The effect of five-membered heterocyclic bridges and ethoxyphenyl substitution on the performance of phenoxazine-based dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2013.12.022

Phenoxazine derivatives with heterocyclic bridge units and an additional donor were synthesized and applied to dye-sensitized solar cells. To study the effects of these substituents on the DSSC performance, the photophysical, electrochemical, and photovoltaic properties of the dyes were investigated. The introduced heterocyclic bridge units furan and thiophene improved the short-circuit current due to the red-shifted absorption spectra of the dyes. The ethoxyphenyl ring introduced to the phenoxazine moiety as an additional donor broadened the spectrum of the dye, while the reduced adsorption of the dye caused by its non-planar structure limited the enhancement of the short-circuit current. Among the synthesized dyes, the one with furan as a bridge unit showed the best overall conversion efficiency of 5.26%.

Full text of DOI:10.1016/j.dyepig.2013.12.022

Dyes and Pigments 104 (2014) 185e193
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The effect of five-membered heterocyclic bridges and ethoxyphenyl
substitution on the performance of phenoxazine-based dye-sensitized
solar cells
Woosung Lee a, Jun Choi a, Jin Woong Namgoong a, Se Hun Kim a, Kyung Chul Sun b,
,
Sung Hoon Jeong c, Kicheon Yoo d, Min Jae Ko d, Jae Pil Kim a
*
a Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, South Korea
b Department of Fuel Cells and Hydrogen Technology, Hanyang University, Seongdong-gu, Seoul 133-791, South Korea
c Department of Organic and Nano Engineering, College of Engineering, Hanyang University, Seongdong-gu, Seoul 133-791, South Korea
d Photo-Electronic Hybrids Research Center, National Agenda Research Division, Korea Institute of Science and Technology (KIST), Seoul 136-791, South
Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Phenoxazine derivatives with heterocyclic bridge units and an additional donor were synthesized and
applied to dye-sensitized solar cells. To study the effects of these substituents on the DSSC performance,
the photophysical, electrochemical, and photovoltaic properties of the dyes were investigated. The
introduced heterocyclic bridge units furan and thiophene improved the short-circuit current due to the
red-shifted absorption spectra of the dyes. The ethoxyphenyl ring introduced to the phenoxazine moiety
as an additional donor broadened the spectrum of the dye, while the reduced adsorption of the dye
caused by its non-planar structure limited the enhancement of the short-circuit current. Among the
synthesized dyes, the one with furan as a bridge unit showed the best overall conversion efficiency of
5.26%.
Received 7 October 2013
Accepted 20 December 2013
Available online 31 December 2013
Keywords:
Dye-sensitized solar cells
Phenoxazine dyes
Five-membered heterocyclic bridges
Ethoxyphenyl substitution
Dihedral angle
Ó 2014 Elsevier Ltd. All rights reserved.
Dye adsorption
1. Introduction
For efficient DSSCs, organic dyes should have broad and red-
shifted absorptions in the visible region. Accordingly, most
Dye-sensitized solar cells (DSSCs) have attracted considerable
attention as promising solar devices since Grätzel et al. reported
Ru-based photosensitizers in 1991 [1]. The Ru complex dyes typical
used as sensitizers in DSSCs have shown high electronic conversion
efficiencies of over 11% with good stability [2]. However, high
production cost and difficulties in purification have limited their
development for large-scale applications. Recently, more attention
has been paid to sensitizers without Ru (metal-free organic dyes
and organometallic dyes) due to their lower cost, easier modifica-
tion and purification, high molar extinction coefficient, and envi-
ronmental friendliness. As such, sensitizers without Ru such as
triphenylamine [3], indoline [4], cyanine [5], coumarin [6], perylene
[7], porphyrin [8], phthalocyanine [9], and phenothiazine [10] have
been extensively studied. Among these, porphyrin derivatives have
shown high electronic conversion efficiency (12.3%) [7].
organic dyes have the structure of donoreconjugated bridgese
acceptor (De eA) to obtain a broad and red-shifted absorption
p
spectrum. Among the various conjugated bridges, furan and thio-
phene have displayed the most remarkable results, showing wide
and red-shifted absorption spectra, as well as high molar extinction
coefficients [11]. In addition, to achieve enhanced photovoltaic
performance, organic dyes with an additional donor (DeDeA or De
DepeA) have been suggested [12]. The introduction of an addi-
tional donor group could increase the electron-donating capability,
which would improve electron injection and charge separation.
Phenoxazine (POZ) includes electron-rich oxygen and nitrogen
atoms in a heterocyclic ring, which displays high electron-donating
ability [13]. It also shows sufficient electrochemical properties,
which implies that POZ could be a promising sensitizer in DSSCs
[14]. However, despite these advantages, POZ-based sensitizers
have not been reported extensively.
In this research, to study effects of conjugated bridges with a
POZ moiety on photovoltaic performance, five-membered hetero-
cyclic rings were introduced as a conjugated bridge unit to POZ
* Corresponding author. Tel.: þ82 2 880 7187; fax: þ82 2 880 7238.
E-mail address: jaepil@snu.ac.kr (J.P. Kim).
0143-7208/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.12.022

186
W. Lee et al. / Dyes and Pigments 104 (2014) 185e193
molecules. The addition of these bridge units could extend the
conjugation of the dye molecule, which red-shifted the absorption
spectrum and increased the molar absorptivity of the dyes.
Furthermore, to improve the donating power and molar extinction
coefficient, an ethoxy phenyl ring was substituted in the 7 position
of the POZ-furan dye as an additional donor.
