Molecular design and photovoltaic performance of organic dyes containing phenothiazine for dye-sensitized solar cells

Article abstract of DOI:10.1166/jnn.2014.9450

We synthesized novel organic photosensitizers based on fluorine-substituted phenothiazine with thiophene bridge units in the chromophore for application in dye-sensitized solar cells (DSSCs). Furthermore, organic dyes with different acceptors exhibited higher molar extinction coefficients, and better light absorption at longer wavelengths. The photovoltaic properties of organic dyes composed of different acceptors in their chromophores were measured to identify their effects on the DSSC performance. The organic dye, PFSCN2 containing multi-cyanoacrylic acid as the electron acceptor, showed a power conversion efficiency of 4.67% under AM 1.5 illumination (100 mW/cm²). The retarded recombination kinetics from TiO₂ electrode to electrolyte enhanced the electron life time of the organic dye, PFSCN2 in the photoanode of the DSSC. This was confirmed with impedance analysis.

Full text of DOI:10.1166/jnn.2014.9450

Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 7938–7942, 2014
Copyright © 2014 American Scientific Publishers
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Printed in the United States of America
Molecular Design and Photovoltaic Performance
of Organic Dyes Containing Phenothiazine
for Dye-Sensitized Solar Cells
Hyo Jeong Jo1, Jung Eun Nam1, Kyoseung Sim1, Dae-Hwan Kim1,
Jae Hong Kim2, and Jin-Kyu Kang1ꢀ∗
1Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu, 711-873, Korea
2Department of Display and Chemical Engineering, Yeungnam University, Gyeongbuk-Do, 712-749, Korea
We synthesized novel organic photosensitizers based on fluorine-substituted phenothiazine with
thiophene bridge units in the chromophore for application in dye-sensitized solar cells (DSSCs).
Furthermore, organic dyes with different acceptors exhibited higher molar extinction coefficients, and
better light absorption at longer wavelengths. The photovoltaic properties of organic dyes composed
of different acceptors in their chromophores were measured to identify their effects on the DSSC
performance. The organic dye, PFSCN2 containing multi-cyanoacrylic acid as the electron acceptor,
showed a power conversion efficiency of 4.67% under AM 1.5 illumination (100 mW/cm2ꢀ. The
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retarded recombination kinetics from TiO2 electrode to electrolyte enhanced the electron life time
IP: 117.243.181.251 On: Tue, 03 Nov 2015 15:48:46
of the organic dye, PFSCN2 in the photoanode of the DSSC. This was confirmed with impedance
Copyright: American Scientific Publishers
analysis.
Keywords: Dye-Sensitized Solar Cell, Organic Dyes, Fluorine-Substituted Phenothiazine Dyes,
Photovoltaic Performance.
1. INTRODUCTION
In spite of their low conversion efficiency in DSSCs,
organic dyes have been employed recently as promis-
ing candidates for the photosensitizers because of their
significant advantages (e.g., high molar extinction coef-
ficient, convenient synthetic process, customized molecu-
lar design), interesting photophysical and photochemical
properties, and low production-cost.8–15 To maximize the
efficiency of DSSCs, the photosensitizers should diminish
the charge recombination between the injected electrons
in TiO2 and the dye cations ꢁdye+ꢂ−, this recombination
is considered to be one of the factors decreasing the pho-
tovoltaic performance of DSSCs based on organic dyes.
In addition to increase the photon-to-current efficiency
(PCE), the organic photosensitizer should also absorb a
broad range of wavelengths of light in the solar spec-
trum. In this work, we have designed and synthesized a
fluorine-substituted phenothiazine, which can be controlled
by the modification of the molecular structure of the dye to
increase the electron injection efficiency between electron-
withdrawing group (−CF3) present in the donor and elec-
trolyte. We also introduced a different anchoring group in
Renewable energy sources have been considered as one
of the most significant research topics. Solar energy is a
promising candidate as it is renewable, clean, and inex-
haustible. Improvement in the solar-energy-to-electricity
conversion efficiency continues to be an important research
area for various kinds of solar cells, including silicon-
based solar cells, organic solar cells and dye-sensitized
solar cells (DSSCs).1ꢀ2 A typical DSSC is constructed with
a dye-absorbed wide-band-gap semiconductor electrode
such as TiO2 or ZnO, an electrolyte containing I−/I3−
redox couples, and a Pt-coated counter electrode.3–7 The
photo-sensitizing dye plays an important role in captur-
ing the photons, generating the electron and hole pairs,
and transferring them to the interface of the semiconductor
and the electrolyte, respectively. The most successful pho-
tosensitizers for DSSCs are ruthenium polypyridyl com-
plexes (Ru-complex), which yield solar-to-electric power
conversion efficiencies of 9–11% under AM 1.5.
