Effects of introducing functional groups on the performance of phenoxazine-based dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2018.11.010

Phenoxazine dyes with a hexyloxy chain and furan moiety were synthesized and used in dye-sensitized solar cells. The dyes were designed to investigate the effects of these substituents on the photophysical and electrochemical properties of the dyes and on cell photovoltaic performance. The introduced hexyloxy moiety, as an additional donor, improved the short-circuit current and open-circuit voltage of the cell by increasing light absorption and steric hindrance, respectively. The furan moiety introduced in the phenoxazine dye as a bridge unit led to a bathochromic shift of the absorption band and a large amount of dye adsorption, increasing the short-circuit current of the cell. Among the synthesized dyes, those with the furan moiety showed the best conversion efficiency of 6.34%.

Full text of DOI:10.1016/j.dyepig.2018.11.010

Accepted Manuscript
Effects of introducing functional groups on the performance of phenoxazine-based
dye-sensitized solar cells
Jong Hyuk Bae, Seung Ju Lim, Jun Choi, Sim Bum Yuk, Jin Woong Namgoong, Jae
Hoon Ko, Woosung Lee, Jae Pil Kim
PII:
S0143-7208(18)31861-8
https://doi.org/10.1016/j.dyepig.2018.11.010
DYPI 7154
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 31 August 2018
Revised Date: 11 October 2018
Accepted Date: 5 November 2018
Please cite this article as: Bae JH, Lim SJ, Choi J, Yuk SB, Namgoong JW, Ko JH, Lee W, Kim JP,
Effects of introducing functional groups on the performance of phenoxazine-based dye-sensitized solar
cells, Dyes and Pigments (2018), doi: https://doi.org/10.1016/j.dyepig.2018.11.010.
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ACCEPTED MANUSCRIPT
Effects of introducing functional groups on the performance of phenoxazine-
based dye-sensitized solar cells
Jong Hyuk Baea, Seung Ju Lima, Jun Choib, Sim Bum Yukc, Jin Woong Namgoongc, Jae
Hoon Koa, Woosung Leea,*, and Jae Pil Kimc,**
aSmart Textiles R&D Group, Korea Institute of Industrial Technology (KITECH), 143 Hanggaulro, Sangnokgu,
Ansan-si, Gyeonggi-do 426-910, Korea
bHuman Convergence Technology Group, Korea Institute of Industrial Technology (KITECH), 143 Hanggaulro,
Sangnokgu, Ansan-si, Gyeonggi-do 426-910, Korea
cDepartment of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea
* Corresponding author. Tel.: +82 31 8040 6128; fax: +82 31 8040 6120
E-mail address: wslee@kitech.re.kr (W. Lee)
** Corresponding author. Tel.: +82 2 880 7187; fax: +82 2 880 7238
E-mail address: jaepil@snu.ac.kr (J. P. Kim)
Abstract
Phenoxazine dyes with a hexyloxy chain and furan moiety were synthesized and used in
dye-sensitized solar cells. The dyes were designed to investigate the effects of these
substituents on the photophysical and electrochemical properties of the dyes and on cell
photovoltaic performance. The introduced hexyloxy moiety, as an additional donor, improved
the short-circuit current and open-circuit voltage of the cell by increasing light absorption and
steric hindrance, respectively. The furan moiety introduced in the phenoxazine dye as a
bridge unit led to a bathochromic shift of the absorption band and a large amount of dye
adsorption, increasing the short-circuit current of the cell. Among the synthesized dyes, those
with the furan moiety showed the best conversion efficiency of 6.34%.

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Keywords: Dye-sensitized solar cells; Phenoxazine; π-conjugation; Aggregation; Dye
adsorption; Charge recombination.
1. Introduction
Dye-sensitized solar cells (DSSCs) have attracted considerable attention as promising
alternatives to conventional inorganic solar devices since Grätzel et al. reported on ruthenium
(Ru)-based photosensitizers in 1991 and their high efficiency and low-cost fabrication [1].
Typical DSSCs consist of a dye-coated nanoporous semiconductor film as the anode
electrode, a redox shuttle, and a Pt counter electrode [2]. In such devices, as an organic
sensitizer, a dye is one of the key components responsible for high light-harvesting and power
conversion efficiency and has been extensively investigated over the past decade [3]. The
widely developed dyes in DSSCs are Ru complexes, which have shown high photoelectric
conversion efficiencies of over 11% under AM 1.5 conditions [3c]. However, high production
costs and difficulties in purification have limited the widespread application of Ru complexes.
Hence, to overcome the shortcomings of Ru-based dyes, metal-free organic dyes and
organometallic dyes have been recently developed because of their lower cost, easier
modification and purification, environmental friendliness, and high molar extinction
coefficient [4]. The highest conversion efficiencies of organic dyes (>14%, [5]) and
organometallic dyes (13%, [5a], [6]) indicate that non-Ru dyes could be promising sensitizers
for high-efficiency DSSCs. As such, non-Ru sensitizers such as coumarin [7], carbazole [8],
fluorine [9], hemicyanine [10], indoline [11], merocyanine [12], perylene [13], polyene [14],
porphyrin [4a], [6], [15], phthalocyanine [16], triphenylamine [4d], [17], and phenothiazine
[18] have been extensively studied.
A basic design concept of metal-free organic sensitizers is characterized by an electron

