Modifying organic phenoxazine dyes for efficient dye-sensitized solar cells

Article abstract of DOI:10.1039/c1jm12071a

Four organic dyes bearing the phenoxazine chromophore have been synthesized and applied in dye-sensitized solar cells (DSCs). The effect of different dye structures on the performance of the DSCs was investigated systematically with photophysical, photovoltaic as well as photoelectrochemical methods. Due to the slow recombination process between injected electrons and electrolyte, the IB3 dye with two 2,4-dibutoxyphenyl units showed the best efficiency of 7.0% under 100 mW cm^-2 light illumination in the liquid state-DSCs. Moreover, the phenoxazine dyes-based solid state-DSCs were fabricated for the first time. With the IB4 dye, a higher efficiency of 3.2% has been achieved under the same light intensity.

Full text of DOI:10.1039/c1jm12071a

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 12462
www.rsc.org/materials
PAPER
Modifying organic phenoxazine dyes for efficient dye-sensitized solar cells†
Haining Tian,a Ilkay Bora,ac Xiao Jiang,b Erik Gabrielsson,a Karl Martin Karlsson,a Anders Hagfeldtb
ad
*
and Licheng Sun
Received 10th May 2011, Accepted 9th June 2011
DOI: 10.1039/c1jm12071a
Four organic dyes bearing the phenoxazine chromophore have been synthesized and applied in dye-
sensitized solar cells (DSCs). The effect of different dye structures on the performance of the DSCs was
investigated systematically with photophysical, photovoltaic as well as photoelectrochemical methods.
Due to the slow recombination process between injected electrons and electrolyte, the IB3 dye with two
2,4-dibutoxyphenyl units showed the best efficiency of 7.0% under 100 mW cmꢀ2 light illumination in
the liquid state-DSCs. Moreover, the phenoxazine dyes-based solid state-DSCs were fabricated for the
first time. With the IB4 dye, a higher efficiency of 3.2% has been achieved under the same light intensity.
phenoxazine)43–48 have also been successfully utilized for the
Introduction
application of DSCs. In order to absorb the sun light as much as
possible, a broad absorption spectrum of the dye is desirable.
Usually, extension of the p-conjugated system (introducing more
p units) is a feasible strategy to complete the mission. However,
the passive effect is that the large p-conjugated system leads to
poor photovoltaic properties due to dye aggregation as well as
electron recombination issues. To diminish the dye aggregation
and the electron recombination, the alkyl chain was often
introduced into linker or donor subunits; consequently, the
efficiency can be greatly improved. In this paper, we further
develop this concept to design the phenoxazine (POZ) dyes
(IB1ꢁIB4, see Fig. 1) through simultaneously modifying both
donor and linker units with the same aromatic groups and also
investigate the effect of different substituents and linker positions
on the photophysical, photochemical and photovoltaic
properties.
Dye-sensitized solar cells (DSCs) provide a kind of more
economical and effective method to carry out the energy
conversion from solar light to electricity since O’Regan and
€
1
Gratzel reported that in 1991. As one of the crucial components
in DSCs, the photosensitizers have always attracted ever-
increasing attention in the past decades. Due to the better cost-
performance compared to metal-complex sensitizers,2–7 the
organic dyes exhibit the more brilliant future in commercial
application. The traditional design concept of the organic dyes is
the donor–linker–acceptor structure, also named the D–p–A
configuration. The electron can be facile to transfer from the
donor to the acceptor part via the linker unit, and then finish the
electron injecting process from the dye to the conduction band
(CB) of the semiconductor. Up to now, many kinds of organic
dyes based on coumarin,8–11 perylene,12–15 triarylamine,16–25
carbazole,26–29 indoline30–32 and other chromophores33–40 as
donating groups have been developed and have shown potential
commercial application for DSCs. In addition, due to the strong
electron donating ability and facile structure modification, tet-
rahydroquinoline41,42 and heteroanthracene (phenothiazine and
Experiments
General synthetic procedure
NMR spectra were measured on a Bruker AVANCE 500 MHz
spectrometer, using TMS as the internal standard. HR-MS was
performed using a Q-Tof micro (micromass Inc., Manchester,
England) mass spectrometer equipped with a Z-spray ionization
source. Commercial reagents and solvents were used as received
without further purification. 10-Octyl-10H-phenoxazine (2),
3-bromo-10-octyl-10H-phenoxazine (3), 10-octyl-10H-phenox-
azine-3-carbaldehyde (8) and 7-bromo-10-octyl-10H-phenox-
azine-3-carbaldehyde (9) were synthesized according to our
previous publication.48
aDepartment of Chemistry, School of Chemical Science and Engineering,
Center of Molecular Devices, Royal Institute of Technology (KTH),
10044 Stockholm, Sweden. E-mail: lichengs@kth.se
bDepartment of Physical and Analytical Chemistry, Center of Molecular
Devices, Uppsala University, 75105 Uppsala, Sweden
cInstitute of Organic Chemistry, University of Stuttgart, 70049 Stuttgart,
Germany
dState Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and
Research Center for Molecular Devices, Dalian University of Technology
(DUT), 116024 Dalian, China
† Electronic supplementary information (ESI) available: NMR spectra of
IB dyes; the NOESY spectrum of compound 11; the absorption spectra of
IB4-sensitized TiO2 with different ratios of IB4/CDCA (mol/mol); the
trend of photovoltaic properties of IB4 with different amounts of
CDCA. See DOI: 10.1039/c1jm12071a
5-(10-Octyl-10H-phenoxazin-3-yl)thiophene-2-carbaldehyde (4).
Compound (3) (mono- and di-brominated product mixture,
1.00 g, 2.5 mmol), (5-formylthiophen-2-yl)boronic acid (0.62 g,
12462 | J. Mater. Chem., 2011, 21, 12462–12472
This journal is ª The Royal Society of Chemistry 2011

View Article Online
Fig. 1 The structures of phenoxazine IB1ꢁIB4 dyes.
4 mmol), potassium carbonate (1.80 g, 13 mmol) and [1,10-bis
(diphenylphosphino)ferrocene]-dichloropalladium(II) (0.20 g,
13 mmol) are dissolved in 12 ml of toluene/MeOH (1 : 1; v/v).
The final solution was placed in two sealable tubes (6 ml per tube)
and stirred at 80 ꢂC for 3.5 h. After the reaction, 25 ml of water
are added and the solution is extracted with dichloromethane
(DCM) to give the crude product. Column purification with
silica gel and DCM/MeOH (30 : 1; v/v) gives (4) as orange
a sealable flask and dissolved in 6 ml of toluene/MeOH (1 : 1;
v/v) and stirred at 75 ꢂC for 3 h. Then 25 ml of water are added
and the solution extracted with DCM. Column purification with
silica gel and DCM as eluent gives (6) (160 mg, 89%) as dark
1
orange crystals. H NMR (500 MHz, d6-acetone): d ¼ 9.94 (s,
1H), 7.98 (s, 1H), 7.54 (d, J ¼ 8.65 Hz, 2H), 7.27–7.32 (m, 10H),
7.17 (dd, J ¼ 8.36, 2.07 Hz, 1H), 7.02–7.17 (m, 17H), 6.96 (d, J ¼
2.09 Hz, 1H), 6.79 (d, J ¼ 8.42 Hz, 1H), 6.72 (d, J ¼ 8.39 Hz, 1H),
6.54 (d, J ¼ 2.05 Hz, 1H), 3.63–3.69 (m, 2H), 1.68–1.75 (m, 2H),
1.46–1.53 (m, 2H), 1.30–1.45 (m, 8H), 0.88 ppm (t, J ¼ 6.91, 6.91
Hz, 3H).
