The effects of the number of anchoring groups and N-substitution on the performance of phenoxazine dyes in dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2013.10.005

Four novel phenoxazine-based organic sensitizers were synthesized and applied to dye-sensitized solar cells. Cyanoacrylic acid as an additional anchoring group was introduced to the phenoxazine for efficient electron extraction from the donor part, and an N-substituent was added to suppress dye aggregation. The photophysical, electrochemical and photovoltaic properties of the solar cells based on these dyes were investigated to study the effects of these substituents on the performance of the dye-sensitized solar cells. The additional cyanoacrylic acid acceptor improved the short-circuit current because it widened the absorption ranges of the dyes, although it also increased the recombination rate. The N-methoxyphenyl unit decrease charge recombinations, resulting in higher open-circuit voltage. However, the bulky substituent decreased the amount of dye absorbed on the TiO₂. As a result, the fabricated cells with the four dyes exhibited similar overall conversion efficiencies and, of these cells, the solar cell based on the N-4-methoxyphenyl mono-cyanoacrylate substituted dye showed the highest conversion efficiency of 5.09% (short-circuit current = 10.11 mA/cm², open-circuit voltage = 690 mV, fill factor = 72.23%, an overall conversion efficiency = 5.09% under AM 1.5G conditions).

Full text of DOI:10.1016/j.dyepig.2013.10.005

Dyes and Pigments 102 (2014) 13e21
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The effects of the number of anchoring groups and N-substitution on
the performance of phenoxazine dyes in dye-sensitized solar cells
Woosung Lee a, Sim Bum Yuk a, Jun Choi a, Hae Jung Kim a, Hyun Woo Kim a, Se Hun Kim a,
,
Boeun Kim b, Min Jae Ko b, Jae Pil Kim a
*
a Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea
b Solar Cell Research Center, Materials Science and Technology Division, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Four novel phenoxazine-based organic sensitizers were synthesized and applied to dye-sensitized solar
cells. Cyanoacrylic acid as an additional anchoring group was introduced to the phenoxazine for efficient
electron extraction from the donor part, and an N-substituent was added to suppress dye aggregation.
The photophysical, electrochemical and photovoltaic properties of the solar cells based on these dyes
were investigated to study the effects of these substituents on the performance of the dye-sensitized
solar cells. The additional cyanoacrylic acid acceptor improved the short-circuit current because it
widened the absorption ranges of the dyes, although it also increased the recombination rate. The N-
methoxyphenyl unit decrease charge recombinations, resulting in higher open-circuit voltage. However,
the bulky substituent decreased the amount of dye absorbed on the TiO2. As a result, the fabricated cells
with the four dyes exhibited similar overall conversion efficiencies and, of these cells, the solar cell based
on the N-4-methoxyphenyl mono-cyanoacrylate substituted dye showed the highest conversion effi-
ciency of 5.09% (short-circuit current ¼ 10.11 mA/cm2, open-circuit voltage ¼ 690 mV, fill factor ¼ 72.23%,
an overall conversion efficiency ¼ 5.09% under AM 1.5G conditions).
Received 5 August 2013
Received in revised form
1 October 2013
Accepted 1 October 2013
Available online 18 October 2013
Keywords:
Dye-sensitized solar cells
Phenoxazine dyes
Di-anchor
N-substituent
Dihedral angle
Dye adsorption
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
sensitizers containing coumarin [5], carbazole [6], fluorene [7],
hemicyanine [8], indoline [9], merocyanine [10], perylene [11],
The demand for environmentally friendly energy sources con-
tinues to increase due to depletion of traditional energy sources
and due to environmental consideration. One of the attractive
candidates of these new energy sources is solar energy, which is
clean, renewable and limitless. Dye-sensitized solar cells (DSSCs)
have been considered as one of the promising energy harvesting
devices since the report of Ru-based photosensitizers in 1991 by
Grätzel et al. [1]. The Ru complex sensitizers (N3, N719 and black
dye) have shown high photoelectric conversion efficiencies of over
11% under AM 1.5 conditions [2]. Compared with them, metal-free
organic dyes and organometallic dyes have advantages including
lower cost, easier modification and purification, environmental
friendliness and high molar extinction coefficient. The highest
conversion efficiencies of organic dyes (10% [3]) and organometallic
dyes (12.3% [4]) demonstrated that non-Ru dyes could be promising
sensitizers for realizing highly efficient DSSCs. For these reasons,
polyene [12], porphyrin [4,13], phthalocyanine [14], triphenylamine
[3,15] have been extensively studied.
Phenoxazine (POZ)-based sensitizers have exhibited higher
conversion efficiencies than triphenylamine (TPA) and phenothia-
zine (PTZ)-based sensitizers, which are structurally similar [16e18].
This is because POZ-based sensitizers, with electron-rich nitrogen
and oxygen heteroatoms, have stronger electron-donating ability
than TPA and PTZ-based sensitizers. POZ-based sensitizers also
show sufficient electrochemical properties for use in DSSCs [19].
However, despite their potential for application to DSSCs, POZ-
based sensitizers have not been studied extensively.