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donor-π-conjugated linker-acceptor configuration, which is termed the D-π-A structure. In
general, the D-π-A sensitizer has a rod-like configuration, which may induce unwanted dye
aggregations and charge recombination. The strong dipole-dipole interactions between the
delocalized π-bonds in the dye molecules can lead to self-quenching of the excited state of a
dye, causing inefficient electron injection. To suppress dye aggregation, bulky substituents
are commonly introduced in dye molecules. The introduced bulky moiety can cause steric
hindrances between dye molecules and/or break the planarity of dye molecules, which retards
charge recombination. Furthermore, to achieve higher photovoltaic performance, organic
dyes with an additional donor have been suggested [19]. The introduction of an additional
donor group could increase the electron-donating capability, which would improve electron
injection and charge separation. In other studies, narrow absorption windows of metal-free
dyes compared to Ru-based dyes have been shown to limit light-harvesting in the visible
region and, hence, have produced unfavorable photovoltaic properties. Therefore, to exhibit
broad and red-shifted absorption of dyes, extension of π-conjugated systems is required. One
feasible strategy to extend the conjugation pathway is the use of an electron-rich heterocyclic
aromatic ring as a π-conjugated linker [17c], [20]. In particular, furan and thiophene have
displayed the most promising results, exhibiting wide and red-shifted absorption spectra, as
well as high molar extinction coefficients [21]. It is worth noting that a phenoxazine (POZ)
moiety includes electron-rich oxygen and nitrogen atoms in a heterocyclic ring, and also
displays high electron-donating capability. It also has suitable electrochemical properties,
which implies that POZ could be a promising sensitizer in DSSCs.
In this study, for efficient DSSC performance, a bulky methoxyphenyl ring was introduced
to the POZ nitrogen atom. In our previous research, an N-methoxyphenyl ring caused
significant steric hindrances, which reduced dye aggregation and suppressed the electron
recombination between the electrons injected into the TiO2 and the holes in the electrolyte.

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Furthermore, as an additional donor, a hexyloxy chain was substituted in the 7-position of the
POZ dye. A long alkoxy chain could also reduce dye aggregation to some extent and prevent
electrons in the TiO2 from recombining with the electrolyte. Meanwhile, to achieve an
extended absorption spectrum and high molar absorption coefficient, furan was introduced to
the POZ dye as a π-conjugated linker. As reported in previous studies, furan showed
improved photovoltaic properties compared to thiophene, particularly exhibiting higher
current values [20b].
Based on these strategies, three POZ-based organic dyes (WH1, WS4, and WS5) were
designed and synthesized. The photophysical and electrochemical properties of the
synthesized dyes were investigated 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 cell
2. Experimental
2.1. Materials and reagents
Phenoxazine, 4-iodoanisole, copper-tin alloy, 18-crown-6, 5-formyl-2-furan-boronic acid,
tetrakis(triphenylphosphine) palladium (0), 1-hexanol, potassium phosphate, copper(I) iodide,
phosphorus oxychloride, cyanoacetic acid, and piperidine were purchased from Sigma-
Aldrich and used as received without further purification. N-bromosuccinimide and 8-
hydroxyquinoline were purchased from TCI and used as received without further purification.
All solvents (chloroform, tetrahydrofuran, dimethylformamide, 1,2-dichlorobenznene,
dimethyl sulfoxide, dichloromethane, 1,2-dichloroethane, and acetonitrile) were obtained
from Sigma-Aldrich and used as received. Other chemicals were reagent-grade and used

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without further purification.
2.2. Analytical instruments and measurements
1H NMR and 13C NMR spectra were recorded on a Bruker Avance spectrometer at 500 MHz
and 600 MHz using DMSO and CHCl3, with the chemical shift against tetramethylsilane
(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). UV-vis spectra were
measured with a Hewlett-Packard 8425 A spectrophotometer. 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/Ag+), 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/Fc+) redox couple was
employed to calibrate the oxidation peak. Photocurrent-voltage measurements were
performed using a Keithley Model 2400 source measure unit. A 1000 W xenon (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 DSSCs (PV
Measurements, Inc.). A 75 W Xe lamp was employed as a light source to generate a
monochromatic beam. The electrical impedance spectra of the DSSCs under dark conditions
with 0.60 V forward bias were measured with an impedance analyzer (Compactstat, IVIUM
Tech) at frequencies of 10-1–106 Hz. The magnitude of the alternative signal was 10 mV. The

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impedance parameters were determined by fitting the impedance spectra using Z-view
software.
2.3. Fabrication of dye-sensitized solar cells and measurements
A photoanode paste was prepared for a screen-printing process [22]. The final composition
of the paste consisted of TiO2 nanopowder (1 g), ethyl cellulose (0.5 g), terpineol (3.3 mL),
and acetic acid (0.16 mL). Pre-washed FTO glass was then coated by a doctor blade process
and heated at 70 °C for 30 min for drying. After 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 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 the spin-
coating method using 5 mM H2PtCl6 solution (in ethanol) on one-hole 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 0.5 mM dyes for 48 h at ambient temperature. After
dye adsorption, 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 preparation, 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 onto the one-hole FTO glass using a capillary 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 microscope (Moticam 1000). The TiO2 film thickness was measured by an α-

