Phenothiazine derivatives as organic sensitizers for highly efficient dye-sensitized solar cells

Article abstract of DOI:10.1039/c2jm13961h

A series of organic dyes containing a phenothiazine central unit were synthesized and were used effectively in the fabrication of dye-sensitized solar cells (DSSCs). A cyanoacrylate moiety was added at the C(3) position of the phenothiazine as an electron acceptor, and a triarylamine moiety was attached at the C(7) position as an electron donor. The DSSCs made with these dyes displayed remarkable quantum efficiency, ranging from 4.2-6.2% under an AM 1.5 solar condition (100 mW cm^-2). A variety of substituents, i.e., methyl, hexyl and triphenylamino groups, were added at the N(10) of phenothiazine in order to optimize the incident photon-to-current conversion efficiency. Along the main chromophore a thiophenylene group was inserted at different positions to examine its influence on the properties of devices. The best performance was found in compound NSPt-C6, in which a hexyl group was attached at the N(10) of phenothiazine and a thiophenylene at the C(7) position. It displayed a short-circuit current (J_sc) of 14.42 mA cm ^-2, an open-circuit voltage (V_oc) of 0.69 V, and a fill factor (ff) of 0.63, corresponding to an overall conversion efficiency of 6.22%. Their photophysical properties were analyzed with the aid of a time-dependent density functional theory (TDDFT) model with the B3LYP functional. Their photovoltaic behavior was further elucidated by the electrochemical impedance spectroscopy.

Full text of DOI:10.1039/c2jm13961h

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PAPER
Phenothiazine derivatives as organic sensitizers for highly efficient
dye-sensitized solar cells†
b
a
bc
Chih-Jen Yang,a Yuan Jay Chang,bc Motonori Watanabe, Yung-Son Hon‡ and Tahsin J. Chow
*
Received 15th August 2011, Accepted 18th December 2011
DOI: 10.1039/c2jm13961h
A series of organic dyes containing a phenothiazine central unit were synthesized and were used
effectively in the fabrication of dye-sensitized solar cells (DSSCs). A cyanoacrylate moiety was added at
the C(3) position of the phenothiazine as an electron acceptor, and a triarylamine moiety was attached
at the C(7) position as an electron donor. The DSSCs made with these dyes displayed remarkable
quantum efficiency, ranging from 4.2–6.2% under an AM 1.5 solar condition (100 mW cmꢀ2). A variety
of substituents, i.e., methyl, hexyl and triphenylamino groups, were added at the N(10) of
phenothiazine in order to optimize the incident photon-to-current conversion efficiency. Along the
main chromophore a thiophenylene group was inserted at different positions to examine its influence
on the properties of devices. The best performance was found in compound NSPt-C6, in which a hexyl
group was attached at the N(10) of phenothiazine and a thiophenylene at the C(7) position. It displayed
a short-circuit current (Jsc) of 14.42 mA cmꢀ2, an open-circuit voltage (Voc) of 0.69 V, and a fill factor
(ff) of 0.63, corresponding to an overall conversion efficiency of 6.22%. Their photophysical properties
were analyzed with the aid of a time-dependent density functional theory (TDDFT) model with the
B3LYP functional. Their photovoltaic behavior was further elucidated by the electrochemical
impedance spectroscopy.
organic dipolar dyes composed of an aromatic amine donor (D)
and a cyanoacrylic acid acceptor (A), while the two were sepa-
rated by a linearly connected triarylene bridge to form a D–p–A
system. The performance of DSSCs made with these dyes can
achieve a maximal quantum efficiency of 7%, considerably close
to that of N719.11 To explore the possibility of extending to other
types of structures, we designed a new group of compounds
consisting a phenothiazine as the major bridging unit, onto
which a variety of substituents were attached.12 The structure of
phenothiazine possesses some unique features different from that
of a typical triarylamine. The two phenyl groups of phenothia-
zine are arranged nearly coplanar, so that a p-delocalization can
be extended over the entire chromophore.
Introduction
Dye-sensitized solar cells (DSSCs) have been considered as
a promising alternative energy source to solve the crisis of energy
as well as environmental issues.1 The ruthenium-based
complexes, i.e., N3, N719, and black dye reported by Michael
€
Gratzel, have achieved maximal power conversion efficiencies
over 11%.2 Compared with metal complexes, organic dyes have
the advantages of environmental friendliness, higher structural
flexibility, lower cost, easier preparation and purification, etc.
The reports on organic dyes are abundant, most of which
exhibited energy-to-electricity conversion efficiency in the range
of 5–9%.3–10 In our previous works we have developed a series of
The electron rich nature of a phenothiazine moiety provides
a good relay for the electron migration from D to A. In this
work, a series of phenothiazine derivatives are prepared, and the
DSSCs made with them are examined.13
aDepartment of Chemistry and Biochemistry, National Chung Cheng
University, Minhsiung, Chiayi, 62102, Taiwan. E-mail: a0920584035@
yahoo.com.tw; Fax: +886-5-2721040; Tel: +886-5-2720411
bInstitute of Chemistry, Academia Sinica, No.128, Sec. 2, Academia Rd,
Nankang Dist., Taipei, 11529, Taiwan. E-mail: tjchow@chem.sinica.edu.
tw; Fax: +886-2-27884179; Tel: +886-2-27898552
cDepartment of Chemistry, National Taiwan University, Taipei, 10617,
Taiwan. E-mail: d93223006@ntu.edu.tw; Fax: +886-2-27884179; Tel:
+886-2-27898641
Eight derivatives are designed, i.e., NPt-C1, NPt-C6, NPtS-
C6, NSPt-C6, NPt-T, NPtS-T, NSPt-T and Pt-C6 as shown in
Fig. 1, where the letters N, Pt, S, C1, C6, and T denote triaryl-
amine, phenothiazine, thiophenyl, methyl, hexyl, and phenyl-
ditolylamine moieties, respectively. To each phenothiazine
a cyanoacrylic acid group was attached at the C(3) position as an
electron acceptor, and a triarylamine moiety was attached at the
C(7) position as an electron donor. The phenothiazine moiety
acts as a spacer, although it is not fully p-conjugated in a classical
† Electronic supplementary information (ESI) available: Experimental
data (1-X, 2-X, and Pt-C6), 1H NMR and 13C NMR spectra, UV/Vis
spectra, theoretical calculation data, loading amount data, EIS spectra,
and devices incorporating CDCA. See DOI: 10.1039/c2jm13961h
‡ Deceased on June 16, 2011.
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620 analyzer. All measurements were carried out in THF solu-
tions containing 0.1
M
tetrabutylammonium hexa-
fluorophosphate (TBAPF6) as the supporting electrolyte under
ambient conditions after purging for 10 minutes with N2. The
conventional three electrode configuration was employed, which
consists of a glassy carbon working electrode, a platinum counter
electrode, and a Ag/Ag+ reference electrode calibrated with
ferrocene/ferrocenium (Fc/Fc+) as an internal reference. Mass
spectra were recorded on a VG70-250S mass spectrometer.