We report the introduction of an additional cyanoacrylic acid
moiety in the 7-position of the POZ chromophore as the second
anchoring group. Compared to mono-anchoring sensitizers, this di-
anchoring sensitizer has increased electron pathways and extended
conjugations of the POZ moiety. Therefore, the short-circuit current
(Jsc) can be improved by the bathochromic shift of the absorption
spectrum. However, the di-anchoring dyes have exhibited lower
open-circuit voltages (Voc) compared to the mono-anchoring dyes.
Therefore, to improve Voc of cells based on di-anchoring dyes, a
* 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 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.10.005

14
W. Lee et al. / Dyes and Pigments 102 (2014) 13e21
bulky methoxyphenyl ring was introduced to the POZ nitrogen
atom. The steric hindrance by these bulky groups was expected to
reduce dye aggregation and suppress the electron recombination
between the electrons injected on the TiO2 and the holes in the
electrolyte to result in high Voc.
Four POZ derivatives were designed and synthesized, i.e., POX,
WB, WH1 and WH2 as shown in Fig. 1. To examine the effects of the
additional anchoring group and the N-substituents on the perfor-
mance of DSSCs, photophysical and electrochemical properties of
the dyes and photovoltaic performance of the cells based on these
dyes were analyzed. In addition, electrochemical impedance
spectroscopy (EIS) was used to investigate interfacial charge
transport process. Density functional theory (DFT) calculations
were also performed for further analysis of the results.
Agþ), a glassy carbon and a platinum wire as the reference, working
and counter electrodes, respectively, in the DMF solution contain-
ing 0.1 M tetrabutylammonium tetrafluoroborate (TBATFB) as the
supporting electrolyte. A standard ferrocene/ferrocenium (Fc/Fcþ)
redox couple was used to calibrate the oxidation peak. Photocur-
rentevoltage measurements were performed using a Keithley
model 2400 source measure unit. Incident photon-to-current
conversion efficiency (IPCE) was measured as a function of wave-
length from 300 nm to 1000 nm using a specially designed IPCE
system for dye-sensitized solar cells (PV measurements, Inc.).
Electrical impedance spectra (EIS) of DSSCs under dark with 0.55 V
forward bias and under illumination at an open-circuit voltage
were measured with an impedance analyzer (Compactstat, IVIUM
Tech) at frequencies of 10ꢀ1e106 Hz. The magnitude of the alter-
native signal was 10 mV. Impedance parameters were determined
by fitting the impedance spectra using the Z-view software.
2. Experimental
2.3. Fabrication of dye-sensitized solar cells and measurements
2.1. Materials and reagents
The nanocrystalline TiO2 working electrode was comprised of a
TiO2 transparent layer (20 nm, synthesized) and a TiO2 scattering
layer (250 nm, G1). The Pt-coated counter electrode was prepared
by a reported procedure [20]. TiO2 electrodes were immersed in
THF solution containing the dyes at 0.5 mM for 40 h at ambient
temperature. They were then washed with ethanol and dried under
a stream of nitrogen. The working and counter electrodes were
Phenoxazine, 1-bromobutane, 4-iodoanisole, copper-tin alloy,
18-crown-6, phosphorus oxychloride, cyanoacetic acid and piperi-
dine were purchased from SigmaeAldrich and used as received
without further purification. All solvents (dimethylformamide, 1,2-
dichlorobenznene, dimethyl sulfoxide, dichloromethane, 1,2-
dichloroethane and acetonitrile) were obtained from Sigmae
Aldrich and used as received. Other chemicals were reagent grade
and used without further purification.
sealed with Surlyn (60 mm, Dupont) and electrolyte was injected
through a hole in the counter electrode. The electrolyte was
comprised of 0.7 M 1-propryl-3-methyl-imidazolium iodide (PMII,
synthesized), 0.2 M LiI (Aldrich), 0.05 M I2 (Aldrich), and 0.5 M 4-
tert-butylpyridine (Aldrich) in a mixed solvent of acetonitrile and
valeronitrile (v/v, 85/18). The active area of the dye-coated TiO2 film
was ca. 0.24 cm2, measured by analyzing the images from a CCD
2.2. Analytical instruments and measurements
1H NMR and 13C NMR spectra were recorded on a Bruker
Advance 500 and 600 MHz (Seoul National University National
Center for inter-University Research Facilities) with the chemical
shift against TMS. Mass data were measured with a JEOL JMS 600W
mass spectrometer (Seoul National University National Center for
inter-University Research Facilities). ATR-FTIR spectra were recor-
ded on a Nicolet 6700 spectrometer by using the window ZnSe/
diamond ATR accessory. UVevis spectra and photoluminescence
spectra were recorded on a HewlettePackard 8425A spectropho-
tometer and a Shimadzu RF-5301PC spectrofluorometer, respec-
tively. Cyclic voltammetry spectra were obtained using a three-
camera (moticam 1000). TiO2 film thickness was measured by a
step surface profiler (KLA tencor).
