High performance organic sensitizers based on 11,12-bis(hexyloxy) dibenzo[a,c]phenazine for dye-sensitized solar cells

Article abstract of DOI:10.1039/c2jm33833e

Three new metal-free organic sensitizers containing 11,12-bis(hexyloxy) dibenzo[a,c]phenazine (BPz) units were synthesized and used for dye-sensitized solar cells (DSSCs). The broad absorption spectra indicate that the light harvesting abilities were enhanced by the introduction of the BPz unit in the π-conjugated space, which can also cause an anti-aggregation effect and the suppression of charge recombination. Among these sensitizers, LI-39 showed the best photovoltaic performance: a short-circuit photocurrent density (J _sc) of 14.40 mA cm^-2, an open-circuit photovoltage (V _oc) of 0.74 V, and a fill factor (ff) of 0.67, corresponding to an overall conversion efficiency of 7.18% under standard global AM 1.5 solar light conditions. The result shows that the organic sensitizers based on this bulky fused aromatic rings as well as the auxiliary acceptor are the promising candidates for improvement of the performance of DSSCs. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2jm33833e

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PAPER
High performance organic sensitizers based on 11,12-bis(hexyloxy) dibenzo
[a,c]phenazine for dye-sensitized solar cells
Jie Shi,a Junnian Chen,a Zhaofei Chai,a Heng Wang,b Runli Tang,a Ke Fan,a Mei Wu,a Hongwei Han,b
a
Jingui Qin, Tianyou Peng, Qianqian Li and Zhen Li
a
a
a
*
*
Received 14th June 2012, Accepted 24th July 2012
DOI: 10.1039/c2jm33833e
Three new metal-free organic sensitizers containing 11,12-bis(hexyloxy) dibenzo[a,c]phenazine (BPz)
units were synthesized and used for dye-sensitized solar cells (DSSCs). The broad absorption spectra
indicate that the light harvesting abilities were enhanced by the introduction of the BPz unit in the
p-conjugated space, which can also cause an anti-aggregation effect and the suppression of charge
recombination. Among these sensitizers, LI-39 showed the best photovoltaic performance: a short-
circuit photocurrent density (Jsc) of 14.40 mA cmꢀ2, an open-circuit photovoltage (Voc) of 0.74 V, and a
fill factor (ff) of 0.67, corresponding to an overall conversion efficiency of 7.18% under standard global
AM 1.5 solar light conditions. The result shows that the organic sensitizers based on this bulky fused
aromatic rings as well as the auxiliary acceptor are the promising candidates for improvement of the
performance of DSSCs.
prevent the intermolecular p–p stacking among sensitizers and
Introduction
keep I3ꢀ ions away from the TiO2 electrode surface in the DSSCs
is another important requirement for the design of sensitizers,
besides the above mentioned electronic properties. Thus, it might
be an ideal mode to find a construction block, which could fulfil
all the above requirements, to build new sensitizers.
Dye-sensitized solar cells (DSSCs), which have been investigated
as promising candidates for renewable-energy systems, have
attracted great attention in both academic and industrial
communities due to their high achievable efficiencies at low cost.1
In addition to the metal complexes,2 the metal-free organic
sensitizers have also been desirable due to their wide variety of
structures, facile modification, and high molar absorption coef-
ficient.3 Importantly, there is still much room for chemists to
improve their cell performance by modifying the structure of the
metal-free organic sensitizers. Actually, the p-conjugated spacer,
which links the donor (D) and acceptor (A) moieties in the
general D–p–A structure of metal-free organic sensitizers, plays
an important role in intramolecular charge transfer (ICT) effi-
ciency, partially determining the DSSC performances of the
corresponding cells.4 Thus, many efforts have been made to
modify the intrinsic electronic characteristics of the p-conjugated
spacer.5 To tune the HOMO and LUMO levels, and broaden the
responsive wavelength, an auxiliary acceptor, generally the
electron-deficient segment, was introduced to the p-conjugated
spacer, to construct a so called ‘‘D–A–p–A’’ structure. Applying
this idea, Tian and co-workers have reported some sensitizers
with good DSSC performance.6 As it is well known, how to
11,12-Bis(hexyloxy) dibenzo[a,c]phenazine (BPz), a derivative
of quinoxaline, in which the combination of the bis(hexyloxy)-
linked quinoxaline and phenanthrene rings resulted in the large
extended p-conjugation system on the orthogonal directions
with planar configuration, to favor the light harvesting over the
solar irradiation (Chart 1). Moreover, the extension of the
conjugation system in the orthogonal directions can also act as
the isolation group to prevent the intermolecular p–p aggrega-
tion, and reduce the degree of charge trapping at the electron
deficient moiety.7 With the incorporation of the electron-defi-
cient properties of the quinoxaline unit, this bulky fused
aromatic rings with electron-withdrawing character could lead to
aDepartment of Chemistry, Hubei Key Lab on Organic and Polymeric
Opto-Electronic Materials, Wuhan University, Wuhan 430072, China.
E-mail: qianqian-alinda@163.com; lizhen@whu.edu.cn; lichemlab@163.
com; Fax: +86-27-68755363; Tel: +86-27-62254108
bMichael Gra€tzel Center for Mesoscopic Solar Cells, Wuhan National
Laboratory for Optoelectronics, Huazhong University of Science and
Technology, Wuhan, 430072, China
Chart 1
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the broader light absorption as well as the red-shifted absorption
spectra. Additionally, the hexyloxy chains, linked to the qui-
noxaline moiety in the vertical direction, can not only form a
Instrumentation
1H and13C NMRspectroscopystudywasconductedwith a Varian
Mercury 300 spectrometer using tetramethylsilane (TMS; d ¼
0 ppm) as the internal standard. UV-visible spectra were obtained
using a Shimadzu UV-2550 spectrometer. Cyclic voltammograms
were carried out on a CHI 660 voltammetric analyzer at room
temperature in nitrogen-purged anhydrous dichloromethane with
tetrabutylammonium hexafluorophosphate (TBAPF6) as the
supporting electrolyteata scanningrate of100 mVsꢀ1. A platinum
disk and a Ag/AgCl electrode were used as the working electrode
and quasi-reference electrode, respectively. The ferrocene/ferro-
cenium redox couple was used for potential calibration. Elemental
analyses were performed by a 73 CARLOERBA-1106 micro-
elemental analyzer. EI-MS spectra were recorded with a Finnigan
PRACE mass spectrometer. Matrix-assisted laser desorption
ionization time-of-flight mass spectra were measured on a
Voyager-DE-STR MALDI-TOF mass spectrometer (MALDI-
TOF MS; ABI, American) equipped with a 337 nm nitrogen laser
and a 1.2 m linear flight path in positive ion mode.