1
crystals (0.50 g). H NMR (500 MHz, d6-acetone): d ¼ 9.89 (s,
1H), 7.91 (d, J ¼ 3.94 Hz, 1H), 7.51 (d, J ¼ 3.92 Hz, 1H), 7.27
(dd, J ¼ 8.41, 2.16 Hz, 1H), 7.03 (d, J ¼ 2.13 Hz, 1H), 6.84–6.88
(m, 1H), 6.65–6.68 (m, 4H), 3.63–3.68 (m, 2H), 1.67–1.73 (m,
2H), 1.45–1.52 (m, 2H), 1.28–1.44 (m, 8H), 0.88 ppm (t, J ¼ 6.87,
6.87 Hz, 3H).
4-(2,4-Dibutoxyphenyl)-5-(7-(2,4-dibutoxyphenyl)-10-octyl-10H-
A mixture of
phenoxazin-3-yl)thiophene-2-carbaldehyde (7).
compound (5) (185 mg, 3.3 mmol), (2,4-dibutoxyphenyl)boronic
acid (350 mg, 1.3 mmol), potassium carbonate (230 mg,
1.6 mmol) and [1,10-bis(diphenylphosphino)ferrocene]-dichlor-
opalladium(II) (60 mg, 0.07 mmol) are placed in a sealable flask
and dissolved in 6 ml of toluene/MeOH (1 : 1; v/v) and stirred at
4-Bromo-5-(7-bromo-10-octyl-10H-phenoxazin-3-yl)thiophene-
2-carbaldehyde (5). Compound (4) (450 mg, 1.1 mmol) is dis-
solved in 10 ml DCM in an ice bath and bromine (360 mg,
2.2 mmol), dissolved in 4 ml of DCM, is added to the solution
dropwise over 30 min. Stirring of the reaction mixture at room
temperature (RT) is continued for 2.5 h. The reaction is
quenched by adding 25 ml of dilute sodium hydroxide solution
and extracted with DCM. Column purification with silica gel and
DCM gives (5) (185 mg, 30%). 1H NMR (500 MHz, d6-acetone):
d ¼ 9.91 (s, 1H), 7.98 (s, 1H), 7.27 (dd, J ¼ 8.42, 2.14 Hz, 1H),
7.06 (d, J ¼ 2.16 Hz, 1H), 7.01 (dd, J ¼ 8.64, 2.30 Hz, 1H), 6.85
(d, J ¼ 8.54 Hz, 1H), 6.83 (d, J ¼ 2.26 Hz, 1H), 6.71 (d, J ¼ 8.64
Hz, 1H), 3.64–3.69 (m, 2H), 1.67–1.74 (m, 2H), 1.45–1.52 (m,
2H), 1.30–1.44 (m, 8H), 0.88 ppm (t, J ¼ 6.91, 6.91 Hz, 3H).
ꢂ
75 C for 3 h. Then 25 ml of water are added and the solution
extracted with DCM. Column purification with silica gel and
pentane/DCM (1 : 20; v/v) as eluent gave (7) (233 mg, 83.6%) as
dark orange crystals. 1H NMR (500 MHz, d6-acetone): d ¼ 9.87
(s, 1H), 7.77 (s, 1H), 7.14 (d, J ¼ 8.40 Hz, 1H), 7.07 (d, J ¼ 8.35
Hz, 1H), 6.97 (dd, J ¼ 8.30, 1.89 Hz, 1H), 6.84 (dd, J ¼ 8.40, 1.96
Hz, 1H), 6.80 (d, J ¼ 1.90 Hz, 1H), 6.63 (d, J ¼ 8.40 Hz, 1H),
6.49–6.59 (m, 6H), 3.96–4.00 (m, 6H), 3.88 (t, J ¼ 6.49, 6.49 Hz,
2H), 3.54–3.57 (m, 2H), 1.63–1.76 (m, 8H), 1.43–1.53 (m, 10H),
1.26–1.37 (m, 10H), 0.83–0.98 ppm (m, 15H).
4-(4-(Diphenylamino)phenyl)-5-(7-(4-(diphenylamino)phenyl)-
10-Octyl-7-(thiophen-2-yl)-10H-phenoxazine-3-carbaldehyde (10).
Compound (9) (1.30 g, 3.2 mmol), thiophen-2-ylboronic acid
(0.87 g, 6.5 mmol), potassium carbonate (2.23 g, 5.0 mmol) and
[1,10-bis(diphenylphosphino)ferrocene]-dichloropalladium(II) (400
mg, 0.5 mmol) are placed in two sealable flasks and dissolved in
6 ml of toluene/DCM (1 : 1; v/v) each flask. The mixtures are
10-octyl-10H-phenoxazin-3-yl)thiophene-2-carbaldehyde (6).
A
mixture of compound (5) (115 mg, 0.2 mmol), (4-(diphenyla-
mino)phenyl)boronic acid (236 mg, 0.8 mmol), potassium
carbonate (280 mg, 2.0 mmol) and [1,10-bis(diphenylphosphino)
ferrocene]-dichloropalladium(II) (23 g, 0.03 mmol) are placed in
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 12462–12472 | 12463

View Article Online
then stirred at 80 ꢂC for 3.5 h. After the reaction 30 ml of water
are added and the solution extracted with DCM. Column puri-
fication with silica gel and petroleum ether/DCM (v/v) 1 : 1 and
later 1 : 2 as eluent gives (10) (0.97 g, 74%) as dark oil. 1H NMR
(500 MHz, d6-acetone): d ¼ 9.68 (s, 1H), 7.40 (dd, J ¼ 8.29, 1.78
Hz, 1H), 7.37 (d, J ¼ 5.08 Hz, 1H), 7.34 (d, J ¼ 3.58 Hz, 1H), 7.13
(dd, J ¼ 8.35, 2.07 Hz, 1H), 7.07 (dd, J ¼ 5.02, 3.66 Hz, 1H), 7.04
(d, J ¼ 1.79 Hz, 1H), 6.95 (d, J ¼ 2.08 Hz, 1H), 6.82 (d, J ¼ 8.30
Hz, 1H), 6.78 (d, J ¼ 8.39 Hz, 1H), 3.65–3.74 (m, 2H), 1.66–1.75
(m, 2H), 1.44–1.51 (m, 2H), 1.27–1.43 (m, 7H), 0.88 ppm (t, J ¼
6.80, 6.80 Hz, 3H).
2H), 1.53–1.59 (m, 2H), 1.37–1.41 (m, 2H), 1.26–1.33 (m, 8H),
0.87 ppm (t, J ¼ 6.89 Hz, 3H); 13C NMR (126 MHz, d6-DMSO):
d ¼ 163.8, 152.5, 146.4, 144.4, 143.7, 141.6, 134.4, 133.1, 131.7,
124.8, 124.3, 123.8, 122.6, 121.6, 116.7, 115.2, 112.4, 112.3, 112.2,
31.2, 28.8, 28.7, 26.0, 24.5, 22.1, 13.9 ppm; ESI-HRMS calcd. for
C28H28N2O3S [M+] 472.1815, found 472.1831.
(E)-2-Cyano-3-(4-(4-(diphenylamino)phenyl)-5-(7-(4-(diphenylamino)-
phenyl)-10-octyl-10H-phenoxazin-3-yl)thiophen-2-yl)acrylic acid
(IB2). Compound (10) (160 mg, 0.2 mmol) and 2-cyanoacetic
acid (60 mg, 0.7 mmol) are dissolved in 20 ml acetonitrile. 4 drops
of piperidine are added and the mixture is then refluxed for 1 h.