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step surface profiler (KLA Tencor).
Photocurrent–voltage (I–V) measurements were performed using a Keithley Model 2400
source measurement 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 using a KG-5 filter to approximate the light intensity of one sun. Photocurrent–
voltage measurements of the DSSCs were performed with an aperture mask by following a
reported method. The IPCE was measured as a function of wavelength from 300 to 1000 nm
using a specially designed IPCE system for DSSCs (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 the Gaussian 09 software.
Optimized geometries, energy levels, and frontier molecular orbitals of the dyes’ highest
occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs)
were calculated at the B3LYP/6-31G (d,p) level.
2.4. Synthesis of dyes
2.4.1. 10-(4-Methoxyphenyl)-10H-phenoxazine (1)
Under nitrogen atmosphere, phenoxazine (1.5g, 0.0082mol), 4-iodoanisole (2.88g,
0.0123mol), CuSn (1.49g, 0.0082mol), K2CO3 (3.4g, 0.0246mol) and 18-crown-6 (0.38g,
0.00145mol) were dissolved in dry 1,2-dichlorobenznene (60mL). The mixture was heated
under refluxed for 48h under nitrogen atmosphere. Then the reaction mixture was filtered and
washed with dichloromethane (DCM). The filtrate was extracted with DCM, water and
NH4OH. The organic phase was collected and dried over anhydrous MgSO4. After removing

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solvent, the residue was purified by column chromatography using DCM-hexane (1:3; v/v) to
give 1, viscous pale yellow liquid (1.42g, 81.6%).
1H NMR (300MHz, d6-DMSO): δ = 7.30 (d, J = 8.6 Hz, 2H), 7.18 (d, J = 8.6 Hz, 2H), 6.62-
6.72 (m, 6H), 5.85 (d, J = 9.2 Hz, 2H), 3.83 ppm (s, 3H).
13C NMR (125MHz, d6-DMSO): δ = 158.5, 142.6, 133.7, 130.9, 130, 123, 120.6, 115.9,
114.6, 112.5, 54.9 ppm;
m/z (FAB) 289.1097 ((M +), C19H15NO2 requires 289.1103).
2.4.2. 10-(4-Methoxyphenyl)-10H-phenoxazine-3-carbaldehyde (2)
POCl3 (1.1mL, 0.012mol) was added dropwise to a solution of 1 (0.48g, 0.0017mol) and dry
DMF (0.39mL) in dry1,2-dichloroethane (21.4mL) in an ice water bath with temperature
below 15℃. The reaction was heated to room temperature and refluxed at 90℃for 48h. 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 DCM to give 2, yellow solid
(0.41g, 76.2%).
1H NMR (500MHz, d6-DMSO): δ = 9.62 (s, 1H), 7.36 (d, J = 9 Hz, 2H), 7.21-7.25 (m, 3H),
7.09 (s, 1H), 6.67-6.77(m, 3H), 5.96 (d, J = 8.5 Hz, 1H), 5.89 (d, J = 8 Hz, 1H), 3.85 ppm (s,
3H).
13C NMR (125MHz, d6-DMSO): δ = 189.9, 159.3, 143.3, 142.9, 139.8, 132.6, 131, 129.9,
129.3, 128.1, 123.9, 122.6, 116.6, 115.3, 113.8, 112.6, 55.4 ppm;
m/z (FAB) 317.1055 ((M +), C20H15NO3 requires 317.1052).
2.4.3. 3-Bromo-10-(4-Methoxyphenyl)-10H-phenoxazine (3)

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2 (1.79g, 0.0062mol) and N-bromosuccinimide (1.106g, 0.0062mol) were dissolved in
chloroform (30mL), and the reaction was stirred for 1h 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 DCM-hexane (1:3; v/v) to give 3, white solid (1.64g, 72%).
1H NMR (500MHz, CDCl3): δ = 7.23 (d, J = 9 Hz, 2H), 7.07 (d, J = 8.5 Hz, 2H), 6.56-6.67
(m, 5H), 5.91 (d, J = 7.5 Hz, 2H), 3.87 ppm (s, 3H).
13C NMR (125MHz, CDCl3): δ = 159.3, 144, 134.7, 131.7, 123.1, 121, 116.1, 115.2, 113.2,
55.4 ppm;
m/z (FAB) 367.0210 ((M +), C19H4BrNO2 requires 367.0208).
2.4.4. 5-(10-(4-Methoxyphenyl)-10H-phenoxazin-3-yl)furan-2-carbaldehyde (4)
Under nitrogen atmosphere, a mixture of 3 (3.57g, 0.0097mol), 5-formyl-2-furan-boronic
acid (6.51g, 0.0464mol), 2M aqueous of K2CO3 (48mL), Pd(PPh3)4 (2.24g, 0.00194mol) in
dry THF (100mL) was stirred for 1/2 h and refluxed at 80℃ 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 4, orange solid (1.58g, 43%).
1H NMR (600MHz, d6-DMSO): δ = 9.47 (s, 1H), 7.55 (s, 1H), 7.31 (d, J = 8.7 Hz, 2H),
7.16-7.19 (m, 3H), 7.12 (d, J = 8.4 Hz, 1H), 7.06 (s, 1H), 6.71 (d, J = 7.6 Hz, 1H), 6.62-6.68
(m, 2H), 5.91 (d, J = 8.4 Hz, 1H), 5.83 (d, J = 7.5 Hz, 1H), 3.80 ppm (s, 3H).
13C NMR (150MHz, d6-DMSO): δ = 177.1, 159.2, 157.9, 151.1, 143.5, 142.9, 135.6, 133.4,
131.3, 129.7, 123.9, 121.9, 121.6, 120.9, 116.5, 115.3, 113.4, 113.3, 11.7, 107.5, 55.4 ppm;
m/z (FAB) 383.1162 ((M +), C24H17NO4 requires 383.1158).