ꢀ
blocking layer to keep I3 ions away from the TiO2 electrode
surface, but also increase the solubility of the whole molecule.8
Thus, this aromatic block might be a good construction one.
However, interestingly, there are no reports concerning its utili-
zation in the design of DSSC sensitizers.
In our previous studies, the BPz unit was applied in alternating
copolymers with different connecting points as the acceptor.
Unexpectedly, the different connecting points led to the
completely different photovoltaic performance of the copoly-
mers in the devices of polymer solar cells. These results were very
interesting, exhibited the unique properties of this large conju-
gated system, and hinted that the BPz unit was a good building
block to enhance the electron-optical properties of the resulting
materials.9
Considering all the above points, the BPz unit was incorpo-
rated into the conjugated bridge as an auxiliary acceptor to form
new sensitizers with the D–p–A0–p–A configuration, 3-hexylth-
iopene was chosen as an additional p spacer to link the donor
and the auxiliary acceptor (Chart 2). With the aim to optimize
the performance of these organic sensitizers, and seek the suit-
able donor group to match the electron properties of the p-
conjugated system, the modulation of the donor units was made
to alter the molecular energy levels and the absorption edge. As a
result, these sensitizers show evident bathochromic shifts in
absorption, and thus effectively improve the light-harvesting
range. The best photovoltaic performance of these sensitizers
exhibited an overall conversion efficiency of 7.18% under stan-
dard global AM 1.5 solar light condition. Herein, we report their
syntheses, structural characterization, electrochemical proper-
ties, theoretical calculations, and photovoltaic performance.
Synthesis
10,13-Dibromo-11,12-bis(hexyloxy)dibenzo[a,c]phenazine (2).
9,10-Phenanthraquinone (1.50 g, 7.22 mmol), toluene (80 mL)
and 3,6-dibromo-4,5-bis(hexyloxy)benzene-1,2-diamine (3.36 g,
7.22 mmol) were placed in a round-bottomed flask fixed with a
condenser. The mixture was heated under reflux for 6 h and then
allowed to cool to room temperature. The solvent was removed
under vacuum. The product was purified by column chroma-
1
tography to give the product (3.77 g, 82%) as a yellow solid. H
NMR (CDCl3, 300 MHz) d (ppm): 9.45 (d, J ¼ 8.1 Hz, 2H, ArH),
8.56 (d, J ¼ 7.2 Hz, 2H, ArH), 7.84–7.74 (m, 4H, ArH), 4.29–4.25
(m, 4H, –CH2–), 1.98–1.86 (m, 4H, –CH2–), 1.41 (br, 12H, –
CH2–), 0.96–0.93 (m, 6H, –CH3).
Experimental
11,12-Bis(hexyloxy)-10,13-bis(4-hexylthiophen-2-yl)dibenzo
[a,c]phenazine (3). To a solution of 2 (3.19 g, 5.0 mmol) and
tributyl(4-hexylthiophen-2-yl)stannane (4.57 g, 10.0 mmol) in
toluene (30 mL) was added Pd(PPh3)4 (24 mg, 0.02 mmol) under
an atmosphere of dry nitrogen. After refluxing for 12 h, the
mixture was cooled to room temperature and then poured into
water, the organic layer was extracted by CH2Cl2 and dried over
anhydrous Na2SO4. The crude product was purified through a
silica gel chromatography column to give the product (3.46 g,
85%) as an orange red oil. 1H NMR (CDCl3, 300 MHz) d (ppm):
9.38 (d, J ¼ 6.0 Hz, 2H, ArH), 8.56 (d, J ¼ 4.8 Hz, 2H, ArH), 7.95
(s, 2H, ArH), 7.76–7.72 (m, 4H, ArH), 7.25 (s, 2H, ArH), 4.08
(br, 4H, –CH2–), 2.78 (br, 4H, –CH2–), 1.82 (br, 8H, –CH2–),
1.45–1.25 (m, 24H, –CH2–), 0.92 (br, 12H, –CH3).
Materials
Tetrahydrofuran (THF) and toluene were dried over and
distilled from K–Na alloy under an atmosphere of dry nitrogen.
N,N-Dimethylformamide (DMF) was dried over and distilled
from CaH2 under an atmosphere of dry nitrogen. 1,2-Dichlo-
roethane was dried over and distilled from phosphorus pent-
oxide. Phosphorus oxychloride was freshly distilled before use.