Based on these strategies, three organic dyes (WS1, WS2 and
WS3) were designed and synthesized. The photophysical and
electrochemical properties of the synthesized dyes were investi-
gated in detail and density functional theory (DFT) calculations
were also performed. Photovoltaic cells were assembled with the
synthesized dyes and their photovoltaic properties were analyzed.
In addition, electrochemical impedance spectroscopy (EIS) was
used to study the interfacial charge transport process in the
photovoltaic cells.
2.3. Fabrication of dye-sensitized solar cells and measurements
A Photoanode paste was prepared for a screen-printing process.
The final composition of the paste comprised TiO2 nanopowder
(1 g), ethyl cellulose (0.5 g), terpineol (3.3 mL), and acetic acid
(0.16 mL). After that, pre-washed FTO glass was coated by a doctor
blade process and then heated at 70 ꢁC for 30 min for drying. After
the printing, the TiO2 films were heated in four steps of 325 ꢁC,
375 ꢁC, 450 ꢁC, and 500 ꢁC for 5, 5, 15, and 15 min, respectively,
using a high-temperature furnace (Lab house Co.). For the post-
treatment, the coated and sintered TiO2 films were immersed in
TiCl4 solution (40 mM in water) for 30 min at 70 ꢁC. After washing,
the films were annealed at 500 ꢁC for 30 min. Counter electrodes
were prepared by spin coating method using 5 mM H2PtCl6 solu-
tion (in ethanol) on one-holed FTO glass and heated at 400 ꢁC for
20 min. After cooling at 60 ꢁC, the TiO2 electrodes were immersed
in EtOH/CH2Cl2 solution containing the dyes at 0.5 mM for 48 h at
ambient temperature. After dye absorption, the photoanodes were
washed using anhydrous ethanol and dried under nitrogen flow.
The dye-covered photoelectrode and Pt-electrode were assembled
using ionomer surlyn with a hot-press at 80 ꢁC. After assembling,
the electrolyte solution (composed of 0.6 M BMII, 0.05 M I2, 0.1 M
LiI, and 0.5 M TBP in acetonitrile solvent) was injected into the one-
holed FTO glass using a capillarity vacuum technique, and the hole
was sealed with a cover-glass using the same surlyn. A black mask
aperture was placed on the front electrode for better analysis of the
photovoltaic characteristics. The active area of the dye-coated TiO2
film was ca. 0.24 cm2, which was measured by analyzing the images
from a CCD camera (moticam 1000). The TiO2 film thickness was
2. Experimental
2.1. Materials and reagents
Phenoxazine, N-bromosuccinimide and 4-ethoxyphenylboronic
acid were purchased from TCI and used as received without further
purification. 1-bromobutane, 5-formyl-2-furan-boronic acid, 5-
formyl-2-thiophene-boronicacid,
tetrakis(triphenylphosphine)
palladium(0), phosphorus oxychloride, 4-ethoxyphenylboronic
acid, cyanoacetic acid and piperidine were purchased from
SigmaeAldrich and used as received without further purification.
All solvents (chloroform, tetrahydrofuran, dimethylformamide,
dimethyl sulfoxide, dichloromethane, 1, 2-dichloroethane and
acetonitrile) were obtained from SigmaeAldrich and used as
received. Other chemicals were reagent grade and used without
further purification.
measured by an a-step surface profiler (KLA Tencor).
Photocurrentevoltage (IeV) measurements were performed
using a Keithley model 2400 source measure unit. A class-A solar
simulator (Newport) equipped with a 150 W Xe lamp was used as
the light source. The light intensity was adjusted with an NREL-
calibrated Si solar cell with a KG-5 filter for approximating the
light intensity of 1 sun. Photocurrentevoltage measurements of the
dye-sensitized solar cells were performed with an aperture mask
by following a reported method. Incident photon-to-current con-
version efficiency (IPCE) was measured as a function of wavelength
from 300 to 1000 nm using a specially designed IPCE system for
dye-sensitized solar cells (PV measurements, Inc.). A 75 W xenon
lamp was used as the light source for generating monochromatic
beams. Calibration was performed using a silicon photodiode,
which was calibrated based on the NIST-calibrated photodiode
G425 standard. The IPCE values were measured under halogen bias
light at a low chopping speed of 10 Hz. All calculations were carried
out using Gaussian 09 software. Optimized geometries, energy
levels, and frontier molecular orbitals of the dyes’ HOMOs and
LUMOs were calculated at the B3LYP/6-31G (d,p) level.
2.2. Analytical instruments and measurements
1H NMR and 13C NMR spectra were recorded on a Bruker Avance
300, 500 and 600 MHz using DMSO with the chemical shift against
TMS (Seoul National University National Center for Inter-University
Research Facilities). Mass data were measured using a JEOL JMS
600 W mass spectrometer (Seoul National University National
Center for Inter-University Research Facilities). UVevis spectra
were measured with a HewlettePackard 8425A spectrophotom-
eter. Cyclic voltammetry spectra were obtained using a three-
electrode cell with
a 273 A potentiostat (Princeton applied
research, Inc.). Measurements were performed using Ag wire (Ag/
Agþ), glassy carbon and platinum wire as the reference, working
and counter electrodes, respectively, in CH2Cl2 solution containing
0.1 M tetrabutylammonium tetrafluoroborate (TBATFB) as the
supporting electrolyte. A standard ferrocene/ferrocenium (Fc/Fcþ)
redox couple was employed to calibrate the oxidation peak.