The chemicals, i.e., phenothiazine, iodomethane, 1-bromo-
hexane, butyllithium (1.6 M in hexane), trans-dichlorobis-
(triphenylphosphine)palladium(II) (PdCl2(PPh3)2), N,N-dime-
thylformamide, palladium(II) acetate (Pd(OAc)2), N-bromo-
succinimide
(NBS),
1,10-bis(diphenylphosphanyl)ferrocene
(dppf), phosphoryl chloride (POCl3), cyanoacetic acid, ammo-
nium acetate, and acetic acid, were purchased from ACROS,
Alfa, Merck, Lancaster, TCI, Showa, and Sigma-Aldrich, sepa-
rately. Chromatographic separations were carried out by using
silica gel from Merck, Kieselgel si 60 (40–63 mm). Synthetic
procedures for compounds 1-X, 2-X, and Pt-C6 are shown in the
ESI†.
Fabrication and characterization of DSSCs
The FTO conducting glass (FTO glass, fluorine doped tin oxide
over-layer, transmission > 90% in the visible, sheet resistance 8 U
squareꢀ1), titania-oxide pastes of Ti-nanoxide T/SP and Ti-
nanoxide R/SP were purchased from Solaronix. A thin film of
TiO2 (16–18 mm) was coated on a 0.25 cm2 FTO glass substrate.
It was immersed in a THF solution containing 3 ꢂ 10ꢀ4 M dye
sensitizers for at least 12 h, then rinsed with anhydrous aceto-
nitrile and dried. Another piece of FTO with sputtering 100 nm
thick Pt was used as a counter electrode. The active area was
controlled at a dimension of 0.25 cm2 by adhering 60 mm thick
polyester tape on the Pt electrode. The photocathode was placed
on top of the counter electrode and was tightly clipped together
to form a cell. Electrolyte was then injected into the seam
between two electrodes. An acetonitrile solution containing LiI
(0.5 M), I2 (0.05 M) and 4-tert-butylpyridine (0.5 M) was used as
the electrolyte. Devices made of a commercial dye N719 under
the same condition (3 ꢂ 10ꢀ4 M, Solaronix S.A., Switzerland)
was used as a reference. The cell parameters were obtained under
an incident light with intensity 100 mW cmꢀ2 measured by
a thermopile probe (Oriel 71964), which was generated by
a 300 W Oriel Class A Solar Simulator 91160A-1000 (Newport)
passing through a AM 1.5 filter (Oriel 74110). The light intensity
was further calibrated by an Oriel reference solar cell (Oriel
91150) and adjusted to be 1.0 sun. The monochromatic quantum
efficiency was recorded through a monochromator (Oriel 74100)
under short circuit conditions. Electrochemical impedance
spectra of DSSCs were recorded by an Impedance/Gain-Phase
analyzer (SI 1260, Solartron).
Fig. 1 The structure of organic dyes.
sense. The addition of a thiophene unit in some of the structures
extends the range of p-electron delocalization, therefore red-
shifts the absorption wavelength and increases the molar
absorptivity of the materials.14 An alkyl chain was linked at the
N(10) of phenothiazine to help reducing the leakage of dark
current, which usually appears while the surface of TiO2 is not
ꢀ
completely shielded from the Iꢀ/I3 electrolyte. The same effect
may be achieved by substituting the nitrogen atom with another
triphenylamine group, such as the structures of NPt-T, NPtS-T,
and NSPt-T. According to a few recent reports by Tian et al.,
inserting an extra arylamine group can stabilize the charge
separated state and therefore enhance the value of open-circuit
photovoltage (Voc).20b,d It is therefore anticipated that the Voc
values of the T-series compounds can be improved.
Experimental
General information
All reactions and manipulations were carried out under
a nitrogen atmosphere. Solvents were distilled freshly according
to standard procedures. 1H and 13C NMR spectra were recorded
on a Bruker (AV 400/AV 500 MHz) spectrometer in CDCl3, and
DMSO-d6 as a solvent. Chemical shifts are reported in the d scale
downfield from the peak for tetramethylsilane. Absorption
spectra were recorded on a Jasco-550 spectrophotometer.
Emission spectra were obtained from a Hitachi F-4500 spectro-
fluorimeter. The emission spectra in solutions were measured in
spectral grade solvent by a 90ꢁ angle detection. The redox
potentials were measured by using cyclic voltammetry on a CHI
N-Methyl-7-(p-(naphthylphenylamino)phenyl)phenolthiazine-3-
carbaldehyde (3-C1)
To a three-necked flask containing a mixture of 1-C1 (1.53 g, 4.81
mmol), PdCl2(PPh3)2 (112 mg, 0.16 mmol), and p-(tributyl-
stannyl)phenylnaphthylphenylamine (3.35 g, 5.72 mmol) was
This journal is ª The Royal Society of Chemistry 2012
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added DMF (20 mL). The reaction mixture was stirred at 90 ꢁC
for 12 h. After cooling, the reaction was quenched by adding
methanol and KF(aq) (saturated 15 mL). The mixture was
extracted with dichloromethane and the organic layer dried over
anhydrous MgSO4. Evaporation of the solvent gave the crude,
which was purified by silica gel with dichloromethane/hexane (1/
5) as eluent. Yellow solid of 3-C1 was obtained in 64% yield (1.64
g, 3.08 mmol). dH (400 MHz, CDCl3) 9.79 (s, 1H), 7.96 (d, 1H,
J ¼ 8.4 Hz), 7.90 (d, 1H, J ¼ 8.4 Hz), 7.79 (d, 1H, J ¼ 8.0 Hz),
7.65 (dd, 1H, J ¼ 1.6, 8.4 Hz), 7.63 (d, 1H, J ¼ 1.6 Hz), 7.45–7.51
(m, 2H), 7.20–7.37 (m, 8H), 7.03–7.10 (m, 4H), 6.97 (t, 1H, J ¼
7.2 Hz), 6.84 (d, 2H, J ¼ 8.4 Hz), 3.41 (s, 3H); dC (100 MHz,
CDCl3) 189.9, 150.7, 148.0, 147.7, 143.1, 142.4, 136.2, 135.2,
132.2, 131.1, 130.9, 130.4, 129.1, 128.3, 127.8, 127.2, 126.9, 126.5,
126.4, 126.3, 126.1, 125.5, 124.9, 124.1, 123.5, 122.7, 122.1, 121.9,
121.5, 114.8, 113.5, 35.7; m/z (FAB) 535.1844 ((M + H)+,
C36H27ON2S requires 535.1844).