All reagents were purchased from Alfa Aesar or Aldrich and used
as received. 3,6-Dibromo-4,5-bis(hexyloxy)benzene-1,2-diamine
(compound 5) was prepared following the procedure reported in
the literature.10
5-(11,12-Bis(hexyloxy)-13-(4-hexylthiophen-2-yl)dibenzo[a,c]
phenazin-10-yl)-3-hexylthiophene-2-carbaldehyde (4). DMF
(1.07 g, 14.76 mmol) was added to freshly distilled POCl3 (1.13 g,
7.38 mmol) under an atmosphere of dry nitrogen at 0 ꢁC, and the
resultant solution was stirred until its complete conversion into a
glassy solid. After the addition of compound 3 (3.0 g, 3.69 mmol)
in 1,2-dichloroethane (20 mL) dropwise, the mixture was stirred
at room temperature overnight, then poured into an aqueous
Chart 2
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solution of sodium acetate (1 M, 200 mL), and stirred for another
2 h. The mixture was extracted with chloroform several times, the
organic fractions were combined and dried over anhydrous
Na2SO4. After removing the solvent under vacuum, the crude
product was purified through a silica gel chromatography column
to give the product (2.04 g, 65%) as a red oil. 1H NMR (CDCl3,
300 MHz) d (ppm): 10.23 (s, 1H, –CHO), 9.38 (d, J ¼ 6.3 Hz, 1H,
ArH), 9.29 (d, J ¼ 7.2 Hz, 1H, ArH), 8.57 (d, J ¼ 6.9 Hz, 2H,
ArH), 7.98 (d, J ¼ 4.8 Hz, 2H, ArH), 7.79–7.74 (m, 4H, ArH), 7.25
(s, 1H, ArH), 4.12 (br, 4H, –CH2–), 2.74 (br, 4H, –CH2–), 1.85 (br,
8H, –CH2–), 1.48–1.37 (m, 24H, –CH2–), 0.92 (br, 12H, –CH3).
5-(13-(5-(4-(Bis(4-methoxyphenyl)amino)phenyl)-4-hexylthio-
phen-2-yl)-11,12-bis(hexyloxy)dibenzo[a,c]phenazin-10-yl)-3-hex-
ylthiophene-2-carbaldehyde (6c). Dark red solid. 0.54 g. Yield:
72%. 1H NMR (CDCl3, 300 MHz) d (ppm): 10.21 (s, 1H, –CHO),
9.45 (d, J ¼ 7.6 Hz, 1H, ArH), 9.40 (d, J ¼ 7.2 Hz, 1H, ArH), 8.67
(d, J ¼ 6.5 Hz, 2H, ArH), 8.12 (s, 1H, ArH), 7.99 (s, 1H, ArH),
7.76–7.71 (m, 4H, ArH), 7.46 (d, J ¼ 7.8 Hz, 2H, ArH), 7.34
(d, J ¼ 7.8 Hz, 1H, ArH), 7.12–6.93 (m, 7H, ArH), 6.74 (d, J ¼
8.7 Hz, 2H, ArH), 4.18 (br, 4H, –CH2–), 3.82 (br, 6H, –CH3),
2.85 (br, 4H, –CH2–), 1.82 (br, 8H, –CH2–), 1.43–1.34 (m, 24H, –
CH2–), 0.90 (br, 12H, –CH3).
5-(13-(5-Bromo-4-hexylthiophen-2-yl)-11,12-bis(hexyloxy)
dibenzo[a,c]phenazin-10-yl)-3-hexylthiophene-2-carbaldehyde (5).
To a solution of compound 4 (1.99 g, 2.4 mmol) in CHCl3
(20 mL) was added N-bromosuccinimide (0.45 g, 2.5 mmol). This
mixture was stirred for 10 h in the absence of light at room
temperature and then the solvent was removed under vacuum.
The crude product was purified through a silica gel chromatog-
raphy column to give the product (1.75 g, 79%) as a red solid. 1H
NMR (CDCl3, 300 MHz) d (ppm): 10.20 (s, 1H, –CHO), 9.31 (d,
J ¼ 6.1 Hz, 1H, ArH), 9.25 (d, J ¼ 6.5 Hz, 1H, ArH), 8.54 (d, J ¼
6.4 Hz, 2H, ArH), 7.90 (d, J ¼ 5.3 Hz, 2H, ArH), 7.71–7.79 (m,
4H, ArH), 4.18 (br, 4H, –CH2–), 2.71 (br, 4H, –CH2–), 1.88 (br,
8H, –CH2–), 1.42–1.36 (m, 24H, –CH2–), 0.91 (br, 12H, –CH3).
General synthesis of sensitizers. A mixture of 6 (0.2 mmol)
and cyanoacetic acid (1.0 mmol) were vacuum-dried, then MeCN
(15 mL), THF (5 mL) and piperidine (10 mL) were added. The
solution was refluxed for 8 h. After the solution was cooled, the
organic layer was removed under vacuum. The product was
purified by column chromatography.
2-Cyano-3-(5-(13-(5-(4-(diphenylamino)phenyl)-4-hexylthio-
phen-2-yl)-11,12-bis(hexyloxy)dibenzo[a,c]phenazin-10-yl)-3-hex-
ylthiophen-2-yl)acrylic acid (LI-37). Dark red solid. 0.16 g. Yield:
68%. 1H NMR (DMSO-d6, 300 MHz) d (ppm): 9.01 (br, 1H,
ArH), 8.91 (br, 1H, ArH), 8.40 (br, 2H, ArH), 8.25 (s, 1H, –
CH]), 7.89 (s, 1H, ArH), 7.85 (s, 1H, ArH), 7.64–7.54 (m, 4H,
ArH), 7.32 (br, 6H, ArH), 7.10 (br, 8H, ArH), 3.83 (br, 4H, –
CH2–), 2.65 (br, 4H, –CH2–), 1.70 (br, 8H, –CH2–), 1.35–1.26
(m, 24H, –CH2–), 0.85 (br, 12H, –CH3). 13C NMR (DMSO-d6,
75 MHz) d (ppm): 169.4, 154.6, 152.3, 147.9, 147.3, 145.6, 144.7,
141.5, 140.7, 137.8, 135.1, 134.1, 132.3, 131.1, 130.1, 129.6, 129.2,
127.4, 126.4, 124.9, 123.4, 122.9, 121.9, 116.9, 95.7, 75.1, 74.4,
32.1, 31.7, 30.9, 29.8, 29.6, 26.2, 22.9, 14.4. MALDI-TOF MS
calcd for C74H78N4O4S2 [M+] m/z: 1151.55 [M+] found.