Photocurrentevoltage measurements were performed using a
Keithley model 2400 source measure unit. A 1000 W Xe lamp
(Spectra-physics) served as a light source, and it was adjusted using
an NREL-calibrated silicon solar cell equipped with a KG-5 filter to
approximate AM 1.5G sunlight intensity. The incident photon-to-
current conversion efficiency (IPCE) was measured as a function
of the wavelength from 300 nm to 800 nm using a specially
designed IPCE system for dye-sensitized solar cells (PV measure-
ments, Inc.). A 75 W Xe lamp was employed as a light source to
generate a monochromatic beam. The electrical impedance spectra
(EIS) of the DSSCs under dark with 0.60 V forward bias were
measured with an impedance analyzer (Compactstat, IVIUM Tech)
at frequencies of 10ꢀ1e106 Hz. The magnitude of the alternative
signal was 10 mV. The impedance parameters were determined by
fitting the impedance spectra using Z-view software.
2.4. Synthesis of dyes
2.4.1. 10-Butyl-10H-phenoxazine (1)
Sodium hydroxide (7.36 g, 0.184 mol) and 1-bromobutane
(6.62 g, 0.048 mol) were slowly added to a phenoxazine (4.0 g,
0.022 mol) solution in dry DMSO (50 mL) at room temperature and
stirred for 24 h. Then, the reaction mixture was poured into water
and extracted with ethyl acetate. The organic phase was separated
and dried over anhydrous MgSO4. After removing the solvent, the
residue was purified by column chromatography using ethyl ace-
tateehexane (1:10; v/v) as the eluent to give 1, colorless viscous
liquid (4.78 g, 91%).
1H NMR (500 MHz, d6-DMSO):
d
¼ 6.81 (d, J ¼ 8.7 Hz, 2H), 6.63e
6.67 (m, 4H), 3.53 (t, J ¼ 7.7 Hz, 2H), 1.50-1.54 (m, 2H), 1.38-1.43 (m,
2H), 0.94 ppm (t, J ¼ 7.3 Hz, 3H).

W. Lee et al. / Dyes and Pigments 104 (2014) 185e193
187
2.4.2. 10-Butyl-10H-phenoxazine-3-carbaldehyde (2)
2.4.7. 3-(5-Formyl-2-furan)-7-bromo-10-butyl-10H-phenoxazine
(4c)
POCl3 (0.55 mL, 0.006 mol) was added dropwise to a solution of
1 (1.29 g, 0.005 mol) and dry DMF (5 mL) in dry1,2-dichloroethane
(10.7 mL) in an ice water bath with temperature below 15 ꢁC. The
reaction was heated to room temperature and refluxed at 90 ꢁC for
48 h. The mixture was quenched with dilute NaOH (aq) and
extracted with water and dichloromethane (DCM). The organic
phase was dried with anhydrous MgSO4, and then the solvent was
removed in vacuo. The residue was purified by column chroma-
tography using ethyl acetateehexane (1:6; v/v) to give 3, yellow oil
(0.957 g, 71.6%).
4c as an orange oil (1.03 g, 41%) was synthesized according to
the procedure described for the synthesis of 4a. 5-formyl-2-furan-
boronic acid (1.02 g, 0.0072 mol) was added under nitrogen at-
mosphere to a solution of 3b (2.41 g, 0.0061 mol), 2 M aqueous
K2CO3 (15.25 mL), Pd(PPh3)4 (0.71 g, 0.00061 mol) in dry THF
(100 mL). Eluent: DCM-hexane (5:1; v/v).
1H NMR (500 MHz, d6-DMSO):
d
¼ 9.53 (s, 1H), 7.60 (s, 1H), 7.35
(d, J ¼ 8.4 Hz, 1H), 7.13 (m, 2H), 7.02 (d, J ¼ 8.4 Hz, 1H), 6.85 (s, 1H),
6.82 (d, J ¼ 8.6 Hz,1H), 6.68 (d, J ¼ 8.7 Hz,1H), 3.58 (t, J ¼ 7.5 Hz, 2H),
1.54-1.50 (m, 2H), 1.43-1.39 (m, 2H), 0.94 ppm (t, J ¼ 7.3 Hz, 3H).
1H NMR (500 MHz, d6-DMSO):
1.8 Hz, 1H), 7.00 (s, 1H), 6.68-6.87 (m, 5H), 3.62 (t, J ¼ 7.9 Hz, 2H),
1.52-1.56 (m, 2H), 1.40-1.45 (m, 2H), 0.95 ppm (t, J ¼ 7.3 Hz, 3H).
d
¼ 9.64 (s, 1H), 7.41 (dd, J ¼ 8.3,
2.4.8. 5-(3-(4-Ethoxyphenyl)-10-butyl-10H-phenothiazin-7-yl)
furan-2-carbaldehyde (5a)
2.4.3. 3-Bromo-10-butyl-10H-phenoxazine (3a)
A mixture of 4c (0.5 g, 0.0012 mol), 2 M aqueous K2CO3 (6 ml),
Pd(PPh3)4 (0.07 g, 0.00006 mol) in dry THF (15 mL) was refluxed for
1/2 h. 4-ethoxyphenylboronic acid (0.28 g, 0.0017 mol) dissolved in
dry THF (5 mL) was added to the reaction mixture and refluxed for
15 h. The mixture was quenched with water and extracted with
DCM. The organic phase was dried with anhydrous MgSO4, and
then the solvent was removed in vacuo. The residue was purified by
column chromatography using DCM-ethyl acetate (10:1; v/v) to
give 5a, orange oil (0.29 g, 53%).