After evaporation of the solvent the crude product is purified by
column chromatography with silica gel and DCM/MeOH (20 : 1;
v/v) as eluent to give IB2 (126 mg, 73%) as a deep red powder. 1H
NMR (500 MHz, d6-DMSO): d ¼ 8.42 (s, 1H), 7.97 (s, 1H), 7.49
(d, J ¼ 8.5 Hz, 2H), 7.25–7.31 (m, 8H), 7.28 (d, J ¼ 8.5 Hz, 2H),
7.12 (d, 3.26 Hz, 1H), 6.89–7.09 (m, 18 H), 6.79 (d, J ¼ 8.46 Hz,
1H), 6.74 (d, J ¼ 8.31 Hz, 1H), 6.56 (d, J ¼ 1.86 Hz, 1H), 3.52–
3.55 (m, 2H), 1.53–1.46 (m, 2H), 1.45–1.38 (m, 2H), 1.35–1.29
(m, 8H), 0.88 ppm (t, J ¼ 6.88, 6.88 Hz, 3H); 13C NMR (126
MHz, d6-DMSO): d ¼ 163.5, 147.0, 146.9, 146.2, 144.0, 143.7,
138.0, 133.5, 132.8, 132.6, 130.8, 130.0, 129.5, 128.5, 126.5, 124.8,
124.6, 124.0, 123.4, 123.3, 123.1, 122.8, 121.6, 116.6, 114.7, 112.7,
112.6, 112.1, 31.2, 28.8, 28.7, 26.0, 24.5, 22.1, 13.4 ppm; ESI-
HRMS calcd. for C64H54N4O3S [M+] 958.3911, found 958.3909.
7-(3,5-Dibromothiophen-2-yl)-10-octyl-10H-phenoxazine-3-car-
baldehyde (11). Compound (10) (0.49 g, 1.2 mmol) is dissolved in
5 ml DCM in an ice bath and bromine (0.38 g, 2.4 mmol) dis-
solved in 4 ml DCM is added to the solution over 20 min. The
mixture is stirred at 0 ꢂC for additional 30 min and then removed
from the ice bath to continue stirring for 2.5 h. When the reaction
is finished, 20 ml of dilute sodium hydroxide solution is added
and the solution extracted with DCM. Column purification with
silica gel and DCM as eluent gives (11) (0.52 g, 77.5%) as brown
solid. 1H NMR (500 MHz, d6-acetone): d ¼ 9.60 (s, 1H), 7.28 (dd,
J ¼ 8.27, 1.71 Hz, 1H), 7.03 (s, 1H), 6.98 (dd, J ¼ 8.37, 2.09 Hz,
1H), 6.92 (d, J ¼ 1.73 Hz, 1H), 6.79 (d, J ¼ 2.13 Hz, 1H), 6.66 (d,
J ¼ 8.33 Hz, 1H), 6.61 (d, J ¼ 8.45 Hz, 1H), 3.48–3.53 (m, 2H),
1.55–1.62 (m, 2H), 1.25–1.43 (m, 8H), 0.86 ppm (t, J ¼ 6.85, 6.85
Hz, 3H).
(E)-2-Cyano-3-(4-(2,4-dibutoxyphenyl)-5-(7-(2,4-dibutoxyphenyl)-
10-octyl-10H-phenoxazin-3-yl)thiophen-2-yl)acrylic acid (IB3).
Compound (9) (230 mg, 0.28 mmol) and 2-cyanoacetic acid
(90 mg, 1.1 mmol) are dissolved in 12 ml acetonitrile. 10 drops of
piperidine are added and the mixture is then refluxed for 2.5 h.
After evaporation of the solvent the crude product is purified by
column chromatography with silica gel and DCM/MeOH (20 : 1;
v/v) as eluent to give IB3 (190 mg, 75%) as a deep red powder. 1H
NMR (500 MHz, d6-acetone): d ¼ 8.35 (s, 1H), 7.81 (s, 1H), 7.14
(d, J ¼ 8.40 Hz, 1H), 7.09 (d, J ¼ 8.35 Hz, 1H), 6.96 (dd, J ¼ 8.30,
1.89 Hz, 1H), 6.87 (dd, J ¼ 8.40, 1.96 Hz, 1H), 6.80 (d, J ¼ 1.90
Hz, 1H), 6.49–6.63 (m, 7H), 3.99–3.95 (m, 6H), 3.86 (t, J ¼ 6.49,
6.49 Hz, 2H), 3.57–3.53 (m, 2H), 1.60–1.77 (m, 8H), 1.54–1.39
(m, 10H), 1.24–1.39 (m, 10H), 0.84–0.98 ppm (m, 15H); 13C
NMR (126 MHz, d6-acetone): d ¼ 164.4, 161.6, 160.6, 158.2,
157.8, 149.4, 147.1, 145.5, 144.6, 144.5, 136.2, 134.9, 133.0, 132.7,
132.4, 131.3, 131.1, 127.2, 125.4, 124.8, 122.7, 117.6, 117.2, 117.1,
115.0, 112.3, 112.3, 106.6, 106.5, 101.1, 100.9, 98.7, 68.6, 68.4,
68.3, 44.3, 32.6, 32.2, 32.1, 31.8, 27.4, 23.4, 20.0, 19.9, 19.8, 14.4,
14.2 ppm; ESI-HRMS calcd. for C56H68N2NaO7S [M + Na+]
935.4639, found 935.4597.
7-(3,5-Bis(4-(diphenylamino)phenyl)thiophen-2-yl)-10-octyl-10H-
phenoxazine-3-carbaldehyde (12). The compound (11) (250 mg,
0.4 mmol), (4-(diphenylamino)phenyl)boronic acid (500 mg,
1.8 mmol), potassium carbonate (300 mg, 2.2 mmol) and [1,10-bis
(diphenylphosphino)ferrocene]-dichloropalladium(II) (80 mg,
0.1 mmol) are placed in two sealable flasks and dissolved in 6 ml
of toluene/DCM (1 : 1; v/v) each flask. The mixtures are then
stirred at 80 ꢂC for 3 h. After the reaction 30 ml of water are
added and the solution is extracted with DCM. Column purifi-
cation with silica gel and petroleum ether/DCM (v/v) 1 : 1 and
later 1 : 2 as eluent gives (12) (290 mg, 73%) as a yellow solid. 1H
NMR (500 MHz, d6-acetone): d ¼ 9.70 (s, 1H), 7.62 (d, J ¼ 8.63
Hz, 2H), 7.40–7.43 (m, 2H), 7.27–7.35 (m, 10H), 7.08–7.12 (m,
10H), 7.00–7.06 (m, 7H), 6.91 (dd, J ¼ 8.33, 2.04 Hz, 1H), 6.84
(d, J ¼ 8.33 Hz, 1H), 6.75 (d, J ¼ 8.42 Hz, 1H), 6.55 (d, J ¼ 2.03
Hz, 1H), 3.66–3.70 (m, 2H), 1.67–1.74 (m, 2H), 1.45–1.52 (m,
2H), 1.29–1.44 (m, 8H), 0.87 ppm (t, J ¼ 6.84, 6.84 Hz, 3H).