6.74 (d, 1H, J ¼ 8.4 Hz), 3.77 (t, 2H, J ¼ 7.1 Hz), 1.71–1.78
(m, 2H), 1.39–1.42 (m, 2H), 1.25–1.29 (m, 4H), 0.85 (t, 3H, J ¼
6.4 Hz); dC (100 MHz, CDCl3) 189.7, 150.0, 147.8, 143.2, 143.0,
142.1, 140.7, 135.2, 131.1, 130.9, 130.1, 130.0, 129.1, 128.4, 128.2,
127.2, 126.6, 126.4, 126.3, 126.2, 126.1, 124.5, 124.1, 124.0, 124.0,
123.4, 122.8, 122.3, 122.2, 121.2, 115.9, 114.5, 47.9, 31.3, 26.5,
26.4, 22.5, 13.9; m/z (FAB) 686.2424 (M+, C45H38ON2S2 requires
686.2426).
7-(5-(p-(Naphthylphenylamino)phenyl)thiophen-2-yl)-N-(p-
ditolylaminophenyl)phenothiazine-3-carbaldehyde (4-T)
Compound 4-T was synthesized according to the same procedure
as that of 4-C6, as yellow solid in 56% yield. dH (500 MHz,
CDCl3) 9.70 (s, 1H), 7.92 (d, 1H, J ¼ 8.5 Hz), 7.89 (d, 1H, J ¼ 8.5
Hz), 7.79 (d, 1H, J ¼ 8.0 Hz), 7.45–7.50 (m, 3H), 7.32–7.41 (m,
5H), 7.08–7.23 (m, 20H), 6.96–6.99 (m, 3H), 6.31 (d, 1H, J ¼
8.5 Hz), 6.24 (d, 1H, J ¼ 9.0 Hz), 2.35 (s, 6H); dC (125 MHz,
CDCl3) 189.6, 149.0, 148.7, 148.0, 144.5, 143.3, 143.1, 141.8,
140.8, 135.3, 133.8, 131.5, 131.1, 130.9, 130.8, 130.2, 130.1, 129.9,
128.4, 127.4, 127.3, 127.2, 126.6, 126.4, 126.3, 126.1, 125.5, 124.2,
124.1, 123.3, 123.2, 122.8, 122.4, 122.3, 122.2, 121.3, 119.6, 119.5,
116.7, 115.1, 20.9; m/z (FAB) 874.2926 ((M + H)+, C59H44ON3S2
requires 874.2926).
N-Hexyl-7-(p-(naphthylphenylamino)phenyl)phenothiazine-3-
carbaldehyde (3-C6)
Compound 3-C6 was synthesized according to the same proce-
dure as that of 3-C1, as yellow liquid in 62% yield. dH (400 MHz,
CDCl3) 9.77 (s, 1H), 7.94 (d, 1H, J ¼ 8.4 Hz), 7.89 (d, 1H, J ¼ 8.2
Hz), 7.78 (d, 1H, J ¼ 8.2 Hz), 7.62 (dd, 1H, J ¼ 1.6, 8.4 Hz), 7.57
(d, 1H, J ¼ 1.6 Hz), 7.44–7.50 (m, 2H), 7.19–7.38 (m, 8H), 7.02–
7.09 (m, 4H), 6.96 (t, 1H, J ¼ 7.2 Hz), 6.88 (d, 2H, J ¼ 8.4 Hz),
3.88 (t, 2H, J ¼ 7.2 Hz), 1.78–1.86 (m, 2H), 1.41–1.46 (m, 2H),
1.30–1.33 (m, 4H), 0.88 (t, 3H, J ¼ 6.8 Hz); dC (100 MHz, CDCl3)
189.9, 150.4, 148.0, 147.7, 143.2, 141.8, 136.2, 135.2, 132.3, 131.2,
130.9, 130.0, 129.1, 128.4, 127.2, 127.0, 126.6, 126.4, 126.3, 126.1,
125.4, 125.2, 124.4, 124.1, 123.9, 122.1, 122.0, 121.5, 116.0, 114.5,
48.0, 31.3, 26.6, 26.4, 22.5, 13.9; m/z (FAB) 605.2627 ((M + H)+,
C41H37ON2S requires 605.2626).
N-Hexyl-5-(3-(p-(naphthylphenylamino)phenyl)phenothiazin-7-yl)
thiophene-2-carbaldehyde (5-C6)
Compound 5-C6 was synthesized from 1-C6 and p-(tributyl-
stannyl)phenylnaphthylphenylamine according to the same
procedure as that of 3-C1, as yellow liquid in 38% yield of 2 steps.
dH (400 MHz, CDCl3) 9.83 (s, 1H), 7.96 (d, 1H, J ¼ 8.4 Hz), 7.87
(d, 1H, J ¼ 8.4 Hz), 7.77 (d, 1H, J ¼ 8.4 Hz), 7.66 (d, 1H, J ¼
3.9 Hz), 7.18–7.49 (m, 13H), 7.02–7.08 (m, 4H), 6.95 (t, 1H, J ¼
7.3 Hz), 6.81–6.87 (m, 2H), 3.84 (t, 2H, J ¼ 7.2 Hz), 1.79–1.82
(m, 2H), 1.42–1.46 (m, 2H), 1.30–1.32 (m, 4H), 0.87 (t, 3H, J ¼
4.5 Hz); dC (100 MHz, CDCl3) 182.5, 153.6, 148.1, 147.6, 146.1,
143.3, 142.8, 141.5, 137.6, 135.6, 135.3, 132.6, 131.2, 129.1, 128.4,
127.3, 127.2, 127.0, 126.6, 126.4, 126.3, 126.2, 125.6, 125.5, 125.2,
125.1, 124.9, 124.2, 123.9, 123.0, 122.1, 121.9, 121.7, 115.6, 115.3,
47.7, 31.4, 26.7, 26.6, 22.6, 14.0; m/z (FAB) 686.2426
(M+, C45H38ON2S2 requires 686.2426).
7-(p-(Naphthylphenylamino)phenyl)-N-(p-ditolylaminophenyl)
phenothiazine-3-carbaldehyde (3-T)
Compound 3-T was synthesized according to the same procedure
as that of 3-C1, as yellow solids in 60% yield. dH (400 MHz,
CDCl3) 9.69 (s, 1H), 7.93 (d, 1H, J ¼ 8.4 Hz), 7.88 (d, 1H, J ¼ 8.2
Hz), 7.78 (d, 1H, J ¼ 8.2 Hz), 7.44–7.50 (m, 3H), 7.28–7.38 (m,
5H), 7.00–7.23 (m, 20H), 6.95 (t, 1H, J ¼ 7.3 Hz), 6.29 (t, 2H, J ¼
8.7 Hz), 2.34 (s, 6H); dC (100 MHz, CDCl3) 189.6, 149.3, 148.6,
148.0, 147.7, 144.5, 143.2, 141.4, 136.1, 135.2, 133.7, 132.2, 131.6,
131.2, 130.9, 130.8, 130.2, 129.9, 129.1, 128.4, 127.4, 127.2,
126.89, 126.6, 126.4, 126.3, 126.1, 125.5, 124.9, 124.4, 124.1,
122.2, 122.1, 122.0, 121.5, 119.7, 119.3, 116.7, 115.0, 20.8; m/z
(FAB) 792.3049 ((M + H)+, C55H42ON3S requires 792.3048).