1 (1.488 g, 0.0062 mol) and N-bromosuccinimide (1.106 g,
0.0062 mol) were dissolved in chloroform (30 mL), and the reaction
was stirred for 1 h at ambient temperature. The reaction was
quenched with water and extracted with water and DCM. The
organic phase was collected and the solvent was removed by rotary
evaporation. The residue was purified by column chromatography
using hexane to give 3a, white solid (1.48 g, 75%).
1H NMR (500 MHz, d6-DMSO):
(m, 2H), 6.69-6.59 (m, 4H), 3.52-3.47 (m, 2H), 1.53-1.46 (m, 2H),
d
¼ 7.0-6.96 (m, 1H), 6.86-6.80
1H NMR (500 MHz, d6-DMSO):
d
¼ 9.52 (s, 1H), 7.57 (s, 1H), 7.49
1.44-1.35 (m, 2H), 0.90e0.91 ppm (m, 3H).
(d, J ¼ 8.7 Hz, 2H), 7.34 (d, J ¼ 8.4 Hz, 2H), 7.13-7.07 (m, 3H), 6.94 (d,
J ¼ 8.7 Hz, 2H), 6.91 (s, 1H), 6.77 (t, J ¼ 8.7 Hz, 2H), 4.06-4.00 (m,
2H), 3.62 (t, J ¼ 7.5 Hz, 2H), 1.58-1.56 (m, 2H), 1.45-1.43 (m, 2H), 1.34
(t, J ¼ 7 Hz, 3H), 0.97 ppm (t, J ¼ 7.3 Hz, 3H).
2.4.4. 3,7-Dibromo-10-butyl-10H-phenoxazine (3b)
3b as a white solid (1.72 g, 70%) was synthesized according to
the procedure described for the synthesis of 3a. N-bromosuccini-
mide (2.21 g, 0.0124 mol) was added to a solution of 1 (1.49 g,
0.0062 mol) in chloroform (30 mL) at ambient temperature. Eluent:
hexane.
2.4.9. (E)-(10-Butyl-10H-phenoxazin-3-yl)-2-cyanoacrylic acid
(POX)
3 (0.23 g, 0.00086 mol), cyanoacetic acid (0.22 g, 0.0026 mol)
and piperidine (0.18 mL, 0.00347 mol) were added to anhydrous
CH3CN (100 mL). After the mixture was refluxed for 8 h, the solu-
tion was extracted with DCM and 0.1 M HCl aqueous solution. The
organic phase was dried over anhydrous MgSO4, and the solvent
was removed in vacuo The crude product was purified by column
chromatography using DCM-methanol (5:1; v/v) to give POX, red
solid (0.22 g, 75%).
1H NMR (500 MHz, d6-DMSO):
d
¼ 7.0 (d, J ¼ 8.6 Hz, 2H), 6.83 (s,
2H), 6.54 (d, J ¼ 8.7 Hz,1H), 3.50 (t, J ¼ 7.5 Hz, 2H),1.50-1.45 (m, 2H),
1.40-1.36 (m, 2H), 0.92 ppm (t, J ¼ 7.3 Hz, 3H).
2.4.5. 5-(10-Butyl-10H-phenoxazin-3-yl)furan-2-carbaldehyde
(4a)
Under nitrogen atmosphere,
a mixture of 3a (2.21 g,
0.0069 mol), 5-formyl-2-furan-boronic acid (1.12 g, 0.0080 mol),
2 M aqueous of K2CO3 (8.66 mL), Pd(PPh3)4 (0.4 g, 0.00035 mol) in
dry THF (100 mL) was stirred for 1/2 h and refluxed at 80 ꢁC
overnight. The reaction was extracted with DCM, water and brine.
The organic phase was dried with anhydrous MgSO4, and then the
solvent was removed in vacuo. The residue was purified by column
chromatography using DCM-hexane (5:1; v/v) to give 4a, orange oil
(1.13 g, 49%).
1H NMR (600 MHz, d6-DMSO):
d
¼ 7.97 (s,1H), 7.48 (d, J ¼ 8.5 Hz,
1H), 7.36 (s, 1H), 6.85 (t, J ¼ 7.6 Hz, 1H), 6.69-6.78 (m, 4H), 3.59 (t,
J ¼ 7.6 Hz, 2H), 1.50-1.55 (m, 2H), 1.37-1.43 (m, 2H), 0.93 ppm (t,
J ¼ 7.3 Hz, 3H).
13C NMR (150 MHz, d6-DMSO):
d
¼ 164.1, 152.2, 143.8, 143.6,
137.8, 130.9, 130.6, 124.3, 123.7, 122.6, 117.2, 115.3, 114.5, 112.9, 111.7,
98.0, 42.9, 26.7, 19.2, 13.7 ppm; m/z (FAB) 334.1316 ((M þ),
C20H18N2O3 requires 334.1317).
1H NMR (500 MHz, d6-DMSO):
d
¼ 9.53 (s, 1H), 7.60 (s, 1H), 7.33
(d, J ¼ 8.4 Hz, 1H), 7.12 (s, 1H), 7.11 (s, 1H), 6.86 (t, J ¼ 7.9 Hz, 1H),
6.78 (d, J ¼ 8.5 Hz, 1H), 6.74-6.66 (m, 3H), 3.59 (t, J ¼ 7.6 Hz, 2H),
1.57-1.52 (m, 2H), 1.45-1.40 (m, 2H), 0.95 ppm (t, J ¼ 7.3 Hz, 3H).