(E)-2-Cyano-3-(5-(10-octyl-10H-phenoxazin-3-yl)thiophen-2-yl)
acrylic acid (IB1). Compound (6) (62 mg, 0.15 mmol) and
2-cyanoacetic acid (50 mg, 0.6 mmol) are dissolved in 10 ml
acetonitrile. 4 drops of piperidine are added and the mixture is
then refluxed for 2 h. After evaporation of the solvent the crude
product is purified by column chromatography with silica gel and
DCM as eluent at first, then change to DCM/ethyl acetate (v/v)
5 : 1 and finally 10 : 1. To get rid of remained impurities the dye
is recrystallized with 5 ml acetone and 5 ml pentane to finally give
IB1 (40 mg, 56%) as a deep red powder. 1H NMR (500 MHz, d6-
DMSO): d ¼ 8.44 (s, 1H), 7.96 (d, J ¼ 4.03 Hz, 1H), 7.63 (d, J ¼
4.06 Hz, 1H), 7.25 (dd, J ¼ 8.38, 2.13 Hz, 1H), 7.09 (d, J ¼ 2.14
Hz, 1H), 6.85–6.88 (m, 1H), 6.76–6.97 (m, 4H), 3.58–3.61 (m,
(E)-3-(7-(3,5-Bis(4-(diphenylamino)phenyl)thiophen-2-yl)-10-
octyl-10H-phenoxazin-3-yl)-2-cyanoacrylic acid (IB4). The
compound (13) (220 g, 2.4 mmol) and 2-cyanoacetic acid (80 mg,
1.0 mmol) are dissolved in 15 ml acetonitrile. 10 Drops of
piperidine are added and the mixture is then refluxed for 2.5 h.
After evaporation of the solvent the crude product is purified by
column chromatography with silica gel and DCM/MeOH (10 : 1;
1
v/v) as eluent to give IB4 (113 mg, 49%) as a dark powder. H
NMR (500 MHz, d6-acetone): d ¼ 7.99 (s, 1H), 7.58 (d, J ¼ 8.65
Hz, 2H), 7.53 (dd, J ¼ 8.57, 2.01 Hz, 1H), 7.46 (s, 1H), 7.37 (d,
12464 | J. Mater. Chem., 2011, 21, 12462–12472
This journal is ª The Royal Society of Chemistry 2011

View Article Online
J ¼ 2.05 Hz, 1H), 7.33–7.24 (m, 10H), 7.07–7.10 (m, 12H), 7.06–
7.00 (m, 4H), 6.87 (dd, J ¼ 8.34, 2.03 Hz, 1H), 6.80 (d, J ¼ 8.62
Hz, 1H), 6.70 (d, J ¼ 8.48 Hz, 1H), 6.54 (d, J ¼ 2.04 Hz, 1H),
3.63–3.63 (m, 2H), 1.64–1.71 (m, 2H), 1.43–1.50 (m, 2H), 1.28–
1.42 (m, 8H), 0.86 ppm (t, J ¼ 6.80, 6.80 Hz, 3H); 13C NMR (126
MHz, d6-DMSO): d ¼ 165.6, 146.9, 146.8, 143.5, 129.6, 129.5,
124.3, 124.1, 123.5, 123.2, 122.9, 122.9, 115.6, 31.2, 28.7, 28.3,
24.6, 22.4, 22.0, 13.9 ppm; ESI-HRMS calcd. for C64H54N4O3S
[M+] 958.3911, found 958.3920.
(4 : 1; v/v) and kept for 16ꢁ17 h in dark at room temperature.
After the adsorption of the dyes, the electrodes were rinsed with
the same solvent. The hole transporting material (HTM) solution
containing 120 mM Spiro-OMeTAD, 60 mM 4-TBP, and 15 mM
LiN(CF3SO2)2 (Li-TFSI) in chlorobenzene was deposited onto
the dye-coated TiO2 film and left stationary for 1 min in order to
penetrate the HTM solution into TiO2 pores prior to spin
coating. Then, the substrate was spun up to 1400 rpm for 30 s.
Finally, a 50 nm of Ag was thermally evaporated on top of the
HTM as counter electrode to form our test cells.
Preparation of liquid-solar cells
Photocurrent density–voltage (J–V) measurements
Fluorine-doped tin oxide (FTO) glass plates (Pilkington-TEC15)
were cleaned in subsequent order of detergent solution, water
and EtOH using an ultrasonic bath overnight. The conducting
glass substrates were immersed into a 40 mM aqueous TiCl4
solution at 70 ꢂC for 30 min and washed with water and ethanol.
The screen-printing procedure was repeated (layer of 2 mm) with
TiO2 paste (Dyesol, 18 NR-T) to obtain a transparent nano-
crystalline film of a thickness around 10 mm and an area of
0.25 cm2. Two scattering layers (ꢁ4 mm, solaronix R/SP) were
deposited on the top of a 10 mm transparent layer to form the
10 + 4 mm TiO2 film. The TiO2 electrodes were gradually heated
in an oven (Nabertherm Controller P330) in an ambient air
atmosphere. The temperature gradient program used included
two stationary plateaus at 325 ꢂC (10 min), and 450 ꢂC (30 min).
After sintering the electrodes once again underwent, as described
above, a post-TiCl4 treatment. A second and final sintering, at
450 ꢂC for 30 min, was performed. When the temperature
decreased to 30ꢁ40 ꢂC after the sintering the electrodes were
immersed into 0.2 mM dye solutions with 2 mM chenodeoxy-
cholic acid (CDCA) and kept for 16ꢁ17 h in dark at room
temperature. The mixture of EtOH (99.5%)/DCM (4 : 1; v/v) was
used as solvent for all dyes. After the adsorption of the dyes, the
Prepared solar cells were characterized by current–voltage
characteristics and incident photon-to-current conversion effi-
ciency (IPCE). Current–voltage characteristics were carried out
with an AM 1.5G solar simulator, 300 W xenon (ozone free). The
incident light intensity was 1000 W mꢀ2 calibrated with a stan-
dard Si solar cell. For the J–V curves of liquid DSCs, the active
area is 0.25 cm2 and a black mask 0.64 cm2 (8 mm ꢃ 8 mm) was
used in the subsequent photovoltaic studies. For solid state
DSCs, the active area is determined to 0.16 cm2 (4 mm ꢃ 4 mm)
with a back mask. IPCE measurements were carried out with
a computerized set-up consisting of a xenon arc lamp (300 W
Cermax, ILC Technology), followed by a 1/8 m monochromator
(CVI Digikrom CM 110). The data collection was done with
a Keithley 2400 source/meter, and a Newport 1830-C power
meter with 818-UV detector head.
Photophysical measurements
The UV-Vis absorption spectra of the dye-loaded transparent
film (2 mm, sensitized 30 min in dye bath) and the dye solution
(0.02 mM, in EtOH/DCM (4 : 1, v/v) in a normal quartz sample
cell (1 cm path length) were recorded on a Lambda 750 spec-
trophotometer. The fluorescence spectra of dye solution were
recorded on Cary Eclipse fluorescence spectrophotometer using
the same concentration as the UV-Vis measurement.
electrode was further rinsed with the EtOH. The electrodes were
49
assembled with a platinized counter electrode (ꢁ8.6 mg cmꢀ2
)
using a 25 mm thick hot-melt film (Surlyn, Solaronix). The redox
electrolyte containing 0.04 M I2 (99.9%), 0.06 M LiI (99.9%),
0.4 M 4-tert-butylpyridine (4-TBP, 99%), 0.6 M 3-hexyl-1,2-
dimethylimidazolium iodide (DMHII, 98%), in MeCN/valer-
onitrile (VN) (85 : 15; v/v) was introduced through a hole drilled
in the back of the counter electrode. Finally, the hole was also
sealed with the Surlyn film.