C74H78N4O4S2 (M+): 1151.37. Anal. calcd for C74H78N4O4S2: C,
77.18; H, 6.83; N, 4.87. Found: C, 77.57; H, 7.13; N, 5.05%
General synthesis of 6. A mixture of 5 (0.60 g, 0.65 mmol), 4-
(diphenylamino) phenylboronic acid, 4-(diptolylamino) phenyl-
boronic acid or 4-(bis(4-methoxyphenyl)amino) phenylboronic
acid (1.0 mmol), sodium carbonate (0.69 g, 6.5 mmol), THF
(monomer concentration about 0.025 M)/water (2 : 1 in volume),
and Pd(PPh3)4 (10 mg) was carefully degassed and charged with
nitrogen. The reaction was stirred for 24 h at 80 ꢁC. After cooled
to room temperature, the organic layer was separated, dried over
anhydrous Na2SO4, and evaporated to dryness. The crude
product was purified by column chromatography over silica gel
to give product.
2-Cyano-3-(5-(13-(5-(4-(dip-tolylamino)phenyl)-4-hexylthio-
phen-2-yl)-11,12-bis(hexyloxy)dibenzo[a,c]phenazin-10-yl)-3-hex-
ylthiophen-2-yl)acrylic acid (LI-38). Dark red solid. 0.14 g. Yield:
5-(13-(5-(4-(Diphenylamino)phenyl)-4-hexylthiophen-2-yl)-11,12-
1
bis(hexyloxy)dibenzo[a,c]phenazin-10-yl)-3-hexylthiop-hene-2-
1
carbaldehyde (6a). Dark red solid. 0.52 g. Yield: 74%. H NMR
59%. H NMR (DMSO-d6, 300 MHz) d (ppm): 9.30 (d, J ¼ 7.5
Hz, 1H, ArH), 9.21 (d, J ¼ 7.2 Hz, 1H, ArH), 8.57 (d, J ¼ 9.0 Hz,
2H, ArH), 8.42 (s, 1H, –CH]), 8.07 (s, 1H, ArH), 8.01 (s, 1H,
ArH), 7.74–7.60 (m, 4H, ArH), 7.39 (d, J ¼ 8.4 Hz, 2H, ArH),
7.13 (d, J ¼ 8.4 Hz, 4H, ArH), 7.03 (d, J ¼ 7.8 Hz, 6H, ArH), 4.06
(br, 4H, –CH2–), 2.87 (br, 4H, –CH2–), 2.32 (br, 6H, –CH3), 1.86
(br, 8H, –CH2–), 1.44–1.34 (m, 24H, –CH2–), 0.89 (br, 12H, –
CH3). 13C NMR (DMSO-d6, 75 MHz) d (ppm): 169.8, 154.5,
152.2, 150.7, 147.6, 145.4, 141.6, 140.5, 137.6, 135.2, 133.9, 133.0,
131.8, 130.8, 130.2, 129.9, 128.1, 126.2, 125.2, 123.9, 122.2, 120.8,
116.1, 101.2, 74.9, 74.3, 32.1, 30.9, 29.8, 29.5, 26.2, 22.9, 21.1,
14.4. MALDI-TOF MS calcd for C76H82N4O4S2 [M+] m/z:
1179.58 [M+] found. C76H82N4O4S2 (M+): 1179.49. Anal. calcd
for C76H82N4O4S2: C, 77.38; H, 7.01; N, 4.75. Found: C, 77.62;
H, 7.26; N, 4.67%.
(CDCl3, 300 MHz) d (ppm): 10.22 (s, 1H, –CHO), 9.48 (d, J ¼ 7.8
Hz, 1H, ArH), 9.32 (d, J ¼ 8.7 Hz, 1H, ArH), 8.59 (d, J ¼ 7.8 Hz,
2H, ArH), 8.10 (s, 1H, ArH), 7.99 (s, 1H, ArH), 7.79–7.71 (m,
4H, ArH), 7.52 (d, J ¼ 8.7 Hz, 2H, ArH), 7.31 (d, J ¼ 7.8 Hz, 1H,
ArH), 7.10–6.94 (m, 9H, ArH), 6.77 (d, J ¼ 9.0 Hz, 2H, ArH),
4.23–4.12 (m, 4H, –CH2–), 2.85 (br, 4H, –CH2–), 1.88 (br, 8H, –
CH2–), 1.46–1.34 (m, 24H, –CH2–), 0.91 (br, 12H, –CH3).
5-(13-(5-(4-(Dip-tolylamino)phenyl)-4-hexylthiophen-2-yl)-11,12-
bis(hexyloxy)dibenzo[a,c]phenazin-10-yl)-3-hexylthio-phene-2-
1
carbaldehyde (6b). Dark red solid. 0.48 g. Yield: 66%. H NMR
(CDCl3, 300 MHz) d (ppm): 10.22 (s, 1H, –CHO), 9.49 (d, J ¼ 7.2
Hz, 1H, ArH), 9.32(d,J¼ 8.1 Hz, 1H, ArH), 8.61 (d, J¼ 6.3Hz, 2H,
ArH), 8.10 (s, 1H, ArH), 7.99 (s, 1H, ArH), 7.78–7.73 (m, 4H, ArH),
7.47 (d, J ¼ 7.6 Hz, 2H, ArH), 7.33 (d, J ¼ 7.8 Hz, 1H, ArH), 7.13–
6.91 (m, 7H, ArH), 6.78 (d, J ¼ 8.1 Hz, 2H, ArH), 4.13 (br, 4H, –
CH2–), 2.85 (br, 4H, –CH2–), 2.34 (br, 6H, –CH3), 1.86 (br, 8H, –
CH2–), 1.46–1.34 (m, 24H, –CH2–), 0.91 (br, 12H, –CH3).
3-(5-(13-(5-(4-(Bis(4-methoxyphenyl)amino)phenyl)-4-hexyl-
thiophen-2-yl)-11,12-bis(hexyloxy)dibenzo[a,c]phenazin-10-yl)-3-
hexylthiophen-2-yl)-2-cyanoacrylic acid (LI-39). Dark red solid.
