Efficient and stable dye-sensitized solar cells based on phenothiazine sensitizers with thiophene units

Article abstract of DOI:10.1039/b918282a

Three new phenothiazine organic dyes containing thiophene, 3-(5-(3-(4-(bis(4-methoxy phenyl)amino)phenyl)-10-octyl-10H-phenothiazin-7-yl) thiophen-2-yl)-2-cyanoacrylic acid (P1), 3-(5-(3-(4-(diphenylamino)phenyl)-10- octyl-10H-phenothiazin-7-yl)thiophen-2-yl)-2-cyanoacrylic acid (P2) and 2-cyano-3-(5-(10-octyl-3-(4-(2,2-diphenylvinyl)phenyl)-10H-phenothiazin-7-yl) thiophen-2-yl) acrylic acid (P3) were designed and synthesized as sensitizers for application in dye-sensitized solar cells (DSSCs). For these dyes, the phenothiazine derivative moiety and the cyanoacetic acid take the roles of electron donor and electron acceptor, respectively. The absorption spectra, electrochemical and photovoltaic properties of P1-P3 and the cell long-term stability were extensively investigated. It was found that HOMO and LUMO energy level tuning can be conveniently accomplished by alternating the donor moiety. The DSSCs based on dye P2 showed the best photovoltaic performance: a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 84.9%, a short-circuit photocurrent density (J_sc) of 10.84 mA cm ^-2, an open-circuit photovoltage (V_oc) of 592 mV, and a fill factor (ff) of 0.69, corresponding to an overall conversion efficiency of 4.41% under standard global AM 1.5 solar light conditions. These results demonstrated that the DSSCs based on phenothiazine dyes could achieve both high performance and good stability.

Full text of DOI:10.1039/b918282a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Efficient and stable dye-sensitized solar cells based on phenothiazine
sensitizers with thiophene units†
*
Wenjun Wu, Jiabao Yang, Jianli Hua, Jin Tang, Lei Zhang, Yitao Long and He Tian
*
Received 4th September 2009, Accepted 30th November 2009
First published as an Advance Article on the web 14th January 2010
DOI: 10.1039/b918282a
Three new phenothiazine organic dyes containing thiophene, 3-(5-(3-(4-(bis(4-methoxy
phenyl)amino)phenyl)-10-octyl-10H-phenothiazin-7-yl)thiophen-2-yl)-2-cyanoacrylic acid (P1),
3-(5-(3-(4-(diphenylamino)phenyl)-10-octyl-10H-phenothiazin-7-yl)thiophen-2-yl)-2-cyanoacrylic acid
(P2) and 2-cyano-3-(5-(10-octyl-3-(4-(2,2-diphenylvinyl)phenyl)-10H-phenothiazin-7-yl)thiophen-2-yl)
acrylic acid (P3) were designed and synthesized as sensitizers for application in dye-sensitized solar cells
(DSSCs). For these dyes, the phenothiazine derivative moiety and the cyanoacetic acid take the roles of
electron donor and electron acceptor, respectively. The absorption spectra, electrochemical and
photovoltaic properties of P1–P3 and the cell long-term stability were extensively investigated. It was
found that HOMO and LUMO energy level tuning can be conveniently accomplished by alternating
the donor moiety. The DSSCs based on dye P2 showed the best photovoltaic performance: a maximum
monochromatic incident photon-to-current conversion efficiency (IPCE) of 84.9%, a short-circuit
photocurrent density (Jsc) of 10.84 mA cmꢀ2, an open-circuit photovoltage (Voc) of 592 mV, and a fill
factor (ff) of 0.69, corresponding to an overall conversion efficiency of 4.41% under standard global
AM 1.5 solar light conditions. These results demonstrated that the DSSCs based on phenothiazine dyes
could achieve both high performance and good stability.
presenting unique electronic and optical properties.27 Therefore,
phenothiazine-based polymers have been widely employed in
thin film transistor, photovoltaic cell, and light-emitting diode
applications. However, to the best of our knowledge, there was
only one report on the application of a small molecular dye
containing a phenothiazine moiety in DSSCs.28 On the other
hand, Haque and co-workers have reported a group of hole
transport material (HTM) which can indeed slow the recombi-
nation between the photoinjected electrons and oxidized HTM
through separating the holes spatially away from the electrons
in the semiconductor, and thus improve the efficiency of
DSSCs.29 Triarylamine has been widely used in opto- and
electro-active materials for its good electron donating and
transporting capability, as well as its special propeller starburst
molecular structure. Recently, organic photovoltaic functional
materials with triarylamine as electron donor have aroused
great interest and become the focus of intensive research in the
field of solar cells. These materials have significantly reinforced
the conversion efficiency of next-generation solar cells, espe-
cially dye sensitized solar cells.3a The methoxy groups attached
to the triphenylamine can enhance the extent of electron delo-
calization and the ability to donate the electron of the sensitizer.