Adsorption amount of dyes on TiO2
The dye-loading on TiO2 film was estimated from the dye
desorption in 0.1 M tetrabutylammonium hydroxide (TBAOH)
MeOH solution by means of UV-Vis absorption spectroscopy,
two pieces TiO2 film with total 0.5 cm2 (0.25 cm2 per piece) were
used for all dyes.
Preparation of solid state-solar cells
A compact TiO2 layer (TiO2 underlayer) was firstly deposited
onto the FTO substrate by spray pyrolysis, and then TiO2
particles (Dyesol, 18 NR-T) were deposited by doctor-blading
method to obtain a ꢁ2.2 mm thick mesoporous film including the
TiO2 underlayer. The TiO2 electrodes were gradually heated in
Electrochemical measurements
Electrochemical experiments were performed with a CH Instru-
ments electrochemical workstation (model 660A) using
a conventional three-electrode electrochemical cell. Measure-
ments were obtained with 1 mM solution of dyes in DCM by
ꢂ
an oꢂven at air atmosphere to two levels, at 250 C (20 min) and
500 C (30 min). After sintering, the electrodes were immersed
into 40 mM aqueous TiCl4 solution at 70 ꢂC for 30 min and
washed with water and ethanol. A second and final sintering, at
500 ꢂC for 30 min, was performed. When the temperature
decreased to 30ꢁ40 ꢂC after the sintering, the electrodes were
immersed into 0.2 mM dye solutions with the addition of 2 mM
co-adsorbent chenodeoxycholic acid (CDCA) in EtOH/DCM
using 0.1
M
tetrabutylammonium hexafluorophosphate
(TBAPF6) as supporting electrolyte. A glass carbon disk was
used as the working electrode, a platinum wire served as
a counter electrode, a Ag/Ag+ electrode was utilized as a refer-
ence electrode, and the scan rate used was 50 mV sꢀ1. All redox
potentials were calibrated versus a normal hydrogen electrode
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 12462–12472 | 12465

View Article Online
(NHE) by the addition of ferrocene as an internal standard
taking E(Fc/Fc+) ¼ 630 mV vs. NHE.16
with a current amplifier (Stanford Research SR570). The relation
between potential and charge was studied using a combined
voltage decay/charge extraction method. Charge extraction
measurements were carried out as follows: the solar cell was
illuminated for 5 s under open-circuit conditions, and then the
light was switched off and the voltage was let to decay to
a voltage V. At a certain voltage V, the cell was short-circuited,
and the current was measured under 10 s and then integrated to
obtain Qoc (V). Electron lifetime was tested by monitoring
transient photovoltage response after a small perturbation of
light intensity.
Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) tests were carried
out using an Autolab PGstat12 potentiostat with an impedance
module in the frequency range 10 kHz to 0.1 Hz, using 20 mV AC
amplitude, under dark conditions with an applied bias voltage
ꢀ0.68 V potential.
Photoelectrochemical measurements
Electron lifetimes for the solar cells were measured using a red-
light-emitting diode (Luxeon, 1 W) as the light source. Voltage
and current traces were recorded by a 16-bit resolution data
acquisition board (DAQ National Instruments) in combination
Molecular orbital calculation
The structural and electronic properties of the dyes were
calculated with the B3LYP hybrid functional at the 6-31G
Fig. 2 Synthetic routes of IB1ꢁIB4 dyes: (a) C8H17Br, NaOH, C16H33NMe3Br, acetone, 12 h, reflux; (b) 1 equiv. Br2, DCM, 2 h; (c) (5-formylthiophen-
2-yl)boronic acid, K2CO3, Pd(dppf)Cl2, toluene/MeOH, 3.5 h, 80 ꢂC; (d) 2 equiv. Br2, DCM, 0ꢂ to RT, 2 h; (e) (2,4-dibutoxyphenyl)boronic or (4-
(diphenylamino)phenyl)boronic acid, K2CO3, Pd(dppf)Cl2, toluene/MeOH, 3h, 75 ꢂC; (f) POCl3, DMF, CHCl3, 75 ꢂC, 15 h; (g) 2-cyanoacetic acid,
piperidine, MeCN, 2 h.
12466 | J. Mater. Chem., 2011, 21, 12462–12472
This journal is ª The Royal Society of Chemistry 2011

View Article Online
(d) level in gas phase using the Gaussian 03 program
package.50 Long alkyl substituents, which have no significant
impact on the frontier orbitals of the chromophores, were
truncated to methyl groups to hasten the convergence of
optimizations.
Calculation study
In order to explain the experimental spectra and gain insights
into the structural properties of the chromophores, density
functional theory (DFT) calculations were performed. The
optimized geometries and isodensity plots of the frontier orbitals
of the chromophores are shown in Table 1. The isodensity plots
were generated in Avogadro51 using an isovalue of 0.02.
A first insight gained from the calculations is that introducing
a substituent on the thiophene in a position adjacent to the
phenoxazine will disrupt the dihedral angle between the POZ and
thiophene. As the dihedral angle between these two aromatic
units in the chromophore grows from 20ꢂ in IB1 to 36–39ꢂ in IB2
and IB4, conjugation over the p-system is disrupted. This
disruption of the conjugation is reflected in the experimental
optical spectra of the chromophores in that IB1 has the strongest
absorption.
Results and discussion
Synthesis
The detailed synthetic routes of IB1ꢁIB4 dyes are depicted in
Fig. 2. All of the dyes were synthesized according to the
traditional methods: alkylation, bromination, Vilsmeyer–Haack
formylation, Suzuki-coupling and Knoevenagel condensation.
The N-alkylation was performed in acetone with 1-bro-
mooctane, NaOH as the base and palmityl trimethylammo-
nium bromide as the phase transfer catalyst. Formylation was
accomplished by the Vilsmeyer–Haack reaction. Mono-
bromination and dibromination were controlled by the amount
of bromine added in the reaction. The dibromination products
(compounds 5 and 11) are very easy to be purified compared
with the mono-brominated product due to much lower
polarity. The position of the Br atom in the dibrominated
compounds was further proved by NOESY spectra. The final
reaction for all dyes was the condensation of corresponding
aldehyde with 2-cyanoacetic acid by the Knoevenagel reaction
in the presence of piperidine. All the compounds were char-
acterized by 1H- and 13C-NMR spectroscopy. The final dyes
were also characterized by HR-MS.
Photophysical properties
The absorption spectra of IB1ꢁIB4 dyes in EtOH/DCM (4 : 1;
v/v) solution and on TiO2 films are illustrated in Fig. 3. The
corresponding photophysical data are collected in Table 2. IB2
and IB3 dyes show slightly blue-shifted absorption peaks (lmax
)
and lower extinction coefficient comparing with IB1 due to the
weak p-conjugated system caused by the increased twist angle
between the POZ unit and the thiophene group, which has been
proved with the DFT calculation. Interestingly, when the thio-
phene unit in IB2 was changed from right to left of the POZ unit
Table 1 The optimized structures and electron distribution in HOMO and LUMO levels of the IB dyes
Dye
Optimized structure
HOMO
LUMO
IB1
IB2
IB3
IB4
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 12462–12472 | 12467

View Article Online
Table 3 The adsorption amount of IB1ꢁIB4 dyes on TiO2 filmsa
Dye-load/mol cmꢀ3
Dye
(ꢃ 10ꢀ8
)
IB1
IB2
IB3
IB4
2.0
1.8
1.8
2.5
a
The dye load on the TiO2 film was estimated from the dye desorption in
0.1 M tetrabutylammonium hydroxide methanol solution by means of
UV-Vis absorption spectroscopy.