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2.4.5. 7-Hexyloxy-10-(4-Methoxyphenyl)-10H-phenoxazine (5)
Under nitrogen atmosphere, a mixture of 3 (1.51g, 0.0041mol), CuI (0.04g, 0.0002mol), 8-
hydroxyquinoline (0.06g, 0.00041mol), and K3PO4 (1.75g, 0.0082mol) was stirred for 1/2 h.
1-hexanol (6 mL) was added to the mixture by syringe and refluxed at 110℃ for 24h. The
reaction mixture was extracted with DCM, water and brine. The residue was purified by
column chromatography using DCM-hexane (1:3; v/v) to give 5, pale yellow solid (1.13g,
49%).
1H NMR (500MHz, d6-DMSO): δ = 7.28 (d, J = 8.5 Hz, 2H), 7.18 (d, J = 8.5 Hz, 2H), 6.70
(d, J = 8 Hz, 1H), 6.63-6.66 (m, 2H), 6.38 (s, 1H), 6.24 (d, J = 7.5 Hz, 1H), 5.86 (s, 1H), 5.78
(d, J = 8.5 Hz, 1H), 3.82-3.84 (m, 5H), 1.60-1.66 (m, 2H), 1.33-1.39 (m, 2H), 1.28-1.29 (m,
4H), 0.86-0.88 ppm (m, 3H).
13C NMR (125MHz, d6-DMSO): δ = 158.3, 153.1, 143, 142, 133.9, 131, 130.6, 127.2, 123.2,
120.1, 115.8, 114.6, 112.9, 112.3, 107.9, 102.3, 67.2, 54.8, 30.4, 28, 24.5, 21.4, 13.3 ppm;
m/z (FAB) 389.1190 ((M +), C25H27NO3 requires 389.1191).
2.4.6. 7-Hexyloxy-10-(4-Methoxyphenyl)-10H-phenoxazine-carbaldehyde (6)
6 as an yellow solid (0.35g, 45%) was synthesized according to the procedure described for
the synthesis of 2. POCl3 (0.525mL, 0.0114mol) was added under nitrogen atmosphere to a
solution of 5 (0.744g, 0.0019mol) and dry DMF (15mL) in dry1,2-dichloroethane (32 mL).
Eluent: DCM.
1H NMR (500MHz, d6-DMSO) : δ = 9.60 (s, 1H), 7.34 (d, J = 7 Hz, 2H), 7.20-7.24 (m, 3H),
7.08 (s, 1H), 6.42 (s, 1H), 6.27 (d, J = 8.5 Hz, 1H), 5.94 (d, J = 8 Hz, 1H), 5.80 (d, J = 9 Hz,
1H), 3.82-3.85 (m, 5H), 1.60-1.65 (m, 2H), 1.34-1.37 (m, 2H), 1.26-1.29 (m, 4H), 0.85-0.87
ppm (m, 3H).

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13C NMR (125MHz, d6-DMSO): δ = 158.8, 154.2, 142.9, 142.2, 139.4, 130.6, 129.1, 128.8,
127.8, 125.5, 116, 113.8, 113.3, 111.6, 108.2, 102.4, 67.3, 54.9, 30.4, 30.1, 28, 24.5, 21.5,
13.3 ppm;
m/z (FAB) 417.1935 ((M +), C26H27NO4 requires 417.1940).
2.4.7. (E)-10-((4-Methoxyphenyl)-10H-phenoxazin-3-yl)-2-cyanoacetic acid (WH1)
2 (0.4g, 0.00126mol), cyanoacetic acid (0.32g, 0.0038mol) and piperidine (0.43mL,
0.00504mol) were added to anhydrous CH3CN (100mL). After the mixture was refluxed for
8h, the solution was extracted with DCM and 0.1M 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 (20:1; v/v) to give WH1, red
solid (0.43g, 75%).
1H NMR (600MHz, d6-DMSO): δ = 7.97 (s, 1H), 7.49 (s,1H), 7.36 (d, J = 8.6 Hz, 2H), 7.30
(d, J = 8.6 Hz, 1H), 7.20 (d, J = 8.6 Hz, 2H), 6.79 (d, J = 7.9 Hz, 1H), 6.74 (t, J = 7.5 Hz, 1H),
6.68 (t, J = 7.7 Hz, 1H), 5.89-5.91 (m, 2H), 3.84 ppm (s, 3H).
13C NMR (150MHz, d6-DMSO): δ = 163.9, 159.4, 152.4, 143.1, 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 ((M +), C23H16N2O4 requires 384.1110).
ATR-FTIR (cm-1): 2225 (cyano, C≡N stretching band), 1679 (carbonyl, C=O stretching
band).
2.4.8. (E)-3-(7-hexyloxy-10-(4-Methoxyphenyl)-10H-phenoxazin-3-yl)-2-cyanoacrylic acid
(WS4)