The 1,1,2-triphenylethene unit as a donor has been introduced
in indoline dyes with high-performance dye-sensitized solar
cells.29c Also, the introduction of thiophene moieties was
expected to allow a red-shift of the spectra, and broaden the
spectral region of absorption. Based on these studies, three new
organic dyes (P1–P3 shown in Scheme 1) have been designed
and synthesized through the modification of simple phenox-
azine-thiophene dyes (6–8 shown in Scheme 2) with hole
transport group, corresponding molecular structures of the
1. Introduction
Dye-sensitized solar cell (DSSC) systems based on highly porous
nanocrystalline films of titanium dioxide are of considerable
technological interest because of their demonstrated high-power
conversion efficiency, potential low cost, and high semiconductor
stability.1–26 As known to all, the sensitizer is a critical compo-
nent in DSSCs. To overcome the prohibitive issues of ruthenium
metal complexes, organic DSSCs have received more and more
attention because of their ease of synthesis, high molar extinction
coefficients and simple preparation process of low cost in
comparison to Ru complexes. Coumarin,9–11 merocyanine,12
indoline,13,14 polyene,15 hemicyanine,16,17 triphenylamine,18–22
fluorene23–25 and tetrahydroquinoline26 based-organic dyes have
been developed and showed good performance. Recently, great
progress has been made in this field and organic dyes would be
a promising type of sensitizer for dye-sensitized solar cells.
Phenothiazine is a well-known heterocyclic compound with
electron-rich sulfur and nitrogen heteroatoms, and the pheno-
thiazine ring is nonplanar with a butterfly conformation in the
ground state, which can impede the molecular aggregation and
the formation of intermolecular excimers. Thus, phenothiazine
is a potential hole transport semiconductor in organic devices,
Key Laboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science & Technology, 130 Meilong Road,
Shanghai, 200237, PR China. E-mail: tianhe@ecust.edu.cn; jlhua@ecust.
edu.cn; Fax: +86-21-64252756; Tel: +86-21-64252756; +86-21-
64252758; +86-21-64250940
† Electronic supplementary information (ESI) available: Full synthetic
procedures for the important intermediate compounds; Mott–Schottky
plots of P1–P3 sensitized TiO2 electrodes. See DOI: 10.1039/b918282a
1772 | J. Mater. Chem., 2010, 20, 1772–1779
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procedures.31–34 All other chemicals were purchased from Aldrich
and used as received without further purification.
2.2 Preparation of photovoltaic devices
A screen-printed double layer of TiO2 particles was used as
photoelectrode. A 10 mm thick film of 13 nm sized TiO2 particles
(Ti-Nanoxide T/SP) was first printed on the FTO conducting
glass and further coated with a 4 mm thick second layer of 400 nm
light-scattering anatase particles (Ti-Nanoxide 300). Sintering
was carried out at 450 ꢁC for 30 min. Before immersion in the dye
solution, these films were immersed into a 40 mM aqueous TiCl4
solution at 70 ꢁC for 30 min and washed with water and ethanol.
Then the films were heated again at 450 ꢁC for 30 min followed
Scheme 1 Molecular structures of dyes (P1–P3).
ꢁ
ꢀ4
by cooling to 80 C and dipping into a 3 ꢂ 10 M solution of
dye in acetonitrile for 12 h at room temperature. To prepare the
counter electrode, the Pt catalyst was deposited on the cleaned
FTO glass by coating with a drop of H2PtCl6 solution (0.02 M in
2-propanol solution) with the heat treatment at 400 ꢁC for
15 min. A hole (0.8 mm diameter) was drilled on the counter
electrode using a drill-press. The perforated sheet was cleaned
with ultrasound in an ethanol bath for 10 min. For the assembly
of DSSCs, the dye-covered TiO2 electrode and Pt-counter elec-
trode were assembled into a sandwich type cell and sealed with
a hot-melt gasket of 25 mm thickness made of the ionomer Surlyn
1702 (DuPont). The size of the TiO2 electrodes used was 0.25 cm2
(i.e., 5 mm ꢂ 5 mm). A drop of the electrolyte was put on the hole
in the back of the counter electrode. It was introduced into the
cell via vacuum backfilling. The hole in the counter electrode was
sealed with a film of Surlyn 1702 and a cover glass (0.1 mm
thickness) using a hot iron bar.
Scheme 2 Synthesis route to dyes (P1–P3).