Fig. 3 The absorption spectra of IB1ꢁIB4 dyes in EtOH/DCM (4 : 1;
v/v) solution (a) and on TiO2 films (b).
to construct the IB4 dye, the absorption peak is red-shifted from
483 nm to 492 nm, but the absorption region was narrowed due
to the change of the p-conjugated system of the structure.
Compared with the absorption spectra of the dyes in solution,
the absorption spectra of IB2 and IB3 on TiO2 films show almost
no obvious change, probably due to the suppression of dye
aggregation on TiO2 ascribed to the extra units in both the POZ
and thiophene units. However, the IB1 and IB4 dyes have the
obvious blue-shifted absorption spectra on the TiO2 surface. The
effect of different ratios of IB4/CDCA on the absorption spectra
(see Fig. S1†) was performed to study if the aggregation causes
the blue-shift phenomenon. However, no obvious red-shift of
absorption spectrum was observed when the concentration of
CDCA in the dye bath was increased. Therefore, we consider
Fig. 4 The J–V curves (a) and IPCE spectra (b) of DSCs based on IB
dyes.
Table 2 Photophysical and electrochemical properties of IB1ꢁIB4 dyes
Absorption
Potentials
Emission
lemi/nm
Dye
lmax/nm
3/Mꢀ1 cmꢀ1
lmax on TiO2/nm
E0–0/V (vs. NHE)
HOMO/V (vs. NHE)
LUMO/V (vs. NHE)
IB1
IB2
IB3
IB4
486
483
482
492
21 000
15 000
17 000
19 000
466
483
480
475
638
650
650
605
2.19
2.17
2.17
2.28
0.91
0.81
0.79
0.93
ꢀ1.28
ꢀ1.36
ꢀ1.38
ꢀ1.35
12468 | J. Mater. Chem., 2011, 21, 12462–12472
This journal is ª The Royal Society of Chemistry 2011

View Article Online
Table 4 Photovoltaic performance of DSCs based on the different
IB1ꢁIB4 dyesa
that the blue-shift of the IB4 dye on TiO2 could be caused by
different configurations of IB4 on the TiO2 surface from that in
solution, not the aggregation. Noticeably, with the same thick-
ness of the TiO2 film (2 mm) and the sensitized time (30 min) in
the same concentration of dye-bath, the IB4 dye shows a stronger
absorbance even than the IB1 dyes (Fig. 3b). According to dye
adsorption amount measurements (Table 3), one finds that IB4
dyes show a much greater adsorption amount on TiO2 film,
2.5 ꢃ10ꢀ8 mol cmꢀ3 than IB1, 2.0 ꢃ 10ꢀ8 mol cmꢀ3, although the
CDCA has been added into the dye-bath to diminish the dye
aggregation. Coincidentally, IB2 and IB3 dyes display the same
adsorption amount on TiO2, 1.8 ꢃ 10ꢀ8 mol cmꢀ3, which is lower
than that of the IB1 dye on TiO2 due to the special molecular
configuration. This result could imply that the IB4 dye has the
fast sensitization rate as compared to other dyes.
Dye b
Jsc/mA cmꢀ2
Voc/V
ff
h (%)
IB1
IB2
IB3
IB4
13.2
12.8
13.7
13.4
0.70
0.76
0.80
0.74
0.68
0.65
0.63
0.66
6.2
6.3
7.0
6.5
a
Light intensity: under standard AM 1.5G illumination (100 mW cmꢀ2);
active area: 0.25 cm2; thickness of TiO2 film: 10 mm transparent layer + 4
mm scattering layer. Dye bath: 0.2 mM in EtOH/DCM (4 : 1; v/v);
b
sensitized time: 16ꢁ17 h; electrolytes: 0.6 M DMPII, 0.06 M LiI, 0.04
M I2, 0.4 M TBP in MeCN/VN (85 : 15; v/v).
same light illumination. The position of thiophene also signifi-
cantly affects the photovoltaic properties. The IB4 dye shows
a lower photovoltage value, 0.74 V, and higher IPCE values,
between 400 and 600 nm, than the IB2 dye.
Electrochemical properties
The electrochemical properties of IB1ꢁIB4 are shown in Table 2.
The oxidation potentials (Eox) of these dyes were measured by
cyclic voltammetry. The first oxidation potential versus normal
hydrogen electrode (vs. NHE) calibrated by Fc/Fc+ (with 630 mV
vs. NHE) corresponds to the HOMO level of the dye. The
HOMO levels of all the dyes are more positive than the redox
potential of Iꢀ/I3ꢀ (0.3ꢁ0.4 V vs. NHE),52 which means that the
oxidized dyes can be regenerated effectively. For IB2 and IB3
dyes, the introduction of an extra electron donor in both POZ
and thiophene makes the HOMO level more negative compared
with IB1 due to the increase of electron donating ability.
However, when the substituted thiophene was moved from the
right side of the POZ (IB2) to the left side of the POZ (IB4), the
HOMO level was significantly changed. IB4 shows more positive
HOMO potential than IB2 due to the weaker p-conjugated
system in molecular structure. The LUMO levels of these dyes
were calculated by Eox ꢀ E0–0, where E0–0 is the zeroth–zeroth
energy of the dyes estimated from the intersection between the
absorption and emission spectra. As shown in Table 2, the
LUMO potentials of these dyes are much more negative than
the conduction band (CB) of the TiO2 level (ꢀ0.5 V vs. NHE),
which means that the electrons could be efficiently injected into
the CB of TiO2 from the excited dyes. Furthermore, the large
difference between the TiO2 CB and the LUMO levels of the dyes
suggests that 4-tert-butylpyridine can be used in the electrolyte to
move the CB negatively, yielding higher photovoltage (V) values.
In order to gain further insight into the reasons for the
increased photovoltage, the lifetime and charge extraction of
these dyes based DSCs were measured. As shown in Fig. 5, all of
the dyes show the same effect on charge extraction experiments,
which means that the DSCs based on these dyes using the same
electrolyte show the same effect on the CB of TiO2. However, the
Photovoltaic and photoelectrochemical properties
The photocurrent density–photovoltage (J–V) curves of DSCs
based on IB1ꢁIB4 dyes performed under standard AM 1.5G
illumination (100 mW cmꢀ2) are shown in Fig. 4. The corre-
sponding photovoltaic data are collected in Table 4. The basic
dye structure IB1 dye gave an efficiency of 6.2% with a short-
circuit photocurrent density (Jsc) of 13.2 mA cmꢀ2, open-circuit
photovoltage (Voc) of 0.7 V and fill factor (ff) of 0.68. Featuring
the TPA groups in the POZ and thiophene units, the IB2 dye
rendered a higher Voc value, 0.76 V. However, replacing the TPA
groups with 2,4-dibutoxyphenyl units to construct the IB3 dye,
the Voc value was further improved to 0.8 V without decrease of
photocurrent, hence yielding an ideal efficiency of 7.0% under
Fig. 5 The electron lifetime (a) and charge extraction (b) of DSCs based
on IB dyes.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 12462–12472 | 12469

View Article Online
measurement of the injected electron lifetime (se) of DSCs based
on different IB dyes presents an interesting result. The se values
of DSCs based on IB dyes are in order of IB1 < IB4 < IB2 < IB3.
The result implies that the introduction of 2,4-dibutoxyphenyl
units can greatly suppress the electron recombination between
the injected electron and the electrolyte or oxidized dye.
Comparing IB2 and IB4 dyes, when the TPA substituted thio-
phene unit is close to the anchor group, in other words, near to
the TiO2 surface, the injected electron will be further protected to
avoid the recombination process mentioned above, rendering the
higher Voc.