2.4.10. (E)-3-(5-(10-Butyl-10H-phenoxazin-3-yl)furan-2yl)-2-
cyanoacrylic acid (WS1)
WS1 (0.13 g, 54%) was synthesized according to the procedure
described for the synthesis of POX. 4a as a dark red solid (0.2 g,
0.0006 mol), cyanoacetic acid (0.15 g, 0.0018 mol), and piperidine
(0.24 mL, 0.0024 mol) were added to anhydrous CH3CN (50 mL).
Eluent: DCM-methanol (9:1; v/v).
2.4.6. 5-(10-Butyl-10H-phenoxazin-3-yl)thiophene-2-
carbaldehyde (4b)
4b as an orange oil (1.53 g, 42%) was synthesized according to
the procedure described for the synthesis of 4a. 5-formyl-2-
thiophene-boronic acid (1.95 g, 0.0124 mol) was added under ni-
trogen atmosphere to a solution of 3a (3.32 g, 0.0104 mol), 2 M
aqueous K2CO3 (13 mL), Pd(PPh3)4 (0.61 g, 0.00053 mol) in dry THF
(100 mL). Eluent : DCM-hexane (5:1; v/v).
1H NMR (500 MHz, d6-DMSO):
d
¼ 7.97 (s, 1H), 7.48 (s, 1H), 7.41
(d, J ¼ 8 Hz, 1H), 7.19-7.17 (m, 2H), 6.87-6.84 (m, 1H), 6.81 (d,
J ¼ 9 Hz,1H), 6.74-6.72 (m, 1H), 6.70-6.68 (m, 2H), 3.60 (t, J ¼ 7.5 Hz,
2H), 1.58-1.52 (m, 2H), 1.46-1.39 (m, 2H), 0.95 ppm (t, J ¼ 7.5 Hz,
3H).
1H NMR (300 MHz, d6-DMSO):
d
¼ 9.84 (s, 1H), 7.96 (s, 1H), 7.59
13C NMR (125 MHz, d6-DMSO):
d
¼ 163.5, 158.0, 146.4, 143.7,
(s, 1H), 7.26 (d, J ¼ 8.3 Hz, 1H), 7.09 (s, 1H), 6.88-6.84 (m, 1H), 6.75-
6.66 (m, 4H), 3.59 (t, J ¼ 7.9 Hz, 2H), 1.55-1.48 (m, 2H), 1.46-1.38 (m,
2H), 0.95 ppm (t, J ¼ 7.2 Hz, 3H).
143.2, 136.4, 133.9, 131.1, 126.2, 123.7, 121.1, 121.0, 120.4, 116.4, 114.6,
111.9, 111.8, 110.6, 108.0, 42.6, 26.1, 18.7, 13.2 ppm; m/z (FAB)
400.1422 ((M þ), C24H20N2O4 requires 400.1423).

188
W. Lee et al. / Dyes and Pigments 104 (2014) 185e193
2.4.11. (E)-3-(5-(10-Butyl-10H-phenoxazin-3-yl)thiophene-2yl)-2-
cyanoacrylic acid (WS2)
WS2 (0.43 g, 79%) as a dark red solid was synthesized according
to the procedure described for the synthesis of POX. 4b (0.46 g,
0.0013 mol), cyanoacetic acid (0.33 g, 0.0039 mol), and piperidine
(0.51 mL, 0.0052 mol) were added to anhydrous CH3CN (100 mL).
Eluent: DCM-methanol (10:1; v/v).
1H NMR (500 MHz, d6-DMSO):
d
¼ 8.43 (s, 1H), 7.96 (s, 1H), 7.63
(s, 1H), 7.25 (d, J ¼ 8.5 Hz, 1H), 7.08 (s, 1H), 6.87 (t, J ¼ 7.5 Hz, 1H),
6.77-6.69 (m, 4H), 3.61 (t, J ¼ 6.5 Hz, 2H), 1.59-1.53 (m, 2H), 1.47-
1.40 (m, 2H), 0.96 ppm (t, J ¼ 7 Hz, 3H).
13C NMR (75 MHz, d6-DMSO):
d
¼ 163.2,151.9,145.8,143.8,143.1,
141.0, 133.8, 132.6131.3, 124.2, 123.8, 123.2, 122.1, 121.0, 116.2, 114.6,
111.9, 111.6, 66.4, 26.0, 18.8, 13.2 ppm; m/z (FAB) 417.1281 ((M þ H
þ), C24H21N2O3 S requires 417.1273).
Fig. 1. Structure of POX, WS1, WS2, and WS3.
2.4.12. (E)-3-(5-(4-Methoxyphenyl)-10-butyl-10H-phenoxazin-7-
yl)furan-2-yl)-2 cyanoacrylic acid (WS3)
to give intermediate 1. The bromination of 1 was performed by
varying the amount of N-bromosuccinimde (NBS) to provide mono-
brominated 3a and dibrominated 3b, respectively. The Suzuki
coupling reactions of 3a were carried out with 5-formyl-2-
furanboronic acid or 5-formyl-2-thiopheneboronic acid, which
produced 4a and 4b, respectively. The Suzuki coupling reaction of
3b to give 4c followed the procedure performed with 3a, and
subsequently, an additional Suzuki coupling reaction of 4c with 4-
ethoxyphenylboronic acid gave 5a. The final compounds (WS1,
WS2, and WS3) were obtained by the Knoevaenagal reactions of
the corresponding aldehydes (4a, 4b, and 5a) with cyanoacetic acid
in the presence of piperidine [15]. The reference dye (POX) was
obtained by previously reported synthetic routes. The structures of
all the synthesized intermediates and dyes were identified by 1H
NMR, and the final products were additionally confirmed by 13C
NMR and HRMS.