a-
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. Light intensity was adjusted with an NREL-
calibrated Si solar cell with KG-5 filter for approximating 1 sun-
light intensity. Photocurrentevoltage measurements of the dye-
sensitized solar cells were performed with an aperture mask
following a reported method. Incident photon-to-current conver-
sion 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. IPCE values were measured under halogen bias
light at a low chopping speed of 10 Hz. All calculations were carried
out using the 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.
electrode cell with
a 273A potentiostat (Princeton applied
research, Inc.). Measurements were taken using an Ag wire (Ag/
2.4. Synthesis of dyes
2.4.1. 10-Butyl-10H-phenoxazine (1)
To a phenoxazine (1.5 g, 0.0082 mol) solution in dry DMSO
(22.5 mL), sodium hydroxide (2.76 g, 0.069 mol) and 1-bromobutane
(1.89 g, 0.0137 mol) were slowly added 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 acetateehexane
Fig. 1. Structure of POX, WB, WH1 and WH2.

W. Lee et al. / Dyes and Pigments 102 (2014) 13e21
15
(1:10; v/v) as the eluent to give 1, colorless viscous liquid (1.8 g, 92%).
1H NMR (500 MHz, d6-DMSO):
(m, 4H), 3.53 (t, J ¼ 7.7 Hz, 2H),1.50e1.54 (m, 2H), 1.38e1.43 (m, 2H),
0.94 ppm (t, J ¼ 7.3 Hz, 3H).
2.4.7. (E)-(10-butyl-10H-phenoxazin-3-yl)-2-cyanoacrylic acid
(POX)
d
¼ 6.81 (d, J ¼ 8.7 Hz, 2H), 6.63e6.67
3 (0.46 g, 0.00173 mol), cyanoacetic acid (0.44 g, 0.0052 mol)
and piperidine (0.35 mL, 0.00693 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 (0.43 g, 75%). mp 232e233 ꢁC.
2.4.2. 10-(4-Methoxyphenyl)-10H-phenoxazine (2)
Under nitrogen atmosphere, phenoxazine (1.5 g, 0.0082 mol), 4-
iodoanisole (2.88 g, 0.0123 mol), CuSn (1.49 g, 0.0082 mol), K2CO3
(3.4 g, 0.0246 mol) and 18-crown-6 (0.38 g, 0.00145 mol) were
dissolved in dry 1,2-dichlorobenznene (60 mL). The mixture was
heated under refluxed for 48 h under nitrogen atmosphere. Then
the reaction mixture was filtered and washed with dichloro-
methane (DCM). The filtrate was extracted with DCM, water and
NH4OH. The organic phase was collected and dried over anhydrous
MgSO4. After removing solvent, the residue was purified by column
chromatography using DCM-hexane (1:3; v/v) to give 2, viscous
pale yellow liquid (1.42 g, 81.6%).
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), 6.69e6.78 (m, 4H), 3.59 (t,
J ¼ 7.6 Hz, 2H), 1.50e1.55 (m, 2H), 1.37e1.43 (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 ((Mþ),
1H NMR (300 MHz, d6-DMSO):
d
¼ 7.30 (d, J ¼ 8.6 Hz, 2H), 7.18
C
20H18N2O3 requires 334.1317).
(d, J ¼ 8.6 Hz, 2H), 6.62e6.72 (m, 6H), 5.85 (d, J ¼ 9.2 Hz, 2H),
ATR-FTIR (cmꢀ1): 2221 (cyano, C^N stretching band), 1686
3.83 ppm (s, 3H).
(carbonyl, C]O stretching band).
2.4.3. 10-Butyl-10H-phenoxazine-3-carbaldehyde (3)
2.4.8. 3,30- (10-Butyl-10H-phenoxazin-3,6-diyl) bis[2-cyanoacrylic
acid] (WB)
WB as a dark red solid (0.56 g, 57.3%) was synthesized according
to the procedure described above for the synthesis of POX. 4 (0.67 g,
0.0023 mol), cyanoacetic acid (1.04 g, 0.0138 mol) and piperidine
(1.35 mL, 0.021 mol) were added to anhydrous CH3CN (75 mL).
Eluent: DCM-methanol (2:1; v/v). mp 279e280 ꢁC.
To a solution of 1 (2.57 g, 0.01 mol) and dry DMF (5 mL) in dry
1,2-dichloroethane (21.4 mL) in an ice water bath, POCl3 (1.1 mL,
0.012 mol) was added dropwise below 15 ꢁC. The reaction was
heated to room temperature and heated under refluxed at 90 ꢁC for
48 h. The mixture was quenched with dilute NaOH (aq) and
extracted with water and 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 ethyl ace-
tateehexane (1:6; v/v) to give 3, yellow oil (1.91 g, 71.6%).
1H NMR (600 MHz, d6-DMSO):
d
¼ 7.97 (s, 2H), 7.48 (d,
J ¼ 8.4 Hz, 2H), 7.36 (s, 2H), 6.85 (t, J ¼ 7.5 Hz, 2H), 6.69e6.78 (m,
8H), 3.59 (t, J ¼ 7.6 Hz, 2H), 1.50e1.55 (m, 2H), 1.37e1.43 (m, 2H),
0.93 ppm (t, J ¼ 7.3 Hz, 3H).