2.3 Electrolyte
The electrolyte employed was a solution of 0.6 M PMII
(1-propyl-3-methylimidazolium iodide), 0.05 M I2, 0.10 M LiI
and 0.5 M 4-tert-butylpyridine in a mixture of acetonitrile and
methoxypropionitrile (volume ratio, 7 : 3).
three dyes are shown in Scheme 1. Finally, three new sensitizers
have been applied successfully to sensitization of nanocrystalline
TiO2-based solar cells and the corresponding photovoltaic
properties, electronic and optical properties are also presented.
For the quasi-solid state electrolyte, fumed silica nanoparticles
(5 wt%) with 12 nm primary particle size, kindly provided by
Degussa, were mixed with BMII (1-butyl-3-methylimidazolium
iodide) based liquid electrolyte containing 0.5 M iodine and
0.45 M BI (benzimidazole) in an agate mortar.
2. Experimental
2.1 Materials
FTO conducting glass (FTO glass, fluorine doped tin oxide over-
layer, transmission > 90% in the visible, sheet resistance 15 U
squareꢀ1) was obtained from Geao Science and Educational Co.
Ltd. of China. Methoxypropionitrile (MPN), 1-methylimidazole
and n-iodobutane were purchased from Aldrich. Titania pastes
of Ti-Nanoxide T/SP and Ti-Nanoxide 300 were from Solaronix.
Lithium iodide was from Fluka and iodine, 99.999%, was from
Alfa Aesar. 1-Propyl-3-methylimidazolium iodide (PMII) and
1-butyl-3-methylimidazolium iodide (BMII) were prepared
according to published procedures.30
2.4 Measurements
Photovoltaic measurements employed an AM 1.5 solar simulator
equipped with a 300 W xenon lamp (Model No. 91160, Oriel).
The power of the simulated light was calibrated to 100 MW cmꢀ2
using a Newport Oriel PV reference cell system (Model 91150V).
I–V curves were obtained by applying an external bias to the cell
and measuring the generated photocurrent with a Keithley model
2400 digital source meter. The voltage step and delay time of
photocurrent were 10 mV and 40 ms, respectively. Cell active
area was tested with a mask of 0.158 cm2. The photocurrent
action spectra were measured with a IPCE test system consisting
of a Model SR830 DSP Lock-In Amplifier and a Model SR540
Optical Chopper (Stanford Research Corporation, USA),
a 7IL/PX150 xenon lamp and power supply, and a 7ISW301
ꢀ
THF was pre-dried over 4 A molecular sieves and distilled
under argon atmosphere from sodium benzophenone ketyl imme-
diately prior to use. Starting materials 10-octyl-phenothiazine,
4-(N,N-diphenylamino)-1-bromo-benzene, 4-(N,N-bis(4-methox-
yphenyl)amino)-1-bromobenzene and 1,1-diphenyl-2-(4-bromo-
phenyl)ethane were prepared according to published
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Spectrometer. Impedance spectra were recorded using
a CHI-660c electrochemical station.
39.5, 39.3, 39.2, 39.0, 31.1, 30.4, 28.7, 28.6, 26.1, 25.8, 22.0, 13.9.
HRMS (m/z): [M]+ calcd for C48H42N2O2S2, 742.2687; found,
742.2653.
3. Synthesis
4. Results and discussion
Full synthesis procedures for all important intermediate
compounds are available in the ESI.†
4.1 Synthesis
The synthesis route to phenothiazine dyes containing thiophene
(P1–P3) is depicted in Scheme 2. The octyl chain on the pheno-
thiazine nucleus can improve the solubility and form a tightly
3-(5-(3-(4-(Bis(4-methoxyphenyl)amino)phenyl)-10-octyl-10H-
phenothiazin-7-yl)thiophen-2-yl)-2-cyanoacrylic acid (P1)
ꢀ
packed insulating monolayer blocking the I3 or cations
Compound 6 (139 mg, 0.19 mmol), 2-cyanoacetic acid (48 mg,
0.57ꢁmmol), ammonium acetate (3 mg) and acetic acid (11 mL) at
120 C for 12 h. After cooling the solution, water was added to
quench the reaction. The precipitate was filtered and washed with
water. The residue was purified by column chromatography on
silica (CH2Cl2–ethanol ¼ 10/1, v/v) to yield 133 mg of red solid
(yield 88.5%). 1H NMR (DMSO-d6, 500 MHz), d: 0.84 (t, J ¼ 6.7
Hz, 3H), 1.25 (m, 8H), 1.46 (m, 2H), 1.78 (m, 2H), 3.75 (s, 6H),
4.32 (m, 2H), 6.85 (d, J ¼ 8.4 Hz, 2H), 6.94 (d, J ¼ 8.7 Hz, 4H),
7.06 (d, J ¼ 8.7 Hz, 4H), 7.62 (d, J ¼ 8.5 Hz, 2H), 7.71 (d, J ¼ 9.0
Hz, 2H), 7.76 (s, 2H), 7.97 (m, 2H), 8.12 (s, 1H), 8.17 (m, 1H),
8.33 (m, 1H). 13C NMR (DMSO-d6, 125 MHz), d: 165.5, 158.0,
150.1, 149.0, 142.7, 142.0, 139.7, 138.6, 138.1, 135.8, 132.6, 132.3,
131.6, 130.3, 129.9, 129.1, 129.0, 128.4, 126.7, 126.5, 121.6, 121.3,
119.4, 119.3, 117.2, 111.1, 110.5, 57.4, 34.5, 33.3, 31.1, 30.9, 30.8,
28.2, 28.0, 24.2, 16.1. HRMS (m/z): [M]+ calcd for
C48H45N3O4S2, 791.2851; found, 791.2834.
