊ of 2.66%) which is attributed to the
higher JSC (5.76 mA/cm2ꢂ and VOC (0.675 V). Figure 4(b)
shows incident photon-to-electron conversion efficiencies
(IPCE) as a function of the wavelength of the DSSCs using
TPAPC and TPAPR as sensitizers in the visible region,
300–800 nm. The IPCE at each incident wavelength was
calculated from Eq. (2), where JSC is the short circuit
current density under monochromatic irradiation, ꢆ is the
wavelength, and ꢎ is the power of the incident radiation
–60
–40
–20
0
0
10 20 30 40 50 60 70 80 90 100
Z' (Ohm)
Figure 5. Electrochemical impedance spectroscopy (EIS) of the DSSCs
under light illumination (AM 1.5, 100 mW/cm2ꢂ (A) and in the dark
Delivered by Ingenta to: State Univecrosnitdyitioonf uNnedewr bYiaos rvkoltaagteB−in0.g7hVa(mB)t.oTnhe insets show the equivalent
per unit area.22
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Copyright: American SecleiectnrotcifhiecmPicualbcliirschuiet rosf the electrochemical system.
IPCE = ꢋ1240 (eV nm)×JSC (mA/cm−2
ꢂ
Electrochemical impedance spectroscopy (EIS) and
stepped-light induced transient measurements (SLIMs)
were conducted to better understand electron injec-
tion and electron transport within the working electrode
DSSCs using two dyes, TPAPC and TPAPR. Figure 5(a)
presents the charge transfer impedance and equivalent
electrochemical circuit of the electrochemical system,
which was measured under 1.5 AM solar light condi-
tions with a frequency range of 0.1 Hz to 100 kHz.
The large semicircle was assigned to the charge transfer
impedance (Rctꢂ at the interface of the TiO2 sensitizer and
electrolyte. The Rct (Table III) of the TPAPC sensitized
solar cell is 28.7 ꢄ, which is much lower than that of
TPAPR (59.3 ꢄꢂ, indicating that stronger electronic cou-
pling between the TPAPC dye and TiO2 than TPAPR leads
to higher electron injection efficiency (ꢐinjꢂ of TPAPC
compared to TPAPR. The stepped-light induced transient
measurements (SLIMs) were carried out at different light
intensities using an on/off laser beam. Under each light
/ꢆ (nm)×ꢎ (mW cm−2
ꢂ
(2)
The TPAPC was active in the wavelength region, 300–
550 nm, with the maximum IPCE value of 60% at 440 nm
while the TPAPR responses were observed in the range,
300–600 nm, with the maximum IPCE of 16% at 460 nm.
As mentioned in the optical properties section, the J-type
aggregation effect of sensitizers on the TiO2 surface results
in red-shifts in the spectral range of IPCE compared to
the absorption band in solution in both cases of TPAPC
and TPAPR. The higher IPCE of TPAPC than TPAPR sen-
sitized solar cell shows correspondence with the higher
produced JSC of TPAPC compared to TPAPR. The IPCE
is also given by Eq. (3), where LHE is the light harvesting
efficiency of the photo electrode (LHE= 1–10−ꢏd, where
ꢏ is given by the product of the optical absorption cross
section and concentration in the meso-porous film of the
dye, and d is the film thickness), ꢐinj is the quantum yield
of electron injection, and ꢐcoll is the electron collection
efficiency.22
Table III. Electrochemical impedance and SLIM-PCV parameters of
the TPAPC and TPAPR sensitized DSSCs.
IPCE = LHE×ꢐinj ×ꢐcoll
(3)
Dye
Rct (ꢄꢂ
Rrec (ꢄꢂ
Ln (×10−6 m)
Equation (3) shows that JSC depends not only on the LHE,
but also on electron injection and electron transport within
the working electrode.
TPAPC
TPAPR
28.7
59.3
64.3
14.0
12.24
10.37
J. Nanosci. Nanotechnol. 17, 3181–3187, 2017
3185

Effect of Different Anchoring Groups in Phenothiazine-Triphenylamine-Based Di-Anchoring Dyes for DSSCs
Hai et al.