1H NMR (500 MHz, d6-DMSO):
d
¼ 9.64 (s, 1H), 7.41 (dd, J ¼ 8.3,
1.8 Hz, 1H), 7.00 (s, 1H), 6.68e6.87 (m, 5H), 3.62 (t, J ¼ 7.9 Hz, 2H),
13C NMR (150 MHz, d6-DMSO):
d
¼ 163.7, 152.0, 143.6, 135.6,
1.52e1.56 (m, 2H), 1.40e1.45 (m, 2H), 0.95 ppm (t, J ¼ 7.3 Hz, 3H).
130.6, 129.4, 125.3, 116.8, 115.4, 112.9, 43.3, 26.8, 19.2, 13.7 ppm; m/z
(FAB) 429.1325 ((Mþ), C24H19N3O5 requires 429.1325).
ATR-FTIR (cmꢀ1): 2220 (cyano, C^N stretching band), 1680
(carbonyl, C]O stretching band).
2.4.4. 10-Butyl-10H-phenoxazine-3,7-dicarbaldehyde (4)
4 as an orange solid (1.46 g, 49.5%) was synthesized according to
the procedure described above for the synthesis of 3. To a solution
of 1 (2.57 g, 0.01 mol) and dry DMF (5 mL) in dry 1,2-dichloroethane
(21.4 mL) in an ice water bath, POCl3 (9.17 mL, 0.1 mol) was added
dropwise below 15 ꢁC. Eluent: DCM-methanol (7:1; v/v).
2.4.9. (E)-10-((4-methoxyphenyl)-10H-phenoxazin-3-yl)-2-
cyanoacetic acid (WH1)
WH1 as a red solid (0.37 g, 77%) was synthesized according to
the procedure described above for the synthesis of POX. 5
(0.4 g, 0.00126 mol), cyanoacetic acid (0.32 g, 0.0038 mol) and
piperidine (0.43 mL, 0.00504 mol) were added to anhydrous
CH3CN (100 mL). Eluent: DCM-methanol (20:1; v/v). mp 266e
267 ꢁC.
1H NMR (500 MHz, d6-DMSO):
d
¼ 9.68 (s, 2H), 7.44 (dd, J ¼ 8.3,
1.8 Hz, 2H), 7.05 (s, 2H), 6.94 (d, J ¼ 8.4 Hz, 2H), 3.68 (t, J ¼ 7.7 Hz, 2H),
1.52e1.57 (m, 2H), 1.41e1.45 (m, 2H), 0.95 ppm (t, J ¼ 7.3 Hz, 3H).
2.4.5. 10-(4-Methoxyphenyl)-10H-phenoxazine-3-carbaldehyde (5)
5 as a yellow solid (0.41 g, 76.2%) was synthesized according to
the procedure described above for the synthesis of 3. To a solution
of 2 (0.48 g, 0.0017 mol) and dry DMF (0.39 mL) in dry1,2-
dichloroethane (21.4 mL) in an ice water bath, POCl3 (1.1 mL,
0.012 mol) was added dropwise below 15 ꢁC. Eluent: DCM.
1H NMR (600 MHz, d6-DMSO):
d
¼ 7.97 (s, 1H), 7.49 (s, 1H), 7.36
(d, J ¼ 8.6 Hz, 2H), 7.30 (d, J ¼ 8.6 Hz, 1H), 7.20 (d, J ¼ 8.6 Hz, 2H),
6.79 (d, J ¼ 7.9 Hz, 1H), 6.74 (t, J ¼ 7.5 Hz, 1H), 6.68 (t, J ¼ 7.7 Hz, 1H),
5.89e5.91 (m, 2H), 3.84 ppm (s, 3H).
13C NMR (150 MHz, d6-DMSO):
d
¼ 163.9, 159.4, 152.4, 143.1,
1H NMR (500 MHz, d6-DMSO):
2H), 7.21e7.25 (m, 3H), 7.09 (s, 1H), 6.67e6.77(m, 3H), 5.96 (d,
J ¼ 8.5 Hz, 1H), 5.89 (d, J ¼ 8 Hz, 1H), 3.85 ppm (s, 3H).
d
¼ 9.62 (s, 1H), 7.36 (d, J ¼ 9 Hz,
143.0, 138.9, 132.3, 131.0, 130.6, 129.0, 124.3, 123.9, 122.9, 116.9,
116.5, 115.4, 114.7, 113.9, 112.7, 98.2, 55.4 ppm; m/z (FAB) 384.1115
((Mþ), C23H16N2O4 requires 384.1110).
ATR-FTIR (cmꢀ1): 2225 (cyano, C^N stretching band), 1679
(carbonyl, C]O stretching band).
2.4.6. 10-(4-Methoxyphenyl)-10H-phenoxazine-3,7-
dicarbaldehyde (6)
6 as an orange solid (0.27 g, 46%) was synthesized according to
the procedure described above for the synthesis of 3. To a solution
of 2 (0.48 g, 0.0017 mol) and dry DMF (1.4 mL) in dry1,2-
dichloroethane (10 mL) in an ice water bath, POCl3 (9.17 mL,
0.1 mol)was added dropwise below 15 ꢁC. Eluent: DCM.