5-(3-(p-(Naphthylphenylamino)phenyl)-N-(p-ditolylaminophenyl)
phenothiazin-7-yl)thiophene-2-carbaldehyde (5-T)
Compound 5-T was synthesized from 1-T according to the same
procedure as that of 5-C6, as yellow liquid in 58% yield. dH (400
MHz, CDCl3) 9.82 (s, 1H), 7.94 (d, 1H, J ¼ 8.5 Hz), 7.88 (d, 1H,
J ¼ 8.2 Hz), 7.77 (d, 1H, J ¼ 8.2 Hz), 7.66 (d, 1H, J ¼ 3.9 Hz),
7.44–7.49 (m, 2H), 7.33 (t, 2H, J ¼ 7.2 Hz), 7.29 (d, 2H, J ¼
8.6 Hz), 7.32–7.41 (m, 5H), 7.01–7.26 (m, 18H), 6.95 (t, 1H, J ¼
7.4 Hz), 6.29 (t, 2H, J ¼ 9.1 Hz), 2.34 (s, 6H); dC (100 MHz,
CDCl3) 182.5, 153.6, 148.5, 148.1, 147.6, 145.4, 144.7, 143.3,
142.3, 141.3, 137.6, 135.5, 135.3, 133.6, 132.5, 132.1, 131.2, 131.1,
130.2, 129.1, 128.4, 127.3, 127.0, 126.8, 126.6, 126.5, 126.4, 126.2,
125.5, 125.2, 125.0, 124.4, 124.2, 124.1, 122.8, 122.5, 122.1, 121.9,
120.2, 119.1, 116.2, 115.8, 20.9; m/z (FAB) 874.2926 ((M + H)+,
C59H44ON3S2 requires 874.2926).
N-Hexyl-7-(5-(p-(naphthylphenylamino)phenyl)thiophen-2-yl)
phenothiazine-3-carbaldehyde (4-C6)
Compound 4-C6 was synthesized from 1-C6 and p-(5-(tributyl-
stannyl)thiophen-2-yl)phenylnaphthylphenylamine according to
the same procedure as that of 3-C1, as yellow liquid in 89% yield.
dH (400 MHz, CDCl3) 9.72 (s, 1H), 7.92 (d, 1H, J ¼ 8.4 Hz), 7.85
(d, 1H, J ¼ 8.4 Hz), 7.74 (d, 1H, J ¼ 8.2 Hz), 7.55 (d, 1H, J ¼ 8.5
Hz), 7.52 (s, 1H), 7.20–7.45 (m, 8H), 7.18 (t, 2H, J ¼ 7.9 Hz),
7.06–7.07 (m, 4H), 6.92–6.97 (m, 3H), 6.79 (d, 1H, J ¼ 8.4 Hz),
4042 | J. Mater. Chem., 2012, 22, 4040–4049
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(E)-(N-Methyl-3-(p-(naphthylphenylamino)phenyl)phenothiazin-
7-yl)-2-cyanoacrylic acid (NPt-C1)
(E)-N-Hexyl-(3-(5-(p-(naphthylphenylamino)phenyl)thiophen-2-
yl)phenothiazin-7-yl)-2-cyanoacrylic acid (NSPt-C6)
A mixture of 3-C1 (1.00 g, 1.87 mmol), cyanoacetic acid (210 mg,
2.43 mmol) and ammonium acetate (44 mg, 0.56 mmol) in acetic
acid was placed in a three-necked flask under a nitrogen atmo-
sphere and was stirred at 100 ꢁC for 12 h. After cooling, the
reaction was quenched by adding water, then was extracted with
CH2Cl2. The organic layer was dried over anhydrous MgSO4 and
concentrated under reduced pressure to give the crude product.
The product was purified by silica gel column chromatograph
eluted with CH2Cl2/acetic acid (19/1). The deeply red solid was
isolated in 68% yield (764 mg, 1.27 mmol), mp: 177–181 ꢁC.
Spectroscopic data of NPt-C1: dH (400 MHz, CDCl3) 8.09 (s,
1H), 7.89–7.96 (m, 3H), 7.79 (d, 1H, J ¼ 7.6 Hz), 7.73 (d, 1H, J ¼
2.0 Hz), 7.45–7.51 (m, 2H), 7.22–7.37 (m, 8H), 7.03–7.09 (m, 4H),
6.97 (t, 1H, J ¼ 7.2 Hz), 6.83–6.87 (t, 2H, J ¼ 8.0 Hz), 3.45 (s,
3H); dC (100 MHz, CDCl3) 167.2, 154.6, 150.5, 148.1, 147.9,
143.3, 142.1, 136.7, 135.3, 132.3, 132.2, 131.2, 130.0, 129.2, 128.4,
127.3, 127.1, 126.7, 126.5, 126.4, 126.2, 125.8, 125.4, 125.2, 124.2,
123.8, 122.6, 122.3, 122.1, 121.6, 115.9, 115.1, 114.0, 97.7, 35.8;
Compound NSPt-C6 was synthesized according to the same
procedure as that of NPt-C1, as red solids in 55% yield, mp: 137–
141 ꢁC. dH (500 MHz, DMSO-d6) 8.46 (s, 1H), 8.00 (d, 1H, J ¼
8.0 Hz), 7.96 (d, 1H, J ¼ 4.0 Hz), 7.91 (d, 1H, J ¼ 8.0 Hz), 7.86 (d,
1H, J ¼ 8.5 Hz), 7.67 (d, 1H, J ¼ 4.0 Hz), 7.35–7.59 (m, 11H),
7.24 (t, 2H, J ¼ 8.0 Hz), 7.03–7.06 (m, 2H), 6.93–6.96 (m, 2H),
6.90–6.92 (m, 2H), 3.89 (t, 2H, J ¼ 6.5 Hz), 1.66–1.67 (m, 2H),
1.37–1.38 (m, 2H), 1.21–1.25 (m, 4H), 0.81 (t, 3H, J ¼ 7.0 Hz); dC
NMR (125 MHz, DMSO-d6) 147.9, 147.4, 147.2, 142.3, 142.2,
141.8, 140.0, 134.9, 131.2, 130.5, 129.4, 129.2, 128.9, 128.6, 127.3,
126.9, 126.7, 126.6, 126.5, 126.3, 126.2, 125.9, 124.6, 124.4, 123.6,
123.3, 122.9, 122.5, 122.4, 122.0, 120.7, 116.7, 115.6, 47.0, 30.7,
26.0, 25.6, 22.0, 13.8; m/z (FAB) 754.2562 ((M + H)+,
C48H40O2N3S2 requires 754.2562).
(E)-(3-(5-(p-(Naphthylphenylamino)phenyl)thiophen-2-yl)-N-(p-
ditolylaminophenyl)phenothiazin-7-yl)-2-cyanoacrylic acid
(NSPt-T)
m/z (FAB) 602.1902 ((M
+
H)+, C39H28O2N3S requires
602.1902).