approaching the TiO2. The thiophenyl unit is attached to the core
phenothiazine by a Suzuki coupling reaction. The reaction
produced the di-substituted side products as well, fortunately
monocapped compounds can be easily separated by column
chromatography. In the next step, this bromo-exposed interme-
diate was reacted with arylphenylboronic acids (3–5) by Suzuki
coupling reaction, respectively. Finally, the target products
(P1–P3) were synthesized via Knoevenagel condensation reac-
tion of the respective carbaldehydes with cyanoacetic acid in the
presence of acetic acid and ammonium acetate. All the key
intermediates and three new organic phenothiazine sensitizers
(P1–P3) were confirmed by H NMR, 13C NMR and HRMS.
1
4.2 Absorption properties in solution and on TiO2 film
UV-Vis absorption spectra of dyes P1–P3 in a diluted solution of
CH2Cl2 are shown in Fig. 1 and their absorption data are listed in
Table 1. In the UV-Vis spectra, P1–P3 exhibit two major
prominent bands, appearing at 300–400 nm and at 400–600 nm,
respectively. The former is ascribed to a localized aromatic p–p*
transition and the later is of charge-transfer character. The
absorption maxima for P1–P3 in CH2Cl2 are at l ¼ 460 nm, 450
nm and 449 nm, respectively. Compared with P2 and P3, P1 has
two methoxy groups attached to the triphenylamine on the
electron donor side, which enhanced the extent of electron
delocalization over the whole molecule, so its maximum
absorption peak was red shifted a little. In comparison with
conventional ruthenium complexes (for example, 1.39 ꢂ 10 4 Mꢀ1
cmꢀ1 at 541 nm for N3)35 the present dye molecules show about
twice as high absorption coefficients. The greater
maximum absorption coefficients of the organic dyes allow
3-(5-(3-(4-(Diphenylamino)phenyl)-10-octyl-10H-phenothiazin-7-
yl)thiophen-2-yl)-2-cyanoacrylic acid (P2)
The synthesis method resembles that of compound P1, and the
compound was purified by column chromatography on silica
(CH2Cl2–ethanol ¼ 10/1, v/v) to yield 55 mg of orange solid
(yield 86.5%). 1H NMR (DMSO-d6, 500 MHz), d: 0.86 (t, J ¼ 6.9
Hz, 3H), 1.28 (m, 8H), 1.48 (m, 2H), 1.82 (m, 2H), 4.38 (m, 2H),
7.08 (m, 8H), 7.34 (t, J ¼ 7.8 Hz, 4H), 7.75 (m, 6H), 8.01 (m, 2H),
8.11 (s, 1H), 8.25 (d, J ¼ 1.6 Hz, 1H), 8.37 (s, 1H). 13C NMR
(DMSO-d6, 125 MHz), d: 165.8, 158.1, 150.2, 149.3, 142.5, 142.1,
139.6, 138.5, 138.3, 135.4, 132.3, 132.1, 131.2, 130.4, 129.7, 129.3,
129.3, 128.2, 126.5, 126.3, 121.2, 121.1, 119.2, 119.5, 117.1, 111.3,
110.6, 34.2, 33.1, 31.2, 30.7, 30.6, 28.1, 28.0, 24.1, 16.3. HRMS
(m/z): [M]+ calcd for C46H41N3O2S2, 731.2640; found, 731.2625.