WS3 (0.25 g, 72%) as a dark purple solid was synthesized ac-
cording to the procedure described for the synthesis of POX. 5a
(0.3 g, 0.00066 mol), cyanoacetic acid (0.17 g, 0.002 mol) and
piperidine (0.26 mL, 0.0026 mol) were added to anhydrous CH3CN
(100 mL). Eluent: DCM-methanol (5:1; v/v).
1H NMR (500 MHz, d6-DMSO):
d
¼ 7.98 (s,1H), 7.54 (d, J ¼ 8.5 Hz,
2H), 7.49 (s, 1H), 7.44(d, J ¼ 8.5, 1H), 7.22-7.20 (m, 2H), 7.13 (d,
J ¼ 8 Hz,1H), 6.95 (m, 3H), 6.86 (d, J ¼ 8.5 Hz,1H), 6.80 (d, J ¼ 8.5 Hz,
1H), 4.06-4.05 (m, 2H), 3.65 (t, J ¼ 7 Hz, 2H), 1.59-1.57 (m, 2H), 1.48-
1.43 (m, 2H), 1.34(d, J ¼ 7 Hz, 3H), 0.97 ppm (t, J ¼ 7 Hz, 3H).
13C NMR (75 MHz, d6-DMSO):
d
¼ 163.5, 158.1, 157.3, 146.4,
143.6, 143.5, 136.5, 133.7, 132.8, 130.5, 129.8, 126.3, 121.3, 121.0,
120.4, 120.4, 116.4, 114.2, 112.3, 112.1, 111.9, 110.6, 108.2, 62.5, 42.2,
26.2, 18.8, 14.1, 13.3 ppm; m/z (FAB) 500.2002 ((M þ), C32H28N2O5
requires 500.1998).
3. Results and discussion
3.2. Photophysical properties
3.1. Synthesis
The absorption spectra of the dyes in EtOH/CH2Cl2 solution and
on the TiO2 surface are shown in Fig. 2, and the corresponding
photophysical data are listed in Table 1. All the dyes exhibited two
major absorption bands, which appeared at below 350 nm and
400e550 nm, respectively. The former band in the UV region is
The synthetic routes of POZ dyes are shown in Scheme 1 and the
synthesized dyes and reference dyes (POX) were shown in Fig. 1.
The POZ syntheses first involved an alkylation reaction on POZ
nitrogen atom and a butyl chain was introduced to the POZ moiety
ascribed to a localized aromatic
pep* transition, and the latter
Scheme 1. Synthesis of POX, WS1, WS2, and WS3: (a) 1-iodobutane, NaOH, DMSO (b) NBS, CHCl3 (c) POCl3, DMF, CHCl3 (d) 20 M aqueous of K2CO3, Pd(pph3)4, THF (e) cyanoacetic
acid, piperidine, acetonitrile.

W. Lee et al. / Dyes and Pigments 104 (2014) 185e193
189
0.30
0.25
0.20
0.15
0.10
0.05
0.00
molar extinction coefficient at lmax of WS1, WS2, and WS3 were
19,063, 19,209, and 26,335 Mꢀ1 cmꢀ1, respectively. These are higher
than those of structurally similar phenothiazine dyes (PT 5,
17,600 Mꢀ1 cmꢀ1) as well as standard ruthenium dye (N719,
14,000Mꢀ1 cmꢀ1), which afford the use of thinner TiO2 film for
efficient electron diffusion. Among these dyes, WS3 exhibited the
highest molar extinction coefficient. This result indicated that the
ethoxy phenyl ring is beneficial to the conjugation and absorption
properties of WS3, despite the large dihedral angle between the
POZ core and ethoxy phenyl ring (Table 2). The red-shifts in the
absorption spectra and high molar extinction coefficients of the
dyes have a tendency to increase with enhancement of the electron
density in the bridge unit and additional donor. Thus, their intro-
duction to the POZ moiety is favorable for enhancing the light
harvest and photocurrent generation of the dyes.
The absorption spectra of the dyes absorbed on TiO2 film are
blue-shifted compared to those in solution, and there is a larger
blue-shift in WS3 (9 nm) than in WS1 and WS2 (3 nm). This might
be due to the H-aggregation of the dyes or the deprotonation of
carboxylic acid upon the adsorption onto the TiO2 surface [16]. WS3
showed almost the same absorbance on the TiO2 surface compared
to WS1 and WS2, although it exhibited a much higher molar
extinction coefficient in solution. As shown in Tables 2 and 3, this is
due to the non-planar structure of WS3 caused by the introduction
of the ethoxy phenyl ring, which inhibits the adsorption of the dye
on the TiO2 surface. The optimized geometry of the dye in the
ground state revealed that the POZ core of all the dyes had planar
structures with small torsion angles (0.82e1.07ꢁ). On the other
hand, there were considerable differences in the dihedral angles
between the POZ core and the substituents. WS1 had almost planar
structure with a small dihedral angle (0.29ꢁ) between the core of
the dye and the furan moiety, while, the planarity of WS2 and WS3
decreased significantly due to the large dihedral angle between the
POZ core and the adjacent bridge unit or donor moiety; the dihedral
angle between thiophene and the POZ core in WS2 was 19.36ꢁ, and
that between the ethoxy phenyl ring and the POZ core in WS3 was
37.35ꢁ. WS3 had the most twisted molecular geometry of the dyes,
which led to less adsorption and decreased absorbance of the dye
on the TiO2 surface (Table 3).