2.4.10. 3,30-10-((4-Methoxyphenyl)-10H-phenoxazin-3,6-diyl) bis
[2-cyanoacrylic acid] (WH2)
WH2 as a dark red solid (0.59 g, 53.7%) was synthesized ac-
cording to the procedure described above for the synthesis of
POX. 6 (0.79 g, 0.0023 mol), cyanoacetic acid (1.06 g, 0.0138 mol)
and piperidine (1.58 mL, 0.02 mol) were added to anhydrous
CH3CN (75 mL). Eluent: DCM-methanol (2:1; v/v). mp 272e
273 ꢁC.
1H NMR (500 MHz, d6-DMSO):
d
¼ 9.67 (s, 2H), 7.42 (d,
J ¼ 8.5 Hz, 2H), 7.29 (dd, J ¼ 8.5, 1.5 Hz, 2H), 7.24 (d, J ¼ 7 Hz, 2H),
7.16 (s, 2H), 6.02 (d, J ¼ 8 Hz, 2H), 3.86 ppm (s, 3H).

16
W. Lee et al. / Dyes and Pigments 102 (2014) 13e21
1H NMR (600 MHz, d6-DMSO):
d
¼ 8.01 (s, 2H), 7.53 (s, 2H), 7.43
functional theory (DFT) at the B3LYP/6-31G(d,p) level. The opti-
mized structures with torsion angle and electron density of the
HOMOs and LUMOs of the dyes are shown in Table 1. In the opti-
mized structures, the POZ moieties of all the dyes exhibited almost
planar structures with small torsion angles (0.06e0.93ꢁ). These
planar structures enhanced the aromatic character of the hetero-
cyclic atom, increasing the degree of electronic resonance between
donor and acceptor moieties in the dye molecules. On the other
hand, the planar structure can increase the stacking of the dye
molecules, inducing more dye aggregation. The methoxyphenyl
substituent located on the POZ nitrogen atom made a large dihedral
angle of about 90ꢁ with the POZ core. This large dihedral angle
caused large steric hindrance that suppressed aggregations among
(d, J ¼ 8.8 Hz, 2H), 7.38 (d, J ¼ 8.6 Hz, 2H), 7.24 (d, J ¼ 8.9 Hz, 2H), 6.0
(d, J ¼ 8.5 Hz, 2H), 3.85 ppm (s, 3H).
13C NMR (150 MHz, d6-DMSO):
d
¼ 163.5, 159.7, 151.8, 143.1,
137.1, 130.7, 129.6, 128.2, 125.8, 116.7, 115.4, 113.8, 55.5 ppm; m/z
(FAB) 479.1120 ((Mþ), C27H17N3O6 requires 479.1117).
ATR-FTIR (cmꢀ1): 2220 (cyano, C^N stretching band), 1679
(carbonyl, C]O stretching band).
3. Results and discussion
3.1. Synthesis of dyes
dye molecules. However, this gave a small
p-system overlap be-
The detailed synthesis routes to the POZ type dyes are shown in
Scheme 1. The POZ dyes were initially modified by N-alkylation or
N-arylation of commercially available POZ. N-alkylation was per-
formed in DMSO with 1-bromobutane and NaOH as the base, giving
intermediate 1. N-arylation was also performed by the Ullmann
reaction, which produced the methoxyphenyl substituted phe-
noxazine, intermediate 2. The intermediates 1 and 2 were subse-
quently formylated via the VilsmeyereHaack reaction to synthesize
intermediates 3e6. During the formylation reactions, the number
of introduced aldehyde groups was controlled by varying the
amount of POCl3. Finally, the dyes (POX, WB, WH1 and WH2) were
synthesized by Knoevenagel condensations of the corresponding
aldehydes on the intermediate moieties with cyanoacetic acid [21].
The structures of all the synthesized intermediates and dyes were
confirmed by 1H NMR and the final products were additionally
identified by 13C NMR and HRMS.
tween the substituent and the POZ core, which limited the HOMO
delocalization over the methoxy phenyl substituent. All of the
electron distributions of the HOMO orbitals of the dyes were mostly
localized over the POZ moiety, whereas those of the LUMO orbitals
were mainly localized in the cyanoacrylic acid and its adjacent
phenyl ring. The results indicated that the HOMOeLUMO excitation
induced by light irradiation can effectively move the electron dis-
tribution from the POZ moiety to the cyanoacrylic acid moiety.
Photoinduced electrons can be efficiently transferred from the dye
to the TiO2 surface by this electron separation.
3.3. Photophysical properties of the dyes in solution and on TiO2
film
The absorption spectra of the four dyes in THF solution and on
the TiO2 surface are shown in Fig. 2 and the corresponding pho-
tophysical data are listed in Table 2. All the dyes exhibited two
major absorption bands at around 310 nm and 480 nm, respec-
tively. The former band at the shorter wavelength is due to the
3.2. Density functional theory (DFT) calculations
To explain the structural properties of the dyes, their geomet-
rically optimized structures were calculated using the density
localized aromatic
pep* transition, and the latter band at the
Scheme 1. Synthesis of POX, WB, WH1 and WH2.