∗Author to whom correspondence should be addressed.
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doi:10.1166/jnn.2014.9450

Jo et al. Molecular Design and Photovoltaic Performance of Organic Dyes Containing Phenothiazine for Dye-Sensitized Solar Cells
a molecule in the dye materials with high molar extinction
coefficient to absorb a wide range of visible light.
2.2.1. Synthesis of PFSCN2
The product obtained was a dark red powder. Yield:
1
(1.3 g, 56.7%), H NMR (300 MHz, DMSO-d6ꢂ: ꢃ 8.33–
8.31 (m, 3H), 8.21 (s, 2H), 7.81 (d, J = 3ꢄ3 Hz, 2H),
7.62–7.59 (m, 4H), 7.39 (d, J = 8ꢄ1 Hz, 2H), 6.29–
6.26 (d, J = 8ꢄ4 Hz, 2H), MALDI-TOF MS: Calcd. for
C36H17F6N3O4S3 m/z:765.03, Found m/z: 764.82 ꢅM +
Hꢆ+. Anal. calcd. for C: 56.47; N: 5.49; S: 12.56; H: 2.24;
Found, C: 56.37; N: 6.02; S: 11.49; H: 3.23%.
2. EXPERIMENTAL DETAILS
2.1. Instrumental Analysis
All commercially available starting materials and solvents
were purchased from Aldrich, TCI, and Alfa aesar and
were used without further purification, unless otherwise
1
stated. Structural analyses were conducted with H NMR
spectra were recorded on Bruker Advance NMR 300
or 400 Hz spectrometer using CDCl3 and DMSO-d6.
MARDI-TOF mass spectra were obtained on a Voyager
DE-STR spectrometer. Elemental analyses were performed
by the center for organic reactions using an EA1112
(Thermo Electron Corp.) elemental analyzer. UV-Vis spec-
tra were recorded on a CARY5000 UV/Vis/NIR. The
redox properties were examined using cyclic voltammetry
(Model: Ivium state). The electrolyte solution was 0.1 M
tetra-butyl ammonium hexafluorophosphate (TBAPF6) in
freshly dried dimethylformamide (DMF). The Ag/AgCl
and Pt wire (0.5 mm in diameter) electrodes were used as
reference and counter electrodes, respectively.
2.2.2. Synthesis of PFSRO2
PFSRO2 was synthesized using a procedure similar to the
one that was used for PFSCN2. The product obtained was
a dark powder. Yield: (0.5 g, 72.4%), 1H NMR (400 MHz,
DMSO-d6ꢂ: ꢃ 8.35–8.33 (m, 3H), 8.11(s, 2H), 7.94 (d,
J = 4ꢄ0 Hz, 2H), 7.71 (d, J = 4ꢄ0 Hz, 2H), 7.66 (s, 2H),
7.44–7.42 (d, J = 8ꢄ4 Hz, 2H), 6.23–6.21 (d, J = 8ꢄ8 Hz,
2H), 4.66–4.65 (s, 4H), MALDI-TOF MS: Calcd. for
C40H21F6N3O6S7 m/z: 976.9, Found m/z: 976.6 ꢅM+Hꢆ+.
Anal. calcd. for C: 49.12; N: 4.30; S: 22.95; H: 2.16;
Found, C: 50.47; N: 3.04; S: 21.65; H: 3.04%.