Compound NSPt-T was synthesized according to the same
procedure as that of NPt-C1, as red solids in 77% yield, mp: 203–
205 ꢁC. dH (500 MHz, DMSO-d6) 8.05 (s, 1H), 8.00 (d, 1H, J ¼
8.5 Hz), 7.91 (d, 1H, J ¼ 8.0 Hz), 7.83 (d, 1H, J ¼ 8.5 Hz), 7.71 (d,
1H, J ¼ 2.0 Hz), 7.61–7.63 (m, 1H), 7.57 (t, 1H, J ¼ 7.5 Hz), 7.51
(t, 1H, J ¼ 8.0 Hz), 7.47 (d, 2H, J ¼ 8.5 Hz), 7.43 (t, 1H, J ¼
8.0 Hz), 7.33–7.38 (m, 3H), 7.29 (d, 1H, J ¼ 4.0 Hz), 7.25 (t, 2H,
J ¼ 8.0 Hz), 7.16–7.20 (m, 7H), 6.98–7.09 (m, 9H), 6.87 (d, 2H,
J ¼ 8.5 Hz), 6.25 (d, 1H, J ¼ 8.5 Hz), 6.19 (d, 1H, J ¼ 8.5 Hz),
2.27 (s, 6H); dC NMR (125 MHz, DMSO-d6) 163.6, 151.8, 148.1,
147.4, 147.2, 146.9, 144.0, 142.3, 142.2, 141.0, 139.8, 134.9, 133.4,
131.2, 130.6, 130.5, 130.2, 129.4, 129.3, 128.5, 128.3, 127.2, 126.9,
126.7, 126.6, 126.5, 126.3, 126.2, 125.7, 125.4, 124.4, 124.2, 123.6,
123.3, 122.6, 122.4, 122.0, 121.5, 120.7, 118.7, 118.3, 116.7, 115.3,
99.9, 20.4; m/z (FAB) 941.2983 ((M + H)+, C62H45O2N4S2
requires 941.2984).
(E)-(N-Hexyl-(3-(p-(naphthylphenylamino)phenyl)phenothiazin-7-
yl)-2-cyanoacrylic acid (NPt-C6)
Compound NPt-C6 was synthesized according to the same
procedure as that of NPT-C1, as red solids in 66% yield, mp:
ꢁ
149–152 C. dH (500 MHz, CDCl3) 8.07 (s, 1H), 7.89–7.95 (m,
3H), 7.78 (d, 1H, J ¼ 6.8 Hz), 7.66 (d, 1H, J ¼ 1.0 Hz), 7.45–7.50
(m, 2H), 7.20–7.38 (m, 8H), 7.02–7.08 (m, 4H), 6.96 (t, 1H, J ¼
7.5 Hz), 6.84–6.88 (m, 2H), 3.87 (t, 2H, J ¼ 7.5 Hz), 1.79–1.85 (m,
2H), 1.42–1.46 (m, 2H), 1.25–1.33 (m, 4H), 0.88 (t, 3H, J ¼
7.0 Hz); dC (125 MHz, CDCl3) 154.4, 154.3, 149.9, 149.8, 147.9,
143.3, 141.2, 136.6, 132.2, 131.9, 131.8, 131.2, 130.4, 129.2, 128.4,
127.3, 127.0, 126.6, 126.5, 126.3, 126.2, 125.5, 125.3, 124.2, 124.1,
123.5, 122.3, 122.1, 121.6, 116.0, 114.8, 48.2, 31.4, 26.7, 26.5,
22.6, 13.9; m/z (FAB) 672.2684 ((M + H)+, C44H38O2N3S
requires 672.2682).
(E)-(5-(N-Hexyl-3-(p-(naphthylphenylamino)phenyl)phenothiazin-
7-yl)thiophen-2-yl)-2-cyanoacrylic acid (NPtS-C6)
(E)-(3-(p-(Naphthylphenylamino)phenyl)-N-(p-ditolylaminophenyl)
phenothiazin-7-yl)-2-cyanoacrylic acid (NPt-T)
Compound NPtS-C6 was synthesized according to the same
procedure as that of NPt-C1, as red solids in 55% yield, mp:
261–264 ꢁC. dH (500 MHz, DMSO-d6) 8.46 (s, 1H), 8.00 (d, 1H,
J ¼ 8.0 Hz), 7.96 (d, 1H, J ¼ 4.0 Hz), 7.91 (d, 1H, J ¼ 8.0 Hz),
7.86 (d, 1H, J ¼ 8.5 Hz), 7.67 (d, 1H, J ¼ 4.0 Hz), 7.35–7.59 (m,
11H), 7.24 (t, 2H, J ¼ 8.0 Hz), 7.03–7.06 (m, 2H), 6.93–6.96 (m,
2H), 6.90–6.92 (m, 2H), 3.89 (t, 2H, J ¼ 6.5 Hz), 1.66–1.67 (m,
2H), 1.37–1.38 (m, 2H), 1.21–1.25 (m, 4H), 0.81 (t, 3H, J ¼ 7.0
Hz); dC NMR (125 MHz, DMSO-d6) 163.6, 152.0, 147.4, 147.0,
146.4, 145.5, 142.5, 142.3, 141.3, 134.9, 134.4, 133.7, 131.6,
130.6, 129.3, 128.5, 127.3, 126.8, 126.7, 126.6, 126.6, 126.5,
126.3, 125.8, 125.3, 124.3, 124.3, 124.2, 123.9, 123.4, 123.0,
122.0, 121.6, 121.0, 116.5, 116.2, 116.0, 97.7, 46.7, 30.7, 26.1,
25.7, 22.0, 13.7; m/z (FAB) 754.2562 ((M + H)+, C48H40O2N3S2
requires 754.2562).
Compound NPt-T was synthesized according to the same
procedure as that of NPt-C1, as red solids in 77% yield, mp: 273–
275 ꢁC. dH (400 MHz, DMSO-d6) 8.05 (s, 1H), 8.00 (d, 1H, J ¼
8.1 Hz), 7.91 (d, 1H, J ¼ 8.2 Hz), 7.85 (d, 1H, J ¼ 8.4 Hz), 7.71 (d,
1H, J ¼ 1.8 Hz), 7.62–7.64 (m, 1H), 7.58 (t, 1H, J ¼ 7.6 Hz), 7.52
(t, 1H, J ¼ 7.6 Hz), 7.44 (d, 3H, J ¼ 8.6 Hz), 7.37 (d, 1H, J ¼ 7.3
Hz), 7.17–7.28 (m, 10H), 7.08–7.10 (m, 6H), 6.97–7.00 (m, 3H),
6.89 (d, 2H, J ¼ 8.6 Hz), 6.22–6.27 (m, 2H), 2.28 (s, 6H); dC
NMR (100 MHz, DMSO-d6) 163.7, 152.0, 148.1, 147.4, 147.3,
147.2, 140.7, 135.2, 134.9, 133.5, 131.4, 131.2, 130.7, 130.6, 130.3,
129.4, 128.6, 128.3, 127.4, 126.8, 126.7, 126.6, 126.3, 125.5, 125.0,
123.8, 123.4, 122.2, 121.8, 121.4, 120.8, 118.6, 118.4, 116.8, 115.3,
99.5, 20.4; m/z (FAB) 859.3106 ((M + H)+, C58H43O2N4S
requires 859.3106).