W. Lee et al. / Dyes and Pigments 102 (2014) 13e21
17
Table 1
Optimized structures, dihedral angles and electronic distributions in HOMO and LUMO levels of the prepared dyes.
Dye
Optimized structure
HOMO
LUMO
POX
WB
WH1
WH2
longer wavelength is attributed to the intramolecular charge-
transfer (ICT) transition from the donor to the acceptor. The ab-
sorption maxima (lmax) of POX, WB, WH1 and WH2 are 464, 498,
458 and 498 nm, respectively. Di-anchoring dyes (WB and WH2)
were red-shifted in their absorption spectra, compared to the cor-
responding mono-anchoring dyes (POX and WH1). This is due to
the extension of electron delocalization over the whole molecule
caused by the introduction of an additional anchoring moiety. WH1
including the N-methoxyphenyl ring showed a slight blue-shift in
its absorption spectrum, compared to POX including the N-butyl
chain. As shown in the calculation study, this is because the N-
methoxyphenyl ring, which is perpendicularly to the POZ plane,
gave a poor orbital overlap, which led to inefficient conjugation
[19]. The molar extinction coefficients at lmax of POX, WB, WH1 and
WH2 were 38,957, 34,229, 37,224 and 47,893 Mꢀ1 cmꢀ1, respec-
tively. These are higher than those of conventional TPA and PTZ
dyes so the light harvesting of the dyes and photocurrent genera-
tion of the cells are increased. As shown in Fig. 2, di-anchoring dyes
showed intermediate shoulders in their absorption spectra at
around 400 nm. This might be attributed to the formation of
localized electron transitions in the dye molecules induced by the
additional anchoring group. This phenomenon is also shown in
absorption spectra of the dyes on TiO2 surface.
The absorption spectra of the dyes absorbed on TiO2 films are
blue-shifted compared to those of the dyes in solution. H-aggre-
gation of the dyes or deprotonation of carboxylic acid upon
adsorption onto the TiO2 surfaces has been suggested as the cause
of a blue-shift of the spectrum [22]. The absorption maxima of the
dyes on TiO2 were similarly ranked as those of the dyes in solution.
The dyes with the N-methoxyphenyl ring showed lower absor-
bance than the dyes with the N-butyl chain. This was ascribed to
the lower adsorption of the dyes with the N-methoxyphenyl ring
due to the large steric hindrance.
3.4. Electrochemical properties
The electrochemical properties and energy levels of the syn-
thesized dyes are provided in Table 2 and Fig. 3. The oxidation
potentials of the dyes (Fig. S1) were measured by cyclic voltam-
metry (CV). The HOMO levels of the four dyes correspond to the
first oxidation potential versus normal hydrogen electrode (vs.
NHE) calibrated by Fc/Fcþ (with 640 mV vs. NHE). All the HOMO
levels of the dyes ranged from 1.01 eV to 1.16 eV (vs. NHE) and were
much more positive than the redox potential of Iꢀ/I3ꢀ (0.4 V vs. NHE)
[24]. Therefore, the oxidized dyes were sufficiently regenerated by
the redox electrolyte. Their LUMO levels, corresponding to excited
state oxidation potentials, were calculated by Eox ꢀ E0e0, where E0e
is the zerothezeroth energy of the dye estimated from the
0
intersection between the normalized absorption and emission
spectra (Figs. S2 and S3). The LUMO levels of the dyes ranged
from ꢀ1.15 to ꢀ1.37 and were more negative than the conduction
bands edge of TiO2 (ꢀ0.5 vs. NHE) [24], thus indicating that the
excited electrons of the dyes can be efficiently injected into the
conduction band of TiO2.
The LUMO levels of the di-anchoring dyes were more positive
than those of mono-anchoring dyes possibly due to the extension of
conjugation induced by the additional anchoring group. Conse-
quently, although the additional anchoring group made the HOMO
level of the di-anchoring group slightly more positive, the HOMOe
LUMO gap decreased and the absorption spectra were red-shifted.
3.5. Photovoltaic properties
DSSCs were fabricated using the synthesized dyes according to the
methods described in the experimental section and their photovol-
taic properties were measured under standard AM 1.5G irradiation
conditions (100 mW cmꢀ2). The incident photon-to-current

18
W. Lee et al. / Dyes and Pigments 102 (2014) 13e21
0.6
0.5
0.4
0.3
0.2
0.1
0.0
POX
WB
(a)
WH1
WH2
300
400
500
600
Wavelength (nm)
Fig. 3. Dyes’ HOMO and LUMO energy levels.
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
(b)
POX
WB
red-shifted spectra could inject more electrons effectively into the
TiO2 conduction band. On the other hand, the N-substituted
methoxyphenyl ring decreased the donating ability of the dyes
because it had a poor orbital overlap with the POZ moiety [19].