(A) 2.5
the EIS were conducted under dark conditions with a for-
ward bias of −0.7 V in the frequency range, 0.1 Hz to
100 kHz (Fig. 5(b)). The impedance spectra were fitted
with the equivalent circuit (Fig. 5(b)) to extract the recom-
bination resistance (Rrecꢂ (Table III) between the injected
electron in TiO2 nanoparticle and oxidized species (I−3 ꢂ
in the electrolyte and the results show a much larger
Rrec in the case of DSSCs using TPAPC (64.3 ꢄꢂ than
DSSCs using TPAPR (14.0 ꢄꢂ. The larger Rrec value indi-
cates the reduced potential lost in the solar cells, which
results in a higher VOC value for TPAPC. These results
show that cyanoacetic acid has stronger electronic coupling
with TiO2 and a lower recombination rate compared to
the rhodanine-3-acetic acid acceptor group, which resulted
from the broken conjugation due to the methylene group in
the rhodanine-3-acetic acid;1 thus, the solar cell based on
TPAPC shows better photovoltaic performance.
TPAPC
TPAPR
2.0
1.5
1.0
0.5
0.0
0.3
0.6
0.9
1.2
1.5
1.8
Current density (×10 A/m2)
(B) 0.20
0.16
TPAPC
TPAPR
0.12
4. CONCLUSION
0.08
Phenothiazine-triphenylamine dyes (TPAPC and TPAPR)
were designed and synthesized, and the effects of differ-
ent acceptor groups on the optical, electrochemical, and
photovoltaic properties were studied. Under illumination
conditions, the DSSCs based on TPAPC showed an overall
conversion efficiency of 2.66% compared to 1.12% for the
DSSCs based on TPAPR. The conversion efficiency was
0.04
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Current density (×10 A/m2)
Figure 6. Electron diffusion coefficient versus photo current den-
sity (A) and electron lifetime versus the photo current density (B) for
DSSCs using TPAPC and TPAPR as sensitizers.
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improved significantly by replacing rhodanine-3-acetic
IP: 188.72.127.123 On: Mon, 06 Mar 2017 08:55:58
acid with cyanoacetic acid, which can be attributed to
the higher JSC and VOC of the TPAPC device. The rea-
son for this is the cyanoacetic acid anchoring group on
TiO2 with the coordinated triphenylamine-phenothiazine
sensitizer showed relatively efficient electron injection, as
evidenced by the electrochemical impedance and stepped
light induced measurement analysis. The results suggest
that cyanoacetic acid acceptor results in DSSCs with bet-
ter properties than the rhodanine-3-acetic acid acceptor in
phenothiazine-triphenylamine dyes.
Copyright: American Scientific Publishers
intensity condition, the laser beam was controlled on/off
to record the decay in the photocurrent density and voltage
as a function of time.23ꢁ24
Figure 6(a) presents the relationship between the diffu-
sion coefficient (Dnꢂ and current density of the TPAPC and
TPAPR sensitized solar cells. The TPAPC devices show
the higher current density and diffusion coefficient under
the same light intensity conditions compared to TPAPR
device. Figure 6(b) shows that TPAPC has a longer elec-
tron lifetime than TPAPR. The higher Dn and the longer
electron lifetime (ꢑeꢂ of the DSSCs using TPAPC as a sen-
sitizer indicate faster electron transport and less electron
recombination, respectively, which result a higher pho-
tocurrent density and photo-voltage. In addition, the elec-
tron diffusion length, Ln, the overall electron collection
efficiency (ꢐcollꢂ, and the electron recombination time were
calculated using Eq. (4),23ꢁ24 which shows that the average
diffusion length of the TPAPC device is 12.24 ꢅm higher
than that of TPAPR 10.37 ꢅm, and the average diffusion
length in both cases is higher than the thickness of the
photo electrode, 7.5 ꢅm (Table III).
Acknowledgment: This study was supported by the
“New and Renewable Energy Core Technology Program”
of the Korea Institute of Energy Technology Evaluation
and Planning (KETEP) granted financial resource from the
Ministry of Trade, Industry and Energy, Republic of Korea
(No. 20133010011750). This study was also supported by
“Human Resources Program in Energy Technology” of the
Korea Institute of Energy Technology Evaluation and Plan-
ning (KETEP), granted financial resource from the Min-
istry of Trade, Industry and Energy, Republic of Korea
(No. 20154030200760).
Ln = ꢌDnꢑeꢂ1/2
These results show good agreement with the much higher
photo current density of the DSSCs employing TPAPC than
that of TPAPR. To determine the reason for the improved
VOC of the TPAPC device compared to the TPAPR device,
(4)
References and Notes
1. A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, and H. Pettersson, Chem.
Rev. 110, 6595 (2010).
2. M. I. Abdullah, M. R. S. A. Janjua, A. Mahmood, S. Ali, and M. Ali,
Bull. Korean Chem. Soc. 34, 2093 (2013).