This journal is ª The Royal Society of Chemistry 2012
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(E)-(5-(3-(p-(Naphthylphenylamino)phenyl)-N-(p-
ditolylaminophenyl)phenothiazin-7-yl)thiophen-2-yl)-2-
cyanoacrylic acid (NPtS-T)
i.e., compounds 1-X and 2-X. N-substituted 7-bromopheno-
thiazine-3-carbaldehyde (1-X) was prepared from 10H-pheno-
thiazine by a sequence of N-alkylation, bromination, followed by
formylation to give a total yield of 31–55%. The presence of an
aldehyde group was verified by an absorption at d 9.7–9.8 ppm in
1H NMR. Compound 3-X was made from p-(tributylstannyl)
phenylnaphthylphenylamine and 1-X by a Stille coupling reac-
tion catalyzed by PdCl2(PPh3)2.15 The same procedure was used
for the preparation of 4-X from a reaction of 1-X and p-(5-
(tributylstannyl)thiophen-2-yl)phenylnaphthylphenylamine, as
well as the preparation of 5-X from p-(tributylstannyl)phenyl-
naphthylphenylamine and 2-X. The final step was a Knoevenagel
condensation between carbaldehyde and cyanoacetic acid in the
presence of ammonium acetate.16 The synthesis of Pt-C6 fol-
lowed a standard procedure in a previous report,12a and is
described in the ESI†. The structures of all new compounds were
characterized by their spectroscopic data.
Compound NPtS-T was synthesized according to the same
procedure as that of NPt-C1, as red solids in 76% yield, mp: 213–
215 ꢁC. dH (500 MHz, DMSO-d6) 8.42 (s, 1H), 8.00 (d, 1H, J ¼
8.2 Hz), 7.94 (d, 1H, J ¼ 4.1 Hz), 7.90 (d, 1H, J ¼ 8.3 Hz), 7.85 (d,
1H, J ¼ 8.4 Hz), 7.61 (d, 1H, J ¼ 4.0 Hz), 7.57 (t, 1H, J ¼ 7.7 Hz),
7.51 (t, 1H, J ¼ 7.3 Hz), 7.41–7.45 (m, 4H), 7.36 (d, 1H, J ¼ 7.0
Hz), 7.30 (dd, 1H, J ¼ 2.1, 8.6 Hz), 7.16–7.25 (m, 11H), 7.04–7.07
(m, 5H), 6.96–6.98 (m, 3H), 6.89 (d, 2H, J ¼ 8.7 Hz), 6.23 (t, 2H,
J ¼ 8.7 Hz), 2.27 (s, 6H); dC NMR (125 MHz, DMSO-d6) 163.6,
147.9, 147.4, 147.0, 144.4, 144.1, 142.4, 141.5, 134.9, 134.4, 133.7,
133.3, 131.5, 131.4, 131.0, 130.6, 130.2, 129.3, 128.5, 127.3, 126.8,
126.7, 126.6, 126.5, 126.4, 126.2, 125.5, 125.3, 124.9, 124.1, 123.7,
123.6, 123.3, 122.0, 121.7, 121.6, 120.9, 119.3, 118.4, 116.6, 116.2,
115.8, 98.1, 20.4; m/z (FAB) 941.2983 ((M + H)+, C62H45O2N4S2
requires 941.2984).
Quantum chemistry computations
The structure of dyes was optimized by using the B3LYP/6-31G*
hybrid functional. For the excited states, a time-dependent
density functional theory (TDDFT) with the B3LYP functional
was employed. All analyses were performed using Q-Chem 3.0
software. The frontier orbital plots of HOMO and LUMO were
drawn by using Gaussian 03.
Results and discussion
Synthesis
The synthetic sequence of eight dyes, i.e., NPt-C1, NPt-C6,
NPtS-C6, NSPt-C6, NPt-T, NPtS-T, NSPt-T, and Pt-C6 is
shown in Scheme 1 (also Scheme S1 in the ESI†). The key
intermediates in all syntheses were 7-bromopheno derivatives,
Fig. 2 Absorption spectra of organic dyes in THF.
Scheme 1 Synthetic route of organic dyes. Reagents and conditions: (i) PdCl2(PPh3)2, p-(tributylstannyl)phenylnaphthylphenylamine or p-(5-(tribu-
tylstannyl)thiophen-2-yl)phenylnaphthylphenylamine, DMF, 90 ꢁC; (ii) (a) PdCl2(PPh3)2, p-(tributylstannyl)phenylnaphthylphenylamine, DMF, 90 ꢁC;
(b) n-BuLi, DMF, THF, ꢀ78 ꢁC; (iii) cyanoacetic acid, NH4OAc, AcOH, 90–100 ꢁC.
4044 | J. Mater. Chem., 2012, 22, 4040–4049
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Table 1 Calculated (TDDFT/B3LYP), absorption, and electrochemical parameters for organic dyesa
(HOMO/
LUMO)b/eV
(labsc/nm)/
(3/Mꢀ1 cmꢀ1
labs
on TiO2/nm
HOMOd
(vs. NHE)/V
LUMOe
(vs. NHE)/V
Dye
Band gapb
fb
)
E0–0/eV
NPt-C1
NPt-C6
NPtS-C6
NSPt-C6
NPt-T
NPtS-T
NSPt-T
Pt-C6
ꢀ4.99/ꢀ2.36
ꢀ4.99/ꢀ2.34
ꢀ4.98/ꢀ2.58
ꢀ4.90/ꢀ2.38
ꢀ4.88/ꢀ2.22
ꢀ4.85/ꢀ2.50
ꢀ4.82/ꢀ2.27
ꢀ5.47/ꢀ2.38
2.63
2.65
2.40
2.52
2.66
2.35
2.55
3.09
0.17
0.14
0.21
0.20
0.29
0.35
0.40
0.23
437(17 100)
453(18 200)
463(22 700)
442(19 300)
474(17 800)
480(22 400)
472(21 100)
438(14 900)
406
450
451
439
462
465
460
392
0.67
0.65
0.55
0.67
0.63
0.52
0.67
0.73
2.22
2.19
2.10
2.23
2.15
2.05
2.13
2.32
ꢀ1.55
ꢀ1.54
ꢀ1.55
ꢀ1.56
ꢀ1.52
ꢀ1.53
ꢀ1.46
ꢀ1.59
a
f: oscillator strength for the lowest energy transition; 3: absorption coefficient; Eox: oxidation potential; E0–0: 0–0 transition energy measured at the
onset of absorption spectra. TDDFT/B3LYP calculated values. Absorptions of charge-transfer transition were measured in THF. Oxidation
b
c
d
potential in THF (10ꢀ3 M) containing 0.1 M (n-C4H9)4NPF6 with a scan rate of 100 mV sꢀ1 (vs. NHE). e LUMO was calculated by HOMO ꢀ E0–0
.