1
0.14 g. Yield: 58%. H NMR (DMSO-d6, 300 MHz) d (ppm):
18832 | J. Mater. Chem., 2012, 22, 18830–18838
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9.39 (br, 1H, ArH), 9.28 (d, J ¼ 8.1 Hz, 1H, ArH), 8.57 (br, 2H,
ArH), 8.45 (s, 1H, –CH]), 8.08 (s, 1H, ArH), 8.04 (s, 1H, ArH),
7.78–7.67 (m, 4H, ArH), 7.39 (d, J ¼ 8.9 Hz, 2H, ArH), 7.13 (d,
J ¼ 8.4 Hz, 4H, ArH), 6.96–6.89 (m, 6H, ArH), 4.09 (br, 4H, –
CH2–), 3.79 (br, 6H, –CH3), 2.91 (br, 4H, –CH2–), 1.85 (br, 8H, –
CH2–), 1.46–1.34 (m, 24H, –CH2–), 0.90 (br, 12H, –CH3). 13C
NMR (DMSO-d6, 75 MHz) d (ppm): 169.0, 156.3, 154.3, 152.2,
148.1, 142.9, 141.9, 141.1, 140.6, 139.0, 137.6, 137.4, 135.0, 133.8,
133.4, 132.8, 132.1, 130.8, 129.9, 137.1, 125.8, 122.1, 120.4, 116.4,
98.5, 74.4, 55.8, 32.1, 31.8, 30.7, 29.8, 29.5, 26.2, 22.9, 14.3.
MALDI-TOF MS calcd for C76H82N4O6S2 [M+] m/z: 1211.57
[M+] found. C76H82N4O6S2 (M+): 1211.52. Anal. calcd for
C76H82N4O6S2: C, 75.34; H, 6.82; N, 4.62. Found: C, 75.03; H,
6.68; N, 4.52%.
measurements mentioned above were taken under ambient
conditions. Incident photon-current conversion efficiency (IPCE)
was recorded on a DC Power Meter (Model 2931-C, Newport
Co.) under irradiation of a 300 W xenon lamp light source with a
motorized monochromator (Oriel). The xenon lamp was powered
by an Arc Lamp Power Supply (Model 69920, Newport Co.).
Results and discussion
Design and synthesis of the sensitizers
The quinoxaline unit was initially utilized to the p-conjugated
space of organic sensitizers with the linkage of the para position
of 2,3-diphenyl rings (Chart 3), as a strong electron-drawing unit.
Lately, with the aim of decreasing electron recombination, this
unit was further modified by the introduction of alkoxy chains to
the phenyl rings (Chart 3).11 When the 2,3-diphenyl rings were
replaced by the phenanthrene unit, and alkoxy chains moved to
the quinoxaline ring, BPz unit was formed, which retained the
intrinsic electron properties of quinoxaline and the suppression
of electron recombination, however, the incorporation of the
phenanthrene unit make the conjugated system more planar and
larger, as mentioned above, which would be beneficial to the
enhancement of the light harvesting abilities.
Device fabrication
A layer of ca. 2 mm TiO2 (TPP-3, Heptachroma, China) was
coated on the FTO conꢁducting glass by screen printing and then
dried for 6 min at 125 C. This procedure was repeated 6 times
(ca. 12 mm) and finally coated by a layer (ca. 4 mm) of TiO2 paste
(TPP-200, Heptachroma, China) as the scattering layer. The
TiO2 electrodes were gradually heated under an air flow at 450 ꢁC
for 30 min. The sintered film was further treated with 0.2 M TiCl4
aqueous solution at room temperature for 12 h, then washed with
water and ethanol, and annealed at 450 ꢁC for 30 min. After the
The sensitizers with the incorporation of BPz unit into the p-
conjugated bridge were synthesized according to the synthetic
routes in Scheme 1. The important intermediate 2, BPz unit with
the linkage of bromine, was obtained by a cyclization reaction
with high yield. After the Stille coupling reaction with tributyl(4-
hexylthiophen-2-yl)stannane, the conjugated bridge was formed
by the incorporation of BPz unit and hexylthiophene. The
introduction of long alkyl chains and alkoxy chains into sensi-
tizers can not only inhibit the dye aggregation but also improve
the solubility, which is of great benefit to the synthesis and
purification. Compound 4 was obtained by the followed Vils-
meier reaction. For further grafting the donor unit, simple
bromination and Suzuki coupling reactions were treated on the
reactive hexylthiophene unit, to produce the aldehyde precursor
6. Finally, the conventional Knoevenagel condensation yielded
the final product with cyanoacetic acid in the presence of
piperidine. The sensitizers and their precursor compounds are
soluble in common organic solvents (i.e., acetone, CH2Cl2,
CHCl3, DMF, DMSO and THF). The structure and purity of the
three compounds were confirmed by 1H NMR, 13C NMR,
MALDI-TOF MS and elemental analysis.
film was cooled to 80 ꢁC, it was immersed in a 3 ꢂ 10ꢀ4
M
sensitizer bath in CH2Cl2 solution and maintained in the dark for
24 h at room temperature. The electrode was then rinsed with
CH2Cl2 and dried. The size of the TiO2 electrodes used was 0.238
cm2. To prepare the counter electrode, the Pt catalyst was
deposited on cleaned FTO glass by coating with a drop of
H2PtCl6 solution (0.02 M 2-propanol solution) with heat treat-
ment 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 sensitizer-covered TiO2 electrode
and Pt-counter electrode 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 electrolyte (DHS-Z23,
Heptachroma, China) was introduced into the cell via vacuum
backfilling from the hole in the back of the counter electrode.
Finally, the hole was sealed using a piece of aluminum foil tape.