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WS4 as a red solid (0.2g, 77%) was synthesized according to the procedure described above
for the synthesis of WH1. 6 (0.23g, 0.00055mol), cyanoacetic acid (0.14g, 0.00165mol) and
piperidine (0.218mL, 0.0033mol) were added to anhydrous CH3CN (50mL).
Eluent: DCM- methanol (10:1; v/v).
1H NMR (500MHz, CDCl3): δ = 7.92 (s, 1H), 7.53 (s, 1H), 7.17-7.26 (m, 3H), 7.10 (d, J =
7.5 Hz, 1H), 6.35 (s, 1H), 6.15 (s, 1H), 5.94 (s, 1H), 5.88 (s, 1H), 3.83-3.89 (m, 5H), 1.70-
1.73 (m, 2H), 1.40-1.43 (m, 2H), 1.30-1.33 (m, 4H), 0.88-0.91 ppm (m, 3H).
13C NMR (125MHz, CDCl3): δ = 168, 159.9, 155.7, 154.4, 144.3, 143.4, 140.4, 131.4,
130.9, 129.6, 125.7, 123.8, 116.4, 116.2, 114.6, 12.3, 108.5, 103.3, 95.3, 68.5, 55.5, 31.5,
29.6, 25.6, 22.5, 14 ppm;
m/z (FAB) 484.1996 ((M +), C29H28N2O5 requires 484.1998).
ATR-FTIR (cm-1): 2215 (cyano, C≡N stretching band), 1684 (carbonyl, C=O stretching
band).
2.4.9. (E)-3-(5-(10-(4-Methoxyphenyl)-10H-phenoxazin-3-yl)furan-2yl)-2-cyanoacrylic acid
(WS5)
WS5 as a red solid (0.35g, 59%) was synthesized according to the procedure described
above for the synthesis of WH1. 4 (0.5g, 0.0013mol), cyanoacetic acid (0.33g, 0.0039mol)
and piperidine (0.496mL, 0.0052mol) were added to anhydrous CH3CN (50mL).
Eluent: DCM- methanol (10:1; v/v).
1H NMR (500MHz, d6-DMSO): δ = 7.99 (s, 1H), 7.49 (s, 1H), 7.36 (d, J = 8.5 Hz, 2H),
7.20-7.27 (m, 4H), 7.17 (s, 1H), 6.77 (d, J = 8.5 Hz, 1H), 6.67-6.79 (m, 2H), 5.93 (d, J = 8.5
Hz, 1H), 5.89 (d, J = 8.5 Hz, 1H), 3.85 ppm (s, 3H).

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13C NMR (125MHz, d6-DMSO): δ = 163.4, 158.7, 158, 146.4, 143, 142.3, 136.8, 135.2,
132.7, 130.7, 129.1, 126.6, 123.3, 121.5, 120.9, 120.8, 116.1, 116, 114.7, 112.9, 112.7, 110.9,
108.2, 95.1, 54.8 ppm;
m/z (FAB) 450.1214 ((M +), C27H18N2O5 requires 450.1216).
ATR-FTIR (cm-1): 2216 (cyano, C≡N stretching band), 1682 (carbonyl, C=O stretching
band).
3. Results and Discussion
3.1. Synthesis
The synthetic routes of POZ dyes are shown in Scheme 1, and the synthesized dyes and
reference dye (WH1) are shown in Fig. 1. The POZ syntheses first involved an N-arylation
on POZ nitrogen atoms by the Ullmann reaction, and a methoxyphenyl ring was introduced
to the POZ moiety to give 1. The bromination of 1 was performed with N-bromosuccinimde
(NBS) to produce mono-brominated 3. To the best of the authors’ knowledge, C-O bond-
forming reactions at the C (7) position of the POZ moiety have not yet been reported in the
literature. Thus, to introduce an alkoxy group at the C (7) position of the POZ, the Cu-
catalyzed coupling reaction of alcohol and aryl bromides reported by Shaojiang Hu et al in
2009 [23] was adopted in our experiments. 5, POZ including the hexyloxy group, was
successfully obtained with good yield by applying the Cu-catalyzed coupling reaction of
hexanol with mono-brominated 3. Subsequently, formylation of 5 via the Vilsmeyer-Haack
reaction was performed to yield 6. The Suzuki coupling reactions of 3 were carried out with
5-formyl-2-furanboronic acid, which produced 4. The final compounds (WS4 and WS5)
were obtained by the Knoevaenagal reactions of the corresponding aldehydes (4 and 6) with