Absorption spectra
compared with others (3–15 nm). The reason can be explained by
the compact geometry of Pt-C6 or the absence of a large
substituent on the nitrogen atom of NPt-C1 that enables the
molecules to pack together more easily on the surface of TiO2.18
The UV-Vis absorption spectra of organic dyes in THF solution
are displayed in Fig. 2 and the parameters are listed in Table 1.
Each of these compounds exhibited two major absorption bands
at 313–334 nm and 437–480 nm, along with a minor one at 381–
384 nm. The two shorter wavelength bands were assigned to the
localized aromatic p–p* transitions, and the long wavelength
one to a charge-transfer (CT) transition. The intensity of p–p*
transitions is higher than that of the charge-transfer transitions
(Fig. 2). The compound with a long alkyl chain or a triphenyl-
amine (TPA) substituent on the nitrogen atom of phenothiazine
displayed a red-shift with an increased absorptivity, i.e., 1.82 ꢂ
104 Mꢀ1 cmꢀ1 for NPt-C6 (453 nm) and 1.78 ꢂ 104 Mꢀ1 cmꢀ1 for
NPt-T (474 nm) compared to 1.71 ꢂ 104 Mꢀ1 cmꢀ1 for NPt-C1
(437 nm). It is known that a wider absorption range is favorable
to the performance of DSSCs, as more photons can be harvested.
Compounds with a thiophene group, i.e., NSPt-C6 and NSPt-T,
also displayed a more extended absorption than those without.
The absorption spectra of all dyes absorbed on the surface of
TiO2 are shown in Fig. 3. The film absorption band displayed
a blue-shift with respect to those in solutions. It appears to be
a common phenomenon for such types of most organic dyes,
which has been ascribed to the deprotonation of carboxylic acid,
as well as to the formation of H-aggregation on the semi-
conductor surface.2c,17 In particular, larger blue-shifts are
observed in the spectra of NPt-C1 (31 nm) and Pt-C6 (46 nm)
Electrochemical properties
The oxidation potentials (Eox), corresponding to the highest
occupied molecular orbital (HOMO) level of dyes, were
measured by cyclic voltammetry (CV) in THF solutions (Fig. 4),
and the results are listed in Table 1. Each compound displayed
two major semi-reversible waves: the lower voltage one is derived
from the oxidation of the triarylamine donor and the higher one
from the phenothiazine moiety. The introduction of an electron-
donating substituent on the phenothiazine p-bridge elevated the
HOMO level, thus decreasing the oxidation potential. The ioni-
zation energy decreased gradually in the order of Pt-C6 (0.73) >
NPt-C1 (0.67) z NSPt-C6 (0.67) z NSPt-T (0.67) > NPt-C6
(0.65) > NPt-T (0.63) > NPtS-C6 (0.55) > NPtS-T (0.52). It is
noteworthy that inserting a thiophene unit close to the acceptor
group promotes the HOMO level, such as NPtS-C6 and NPtS-T.
Suitable HOMO and LUMO (lowest unoccupied molecular
orbital) levels are necessary to match the conduction band
(ꢀ0.5 V vs. NHE) of the TiO2 electrode and the redox potential
(0.4 V vs. NHE) of the iodine/iodide electrolyte. The estimated
HOMO levels were within the range of 0.73–0.52 eV, sufficiently
lower than that of the electrolyte pair Iꢀ/I3ꢀ, thus ensuring
Fig. 3 Absorption spectra of dyes adsorbed on TiO2. The spectra were
subtracted from that of pure TiO2, which was taken as a background.
Fig. 4 Oxidative voltammograms of all dyes.
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a smooth electron flow from the electrolyte to the dyes. The
LUMO levels were obtained by the values of Eox and the 0–
0 band gaps, which was measured at the onset of absorption
spectra in Fig. 2. All levels were considerably higher than the
conduction band of TiO2. Therefore the electron injection from
the excited state of dyes into the conduction band of TiO2 is also
feasible.
Computational analysis
The structure of dyes was analyzed by using the B3LYP/6-31G*
hybrid functional for full geometrical optimization (Fig. 5). In
the ground state the geometry of phenothiazine is not totally
planar, but rather slightly bent in the middle forming the shape of
a butterfly. The two p-chromophores, i.e., the triarylamine
donor and the cyanoacrylate acceptor, are located at different
sides of a phenothiazine and are p-conjugated with the two
phenyl rings. The side chain attached at N(10) of phenothiazine
increases the bulkiness of the molecules, therefore preventing the
formation of excimer and/or aggregates.12c The charge distribu-
tion in the frontier molecular orbitals can be depicted in Fig. 6.
As shown in the graphs, the HOMO level is localized mostly on
the triarylamine (D) moiety, while the LUMO on the cyanoa-
crylic acid (A). The non-planar shape of the phenothiazine
reveals the non-aromatic character of the central heterocycle,
which may reduce the degree of electronic resonance between D
and A.19 In the process of photoexcitation, a key issue of
increasing the quantum efficiency is to reduce the rate of internal
Fig. 6 Computed frontier orbitals of dyes. The left graphs are the
HOMOs, and the right ones are the LUMO.
charge recombination. The non-planar geometry between D and
A may help to slow down the rate, thus increasing the efficiency
of solar cells.
The non-planar geometry of phenothiazine may have a signif-
icant influence on the transition probability of photo-excitation.
The transition probability was estimated by time-dependent
density functional theory (TDDFT) with the B3LYP functional
and the calculated oscillator strength (f) is shown in Table 1. The
calculated potential energy levels of HOMOs and LUMOs
comply well with the experimental values of EHOMO and ELUMO
estimated by CV. The f values of CT transitions are all quite low,
ranging 0.14–0.40. The f values of compounds NPt-T, NPtS-T
and NSPt-T with a TPA substituent at N(10) show slightly
higher f values than others. The results are consistent with the
absorptivity measured in solutions.
Photovoltaic performance of DSSCs
The parameters of DSSCs fabricated with these dyes, i.e., short
circuit current (Jsc), open-circuit photovoltage (Voc), fill factor
(ff), and total power conversion efficiency (h), measured under
AM 1.5 solar light (100 mW cmꢀ2) are summarized in Table 2,
and the photocurrent–voltage (J–V) plots are shown in Fig. 7.
There are two major structural factors worthy of attention: (1)
the effect of inserting a thiophenylene group to the major chro-
mophore near the phenothiazine moiety and (2) the influence
induced by the substituent at the N(10) of phenothiazine.