2-Cyano-3-(5-(10-octyl-3-(4-(2,2-diphenylvinyl)phenyl)-10H-
phenothiazin-7-yl)thiophen-2-yl) acrylic acid (P3)
The synthesis method resembles that of compound P1, and the
compound was purified by column chromatography on silica
(CH2Cl2–ethanol ¼ 10/1, v/v) to yield 69 mg of orange solid
(yield 69.8%). 1H NMR (DMSO-d6, 500 MHz), d: 0.84 (m, 3H),
1.25 (m, 8H), 1.46 (m, 2H), 1.77 (m, 2H), 4.34 (m, 2H), 7.12
(d, J ¼ 8.3 Hz, 2H), 7.15 (s, 1H), 7.22 (d, J ¼ 7.0 Hz, 2H), 7.35
(m, 5H), 7.47 (m, 3H), 7.62 (d, J ¼ 8.1 Hz, 2H), 7.72 (m, 2H), 7.77
(s, 2H), 8.01 (m, 2H), 8.19 (s, 1H), 8.26 (d, J ¼ 1.6 Hz, 1H), 8.35
(d, J ¼ 1.7 Hz, 1H). 13C NMR (DMSO-d6, 125 MHz), d: 163.9,
147.2, 142.4, 141.8, 140.0, 136.6, 136.2, 136.1, 130.0, 129.7, 129.1,
128.6, 128.3, 128.2, 127.7, 127.6, 127.1, 127.0, 125.6, 124.6, 124.4,
117.4, 117.1, 115.2, 79.2, 78.9, 54.9, 46.8, 40.0, 39.9, 39.8, 39.7,
Fig. 1 UV-Vis absorption spectra of P1–P3 in CH2Cl2 (3 ꢂ 10 ꢀ5 M).
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Table 1 Absorption and electrochemical properties of P1–P3
HOMOc/V (vs.NHE)
E0–0d/eV
LUMOe/V (vs. NHE)
a
Dye
lmax /nm (3 ꢂ 10 ꢀ4 Mꢀ1 cmꢀ1
)
lmaxb/nm
P1
P2
P3
460 (2.4)
450 (3.1)
449 (2.3)
476
475
480
0.68
0.90
1.23
2.17
2.26
2.34
ꢀ1.49
ꢀ1.36
ꢀ1.11
a
b
c
ꢀ5
Absorption maximum in CH2Cl2 (3 ꢂ 10
M). Absorption maximum on TiO2 film. HOMO were measured in acetonitrile with 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte (working electrode: FTO/TiO2/dye; reference electrode: SCE; calibrated with
d
ferrocene/ferrocenium (Fc/Fc+) as an external reference. Counter electrode: Pt). E0–0 was estimated from the absorption thresholds from
absorption spectra of dyes adsorbed on the TiO2 film. e LUMO is estimated by subtracting E0–0 from HOMO.
Fig. 3 Cyclic voltammetry plots of P1–P3 attached to a nanocrystalline
Fig. 2 Absorption spectra of P1–P3 adsorbed on TiO2 film.
TiO2 film deposited on conducting FTO glass.
a correspondingly thinner nanocrystalline film so as to avoid the
decrease of the film mechanical strength. This also benefits the
electrolyte diffusion in the film and reduces the recombination
electrode and saturated calomel electrode (SCE) as the reference
electrode. The SCE reference electrode was calibrated using
a ferrocene/ferrocenium (Fc/Fc+) redox couple as an external
standard, and the E1/2 of the Fc/Fc+ redox couple was found to
be 0.37 V versus the SCE reference electrode. It can be speculated
from Fig. 3 that the first half-wave potentials of P1–P3 are 0.28,
0.50 and 0.83 V (vs. Fc/Fc+). Therefore, the ground state
oxidation potential corresponding to the HOMO levels are 0.68,
0.90 and 1.23 V (vs. NHE), respectively. From Fig. 2, we know
that the absorption thresholds for dyes P1–P3 are 572, 550 and
529 nm on TiO2 film, which correspond to the band gap energy
(E0–0) of 2.17, 2.26 and 2.34 eV, respectively. The estimated
excited state potential corresponding to the LUMO levels,
calculated from EHOMO ꢀ E0–0, are ꢀ1.48, ꢀ1.36 and ꢀ1.11 V,
respectively. The examined HOMO levels and the LUMO levels
are listed in Table 1. From these data, we found the HOMO
levels of P1–P3 to be sufficiently more positive than the iodine/
iodide redox potential value (0.4 V), indicating that the oxidized
dyes formed after electron injection into the CB of TiO2 could
thermodynamically accept electrons from Iꢀ ions. The LUMO
levels of these dyes (see Table 1) were sufficiently more negative
than the conduction-band-edge energy level (Ecb) of the TiO2
electrode (ꢀ0.5 V vs. NHE), which implies that electron injection
from the excited dye into the CB of TiO2 is feasible.
possibility
of
the
light-induced
charges
during
transportation.12,36
Fig. 2 shows the absorption spectra of P1–P3 on TiO2 films
after 12 h adsorption. As shown in Fig.2, the maxima absorption
peaks for P1–P3 on the TiO2 film are at l ¼ 476 nm, 475 nm and
480 nm and their absorption bands are red-shifted by 16 nm,
25 nm, 31 nm compared with the solution spectrum, respectively.