2.2.3. Synthesis of PFSOX2
PFSOX2 was synthesized using a procedure similar to the
one that was used for PFSCN2. The product obtained was
a dark purple powder. Yield: (0.25 g, 78%), 1H NMR
2.2. Synthesis
We designed and synthesized a series of fluorine
substituted phenothiazine derivatives containing multi-
acceptors/anchors in a molecule with moderate yields, as
shown in the experimental section and Scheme 1. The
resultant multi-anchored organic dyes PFSCN2, PFSRO2,
and PFSOX2 contain different acceptors in their chro-
mophores. The synthetic procedures were followed using
known methods.16ꢀ17
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(400 MHz, DMSO-d6ꢂ: ꢃ 8.84 (s, 2H), 8.44–8.40 (m,
IP: 117.243.181.251 On: Tue, 03 Nov 2015 15:48:46
3H), 8.27 (d, J = 3ꢄ6 Hz, 2H), 7.94 (d, J = 7ꢄ6 Hz, 4H),
Copyright: American Scientific Publishers
7.81 (d, J = 4ꢄ4 Hz, 2H), 7.69 (s, 2H) 7.53–7.49 (m, 6H),
7.30 (t, 2H), 6.22–6.20 (d, J = 8ꢄ8 Hz, 2H), MALDI-TOF
MS: Calcd. for C50H27F6N5O6S3 m/z:1003.96, Found m/z:
1003.79 ꢅM + Hꢆ+. Anal. calcd. for C: 59.82; N: 6.98;
F3C
CF3
CF3
F3C
CF3
CF3
F3C
F3C
H
N
N
S
N
S
N
S
N
S
S
(a)
(b)
(c)
BR
(d)
H
O
S
H
O
S
S
N
S
BR
HOOC
S
O
O
COOH
NC
N
N
S
COOH
(e)
CF3
F3C
F3C
CF3
F3C
CF3
N
N
S
N
S
S
S
S
S
S
COOH
S
HOOC
S
S
S
O
O
HOOC
S
COOH
O
O
N
S
N
N
N
N
N
NC
CN
HOOC
COOH
PFSOX2
PFSCN2
PFSRO2
Scheme 1. (a) 1-iodo-3, 5-bis(trifluoromethyl)benzene, P-(t-Bu)3, NaO-tBu, Pd2(dba)3, toluene (b) Br2, acetic acid (c) Sn(Bu)3, Pd(PPh3ꢂ2Cl2, THF
Synthesis of organic dyes containing multi-anchors/acceptors. (d) POCl3, DMF, 1, 2-dichloroethane (e) Piperidine, acetonitrile.
J. Nanosci. Nanotechnol. 14, 7938–7942, 2014
7939

Molecular Design and Photovoltaic Performance of Organic Dyes Containing Phenothiazine for Dye-Sensitized Solar Cells Jo et al.
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(a)
S: 9.58; H: 2.71; Found, C: 57.84; N: 6.30; S: 9.29;
H: 3.01%.
PFSCN2
PFSRO2
PFSOX2
2.3. Fabrication of DSSC
The TiO2 paste was coated on the pre-cleaned FTO
(TEC8, Pilkington, 8 ꢇ/cm2, Thickness of 2.3 mm) coated
glass substrate using the doctor-blade coating method, and
ꢀ
sintered at 500 C for 30 min. The other TiO2 paste was
re-coated over the sintered layer using TiO2 particles of
ca. 400 nm size as scattering layer, and then sintered
again at 500 ꢀC for 1 h. The prepared TiO2 film was
ꢀ
dipped in 0.04 M of TiCl4 aqueous solution at 70 C for
350 400 450 500 550 600 650 700 750 800
Wavelength(nm)
30 min. For dye adsorption, the annealed TiO2 electrodes
were immersed in a dye solution (0.3 mM of dye in
THF/Ethanol (1:9, v/v) PFS series) at RT for 24 h. The
dye-adsorbed TiO2 electrode and Pt counter electrode were
assembled using a 60 ꢈm-thick Surlyn (Dupont, 1702) as a
bonding agent. A liquid electrolyte was introduced through
a pre-punctured hole on the counter electrode. The active
areas of the dye-adsorbed TiO2 films were estimated using
a digital microscope camera with image-analysis-software
(Moticam 1000). The photovoltaic I–V characteristics of
the prepared DSSCs were measured under 1 sunlight
intensity (100 mW/cm2, AM 1.5), which was verified
using a standard Si-solar cell (Keithley 2400, ORIEL,
Newport, PV measure ment Inc.). The incident monochro-
matic photon-to-current efficiencies (IPCEs) were plotted
1.5
1.0
0.5
0.0
(b)
PFSCN2
PFSRO2
PFSOX2
350 400 450 500 550 600 650 700 750 800
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Wavelength(nm)
as a function of the wavelength of light using an IPCE
measurement instrument. (PEC-S20, Peccell Technologies,
Inc.).