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cyanoacetic acid in the presence of piperidine. The reference dye (WH1) was obtained by
previously reported synthetic routes [4f]. The structures of all the synthesized intermediates
1
13
and dyes were identified by H NMR, C NMR, and HRMS, and the final products were
additionally confirmed by FT-IR.
3.2. Theoretical calculations
The structural properties of the dyes were analyzed by calculating their geometrically
optimized structures using DFT at the B3LYP/6-31G (d,p) level. The optimized structures
with the torsion angle and electron density of the HOMOs and LUMOs of the dyes are shown
in Table 1. In the optimized geometry, the POZ cores of all the dyes showed fairly planar
structures with small torsion angles (0.06–0.64°). The planar structure of the POZ core
enhances the aromatic characteristics of the oxygen and nitrogen heteroatoms, which
increases the degree of electronic resonance between donor and anchoring moieties in the dye
molecules. The dihedral angle between furan and the POZ core in WS5 was 0.55°, which
indicates an almost planar configuration of WS5. Thus, the linked furan unit is favorable to
charge transfer from donor to acceptor on irradiation during charge transfer transition.
However, the planar characteristics of the POZ core and linked furan unit can induce dye
aggregations because of π-π interactions among dye molecules. The methoxyphenyl ring on
the POZ nitrogen atom made a large dihedral angle of approximately 90° with the POZ core,
while the cyanoacrylic acid anchoring group was in the same plane as the POZ core. This
mutually perpendicular structural characteristic with the methoxyphenyl ring of the POZ core
prevents dye molecules from stacking because of increased steric hindrance. On the other
hand, this indicates that little π-system overlap occurs between the methoxyphenyl ring and
the POZ core, which limits HOMO delocalization over the methoxyphenyl ring. The electron

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distributions of the HOMO of the dyes were mostly localized over the POZ core, while those
of the LUMO lied mainly on the cyanoacrylic acid acceptor and partly on its adjacent
aromatic ring. This result indicates that the HOMO-LUMO excitation induced by light
irradiation can effectively move the electron distribution from the POZ core to cyanoacrylic
acid. This further demonstrates that photoinduced electrons from the dye can be efficiently
injected into the TiO2 conduction band by this electron separation.
3.3. Photophysical properties
The absorption spectra of the three dyes in CH2Cl2 solution and on the TiO2 surface are
shown in Fig. 2, and the corresponding photophysical data are listed in Table 2. All the dyes
exhibited two prominent absorption bands, at λ = 300–400 nm and 450–550 nm. The former
band (at the shorter wavelength) is ascribed to the localized aromatic π-π* electronic
transition and the latter band (at the longer wavelength) is assigned to the intramolecular
charge-transfer (ICT) transition from the donor part to the acceptor group. The intensity of
the ICT transition was higher than that of the π-π* electronic transition. The absorption
maxima (λmax) of WH1, WS4, and WS5 were 484, 500, and 508 nm, respectively. WH1
showed a slight blue-shift in its absorption spectrum compared to basic phenoxazine dye
including N-alkyl chain, because of a poor orbital overlap between N-phenyl ring and
phenoxazine moiety. The absorption spectrum of WS4, which included the hexyloxy group at
the C (7) position of the POZ, was red-shifted by 16 nm compared to that of the reference dye
(WH1). This indicated that the introduced electron-rich hexyloxy moiety enhanced the
donating capability of the POZ core, providing better delocalization of electrons over the π-
conjugated molecules. WS5 containing the furan unit as a π–conjugated linker showed the
highest degree of red-shifting in the absorption peak. As shown in the calculation, this is due

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to the extension of electron delocalization over the entire molecule. In addition, the large red-
shift of WS5 at the shorter wavelength facilitated light-harvesting, which induced high
current density. The molar extinction coefficients at λmax of WH1, WS4, and WS5 were
18857, 16642, and 19250 M-1cm-1, respectively. These are higher than those of conventional
phenothiazine dyes (PT 5, 17600 M-1cm-1) [24] and standard ruthenium dye (N719, 14000 M-
1cm-1), which allow the use of a thinner TiO2 film for efficient electron transport [20b]. When
the dyes are adsorbed onto TiO2 films, the absorption spectrum displays a blue-shift
compared to those of the dyes in solution. This may be mainly ascribed to the H-aggregation
of the dyes or the deprotonation of carboxylic acid upon adsorption onto the TiO2 surface
[25]. The absorption maxima of the dyes on TiO2 were similarly ranked as those of the dyes
in solution. WS4 showed relatively lower adsorbance on the TiO2 surface compared to WH1
and WS5, although all dyes exhibited almost the same molar extinction coefficient in solution.
As shown in Table 1, this could be because of the steric hindrance caused by the bulky
hexyloxy unit, which inhibits the adsorption of the dye on the TiO2 surface [26].
3.4. Electrochemical properties
The electrochemical properties and energy levels of the synthesized dyes are provided in
Table 2 and Fig. 3. The oxidation potentials of the dyes were measured by cyclic voltammetry
(CV), and the CV curves of the dyes are shown in Fig. 4. The HOMO levels of the three dyes
correspond to the first oxidation potential versus normal hydrogen electrode (vs. NHE)
calibrated by Fc/Fc+ (640 mV vs. NHE). All HOMO levels of the dyes ranged from 0.84 eV
to 0.98 eV (vs. NHE) and were sufficiently positive compared to the redox potential of I-/I3-
(0.4 V vs. NHE), which means that the oxidized dyes can be favorably regenerated by the
redox couple. The LUMO levels, corresponding to excited state oxidation potentials, were