The red shifts of the absorption spectra on TiO2 of P1–P3 could
be ascribed to the aggregation of the dyes on the TiO2 surface,
which can occur readily because of the presence of carboxyl
groups in the molecules. The interaction between the carboxylate
group and the surface Ti4+ ions may lead to increased delocal-
ization of the p* orbital of the conjugated framework. The
energy of the p* level is decreased by this delocalization, which
explains the red shift for the absorption spectra.
4.3 Electrochemical properties
Suitable highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) levels of the sensitizer are
required to match the conduction band (CB) edge level of the
TiO2 electrode. To evaluate the possibility of electron transfer
from the excited dye molecule to the conduction band (Ecb) of
TiO2, cyclic voltammograms (Fig. 3) were recorded in acetoni-
4.4 Performances of liquid electrolyte dye-sensitized solar cells
trile solvent using 0.1
M
tetrabutylammonium hexa-
fluorophosphate as supporting electrolyte, TiO2 films stained
with sensitizer as the working electrode, Pt as the counter
Fig. 4 shows the incident monochromatic photon-to-current
conversion efficiencies (IPCEs) obtained with a sandwich-type
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Table 2 Photovoltaic performance of DSSCs based on P1–P3 dye with liquid electrolyte
Amounta/mol cmꢀ2
b
Jsc /mA molꢀ1
Dye
Jsc/mA cmꢀ2
Voc/mV
ff
h (%)
P1
P2
P3
9.43
10.84
7.39
584
592
505
0.69
0.69
0.66
3.78
4.41
2.48
1.12 ꢂ 10ꢀ7
1.41 ꢂ 10ꢀ7
9.76 ꢂ 10ꢀ8
8.39 ꢂ 107
7.70 ꢂ 107
7.57 ꢂ 107
Amount of dye adsorbed on TiO2 film. b The short-circuit photocurrent generated per mole of dye molecules.
a
Fig. 4 Photocurrent action spectra of the TiO2 electrodes sensitized by
Fig. 5 Photocurrent density vs. voltage curves for DSSCs based on P1
(-), P2 (C) and P3 (:) under irradiation of AM 1.5G simulated solar
light (100 mW cmꢀ2) with liquid electrolyte.
P1 (-), P2 (C) and P3 (:) with liquid electrolyte.
two electrode cell. The dye-coated TiO2 film was used as the
working electrode, platinized FTO glass as the counter electrode
and 0.6 M MPII, 0.05 M I2, 0.10 M LiI and 0.5 M 4-tert-
butylpyridine in acetonitrile and methoxypropionitrile (volume
ratio, 7: 3) mixture solution as the redox electrolyte. The losses of
light reflection and absorption by the conducting glass were not
corrected. As shown in Fig. 4, all three dyes can efficiently
convert visible light to photocurrent in the region from 350 nm to
700 nm. The IPCE exceeds 70% in the spectral range 380–520 nm
for P1, which reaches its maximum of 85.3% at 450 nm. The
IPCE of P2 reached a maximum (84.9%) at 510 nm. And the
maximum IPCE of P3 reaches 83.4% at 460 nm. The P2 sensi-
tized TiO2 electrode generates the highest conversion yield
among three dyes (Table 2), which may be due to its broader
photocurrent action spectra. We have noted that the IPCE
spectrum of P1–P3 is red-shifted compared with the absorption
spectrum of the dye adsorbed on TiO2 (Fig. 4). This important
red shift in the photocurrent response is attributed to lateral
interaction in the adsorbed layer of the sensitizer and the detailed
reason for this needs further studies.
Mott–Schottky plot method (see ESI† Fig.S1), which are ꢀ0.31
V, ꢀ0.35 V, ꢀ0.27 V, respectively. It was found that the Voc of
P1–P3 corresponded to the apparent Fermi level and the
different dyes have different apparent Fermi levels. From the
adsorption amounts and the short-circuit photocurrents in Table
2, the contribution of one mole of dye molecules to the short-
circuit photocurrent could be calculated and the values are also
listed in Table 2. About the molecular structure of three dyes, one
can find that they have the same acceptor and conjugation chain
but different donors. The results show that P1 containing the
methoxy group has a broader absorption spectrum than P2 and
P3 in dichloromethane solution because of the larger electron
delocalization, and consequently leads to better photoelectric
conversion properties to some extent. For example, one mole of
P1 produced 8.39 ꢂ 107 mA short-circuit photocurrent, while the
short-circuit photocurrents generated by one mole of P2 and P3
are 7.70 ꢂ 107 and 7.57 ꢂ 107 mA, respectively. However, the
photovoltaic conversion efficiency of P2 is higher than that of P1
due to the lower molar absorption coefficient, adsorption
amount and short-circuit current density of P1. Therefore, it was
found that the increase of electron donating ability is not always
beneficial to improve the photoelectric conversion efficiency of
DSSCs. These results agree well with the corresponding IPCE
spectra.