IP: 117.243.181.251 On: Tue, 03 Nov 2015 15:48:46
Copyright: American Scientific Publishers
Figure 1. (a) Normalized absorption spectra of the organic dyes in the
DMF solution. (b) Normalized absorption spectra of the organic dyes that
were adsorbed onto the TiO2 films.
3. RESULTS AND DISCUSSION
3.1. Optical and Electrochemical Properties
Figure 1 shows the UV-Vis absorption spectra of the syn-
thesized dyes in DMF solution (a) and on the TiO2 film
state (b). The optical and electrochemical properties of the
organic dyes are summarized in Table I.
should be more positive than the redox potential of I−/I3−
in the electrolyte, and the LUMO level must be more neg-
ative than that of the conduction band edge in TiO2. The
HOMO and LUMO energy levels of the dye molecules
were estimated by cyclic voltammetry, as shown in Table I.
The maximum absorption wavelength (ꢉmax) of the
organic dye corresponding to the ꢊ–ꢊ∗ transitions in
the conjugated molecules were red-shifted from 445 to
496 to 516 nm with a molar extinction coefficient (ꢋmaxꢂ
of 2ꢄ5 × 104 M−1 cm−1 to 4ꢄ5 × 104 M−1 cm−1, as seen
in Table I. This shift is due to the longer conjugated ꢊ-
chromophore. The absorption spectra of all dyes absorbed
on the surface of TiO2 are shown in Figure 1(b). The
organic dye with the strongest electron acceptor moieties
in the chromophore, PFSOX2 exhibited stronger red-shift
at the ꢉmax, compared to PFSCN2 and PFSRO2, due to the
increased electron delocalization of the ꢊ∗ level and dye
aggregation (J-aggregation), which can lead to a broaden-
ing of the absorption.18 On the other hand, PFSRO2 and
PFSCN2 exhibited blue-shifts in absorption peaks that may
be caused by the formation of H-aggregation of organic
dyes on the TiO2 surface.19 To ensure regeneration of the
dye molecules, the HOMO level of the dye molecules
3.2. Photovoltaic Performance of DSSCs
The photovoltaic properties of the DSSC based on
PFSCN2, PFSRO2, and PFSOX2 were measured and
compared to those of N719, as shown in Figure 2 and
Table I. Electrochemical parameters of the organic dyes.
ꢋamax
ꢉamax/nm
E0−0 (eV)c HOMO LUMO
(abs.)
Dye
/M−1 cm−1 Soln. (TiO2ꢂb
(eV)d
(eV)
PFSCN2
PFSRO2
PFSOX2
25110
36993
45636
445 (403)
496 (441)
516 (554)
2.25
2.04
1.93
−5.37
−5.31
−5.14
−3.12
−3.27
−3.21
Notes: aMaximum absorption and extinction coefficient at the maximum absorp-
tion of the dyes in the DMF solution; bMaximum absorption of the dyes that were
c
adsorbed onto the TiO2 films; E0−0(band gap) was determined from the inter-
section of the absorption and emission spectra in the DMF solution; dThe oxida-
tion potential (EHOMO) of the dye in the DMF solution measured using the cyclic
voltammogram.
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J. Nanosci. Nanotechnol. 14, 7938–7942, 2014

Jo et al. Molecular Design and Photovoltaic Performance of Organic Dyes Containing Phenothiazine for Dye-Sensitized Solar Cells
15
10
5
20
15
10
5
(a)
(a)
N719
N719
PFSCN2
PFSRO2
PFSOX2
PFSCN2
PFSRO2
PFSOX2
0
0
5
10
15
20
25
30
35
40
0.0
0.2
0.4
0.6
0.8
Z'(ohm)
Potential (V)
–12
–10
–8
–6
–4
–2
0
(b)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(b)
N719
N719
PFSCN2
PFSRO2
PFSOX2
PFSCN2
PFSRO2
PFSOX2
0.1
1
10
100
1000
10000 100000
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300 350 400 450 500 550 600
700 750
Frequency (Hz)
Wavelength(nm)
IP: 117.243.181.251 On: Tue, 03 Nov 2015 15:48:46
Figure 3. (a) Measured impedance spectrum of DSSC at forward Bode-
phase for DSSC cell. (b) Displays of Bode-phase for DSSC cell bias
applied condition under light illumination.