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obtained by subtracting the zeroth-zeroth energy (E0-0) from Eox, where the zeroth–zeroth
energy of the dye is estimated from the intersection between the normalized absorption and
emission spectra (Fig. 5). The LUMO levels of the dyes ranged from -1.28 to -1.39 and were
more negative than the conduction band edge energy level of TiO2 (-0.5 vs. NHE), indicating
that the excited electrons of the dyes can be efficiently injected into the conduction band of
TiO2. It is found that introduction of the hexyloxy group in the POZ core causes the HOMO
energy level to be more negative owing to enhancement of the electron-donating capability.
In addition, elongation of the conjugation length by furan insertion decreased the gap
between the HOMO and LUMO energy level (overall). Consequently, as shown in Fig 2,
WS4 exhibited a red-shifted absorption peak compared to WH1, and WS5 exhibited the most
red-shifted absorption spectrum.
3.5. Photovoltaic properties
DSSCs were fabricated based on the prepared dyes according to the methods described in
the experimental section. To precisely evaluate the effects of the dyes on the cell performance,
the photoanode was fabricated with a TiO2 film of 20 nm particles for the nanoporous layer of
~8 ꢀm.
The photovoltaic properties of the cells were measured under standard AM 1.5G irradiation
conditions (100 mW cm-2). The IPCE spectra and photocurrent-voltage (J-V) curves are
shown in Fig. 6, and the corresponding DSSC performance parameters are summarized in
Table 3 and S1.
As shown by the IPCE curves, despite the use of the thin TiO2 film, all dyes exhibited their
high maximum IPCE values at 400–600 nm, which were 78.3%, 75.3%, and 70.1% for WH1,
WS4, and WS5, respectively. The D-D-A and D-π-A structures of the dyes may be

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responsible for these high maximum IPCEs, inducing favorable charge separation and
electron injection. From the IPCE curves, all dyes could efficiently convert visible light to
photocurrent in the spectral range. The short-circuit current (Jsc) values of the dyes increased
in the order of WH1 < WS4 < WS5< N719, and this tendency coincides with the broadness
of the IPCE spectra. As shown by the absorption spectra of the dyes on TiO2, this is because
of the red-shifted spectra of WS4 and WS5 caused by the introduced hexyloxy moiety as an
additional donor and the furan unit as a bridge unit, respectively. On the other hand, the N-
substituted methoxyphenyl ring decreased the donating ability of the dye, inducing lower Jsc
value of the cell based on WH1. In addition, the IPCE spectra of WS5 showed a much higher
value at approximately 400 nm compared to WH1 and WS4. This is because, as shown in Fig.
2, WS5 exhibited a prominent absorption band at 300-400 nm due to the insertion of furan
unit, which enhanced light-harvesting in this wavelength region and facilitated subsequent
electron injection. The higher and broader IPCE spectra of the cells fabricated with WS4 and
WS5 promote higher Jsc and overall conversion efficiency.
The open-circuit voltage (Voc) values of the dyes were ranked as WS5 < WH1 < WS4, and
Voc values of the cells based on the dyes were higher than for standard ruthenium dye (N719,
720 mV). As discussed in our previous report [4f], these considerably higher Voc values were
mainly attributed to the N-methoxy phenyl ring perpendicularly rotated out of the POZ plane,
causing large steric hindrance, which suppressed dye aggregation and the recombination
between the injected electrons on TiO2 and the electrolyte. In particular, WS4 achieved the
highest Voc values among the dyes owing to the bulky hexyloxy group, which induced
additional steric hindrance among dye molecules and electrolytes. On the other hand, WS5
exhibited the lowest Voc value among the dyes. As shown in previous studies [4f], this may be
explained by the increased interaction caused by the introduction of a heterocyclic bridge unit
from the π-π interaction between the dye molecules and/or the interactions among the

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heteroatoms in the bridge unit and iodine. These interactions enhanced the charge
recombination at the dye/dye or dye/electrolyte interface, resulting in the lower Voc value of
the cell.
To optimize the photovoltaic performances, cells based on a TiO2 photoanode made with 20
nm particles to create the nanoporous layer of ~10 ꢀm, and 500 nm particles (G2) used for
the scattering layer of ~5 ꢀm, were fabricated [27]. The photovoltaic properties of the cells
based on the dyes were measured under standard AM 1.5G irradiation conditions (100 mW
cm-2). The IPCE spectra and J-V curves are shown in Fig. 7, and the corresponding DSSC
performance parameters are listed in Table 4 and S2. The maximum IPCE values of the cells
based on the dyes reached 79.2%, 77.4%, and 76.1% at 400–600 nm for WH1, WS4, and
WS5, respectively. These IPCE values were slightly higher than those of the cells with the
thinner TiO2 film, and WS5 showed the most noticeable increase in IPCE maximum values
among the dyes. The results were ascribed to the greater amount of adsorption of WS5 on the
TiO2 surface, as it is known that a larger amount of dye adsorbed on TiO2 leads to a higher
and broader IPCE spectrum. Both Jsc and Voc values increased simultaneously in all cells
(based on the dyes) by introducing a thicker nanoporous TiO2 layer and scattering layer.
These increases were caused by the increased adsorption amounts of the dyes on TiO2
surfaces and by additional light scattering effects. However, Jsc and Voc trends were almost
identical to those of the cells with thinner TiO2 film. Consequently, all three dyes exhibited
almost the same overall conversion efficiencies, and the η values of WH1, WS4, and WS5
were 4.49%, 5.25%, and 6.34%, respectively. The best overall conversion efficiency was
achieved by the cell fabricated with WS5 (η = 6.34%, Jsc = 11.16 mA/cm2, Voc = 770 mV, and
FF = 74%), reaching about 86.8% of N719 dye’s 7.3%.
3.6. Electrochemical impedance spectroscopy