In addition, electrochemical impedance spectroscopy (EIS)
also was employed to study the electron recombination and
electrolyte reduction process in DSCs based on these IB1ꢁIB4
dyes under ꢀ0.68 V bias applied voltage in the dark. The Nyquist
and Bode plots are shown in Fig. 6. Some important parameters
can be obtained by fitting the EIS spectra to an electrochemical
model.53 RS, Rrec and RCE represent the series resistances, charge-
transfer resistances at the dye/TiO2/electrolyte interface and
counter electrode (CE), respectively. We can find the series
resistance (RS) and electrolyte reduction resistance (RCE) corre-
sponding to the first semicircle show almost same value in all
dyes-based DSCs due to the same electrode material and same
electrolyte. From the EIS measurements, the electron lifetime (se)
expressing the electron recombination between the electrolyte
and TiO2 may be extracted from the angular frequency (urec) at
Fig. 7 The J–V curve and IPCE (inset) of solid state DSCs based on IB4
dyes.
the mid frequency peak in the Bode phase plots using the relation
se ¼ 1/urec. The IB1 dye gives the lowest Rrec value, 60 U, and the
shortest se, 26 ms in TiO2, among these dyes. However, se and
Rrec of the IB3 dye are 455 ms and 1560 U, respectively, which are
much higher than that of the other dyes. The lifetime obtained by
EIS is in order of IB1 < IB4 < IB2 < IB3. This trend is in
accordance with the se vs. Voc results above. This result suggests
that the electron recombination rate between the TiO2 film and
the electrolyte is indeed suppressed by introduction of the extra
electron donor groups on the POZ and thiophene units
efficiently.
We also adopted IB3 and IB4 to fabricate the solid state DSCs
using the Spiro-OMeTAD54 as the hole conductor. The J–V
curve and IPCE spectrum of IB4 based solid state DSCs are
shown in Fig. 7. The IB4 dye showed an efficiency of 3.2% with
a Jsc of 6.4 mA cmꢀ2, Voc of 0.81 V and ff of 0.62 under standard
light illumination. The maximum IPCE value obtained by the
IB4 dye is 63% at 480 nm. However, under the same condition,
the IB3 dye in solid state DSCs gave an efficiency of just 1.6%
Fig. 8 The electron lifetime of solid state DSCs based on IB3 and IB4
Fig. 6 The Nyquist (a) and Bode plots (b) of DSCs based on IB dyes.
12470 | J. Mater. Chem., 2011, 21, 12462–12472
dyes.
This journal is ª The Royal Society of Chemistry 2011

View Article Online
10 K. Hara, Z. S. Wang, T. Sato, A. Furube, R. Katoh, H. Sugihara,
Y. Dan-Oh, C. Kasada, A. Shinpo and S. Suga, J. Phys. Chem. B,
2005, 109, 15476–15482.
11 Z. S. Wang, Y. Cui, K. Hara, Y. Dan-Oh, C. Kasada and A. Shinpo,
Adv. Mater., 2007, 19, 1138–1141.
12 S. Ferrere and B. A. Gregg, New J. Chem., 2002, 26, 1155–1160.
13 T. Edvinsson, C. Li, N. Pschirer, J. Schoneboom, F. Eickemeyer,
R. Sens, G. Boschloo, A. Herrmann, K. Mullen and A. Hagfeldt, J.
Phys. Chem. C, 2007, 111, 15137–15140.
14 Y. Shibano, T. Umeyama, Y. Matano and H. Imahori, Org. Lett.,
2007, 9, 1971–1974.
15 C. Li, J. H. Yum, S. J. Moon, A. Herrmann, F. Eickemeyer,
with much lower Voc and ff values of 0.59 V and 0.34, respec-
tively. According to the lifetime measurement (Fig. 8), interest-
ingly, the IB3 dye shows a much shorter lifetime than IB4. The
trend is totally different from that in liquid DSCs. The serious
electron recombination between the injected electrons and
oxidized dyes or Spiro-OMeTAD will result in the lower Voc
value, probably as well as a low ff value, yielding a disappointing
efficiency of DSCs based on the IB3 dye. The potential of Spiro-
OMeTAD is 0.8 V vs. NHE,55 which means that the regeneration
of oxidized IB3 dye could be difficult by this material. However,
from experiment, the solid state DSCs based on the IB3 dye also
produced a high current of about 8.0 mA cmꢀ2, which proved
that the IB3 dyes also can be regenerated by Spiro-OMeTAD in
solid state DSCs. This result suggests that the redox potentials of
IB3 and/or Spiro-OMeTAD in solid state DSCs are not exactly
same as those in organic solvent.
€
N. G. Pschirer, P. Erk, J. Schoeboom, K. Mullen, M. Gratzel and
M. K. Nazeeruddin, ChemSusChem, 2008, 1, 615–618.
16 D. P. Hagberg, T. Edvinsson, T. Marinado, G. Boschloo, A. Hagfeldt
and L. Sun, Chem. Commun., 2006, 2245–2247.
17 D. P. Hagberg, T. Marinado, K. M. Karlsson, K. Nonomura, P. Qin,
G. Boschloo, T. Brinck, A. Hagfeldt and L. Sun, J. Org. Chem., 2007,
72, 9550–9556.
18 H. Choi, C. Baik, S. O. Kang, J. Ko, M. S. Kang, M. K. Nazeeruddin
€
and M. Gratzel, Angew. Chem., Int. Ed., 2008, 47, 327–330.
19 W. H. Liu, I. C. Wu, C. H. Lai, C. H. Lai, P. T. Chou, Y. T. Li,
C. L. Chen, Y. Y. Hsu and Y. Chi, Chem. Commun., pp. 5152–5154.
20 H. Tian, X. Yang, R. Chen, R. Zhang, A. Hagfeldt and L. Sun, J.
Phys. Chem. C, 2008, 112, 11023–11033.
21 H. Tian, X. Yang, J. Pan, R. Chen, M. Liu, Q. Zhang, A. Hagfeldt
and L. Sun, Adv. Funct. Mater., 2008, 18, 3461–3468.
22 S. Kim, J. K. Lee, S. O. Kang, J. Ko, J. H. Yum, S. Fantacci, F. De
€
Angelis, D. Di Censo, M. K. Nazeeruddin and M. Gratzel, J. Am.
Chem. Soc., 2006, 128, 16701–16707.
23 H. Qin, S. Wenger, M. Xu, F. Gao, X. Jing, P. Wang,
Conclusion
Through modification of the basic dye structure (IB1) from both
the linker and donor units, a series of phenoxazine dyes were
designed and developed for dye-sensitized solar cells (DSCs). An
efficiency of 7.0% has been achieved under the standard light
illumination (AM 1.5G, 100 mW cmꢀ2) by the IB3 dye with
2,4-dibutoxyphenyl units. The detailed photoelectrochemical
study discovers that the introduction of triphenylamine or
2,4-dibutoxyphenyl can decrease the recombination process
between the injected electrons and electrolytes, yielding the high
photovoltage in liquid state DSCs. Furthermore, the solid state
DSCs based on phenoxazine dyes were reported for the first time.
The IB4 dye rendered an efficiency of 3.2% under the standard
light illumination. This strategy of structural modification will
pave a road to develop more efficient organic dyes in the future.
€
S. M. Zakeeruddin and M. Gratzel, J. Am. Chem. Soc., 2008, 130,
9202–9203.
24 J. H. Yum, D. P. Hagberg, S. J. Moon, K. M. Karlsson, T. Marinado,
€
L. C. Sun, A. Hagfeldt, M. K. Nazeeruddin and M. Gratzel, Angew.
Chem., Int. Ed., 2009, 48, 1576–1580.