The addition of a thiophene in the main chain seems to have
exhibited two opposite effects, i.e., it enhanced the DSSC effi-
ciency when the thiophene was located on the side of phenothi-
azine to the donor group, yet it reduced the DSSC efficiency
when it was located on the other side to the acceptor group. The
most apparent difference was observed in the Jsc values of NPtS-
C6 (5.99) and NSPt-C6 (14.42). In both the C6 and T series
Fig. 5 The optimized structure by TDDFT on B3LYP/6-31G* of NPt-
C6, NPt-T, NSPt-C6, and NSPt-T.
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Table 2 Photovoltaic parameters for DSSCs of organic dyesa
Dyes with long alkyl chain have been known to be capable of
reducing dark current by decreasing the rate of charge recom-
bination, and therefore increase the open current voltage (Voc) of
the solar cells. The effect was explicitly displayed by the following
order of dark current, i.e., NPtS-C6 > NPtS-T > Pt-C6 > NPt-
C1 > NPt-C6 > NSPt-C6 > NPt-T z NSPt-T (ESI, Fig. S34†).
The order of open current voltages matched well with the trend
of relative dark current, i.e., NPtS-C6 (0.57) < NPtS-T (0.63) <
Pt-C6 (0.65) < NPt-C1 (0.66) < NPt-C6 (0.67) < NSPt-C6 (0.69)
< NPt-T z NSPt-T (0.71). The values of fill factor (ff) of all dyes
ranged 0.60–0.64, yet did not show much difference among each
other. The performances of all dyes, except NPtS-C6 and NPtS-
T, are better than that of Pt-C6. The incident photon to current
conversion efficiency (IPCE) of all dyes displays a broad band in
the region of 350–545 nm with values mostly >70 to 80% (Fig. 8).
The best performance was observed in the device of NSPt-C6,
which exhibited a Jsc value of 14.42 mA cmꢀ2, a Voc value of 0.69
V, and a ff value of 0.63. The total power conversion efficiency
(h) was estimated to be 6.22%. The IPCE was higher than 80% in
the wavelength region of 350–510 nm. Compared with recent
reports on analogous D–D–p–A systems, this result is superior
based on relatively lower loading amounts.20,21
Dye
Voca/V
ff
hb (%)
sc/ms
a
Jsc /mA cmꢀ2
NPt-C1
NPt-C6
NPtS-C6
NSPt-C6
NPt-T
NPtS-T
NSPt-T
Pt-C6
11.12
13.66
5.99
14.42
11.65
10.90
11.12
10.73
15.50
0.66
0.67
0.57
0.69
0.71
0.63
0.71
0.65
0.75
0.63
0.61
0.60
0.63
0.63
0.62
0.61
0.64
0.59
4.60
5.60
2.04
6.22
5.22
4.24
4.80
4.49
6.85
3.61
4.44
2.47
4.68
5.04
2.71
4.95
3.35
18.8
N719
a
Jsc: short-circuit photocurrent density; Voc: open-circuit photovoltage;
ff: fill factor; and h: total power conversion efficiency. Performance of
b
DSSCs measured in a 0.25 cm2 working area on a FTO (8 U squareꢀ1
)
c
substrate. Lifetime of injected electrons, estimated by fitting the
middle-frequency of the Bode phase plots (ESI) into the expression s ¼
1/(2pf), where f is the frequency.
Fig. 7 J–V curves of the DSSC devices made with the dyes. The plots
were measured under the light intensity of 1.0 sun.
compounds, this trend can be clearly identified (Table 2). The
difference may be resulted from the relatively low loading
amounts of NPtS-C6 and NPtS-T on the TiO2 surface,
compared with the corresponding NSPt-C6 and NSPt-T (ESI†).
A reason for low loading capacity can be explained by examining
the shape of a thiophene. A thiophene substituted at 2,5-posi-
tions tends to bend the molecule to an angular shape, which
makes the whole molecule to occupy a larger surface area than
a molecule of linear shape. Such an effect is more pronounced
when a thiophene unit is located close to the cyanoacrylate
anchoring group, therefore leading to the reduction of loading
capacity (ESI, Fig. S32†). This kind of influence may be dimin-
ished by the addition of a co-absorbent CDCA during film
preparation, while the Jsc was recovered to 6.20 mA cmꢀ2 (ESI†).
The effect of adding a substituent at the N(10) of phenothia-
zine seems to be beneficial. The Jsc values of N-hexyl derivatives
(C6 series, 13.66–14.42) were better than those of N-TPA
derivatives (T series, 11.12–11.65), while both were significantly
better than that with a smaller size substituent, i.e., N-methyl
compound NPt-C1 (11.12). The higher Jsc values of C6 and T
series derivatives consequently led to a better quantum efficiency
of DSSCs.
Fig. 8 IPCE curves of organic dyes.
Fig. 9 EIS Nyquist plots of electrochemical impedance of the DSSCs
made with the dyes at ꢀ0.73 V bias in the dark.
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Electrochemical impedance spectroscopy
Considering all the above factors, the best performance of
DSSCs can be achieved by using compound NSPt-C6, which
possesses a triaryl group at the C(7) position and a hexyl chain at
the N(10). It displayed a Jsc value of 14.42 mA cmꢀ2, a Voc value
of 0.69 V, and a ff value of 0.63, leading to an overall quantum
efficiency of 6.22%.
Electrochemical impedance spectroscopy (EIS) analysis was
performed to further elucidate the photovoltaic property. In
Fig. 9 the electrochemical impedance spectra are shown for the
DSSCs under a forward bias of ꢀ0.73 V in the dark. In the
Nyquist plots a major semicircle was observed for each dye,
which is related to the transport process at the interfaces
between TiO2 and the electrolyte/dye.22 The charge recombi-
nation resistance at the TiO2 surface (Rrec) can be deduced by
fitting the curves using Z-view software. This value is related to
the charge recombination rate, e.g., a smaller Rrec indicates
a faster charge recombination and therefore a larger dark
current (ESI, Fig. S34†). The radius of the semicircles increases
in the order NPtS-C6 (43 ohm) < NPtS-T (50) < Pt-C6 (56) <
NPt-C1 (71) < NPt-C6 (74) < NSPt-C6 (80) < NPt-T (108) <
NSPt-T (112). This trend appears to be consistent with the
values of open circuit voltage. The electron lifetime (s) can also
be estimated in a Bode phase graph (ESI, Fig. 33†), while a shift
to low-frequency corresponds to a longer electron lifetime. The
values, which were obtained by fitting the middle-frequency of
the Bode phase plot into the equation s ¼ 1/(2pf), are given in
Table 2.23 A longer electron lifetime corresponds to a larger
value of Voc, while the longest electron lifetime was found in
NPt-T (5.04 ms).
Acknowledgements
Financial supports from the National Science Council of Taiwan
and Academia Sinica are gratefully acknowledged. Special
thanks to Professor C.-P. Hsu at the Institute of Chemistry for
her assistance on quantum chemistry computations.
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