Photovoltaic properties measurements
Optical properties
The DSSC was illuminated by light with energy of a 100 mW cmꢀ2
from 300 W AM 1.5G simulated sunlight (2 ꢂ 2 beam, w/6258
lamp, Newport, USA). The light intensity was determined using a
SRC-1000-TC-QZ-N reference monocrystalline silicon cell
system (Oriel, USA), which was calibrated by National Renew-
able Energy Laboratory, A2LA accreditation certificate 2236.01.
The current–voltage (J–V) curves for the fabricated DSSCs were
collected by using CHI618 electrochemical analyzer (CH Instru-
ments). Cell active area was tested with a metal mask of 0.16 cm2.
The electrochemical impedance spectra (EIS) measurements were
carried out with applying bias of the open circuit voltage (Voc)
under the conditions and recorded over a frequency range from
0.05 Hz to 105 Hz with an ac amplitude of 10 mV. All of the
The absorption spectra of sensitizers in dilute dichloromethane
solution are displayed in Fig. 1, and the absorption peaks as well
Chart 3
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Scheme 1 Synthetic route to sensitizers.
as their extinction coefficients are summarized in Table 1. In the
visible region, all of these organic sensitizers show two distinct
absorptions around 415 and 510 nm, the absorption band
around 415 nm is assigned to a p–p* transition, while the
absorption band with lmax around 510 nm is corresponded to an
ICT between the triphenylamine donor part and the acceptor end
group. The red-shifted ICT band was obvious due to the strong
electron-drawing ability of the auxiliary acceptor in the p-
conjugated space, which decreases the band group and optimizes
the energy levels, resulting in a broader responsive wavelength
region. In addition, all of the three sensitizers show high molar
coefficients (>40 000 Mꢀ1 cmꢀ1) at the maximum absorption.
The absorption peak of LI-38 (513 nm) and LI-39 (516 nm)
exhibit considerable bathochromic shifts in comparison with that
of LI-37 (504 nm). This result clearly illustrates that the modu-
lation of the optical band-gaps and energy levels of these sensi-
tizers can be partly achieved by altering the electronic nature of
triarylamine donors with the addition of the alkyl or alkoxy
chains, suggesting that LI-38 and LI-39, the sensitizers bearing
stronger donors, possess stronger photoinduced ICT abilities
over the p-conjugation backbone, leading to the red-shifted
absorption spectra. The absorption spectra of the organic
sensitizers anchored onto a transparent nanocrystalline TiO2 film
(6 mm) are presented in Fig. 2. After the sensitizers absorbed on
the surface of TiO2 films, the absorption peaks located at 520,
525 and 526 nm, respectively. Compared to those observed in the
solution, the little red shift of the absorption bands demonstrated
that the intermolecular aggregations can be suppressed in a large
degree by the addition of the BPz unit in the p-conjugated
spacer, which would avoid the unnecessary energy loss by the
molecular interactions. The absorption spectra of the sensitizers
became broadened after adsorption on the TiO2 surface with an
extended absorption onset above 720 nm, which should favor the
light harvesting of the solar cells and thus increase the photo-
current response region, leading to the increase of short-circuit
photocurrent density (Jsc).
Electrochemical properties
In order to study the possibilities of electron injection from the
excited state sensitizers to the conduction band (CB) of the
semiconductor and the sensitizer regenerations, the oxidation
potentials of the sensitizers were determined by cyclic voltam-
mograms (Fig. 3). The oxidation potential vs. NHE (Eox) cor-
responded to the highest occupied molecular orbital (HOMO),
while the reduction potential vs. NHE (Ered), which corre-
sponded to the lowest unoccupied molecular orbital (LUMO),
could be calculated from Eox ꢀ E0–0. The electrochemical data of
the three sensitizers were summarized in Table 1. Two oxidation
waves were observed in the voltammograms. The first quasi-
reversible oxidation wave at lower oxidation potentials is
attributed to the triphenylamine, whereas the higher oxidation
potentials with quasi-reversible behavior, were from the spacer.
LI-39 has the lowest oxidation potential because the oxidized
triphenylamine can be stabilized by the two electron-donating
methoxy moieties and the thiophene unit nearby, and the higher
oxidation potential of LI-37 is ascribed to the poorest ability of
electron-donating among these sensitizers. The lower oxidation
potentials means the HOMO raising upon the introduction of
stronger donors, which agrees well with the calculation and
shows the important contribution of the methyl and methoxy
groups to HOMO distributions. The reduction potentials also
vary significantly with the variation of the donor, which is also
identical to the calculations. As shown in Table 1, the HOMO
levels are more positive than the iodine/iodide redox potential
value (0.4 V vs. NHE), indicating that the oxidized sensitizers
formed after electron injection into the conduction band of TiO2
Fig. 1 UV-vis spectra of sensitizers in CH2Cl2.
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Table 1 Absorbance and electrochemical properties of sensitizers
Sensitizer lmax (nm) 3 ꢂ 104 (Mꢀ1 cmꢀ1
)
lmax (nm) E0–0 (eV) Eox (V) vs. NHE Ered (V) vs. NHE Calcd ELOMO (eV) Calcd EHOMOf (eV)
a
b
c
d
e
f
LI-37
LI-38
LI-39
415, 504
416, 513
417, 516
4.34, 3.04
4.28, 2.99
4.62, 3.21
520
525
526
1.98
2.02
2.04
1.01
0.92
0.83
ꢀ0.97
ꢀ1.10
ꢀ1.21
ꢀ2.72
ꢀ2.70
ꢀ2.66
ꢀ4.91
ꢀ4.80
ꢀ4.66
a
b
Absorption spectra of sensitizers measured in CH2Cl2 with the concentration of 3 ꢂ 10ꢀ5 mol Lꢀ1
. Absorption spectra of sensitizers adsorbed on the
surface of TiO2. The bandgap, E0–0 was derived from the observed optical edge. Eox were measured in CH2Cl2 with 0.1 M (n-C4H9)4NPF6 as
c
d
electrolyte (scanning rate, 100 mV sꢀ1; working electrode and counter electrode, Pt wires; reference electrode, Ag/AgCl)). The oxidation potential
on
e
(Eox) referenced to calibrated Ag/AgCl was converted to the NHE reference scale: Eox ¼ E + 0.2 V. Ered was calculated from Eox ꢀ E0-0
ox
.
f
HOMO and LUMO levels calculated by density functional theory (DFT).