Fig. 5 shows the current–voltage characteristics of dye-sensi-
tized solar cells employing three phenothiazine dyes P1–P3 as
sensitizers under standard global AM 1.5 solar light conditions.
The photovoltaic characteristic parameters of short-circuit
current density (Jsc), open-circuit voltage (Voc), fill factor (ff),
and photovoltaic conversion efficiency (h) are listed in Table 2.
As shown in Fig. 5 and Table 2, we obtained a solar energy to
Electrochemical impedance spectroscopy (EIS) analysis was
performed to elucidate the photovoltaic findings further. Fig. 6
compares the impedance spectra for P1–P3-sensitized cells
measured in the dark under a forward bias of ꢀ0.40 V with
a frequency range of 0.1 Hz to 100 kHz. The important differ-
ences for the these organic dye-sensitized solar cells were found
electricity conversion efficiency of 3.78% (Jsc ¼ 9.43 mA cmꢀ2
,
Voc ¼ 584 mV, ff ¼ 0.69), 4.41% (Jsc ¼ 10.84 mA cmꢀ2, Voc ¼ 592
mV, ff ¼ 0.69) and 2.48% (Jsc ¼ 7.39 mA cmꢀ2, Voc ¼ 505 mV, ff
¼ 0.66) with DSSCs based on P1–P3, respectively. At the same
time we also tested the apparent Fermi levels of P1–P3 dyes by
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Fig. 7 Photocurrent action spectra of the TiO2 electrodes sensitized by
P1 (-), P2 (C) and P3 (:) with quasi-solid state electrolyte.
the IPCE of TiO2 electrodes obtained with quasi-solid-state solar
cells sensitized by dyes P1–P3. From Fig. 7 we can see that
P1–P3 can also efficiently convert visible light to photocurrent in
the region from 400 to 650 nm with a quasi-solid state electrolyte.
The IPCE data exceed 40% for P1, 50% for P2 and 35% for P3 in
the spectral range 400–500 nm, respectively, which reaches its
maximum of 49.4% for P1 at 450 nm, 55.4% for P2 at 450 nm and
47.8% for P3 at 440 nm. The IPCE performance of the quasi-
solid-state DSSC based on P2 is higher than those of P1 and P3,
though the action spectrum for P2 is similar to those for P1 and
P3. Therefore, phenothiazine sensitizer with triphenylamine hole
Fig. 6 Impedance spectra of DSSCs based on P1 (-), P2 (C), P3 (:)
and N3 (B) dye measured at ꢀ0.40 V bias in the dark. (a) Nyquist plots;
(b) Bode phase plots.
Table 3 Performance parameters of quasi-solid-state solar cells sensi-
tized by dyesa
Dye
Jsc/mA cmꢀ2
Voc/mV
ff
h (%)
for the conductivity. The Nyquist plots (Fig. 6a) show the radius
of the middle semicircle to increase in the order P3 < P1 < P2,
indicating that the electron recombination resistance augments
from P3, P1 to P2.37 Under similar conditions black dye N3 was
tested, which is in good accordance with the data previously
reported by Fabregat-Santiago et al.38 The electron lifetime
values derived from curve fitting are 6.07, 6.52 and 5.99 ms for
P1, P2, P3-sensitized solar cells, respectively. The longer electron
lifetime observed with P2- relative to P1- or P3-sensitized cells
indicates more effective suppression of the back reaction of the
injected electron with the I3ꢀ in the electrolyte and is reflected in
the improvements seen in the photocurrent, yielding substan-
tially enhanced device efficiency. The Bode phase plots shown in
Fig. 6b likewise support the differences in the electron lifetime for
TiO2 films derivatized with the three dyes. The middle-frequency
peak of the DSSCs based on P2 shifts to lower frequency relative
to P1 and P3, indicating a longer electron lifetime for P2. This
result is in agreement with the observed shift in the Voc value
under standard global AM 1.5 illumination.
P1
P2
P3
5.95
6.36
5.45
544
551
536
0.66
0.70
0.70
2.13
2.47
2.04
a
The listed data are results from the measurement which has the closest
values to the average of 6 measurements.