25 Z. Ning, Q. Zhang, W. Wu, H. Pei, B. Liu and H. Tian, J. Org. Chem.,
2008, 73, 3791–3797.
26 D. Kim, J. K. Lee, S. O. Kang and J. Ko, Tetrahedron, 2007, 63, 1913–
1922.
27 Y. Ooyama, A. Ishii, Y. Kagawa, I. Imae and Y. Harima, New J.
Chem., 2007, 31, 2076–2082.
28 N. Koumura, Z. S. Wang, S. Mori, M. Miyashita, E. Suzuki and
K. Hara, J. Am. Chem. Soc., 2008, 130, 4202–4202.
29 Z. S. Wang, N. Koumura, Y. Cui, M. Takahashi, H. Sekiguchi,
A. Mori, T. Kubo, A. Furube and K. Hara, Chem. Mater., 2008,
20, 3993–4003.
Acknowledgements
This work was supported by the Swedish Research Council, the
Swedish Energy Agency, and the Knut and Alice Wallenberg
Foundation. We would like to thank Bao-Lin Lee at Stockholm
University for help with the HR-MS measurements and Henrik
Pettersson at Swerea IVF for helpful discussion.
30 T. Horiuchi, H. Miura and S. Uchida, Chem. Commun., 2003, 3036–
3037.
31 T. Horiuchi, H. Miura, K. Sumioka and S. Uchida, J. Am. Chem.
Soc., 2004, 126, 12218–12219.
32 W. Zhu, Y. Wu, S. Wang, W. Li, X. Li, J. Chen, Z.-S. Wang and
H. Tian, Adv. Funct. Mater., 2011, 21, 756–763.
33 Z.-S. Wang, F.-Y. Li, C.-H. Huang, L. Wang, M. Wei, L.-P. Jin and
N.-Q. Li, J. Phys. Chem. B, 2000, 104, 9676–9682.
34 A. Ehret, L. Stuhl and M. T. Spitler, J. Phys. Chem. B, 2001, 105,
9960–9965.
References
35 K. Takechi, P. K. Sudeep and P. V. Kamat, J. Phys. Chem. B, 2006,
110, 16169–16173.
36 S. L. Li, K. J. Jiang, K. F. Shao and L. M. Yang, Chem. Commun.,
2006, 2792–2794.
€
37 A. Burke, L. Schmidt-Mende, S. Ito and M. Gratzel, Chem. Commun.,
2007, 234–236.
38 S. Tatay, S. A. Haque, B. O’Regan, J. R. Durrant, W. J. H. Verhees,
J. M. Kroon, A. Vidal-Ferran, P. Gavina and E. Palomares, J. Mater.
Chem., 2007, 17, 3037–3044.
€
1 B. O’Regan and M. Gratzel, Nature, 1991, 353, 737–740.
2 A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, Chem.
Rev., 2010, 110, 6595–6663.
€
3 A. Hagfeldt and M. Gratzel, Chem. Rev., 1995, 95, 49–68.
4 M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker,
€
E. Mueller, P. Liska, N. Vlachopoulos and M. Gratzel, J. Am.
Chem. Soc., 1993, 115, 6382–6390.
5 M. K. Nazeeruddin, E. Muller, R. Humphry-Baker, N. Vlachopoulos
and M. Gratzel, J. Chem. Soc., Dalton Trans., 1997, 4571–4578.
6 F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing, R. Humphry-
39 J. H. Yum, P. Walter, S. Huber, D. Rentsch, T. Geiger, F. Nuesch,
€
F. De Angelis, M. Gratzel and M. K. Nazeeruddin, J. Am. Chem.
Soc., 2007, 129, 10320–10321.
€
Baker, P. Wang, S. M. Zakeeruddin and M. Gratzel, J. Am. Chem.
Soc., 2008, 130, 10720–10728.
40 Z. Ning, Q. Zhang, H. Pei, J. Luan, C. Lu, Y. Cui and H. Tian, J.
Phys. Chem. C, 2009, 113, 10307–10313.
41 R. Chen, X. Yang, H. Tian, X. Wang, A. Hagfeldt and L. Sun, Chem.
Mater., 2007, 19, 4007–4015.
42 Y. Hao, X. Yang, J. Cong, H. Tian, A. Hagfeldt and L. Sun, Chem.
Commun., 2009, 4031–4033.
7 Z. Ning, Y. Fu and H. Tian, Energy Environ. Sci., 2010, 3, 1170–1181.
8 K. Hara, K. Sayama, Y. Ohga, A. Shinpo, S. Suga and H. Arakawa,
Chem. Commun., 2001, 569–570.
9 K. Hara, T. Sato, R. Katoh, A. Furube, Y. Ohga, A. Shinpo, S. Suga,
K. Sayama, H. Sugihara and H. Arakawa, J. Phys. Chem. B, 2003,
107, 597–606.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 12462–12472 | 12471

View Article Online
43 H. Tian, X. Yang, J. Cong, R. Chen, C. Teng, J. Liu, Y. Hao,
L. Wang and L. Sun, Dyes Pigm., 2010, 84, 62–68.
44 H. Tian, X. Yang, J. Y. Cong, R. Chen, J. Liu, Y. Hao, A. Hagfeldt
and L. C. Sun, Chem. Commun., 2009, 6288–6290.
45 H. Tian, X. Yang, R. Chen, Y. Pan, L. Li, A. Hagfeldt and L. Sun,
Chem. Commun., 2007, 3741–3743.
46 H. Tian, X. Yang, R. Chen, A. Hagfeldt and L. C. Sun, Energy
Environ. Sci., 2009, 2, 674–677.
47 W. Wu, J. Yang, J. Hua, J. Tang, L. Zhang, Y. Long and H. Tian, J.
Mater. Chem., 2010, 20, 1772–1779.
48 K. M. Karlsson, X. Jiang, S. K. Eriksson, E. Gabrielsson,
H. Rensmo, A. Hagfeldt and L. Sun, Chem.–Eur. J., 2011, 17,
6415–6424.
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala,
K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain,
O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford,
J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill,
B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J. A. Pople,
Gaussian, Inc., Wallingford CT, 2004.
51 Avogadro: an open-source molecular builder and visualization tool.
Version 1.01. http://avogadro.openmolecules.net/.
49 H. Tian, Z. Yu, A. Hagfeldt, L. Kloo and L. Sun, J. Am. Chem. Soc.,
2011, 133, 9413–9422.
52 H. Tian and L. Sun, J. Mater. Chem., 2011, DOI: 10.1039/
C1JM10598A.
50 R. D. Gaussian 03, M. J. Frisch, G. W. Trucks, H. B. Schlegel,
G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery,
Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam,
S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada,
M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida,
T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li,
J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo,
53 F. Fabregat-Santiago, J. Bisquert, G. Garcia-Belmonte, G. Boschloo
and A. Hagfeldt, Sol. Energy Mater. Sol. Cells, 2005, 87, 117–131.
54 U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weissortel, J. Salbeck,
€
H. Spreitzer and M. Gratzel, Nature, 1998, 395, 583–
585.
55 X. Jiang, K. M. Karlsson, E. Gabrielsson, E. M. J. Johansson,
M. Quintana, M. Karlsson, L. Sun, G. Boschloo and A. Hagfeldt,
Adv. Funct. Mater., 2011, DOI: 10.1002/adfm.201002319.
12472 | J. Mater. Chem., 2011, 21, 12462–12472
This journal is ª The Royal Society of Chemistry 2011