Fig. 4 Frontier orbitals of the sensitizers optimized at the B3LYP/6-
31+G (D) level.
Fig. 2 UV-vis spectra of the sensitizers on TiO2 films.
BPz unit suggest the well inductive or withdrawing electron
tendency from triphenylamine donor unit to the cyanoacetic acid
unit. After photoexcitation, the electrons in the organic sensi-
tizers loaded on nanocrystalline TiO2 surface could be succes-
sively transferred from the donor to the acceptor unit in spacer,
and then transferred to the cyanoacetic acid subunit, and finally
into TiO2. The calculation of HOMO (Table 1) also reveals the
contribution of methyl and methoxy groups to the HOMO
distributions in LI-38 and LI-39, which is consistent with their
stronger electron-donating nature and the observed bath-
ochromic shifts in absorption spectra. The well-delocalized
LUMO over the A–p–A units indicates that the additional
acceptor of this bulky unit could facilitate the electron transfer
from the donor to the acceptor.
Fig. 3 Cyclic voltammograms of sensitizers in CH2Cl2.
Photovoltaic performance of DSSCs
ꢀ
could thermodynamically accept electrons from I3 ions. The
The photovoltaic characteristics of these sensitizers were
obtained with a sandwich cell using the commercial redox elec-
trolyte (DHS-Z23, Heptachroma, China). The action spectra of
incident photo-to-current conversion efficiency (IPCE) for
DSSCs based on the sensitizers of LI-37, LI-38 and LI-39 are
presented in Fig. 5. The IPCE of the sensitizers illustrated that
the visible light can be converted to photocurrent efficiently in
the range of 400–700 nm, which was in good accordance with
their absorption spectra on the TiO2 film. The IPCE exceeds 50%
in the spectral range 400–570 nm for LI-39 and LI-38, and the
maximum value of LI-39 reaches 65% at 525 nm. From the IPCE
curves, we can infer that LI-38 and LI-39, bearing stronger
electron donors, have shown higher IPCE values than LI-37.
Such broader spectral responses of LI-38 and LI-39 are consis-
tent with the trend of the absorption spectra, implying their
LUMO levels of the sensitizers are 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 sensitizer into the conduction band of TiO2 is
energetically permitted.
Theoretical approach
The structures of the sensitizers were analyzed using the B3LYP
exchange-correlation functional and a 6-31g* basis set for full
geometrical optimization.12 It is clearly seen from Fig. 4 that the
HOMO is distributed along the triphenylamine and first thio-
phene system, the LUMO is delocalized across the entire A–p–A
system. The well-overlapped HOMO and LUMO orbitals on the
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different donors, in Table 2, the Jsc was consistent with the
LUMO energy level of these sensitizers. The reduction potential
(Ered) of the three sensitizers were in the following order: LI-39
(ꢀ1.21 V) < LI-38 (ꢀ1.10 V) < LI-37 (ꢀ0.97 V). LI-39 shows the
most negative reduction potential (Ered), that is to say, in the
process of the electron injection from excited-state sensitizers to
the conductive band of TiO2, LI-39 is the most effective, thus, the
cell based on LI-39 obtains the highest Jsc value. As for the value
of Voc, for the existence of the bulky conjugated unit in the
orthogonal directions as well as the long alkyl chains and alkoxy
chains in the p-conjugated bridge, the dye aggregates and the
charge recombination could be suppressed to a large degree, and
the three sensitizers all give the relatively higher Voc values, in the
range of 0.67–0.74 V. The charge recombination of cells could be
suppressed by the substituent in donor side,13 for this reason, LI-
37, with non-substituted triphenylamine donor, possesses the
highest level of charge recombination and displays the lowest Voc
of the three. In order to further probe the behavior of the
sensitizers on the surface of TiO2, the function of co-adsorbents
was investigated. Chenodeoxycholic acid (CDCA) is the most
popular co-adsorbent, which binds strongly to the surface of
nanostructured TiO2, being able to displace sensitizer molecules
from the semiconductor surface and therefore hindering the
formation of sensitizer aggregates.14 Thus, the performances of
the DSSCs in the presence of CDCA with various concentrations
were studied. The results were collected in Table 2, and the
corresponding J–V curves are shown in Fig. 6. It was found that
Jsc decreased dramatically with the increase of the concentration
of CDCA, whereas Voc remained almost the same. Undoubtedly,
the amount of sensitizer adsorbed on the TiO2 surface was
reduced by the coadsorption of CDCA, resulting in a loss of
active light harvesting, indicating that CDCA was not necessary
for these sensitizers to improve their performance. The result
could be considered as a hint that these sensitizers with the
introduction of bulky fused aromatic rings to the p-conjugated
spacer, did not aggregate on the TiO2 surface.
Fig. 5 Spectra of monochromatic incident photon-to-current conver-
sion efficiency (IPCE) for DSSCs based on these sensitizers.