(a)
WS1
WS2
WS3
400
500
600
700
Wavelength (nm)
4
3
2
1
0
(b)
WS1
WS2
WS3
400
500
600
700
Wavelength (nm)
Fig. 2. Absorbtion spectra of WS1, WS2 and WS3 in (a) EtOH/CH2Cl2 (7:2; v/v)
(10ꢀ5mol Lꢀ1) and (b) on TiO2.
band in the visible region is due to the intramolecular charge-
transfer (ICT) transition from the donor to the acceptor. The ab-
sorption maxima (lmax) of WS1, WS2, and WS3 are 452, 455, and
475 nm, respectively. The absorption spectrum of WS3, which
included both the additional donor and bridge moiety, was red-
shifted more than 10 nm compared to those of WS1 and WS2,
which included the bridge unit only. This was attributed to the
3.3. Electrochemical properties
The electrochemical properties and energy levels of the pre-
pared dyes are provided in Table 1 and Fig. 3. The oxidation po-
tentials of the dyes were measured using cyclic voltammetry (CV),
and the CeV curves of the dyes are shown in Fig. 4. The HOMO
levels of the prepared dyes correspond to the first oxidation po-
tential vs. a normal hydrogen electrode (vs. NHE) calibrated by Fc/
Fcþ (with 640 mV vs. NHE). All of the HOMO levels of the dyes
ranged from 0.81 eV to 0.88 eV (vs. NHE) and were sufficiently
positive compared to the redox potential of Iꢀ/I3ꢀ (0.4 V vs. NHE),
implying that the oxidized dyes can be effectively regenerated by
the redox electrolyte [17]. The LUMO levels of the dyes were ob-
tained by subtracting the zerothezeroth energy (E0e0) from Eox, in
which the zerothezeroth energy of the dyes is determined from the
inflection point at the end of the visible absorption spectrum of the
dyes. All of the LUMO levels of the dyes were more negative than
the conduction band energy level (Ecb) of TiO2 (ꢀ0.5 vs. NHE),
indicating that the excited electrons of the dyes can be efficiently
injected into the conduction band of TiO2, and oxidized dyes are
simultaneously formed [17]. As shown in Table 1, WS3 had a more
negative HOMO level and a more positive LUMO level than WS1
and WS2. This might be attributed to the extension of conjugation
induced by the additional donating group. Consequently, the gap
better delocalization of electrons over the p-conjugated molecules
by both the electron-rich furan and the ethoxy phenyl ring. The
Table 1
Photophysical and electrochemical properties of WS1, WS2 and WS3 dyes.
Dye
Absorptiona
Oxidation potential datac
lmax/nm
3
Eox/V
E0e0d/V
EoxeE0-0/
(at lmax
)
(on TiO2) (vs. NHE)
V (vs. NHE)
WS1 452
WS2 455
WS3 475
19,063
19,209
26,335
449
452
466
0.88
0.89
0.81
2.36
2.33
2.13
ꢀ1.48
ꢀ1.44
ꢀ1.31
a
Measured in 1 ꢂ 10-5 EtOH/CH2Cl2 solutions at room temperature.
b
c
Measured on TiO2 film.
Measured in CH2Cl2 containing 0.1 M tetrabutylammonium tetrafluoroborate.
(TBABF4) electrolyte (working electrode: glassy carbon; counter electrode: Pt;
reference electrode: Ag/Agþ; calibrated with ferrocene/ferrocenium (Fc/Fcþ) as an
internal reference and converted to NHE by addition of 630 mV).
d
Estimated from onset wavelength in absorption spectra.

190
W. Lee et al. / Dyes and Pigments 104 (2014) 185e193
Table 2
Optimized structures, dihedral angles and electronic distributions in HOMO and LUMO levels of the prepared dyes.
Dye
Optimized structure
HOMO
LUMO
WS1
WS2
WS3
1.5G irradiation conditions (100 mW cmꢀ2). The effects of the five-
membered heterocyclic bridges and the ethoxyphenyl substitution
on the performance of the cells were evaluated by measuring their
conversion efficiency relative to the reference dye (POX). The IPCE
spectra and photocurrentevoltage (JeV) curves are shown in Fig. 5,
and the corresponding data are listed in Table 3. All the dyes
showed high maximum IPCE values at 400e500 nm, which were
73.4%, 69.7%, and 73.2% for WS1, WS2, and WS3, respectively. These
values were higher than that of standard ruthenium dye (N719,
69.6%). Therefore, all the dyes can effectively convert visible light
into photocurrent in the absorption ranges. The IPCE spectra of
WS1 and WS2 showed broadened and red-shifted ranges compared
to that of POX, and the onsets of their IPCE spectra were extended
to 750 nm. The bridge units introduced to the POZ moiety red-
shifted the spectra of the dyes due to the extension of conjuga-
tion, increasing light harvest in the long wavelength region. WS3,
which had an additional donor, showed a similar IPCE spectrum to
that of WS1. This might be attributed to the fact that the absorption
spectrum of WS3 on TiO2 was largely blue-shifted compared to that
of WS1. In addition, it is known that a larger amount of dye
adsorbed on TiO2 leads to a broader IPCE spectrum [18]. Thus, the
lesser amount of adsorption of WS3 on the TiO2 surface also
contributed the relatively narrow IPCE spectrum of the dye. The
short-circuit current (Jsc) values increased in the order of
Table 3
DSSC performance parameters of POX, WS1, WS2, and WS3.a
Dyeb
Jsc (mA/cm2)
Voc (mV)
FF (%)
h
(%)
Dye amountc
(10ꢀ7 mol cme2
)
POX
WS1
WS2
WS3
N719
9.88
11.72
11.11
11.35
14.71
663.5
653.0
628.3
639.9
728.3
73.22
68.76
70.38
67.09
72.85
4.80
5.26
4.91
4.87
7.80
e
2.05
1.89
1.34
e
a
Measured under AM 1.5 irradiation G (100 mW cmꢀ1); 0.2 cm2 working area.