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To further investigate the effect of the charge recombination and electron transport on
photovoltaic performance, EIS [28] was carried out in the dark.
Fitted impedance parameters are plotted as a function of bias voltage in Fig 8. As shown in
Fig. 8(a), all the cells showed similar capacitance; thus, it can be concluded that the TiO2
conduction band shift caused by the dyes produces no significant differences. On the other
hand, Fig. 8(b) shows that the Rct values of the cells at the same bias voltage decreased in the
order of WS4 > WH1 > WS5 > N719. As a higher Rct value indicates a lower charge
recombination rate, the charge recombination resistance increased in the order of WS4 >
WH1 > WS5 > N719, which is in accordance with the Voc values measured in the cells made
with the dyes. The cell fabricated with WS4 showed the highest Rct value because the
hexyloxy groups prevented the electrolytes from approaching the surface of the TiO2, leading
to a decrease of recombination between the electrolyte and the injected electrons. In contrast,
the cell fabricated with WS5 showed the lowest Rct value (except for N719) because the
electron recombination between the electrolyte and the surface of the TiO2 was increased by
the furan-electrolyte interaction. All the cells exhibited a lower electron recombination rate
than N719 because the N-methoxyphenyl groups suppressed the electron recombination by
reducing the planarity of the dye molecules.
Figure 8(c) shows the electron lifetime of the fabricated cells, which can be calculated by
the following equation [29];
(1)
electron lifetime = Rct * Capacitance.
In this equation, owing to the similar capacitance values of all cells, the electron lifetime is
proportional to Rct. Therefore, the trend of electron lifetime coincides with Rct, as shown in
Fig. 8(b) (i.e., WS4 > WH1 > WS5 > N719). Consequently, as shown in the photovoltaic
properties, the cell based on WS4 with the longest lifetime showed the highest Voc value,

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while the cell based on WS5 had the lowest Voc value.
The electron density is shown in Fig. 9(a). As electron density is mainly affected by electron
injection, it is thereby correlated to Jsc. Therefore, the higher electron density of the cells
based on N719 and WS5 corresponds with the higher Jsc values of the cells. The diffusion
coefficient and diffusion length according to varying voltage are shown in Fig. 9(b) and (c),
respectively. The diffusion length is expressed by the following equation [30];
(2)
Ld = (Dτ)1/2,
where Ld = electron diffusion length, D = diffusion coefficient, and τ = recombination lifetime
of an electron or the charge lifetime from generation until recombination or extraction. The
diffusion length is the average distance that an injected electron can travel through a
photoelectrode prior to recombination; therefore, it should be longer than the photoelectrode
thickness for the efficient collection of photocurrent. In the case of the cells fabricated with
each dye for the voltage range considered, the diffusion lengths for each case were longer
than the thickness of the TiO2. Therefore, it can be concluded that an injected electron can be
efficiently transferred to the photoelectrode. The electron diffusion length is also a useful tool
to reflect the electron collection efficiency. A greater electron diffusion length indicates
increased electron collection efficiency, with a corresponding increase in Jsc. As shown in Fig.
9(c), the electron diffusion length increased in the order of N719 > WS5 > WS4 > WH1,
which is in accordance with the measured Jsc values of the fabricated cells. Fig. 10(a) and (b)
show the collecting time and collection efficiency, respectively. A shorter collecting time
indicates effective electron injection to the photoelectrode, leading to higher Jsc values.
Therefore, the trends of the collecting times were in good agreement with the Jsc rank of the
dyes. As is well known, charge collection efficiency can be determined by competition
between charge recombination and collection and, thereby, the higher the collection
efficiency of the cell is, the better the photovoltaic performance of a cell. Consequently, as

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shown in Fig. 10(b), the sequence of the electron collection efficiency (N719 > WS5 > WS4
> WH1) coincides with the trends of the overall conversion efficiencies.
4. Conclusion
To study the effects of substituents on the performance of DSSCs, three phenoxazine-based
organic dyes (WH1, WS4, and WS5) were designed and synthesized. The introduction of a
hexyloxy chain as an additional donor red-shifted the absorption spectrum of the dye by
increasing the electron-donating capability, thus improving Jsc. In addition, the hexyloxy
moiety caused steric hindrance due to its bulky structure. Therefore, the electron
recombination between injected electrons and the electrolyte was suppressed, thus improving
Voc. As a result, the DSSC based on WS4 with the hexyloxy moiety achieved higher overall
conversion efficiency in comparison with the reference dye (WH1). The furan moiety
introduced to phenoxazine dye extended π-electron delocalization over the entire molecule,
increasing light-harvesting in the long wavelength region. In addition, its introduction led to
an increase of the amount of dye adsorbed due to π-π interactions among the dye molecules.
Consequently, the DSSCs based on WS5 with the furan moiety showed enhanced Jsc,
achieving the highest solar-to-electricity conversion efficiency of 6.34% (Jsc = 11.16 mA/cm2,
Voc = 770 mV, FF = 74%).
Acknowledgements
This work was supported by the Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future
Planning (Grant number: 2016R1C1B2006941).

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