better light harvesting ability for long-wavelength visible light,
which would be attributed to the strong electron-withdrawing
properties of the BPz unit as well as the good electron–donating
abilities of the methyl or methoxyl-substituted triphenylamine
unit. Fig. 6 shows the current–voltage characteristics of DSSCs
fabricated with these sensitizers under standard global AM 1.5
solar light condition. The detailed parameters of short-circuit
current density (Jsc), open-circuit voltage (Voc), fill factor (ff),
and photovoltaic conversion efficiency (h) are summarized in
Table 2. Each photovoltaic parameter of the devices without
CDCA was obtained from several solar cells, which proved the
excellent repeatability. All of them showed favorable photovol-
taic responses and exhibited the relatively high power conversion
efficiencies. The DSSC based on LI-39 showed the best photo-
voltaic performance: Jsc of 14.40 mA cmꢀ2, Voc of 0.74 V, and ff
of 0.67, corresponding to the h value of 7.18%. With relatively
lower Jsc, solar cells based on LI-37 and LI-38 show relatively
inferior efficiencies of 5.23% and 6.41%. The three sensitizers
have similar chemical structures, but their photovoltaic perfor-
mances had some differences. The ability of electron-donation
could be modified by the methyl or methoxyl-substituted tri-
phenylamine unit. In sensitizers with the D–A0–p–A structure,
the transport of electron from donor to acceptor could be facil-
itated by the introduction of strong donor, and as a result, the
higher values of Jsc could be obtained.6 On the other hand,
the HOMO and LUMO energy levels were also affected by the
Electrochemical impedance spectroscopy
To gain more insight into the DSSCs properties, electrochemical
impedancespectroscopy(EIS) was performedtoanalyzethe effects
on charge recombination, transport and collection. Fig. 7A shows
the Nyquist plots for the DSSCs made with TiO2 electrodes
dipped with the three sensitizers in the dark under a forward bias
of ꢀ0.70 V with a frequency range of 0.1 Hz to 100 kHz. The
ꢀ
interfacial charge recombination reaction of electrons with the I3
ions can be described by a charge transfer resistance (Rct), which
corresponds to the charge transfer processes at the middle
frequency range). Thꢀe interfacial charge recombination reaction of
electrons with the I3 ions can be described by a charge transfer
resistance (Rct), which corresponds to the charge transfer processes
at the TiO2/dye/electrolyte interface (a larger semicircle occurs in
the middle frequency range). By fitting curves using Z-view soft-
ware, the values of Rct were calculated in the order of LI-39
(178.2 U) > LI-38 (102.1 U) > LI-37 (72.87 U). The higher Rct
indicated more effective suppression of the back reaction of the
injected electrons with I3ꢀ in the electrolyte and is reflected in the
improvements seen in the photocurrent or photovoltage, yielding
substantially enhanced device efficiency. Upon illumination under
Fig. 6 Current density–voltage characteristics obtained with a nano-
crystalline TiO2 film supported on FTO conducting glass and derivatized
with monolayer of sensitizers.
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Table 2 DSSCs performance data of sensitizersa
Sensitizer
CDCAb
Jsc (mA cmꢀ2
)
Voc (V)
ff
h (%)
sn (ms)c
LI-37d
None
1 mM
5 mM
None
1 mM
5 mM
None
1 mM
5 mM
11.79 ꢃ 0.15
11.50
9.26
0.680 ꢃ 0.010
0.69
0.68
0.650 ꢃ 0.005
0.67
0.66
5.30 ꢃ 0.07
5.38
4.22
9.8
LI-38d
LI-39d
13.56 ꢃ 0.34
0.725 ꢃ 0.005
0.658 ꢃ 0.005
6.51 ꢃ 0.11
13.6
17.3
13.20
11.25
0.73
0.71
0.66
0.65
6.41
5.27
14.34 ꢃ 0.06
0.741 ꢃ 0.003
0.676 ꢃ 0.003
7.20 ꢃ 0.02
13.83
12.08
0.75
0.74
0.67
0.67
7.01
6.01
a
Illumination: 100 mW cmꢀ2 simulated AM 1.5G solar light. b Expose to CDCA solution for 6 h before the sensitizer bath. c The lifetime (sn) of injected
electrons in the films can be drawn by the positions of the low-frequency peak in Fig. 7 through the expression sn ¼ 1/(2pf), in which f is the frequency of
the superimposed ac voltage. d Each result without CDCA was obtained from three DSSCs.
open-circuit conditions (100 mW cmꢀ2), the radius of the three
intermediate-frequency semicircles in the Nyquist plot (Fig. 7B)
were similar to each other, indicating that the differences of the
electron transport resistance of the three sensitizers were tiny.
However, from the characteristic frequency of the middle
frequency peak in the Bode phase plot (Fig. 7C), the electron life-
time of sensitizers LI-37 to LI-39 is 9.8, 13.6 and 17.3 ms respec-
tively. The relatively long lifetime indicated that four alkyl chains
or alkoxy chains in the spacer of the molecular, blocked the charge
recombination effectively. Longer lifetime resulted in better
performance in Jsc and Voc, the results of the electron lifetime are
consistent with those parameters in photovoltaic characteristics.
Therefore, the larger charge recombination resistance in the dark
andlongelectronlifetimemaybethemainreasonfor the higherVoc
value (0.67–0.75 V) of DSSCs based on these sensitizers.
Conclusion
In summary, we successfully synthesized a series of metal-free
organic sensitizers by exploiting 11,12-bis (hexyloxy) dibenzo
[a,c]phenazine unit as an auxiliary acceptor in the p-conjugated
bridge to form the D–p–A0–p–A system. This planar bulky fused
aromatic rings make the three sensitizers exhibit anti-aggregate
effect, higher open circuit voltage and good visible light spectrum
response. With the proper adjustment of the electron donating
abilities, the level of the bathochromic shift could be enhanced,
and the photovoltaic performance is consistent entirely. Among
them, LI-39 based cell shows the best light to electricity
conversion efficiency of 7.18% (Jsc ¼ 14.40 mA cmꢀ2, Voc ¼ 0.74
V, ff ¼ 0.67) without any co-adsorbent. It is believed that these
results are indicative for the development of highly efficient
metal-free organic sensitizers in the future.
Acknowledgements
We are grateful to the National Science Foundation of China
(no. 21002075), the National Natural Science Foundation of
China (no. 61106056) and the National Fundamental Key
Research Program (no. 2011CB932702) for financial support.
Notes and references
Fig. 7 Electrochemical impedance spectroscopy (EIS) for DSSCs based
on the sensitizers. (A) Nyquist plots in the dark, (B) Nyquist plots under
illumination, (C) Bode phase plots under illumination.
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