Copyright: American Scientific Publishers
Figure 2. (a) Current density–voltage characteristics for DSSCs con-
taining organic dyes under illumination of simulated solar light (AM 1.5,
100 mW/cm2). (b) IPCE curves for DSSCs based on organic dyes.
light illumination condition. Under illumination, a DSSC
is biased by the photovoltage, and thus, the photocurrent
flow depends on the photoelectric performance of the cell.
Figure 3(a) shows the measured IS at a forward bias
applied condition under illumination. Compared to the
results of PFSCN2 dyes, the semicircle size reduces sig-
nificantly in the middle frequency region of PFSRO2 and
PFSOX2 dye, indicating acceleration of the electron trans-
port process in the photo anode. The Bode-phases for
DSSC are also shown in Figure 3(b). The frequency peaks
of DSSC with PFSCN2 appeared in lower a frequency
region than those of DSSC with PFSRO2, PFSOX2, due
to the fact that relatively rapid electron transfer process
occurred in the PFSRO2, PFSOX2. Since the frequency
is related with the inverse of electron life time (ꢌ) in
TiO2 electrodes, higher electron life times were obtained
in PFSCN2, than in PFSRO2 and PFSOX2. Thus, in the in
the case of PFSRO2 and PFSOX2, the lower electron life-
times lead to reduced Voc values in the DSSC, as shown in
Table II. The performances of DSSCs containing N719 and
organic dyes and the electron transport properties of their
photoanodes obtained by the IS analysis are summarized
in Table II.
Table II. Higher efficiencies were achieved in the DSSC
with PFSCN2 containing cyanoacrylic acid as the elec-
tron acceptor as compared to the DSSC with PFSRO2 and
PFSOX2, mainly owing to the higher short-circuit current
(Jsc) value in the former. It is known that the cyanoacrylic
acid moiety of the organic dye exhibits a strong electron-
withdrawing capability and abundant electronic coupling
at the interface of the TiO2 particles, leading to efficient
electron injection from the LUMO level of the dye to
the conduction band of the TiO2. The IPCEs of DSSCs
with organic dyes were also measured and the result is
represented in Figure 2(b).
Furthermore, we attempt to identify the amount of
dye adsorption based on the adsorption ability on the
TiO2 surface, even in a strong NaOH basic aqueous
solution. As a result, we can say that the adsorption
ability of the PFSCN2 dye on the TiO2 surface was
higher than that of others in the same condition, which
could affect the photovoltaic properties in DSSCs. The
DSSCs with the PFSRO2 and PFSOX2 exhibited a slightly
lower open-circuit voltage (Voc) than those with PFSCN2.
The properties of charge transport in DSSCs have been
evaluated by measuring their impedance spectra (IS) under
J. Nanosci. Nanotechnol. 14, 7938–7942, 2014
7941

Molecular Design and Photovoltaic Performance of Organic Dyes Containing Phenothiazine for Dye-Sensitized Solar Cells Jo et al.
Table II. Photovoltaic Performance of DSSCs composed of
N719/Organic dyes and the electron transport properties in their
photoanodes determined by impedance analysis. (cell areas: 0.24 cm2).
References and Notes
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Jsc
(mA/cm2)
Voc
(V)
FF
ꢍ
R1
R2
R3
ꢌ
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PFSCN2 10ꢄ662 0.6185 70.24 4.63 6.45 6.04 14.44
6ꢄ1
1ꢄ9
4ꢄ1
PFSRO2
PFSOX2
N719
2ꢄ985 0.4977 71.80 1.07 6.57 2.49 28.33
5ꢄ025 0.5554 74.55 2.08 6.51 6.88 19.16
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We have designed and synthesized novel organic photosen-
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various multi-electron acceptors. The photovoltaic proper-
ties of organic dyes composed of different acceptors in
their chromophores were measured to identify their effects
on the DSSC performance. By replacing different accep-
tors, the absorption spectrum shows a significant red-shift
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Copyright: American Scientific Publishers
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J. Nanosci. Nanotechnol. 14, 7938–7942, 2014