Therefore, the cells based on the dyes with the N-methoxyphenyl ring
showed narrower IPCEs compared to those with the N-butyl chain,
resulting in lower Jsc values. Photovoltaic properties of DSSCs were
affected by the amount of the dye adsorbed onto the TiO2 surface. As
shown in Table 3, the dyes with the N-methoxyphenyl ring were less
absorbed than those with the N-butyl chain because of their bulky
structures. As a result, the cells with WH1 and WH2 showed nar-
rower IPCE spectra and lower Jsc values than those with POX and WB,
respectively.
WH1
WH2
Voc values increased in the order of WB < WH2 < POX < WH1.
The cells fabricated with the di-anchoring dyes showed lower Voc
values compared to those with the mono-anchoring dyes. The di-
anchoring dyes transfer more protons to the TiO2 surface upon
adsorption [25]. Thus, the high concentration of the protons
changes the TiO2 surface to a more positive state, lowering the
conduction band edges of TiO2 and lowering Voc [26,27]. To confirm
the binding of the di-anchoring dyes, their FT-IR spectra on the TiO2
surface were measured (Fig. S4). The FT-IR spectra of the di-
anchoring dyes showed that the broad band at 1685 cmꢀ1
assigned to the carboxylic groups of free dyes disappeared as the
dyes were adsorbed on the TiO2, indicating their double-anchoring
behavior [28]. The DSSCs based on the dyes with the N-methox-
yphenyl ring showed higher Voc than those with the N-butyl chain.
This was due to the bulky methoxyphenyl ring that suppressed
dye aggregation and the recombination between the injected
electrons on TiO2 and the electrolyte. Consequently WH1, the
400
500
600
Wavelength (nm)
Fig. 2. Absorbtion spectra of POX, WB, WH1 and WH2 in (a) THF (10ꢀ5 mol Lꢀ1) and
(b) on TiO2.
conversion (IPCE) spectra and photocurrentevoltage (JeV) curves are
shown in Fig. 4 and corresponding data are listed in Table 3. The
highest IPCE values of the cells with all dyes were over 79% in the
absorption range ca. 400e600 nm. All the dyes could efficiently
convert visible light to photocurrent in the spectral ranges. The Jsc
values of the dyes were ranked WH1 < WH2 < POX < WB and this
trend was in agreement with the broadness of IPCE spectrum. IPCE
spectra of the cells with the di-anchoring dyes were broader than
those with mono-anchoring dyes, promoting higher Jsc values. This
result indicated that the di-anchoring dyes with broad and
Table 2
Photophysical and electrochemical properties of POX, WB, WH1 and WH2.
Dye
Absorptiona
Emissiona
Oxidation potential datac
lmax/nm
3
/Mꢀ1c mꢀ1 (at lmax
)
labsb/nm (on TiO2)
lmax/nm
Eox/V (vs. NHE)
E0e0d/V
Eox ꢀ E0e0/V (vs. NHE)
POX
WB
WH1
WH2
464
498
458
498
38,957
34,229
37,224
47,893
444
482
426
494
558
565
555
554
1.01
1.14
1.06
1.16
2.38
2.29
2.42
2.33
ꢀ1.37
ꢀ1.15
ꢀ1.36
ꢀ1.17
a
Measured in 1 ꢂ 10ꢀ5 THF solutions at room temperature.
b
c
Measured on TiO2 film.
Measured in DMF 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 [23]).
d
E0e0 was determined from the intersections of absorption and emission spectra.

W. Lee et al. / Dyes and Pigments 102 (2014) 13e21
19
Fig. 5. DSSCs’ JeV curves based on POX, WB, WH1 and WH2 in the dark.
higher dark current is accountable for increased recombination and
low Voc. Thus, as shown in Fig. 5, the low dark current of the cell
with WH1 is consistent with the highest Voc, whereas the high dark
current of the cell with WB is consistent with the lowest Voc
All four dyes exhibited almost the same overall conversion ef-
ficiencies and the values of POX, WB, WH1 and WH2 were 4.97%,
.
h
5.03%, 5.09% and 4.86%, respectively. This similarity is ascribed to
the trade-off relationship between Voc and Jsc for the cells based on
the synthesized dyes. The best overall conversion efficiency was
achieved by the cell fabricated with WH1 (
h
¼ 5.09%, Jsc ¼ 10.11 mA/
cm2, Voc ¼ 690 mV, FF ¼ 72.23%).
3.6. Electrochemical impedance spectroscopy
To elucidate the correlation between the Voc of the cell and the
dye, electrochemical impedance spectroscopy (EIS) [29] was car-
ried out in the dark and under illumination. A Nyquist plot in the
dark under a forward bias of 0.55 V with frequency range of 0.1 Hze
100 kHz and a Bode phase plot under AM 1.5G mW cmꢀ2 illumi-
nation are shown in Fig. 6.