Since long-term stability is a vital parameter for sustained cell
operation, we substituted the liquid electrolyte with a quasi-
solid-state one. P1–P3 organic dyes were evaluated as sensitizers
for the quasi-solid-state dye-sensitized solar cell using 0.5 M
iodine and 0.45 M benzimidazole (BI) in pure 1-butyl-3-meth-
ylimidazolium iodide (BMII) as redox electrolyte which was
solidified with fumed silica nanoparticles (5 wt%).39 Fig. 7 shows
Fig. 8 Photocurrent–voltage curves of P1 (-), P2 (C) and P3 (:)
sensitized TiO2 electrodes under standard global AM 1.5 illumination
(100 mW cmꢀ2) with quasi-solid-state electrolyte.
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transport could become very attractive, because the introduction
of the triphenylamine moiety was indeed to locate the cationic
charge from the TiO2 surface and efficiently restrict recombina-
tion between the conduction band electron and oxidized
sensitizer.3,7
under standard global AM 1.5 solar light conditions (100 mW
cmꢀ2), and the corresponding photocurrent–voltage curves are
shown in Fig. 8. The P2-sensitized cell gave a short circuit
photocurrent density (Jsc) of 6.36 mA cmꢀ2, an open circuit
voltage (Voc) of 551 mV and a fill factor (ff) of 0.70, corre-
sponding to an overall conversion efficiency (h) of 2.47%. Under
the same conditions, the P1-sensitized cell gave a Jsc value of 5.95
mA cmꢀ2, a Voc of 544 mV and an ff of 0.66, corresponding to the
h value of 2.13% and P3 gave a Jsc of 5.45 mA cmꢀ2, a Voc of 536
mV, an ff of 0.70 and h of 2.04%.
4.5 Performances of quasi-solid-state electrolyte dye-sensitized
solar cells
Photovoltaic performances of the P1–P3 sensitized TiO2 film
Fig. 9 shows the photovoltaic performance during long term
accelerated aging of P1–P3-sensitized solar cells using a quasi-
solid-state electrolyte in a solar simulator at full intensity and
55 ꢁC. Values for the short-circuit current (Jsc), open-circuit
potential (Voc), fill factor (ff), and overall efficiency (h) were
recorded over a period of 1000 h. All the solar cells were sub-
jected to accelerated testing in a solar simulator at 100 mW cmꢀ2
intensity at 55 ꢁC. Jsc increased gradually for the first 400 h and
then remained almost constant for 1000 h. The increases in
photocurrent for P1–P3 from the initial value are calculated to
be 26.4%, 42.4% and 32.7%, respectively. Contrasting to the Jsc
increase under illumination, Voc dropped by 48, 50 and 48 mV
for P1–P3 respectively. The decrease in Voc may be due to an
enhancement of the dark current. The Jsc gain compensates the
loss of Voc, resulting in almost constant efficiency during light
soaking. As a consequence, h first increased before 500 h and
then remained almost constant for 1000 h for three dyes.
electrodes with a quasi-solid-state electrolyte are listed in Table 3
5. Conclusions
In summary, three new phenothiazine organic dyes (P1–P3)
containing thiophene have been synthesized for dye-sensitized
solar cells. These dyes are successfully adsorbed on nano-
crystalline anatase TiO2 particles, and subsequently efficient dye-
sensitized solar cells have been fabricated. The absorption
spectra, electrochemical and photovoltaic properties of the three
dyes were extensively studied. It was found that the introduction
of the triphenylamine group as the electron-donor brought about
improved photovoltaic performance compared with methoxy-
triphenylamine and 1,1,2-triphenylethene for phenothiazine
dyes. A maximum solar energy to electricity conversion efficiency
of 4.41% (Jsc ¼ 10.84 mA cmꢀ2, Voc ¼ 592 mV, ff ¼ 0.69) under
simulated AM 1.5 solar light irradiation (100 mW cmꢀ2) with
a DSSC based on P2 was obtained. Most importantly, long-term
stability of the phenothiazine DSSCs under 1000 h light-soaking
was demonstrated. Our findings demonstrate that phenothiazine
organic sensitizers are promising for the further improvement of
the conversion efficiency of DSSCs owing to the original and
versatile molecular design.
Acknowledgements
This work was supported by NSFC/China (20772031), the
National Basic Research 973 Program (2006CB806200) and the
Scientific Committee of Shanghai.
Fig. 9 Stability test photovoltaic parameters (Jsc, Voc, ff, and h) varia-
tions with aging time for the devices based on P1 (a), P2 (b) and P3 (c)
sensitized TiO2 films with quasi-solid-state electrolyte during successive 1
sun visible-light soaking at 55 ꢁC.
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