3186
J. Nanosci. Nanotechnol. 17, 3181–3187, 2017

Hai et al.
Effect of Different Anchoring Groups in Phenothiazine-Triphenylamine-Based Di-Anchoring Dyes for DSSCs
3. S. Agrawal, M. Pastore, G. Marotta, M. A. Reddy,
M. Chandrasekharam, and F. D. Angelis, ACS Appl. Mater. Interface
5, 12469 (2013).
4. S. Hwang, J. H. Lee, C. Park, H. Lee, C. Kim, C. Park, M.-H. Lee,
W. Lee, J. Park, K. Kim, N.-G. Park, and C. Kim, Chem. Commun.
46, 4887 (2007).
5. J. Song and J. Xu, Bull. Korean Chem. Soc. 34, 3211 (2013).
6. W. Wu, J. Yang, J. Hua, J. Tang, L. Zhang, Y. Long, and H. Tian,
J. Mater. Chem. 20, 1772 (2010).
14. Z. Wan, C. Jia, Y. Duan, L. Zhou, J. Zhang, Y. Lin, and Y. Shi, RSC
Advances 2, 4507 (2012).
15. A. Shafiee, M. M. Salleh, and M. Yahaya, Sains Malaysiana 40, 173
(2011).
16. Y. S. Yang, H. D. Kim, J.-H. Ryu, K. K. Kim, S. S. Park, K.-S. Ahn,
and J. H. Kim, Synthetic Metals 161, 850 (2011).
17. X. Chen, C. G. Jia, Z. Wan, J. Zhang, and X. Yao, Spectrochimica
Acta Part A: Molecular and Biomolecular Spectroscopy 123, 282
(2014).
7. S. Nakade, T. Kanzaki, W. Kubo, T. Kitamura, Y. Wada, and
S. Yanagida, J. Phys. Chem. B 109, 3480 (2005).
18. J. H. Park, B. Y. Jang, S. Thogiti, J.-H. Ryu, S.-H. Kim, Y.-A. Son,
and J. H. Kim, S. Met. 203, 235 (2015).
8. T. Marinado, D. P. Hagberg, M. Hedlund, T. Edvinsson, E. M. J.
Johansson, G. Boschloo, H. Rensmo, T. Brinck, L. Sun, and
A. Hagfeldty, Phys. Chem. Chem. Phys. 11, 133 (2009).
9. Z. Wan, C. Jia, Y. Duan, J. Zhang, Y. Lin, and Y. Shi, Dyes and
Pigments 94, 150 (2012).
19. T. Suresh, R. K. Chitumalla, N. T. Hai, J. Jang, T. J. Lee, and J. H.
Kim, RSC Adv. 6, 26559 (2016).
20. P. Wang, C. Klein, R. Humphry-Baker, S. M. Zakeeruddin, and
M. Grätzel, J. Am. Chem. Soc. 127, 808 (2005).
21. D. Kuang, C. Klein, S. Ito, J.-E. Moser, R. Humphry-Baker,
S. M. Zakeeruddin, and M. Grätzel, Adv. Funct. Mater. 17, 154
(2007).
10. H. Shang, Y. Luo, X. Guo, X. Huang, X. Zhan, K. Jiang, and
Q. Meng, Dyes and Pigments 87, 249 (2010).
11. Z. Wan, C. Jia, Y. Duan, L. Zhou, Y. Lin, and Y. Shi, J. Mater.
Chem. 22, 25140 (2012).
22. J. Li, W. Xu, and Y. Shi, Transaction On Control And Mechanical
Systems 1, 235 (2012).
12. M. Liang, W. Xu, F. Cai, P. Chen, B. Peng, J. Chen, and Z. Li,
J. Phys. Chem. C 111, 4465 (2007).
23. S. Nakade, T. Kanzaki, Y. Wada, and S. Yanagida, Langmuir
21, 10803 (2005).
13. Z. Q. Wan, C. Y. Jia, J. Q. Zhang, Y. D. Duan, Y. Lin, and Y. Shi,
Journal of Power Sources 199, 426 (2012).
24. K.-S. Ahn, M.-S. Kang, J.-K. Lee, B.-C. Shin, and J.-W. Lee. Appl.
Phys. Lett. 89, 013103-1 (2006).
Received: 12 January 2016. Accepted: 12 September 2016.
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Copyright: American Scientific Publishers
J. Nanosci. Nanotechnol. 17, 3181–3187, 2017
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