Dyes were maintained at 0.5 mM in EtOH/CH2Cl2 solution, with 10 mM CDCA
b
co-adsorbent. Electrolyte comprised 0.7 M 1-propryl-3-methyl-imidazolium iodide
(PMII), 0.2 M LiI, 0.05 M I2 0.5 M TBP in acetonitrile-valeronitrile (v/v, 85/15) for
organic dyes.
c
Dyes’ adsorption on TiO2 were measured by a colorimetric method using 0.1 M
NaOH aqueous-DMF (1:1) mixed solutions to wash the dyes from 8 mm thick TiO2
film.
between the HOMO and LUMO decreased, and the absorption
spectra became red-shifted.
3.4. Photovoltaic properties
DSSCs were fabricated using the dyes as sensitizers, and their
photovoltaic properties were measured under the standard AM
0.000012
0.000010
0.000008
0.000006
0.000004
0.000002
0.000000
-0.000002
-0.000004
-0.000006
Fer
POX
WS1
WS2
WS3
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Voltage (V)
Fig. 4. CV curves of Fc/Fcþ, POX, WS1, WS2, and WS3 in CH2Cl2.
Fig. 3. Dyes’ HOMO and LUMO energy levels.

W. Lee et al. / Dyes and Pigments 104 (2014) 185e193
191
5
80
70
60
50
40
30
20
10
0
(a)
0
-5
POX
WS1
WS2
WS3
N719
POX
WS1
WS2
WS3
-10
-15
400
500
600
700
800
-20
-0.2
0.0
0.2
0.4
0.6
0.8
Wavelength (nm)
Voltage (V)
20
18
16
14
12
10
8
N719
POX
WS1
WS2
WS3
(b)
Fig. 6. DSSCs’ JeV curves based on POX, WS1, WS2 and WS3 in the dark.
carried out in the dark under a forward bias of ꢀ0.60 V. The Nyquist
plots of the DSSCs fabricated with the dyes are shown in Fig. 7. In
the Nyquist plot, the major semicircle at the intermediate fre-
quency represents the charge transfer impedance at the TiO2/dye/
electrolyte interface. The charge recombination resistance can be
estimated by the radius of the major semicircle at the intermediate
frequency, with a larger radius of the major semicircle meaning a
smaller charge recombination rate [21]. The charge recombination
resistance increased in the order of WS2 < WS3 < WS1 < POX,
which is in accordance with the Voc values in the DSSCs made with
the dyes. As the charge recombination between injected electrons
and the electrolyte increased, Voc increased as well. As shown in
Fig. 8, this result coincides with the electron lifetime vs. dark bias
voltage. The electron lifetime increased in the order of
WS2 < WS3 z WS1 < POX < N719. Longer electron lifetime in-
dicates improved charge recombination resistance between the
injected electrons and electrolyte, with a consequent increase in
the Voc [22]. The results suggest that the introduction of the furan
bridge unit retards the charge recombination more effectively
compared to the thiophene bridge unit, leading to a longer electron
6
4
2
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Voltage (V)
Fig. 5. (a) IPCE spectra DSSCs based on POX, WS1, WS2, WS3 and N719 and (b) the
DSSCs’ JeV curves under AM 1.5G simulated sunlight (100 mW cmꢀ2).
POX < WS2 < WS3 < WS1, and this trend correlated with the
broadness of the IPCE spectrum. Consequently, the introduced
bridge units improved Jsc and the conversion efficiency, while the
introduction of the additional donor group to the POZ moiety had
little effect on Jsc.
lifetime and a higher Voc
.
The open-circuit voltage (Voc) values of the cells fabricated with
the dyes increased in the order of WS2 < WS3 < WS1 < POX. The
Voc values of the cells based on WS1-3 are lower than that based on
POX. As shown in previous studies, this might be explained by the
increased interaction caused by the introduction of a heterocyclic
-25
POX
WS1
WS2
-20
WS3
bridge unit from pep interaction between the dye molecules and/
-15
or the interaction between the heteroatom in the bridge unit and
iodine [19], [11b]. These interactions enhanced the charge recom-
bination at the dye/dye or dye/electrolyte interface, resulting in low
Voc values of the cells. As shown in Fig. 6, this tendency was also
shown in the dark currents of the cells made with the dyes. The
dark current is defined as the relatively small electric current
flowing out of a system without illumination and lower dark cur-
rent indicates decreased recombination and high Voc. Thus, the low
-10
-5
0
dark current of the cell with POX corresponds with the highest Voc
whereas the high dark current of the cell with WS2 is consistent
with the lowest Voc
,
0
5
10
15
20
25
30
Z' (Ohm)
.
To explain the correlation between the Voc of the cells and the
dyes, electrochemical impedance spectroscopy (EIS) [20] was
Fig. 7. Impedance spectra of DSSCs based on POX, WS1, WS2, and WS3; Nyquist plots
measured at 0.60 V forward bias in the dark.

192
W. Lee et al. / Dyes and Pigments 104 (2014) 185e193
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Acknowledgments
This work was supported by a Grant-in-Aid for the Industrial
Strategic Technology Program from the Korea Ministry of Knowl-
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