Fig. 4. (a) IPCE spectra DSSCs based on POX, WB, WH1 and WH2 and (b) the DSSCs’ Je
V curves under AM 1.5G simulated sunlight (100 mW cmꢀ2).
mono-anchoring dye with the N-methoxyphenyl ring, showed the
highest Voc, and WB, the di-anchoring dye with the N-butyl chain,
showed the lowest Voc. The dark currents of all DSSCs were also
measured (Fig. 5); their trends were in good agreement with the Voc
ranks of the dyes. Dark current is defined as the relatively small
electric current flowing out of a system without illumination; a
The major semicircle in the Nyquist plot indicate charge
recombination resistance at the TiO2 surface; a larger radius of the
major semicircle means a larger charge recombination resistance
[30]. The radius of the major semicircle in the Nyquist plot
increased in the order of WB < WH2 < POX < WH1, implying
increasing resistance to charge recombination. This result coincides
with the increase of Voc in the DSSCs based on the dyes, as the
suppression of electron recombination between the injected elec-
trons and electrolyte improves Voc. This trend is in agreement with
results of the electron lifetime vs dark bias voltage (Fig. 7). Electron
lifetime is calculated by multiplying the resistance by the chemical
capacitance (Fig. S5 and Table S1). The chemical capacitances of the
dyes were almost similar, so that electron lifetimes were mainly
dependent on the resistances. Thus, longer electron lifetimes imply
increased resistance between the injected electrons and the elec-
trolyte, which consequently improves the Voc [31].
Table 3
DSSC performance parameters of POX, WB, WH1, and WH2.a
Dyeb
Jsc (mA/cm2)
Voc(V)
FF (%)
h
(%)
Dye amountc
(10ꢀ7 mol cmꢀ2
)
POX
WB
WH1
WH2
N719
11.19
11.79
10.11
10.75
14.89
0.68
0.65
0.69
0.66
0.78
64.97
65.10
72.23
67.5
4.97
5.02
5.09
4.86
7.69
1.4207
1.8653
1.2744
1.2370
e
63.18
a
Measured under AM 1.5 irradiation G (100 mW cmꢀ1); 0.236e0.3 cm2 working
A Bode phase plot is also related to the charge transfer resistance
at the TiO2/dye/electrolyte interface. The frequency of the charac-
teristic peak in the Bode phase plot increased in the order of
WH1 < POX < WH2 < WB. A lower characteristic frequency in the
Bode phase plot means a slower charge recombination rate and
higher Voc. As the reciprocal of the characteristic frequency is
correlated with electron lifetime, electron lifetime increased in the
area.
b
Dyes were maintained at 0.5 mM in THF solution, with 10 mM CDCA 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 acetonitrileevaleronitrile (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.

20
W. Lee et al. / Dyes and Pigments 102 (2014) 13e21
Fig. 6. Impedance spectra of DSSCs based on POX, WB, WH1, and WH2. (a) Nyquist plots measured at 0.55 V forward bias in the dark, (b) Bode phase plots measured under il-
luminations (AM 1.5G).
order of WB < WH2 < POX < WH1 [32]. This sequence of the
electron lifetime calculated from the Bode phase plot also indicated
increasing resistance to recombination. These results suggest that
the introduction of the methoxy phenyl ring improved Voc, whereas
This lowered the Fermi level of the TiO2 and the Voc of the cells. The
adsorption of dyes with the N-methoxyohenyl ring was limited by
their bulky structures. Therefore, the dyes with the N-methox-
yohenyl ring showed lower Jsc values compared to the dyes with the
butyl chain.
the introduction of the additional anchoring group decreased Voc
.
Consequently, introduction of the additional anchoring group
enhanced Jsc, but decreased Voc. To improve the lowered Voc caused
by the di-anchoring system, a methoyphenyl ring was introduced
on the POZ core. The introduction of the 4-methoxyphenyl ring
increased the Voc, but decreased the Jsc simultaneously. Thus, with
the increases and decreases of Jsc and Voc, all DSSCs fabricated with
the dyes showed similar overall conversion efficiencies, and the
cells based on WH1 showed the best overall conversion efficiency
4. Conclusion
Four new organic sensitizers with POZ dyes (POX, WB, WH1 and
WH2) were designed and synthesized to investigate the effects of
an additional anchoring group and the N-substituent on the per-
formance of DSSCs.
The introduction of the additional anchoring group extended
the conjugation of the dyes, leading to the red-shifts of their ab-
sorption spectra. The di-anchoring dyes transferred more electrons
to the TiO2 electrode, increasing the Jsc value. The introduction of
the methoxyphenyl ring located on the POZ nitrogen atom caused
steric hindrance due to the large dihedral angle between the N-
substituent and the POZ core. As a result, the electron recombina-
tion between the injected electron and the electrolyte was
retarded, improving the Voc value.
(h
¼ 5.09%, Jsc ¼ 10.11 mA/cm2, Voc ¼ 690 mV, FF ¼ 72.23%).
Acknowledgments
This work was supported by a Grant-in-Aid for the Industrial
Strategic Technology Program from the Korea Ministry of Knowl-
edge Economy (No. 10038599)
On the other hand, the di-anchoring dyes transferred more
protons to the TiO2 surface compared to the mono-anchoring dyes.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2013.10.005.
10000
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