Efficient and stable organic DSSC sensitizers bearing quinacridone and furan moieties as a planar π-spacer

Article abstract of DOI:10.1039/c2jm31929b

In this work, three new quinacridone-based dyes containing a furan moiety (QA1-3) have been synthesized through simple synthetic routes for the application of dye-sensitized solar cells (DSSCs). Their absorption spectra, electrochemical, photovoltaic properties and the cell long-term stability have been extensively investigated. Electrochemical measurement data indicates that the tuning of the HOMO and LUMO energy levels can be conveniently realized by alternating the donor moiety. The theoretical calculations show that the dihedral angle between the quinacridone moiety and the furan ring is less than 1 degree, indicating excellent planarity between the two groups, which is beneficial for intramolecular charge transfer. All of these dyes performed as sensitizers for DSSCs tested under similar AM 1.5 experimental conditions, and a maximum solar energy to electricity conversion efficiency of 7.70% (J _sc = 13.25 mA cm^-2, V_oc = 804 mV, FF = 0.73) for the 20 mM chenodeoxycholic acid (CDCA) co-adsorbed DSSCs based on QA1 is obtained. Electrochemical impedance experiments indicate that the electron lifetime is improved by co-adsorption of CDCA, accounting for the significant improvement of V_oc. Most importantly, the long-term stability of the QA1-3-based DSSCs with ionic-liquid electrolytes under 1000 h light-soaking has been demonstrated. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm31929b

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PAPER
Efficient and stable organic DSSC sensitizers bearing quinacridone and furan
moieties as a planar p-spacer†
*
Jiabao Yang, Fuling Guo, Jianli Hua, Xin Li, Wenjun Wu, Yi Qu and He Tian
*
Received 28th March 2012, Accepted 24th May 2012
DOI: 10.1039/c2jm31929b
In this work, three new quinacridone-based dyes containing a furan moiety (QA1–3) have been
synthesized through simple synthetic routes for the application of dye-sensitized solar cells (DSSCs).
Their absorption spectra, electrochemical, photovoltaic properties and the cell long-term stability have
been extensively investigated. Electrochemical measurement data indicates that the tuning of the
HOMO and LUMO energy levels can be conveniently realized by alternating the donor moiety. The
theoretical calculations show that the dihedral angle between the quinacridone moiety and the furan
ring is less than 1 degree, indicating excellent planarity between the two groups, which is beneficial for
intramolecular charge transfer. All of these dyes performed as sensitizers for DSSCs tested under
similar AM 1.5 experimental conditions, and a maximum solar energy to electricity conversion
efficiency of 7.70% (Jsc ¼ 13.25 mA cmꢀ2, Voc ¼ 804 mV, FF ¼ 0.73) for the 20 mM chenodeoxycholic
acid (CDCA) co-adsorbed DSSCs based on QA1 is obtained. Electrochemical impedance experiments
indicate that the electron lifetime is improved by co-adsorption of CDCA, accounting for the
significant improvement of Voc. Most importantly, the long-term stability of the QA1–3-based DSSCs
with ionic-liquid electrolytes under 1000 h light-soaking has been demonstrated.
acridine-7,14-dione (quinacridone or QA) and furan units as the
Introduction
p-conjugated system and a cyanoacrylic acid moiety acting as
acceptor. QA and its derivatives are well-known organic
pigments with exceptional light, weather and heat stability,17 and
they are not soluble in most common organic solvents due to the
concurrent strong H-bonding and p–p intermolecular interac-
tions in solid state. Soluble derivatives can be made by attaching
long alkyl chains on the N atom positions of the QA moiety.18,19
Solar energy, a renewable energy source, is generally considered
as the most promising way to solve the global energy crisis. A
variety of other light-harvesting devices were developed rapidly,
including dye-sensitized solar cells (DSSCs),1 which attracted
great worldwide attention. As potential candidates for the future
modality of photovoltaic cells, DSSCs manifest considerable
efficiency and low cost, in which the sensitizer is deemed as the
crucial component. At present, DSSC sensitizers based on Ru(II)-
polypyridyl complexes have achieved power conversion effi-
ciencies of nearly 12%.2,3 However, the large-scale application of
Ru-complexes has become a critical problem due to limited
resources and the hefty purification steps needed.4 Recently, a
number of novel Ru-free sensitizers, such as phenoxazine,5,6
phenothiazine,7–9 diketo-pyrrolo-pyrrole10,11 and indoline,12–15
have been investigated and applied in DSSCs successfully. So far,
impressive photovoltaic performances have been obtained using
zinc porphyrin dye based DSSCs, which showed a power
conversion efficiency exceeding 12%.16
Here we present three novel efficient dyes (QA1–3) (Scheme 1)
with triarylamine as the donor, 5,12-dihydroquino[2,3-b]
Key Laboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science & Technology, Shanghai 200237,
P.R. China. E-mail: jlhua@ecust.edu.cn; tianhe@ecust.edu.cn; Fax: +86-
21-64252758; Tel: +86-21-64250940
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm31929b
Scheme 1 Molecular structures of QA1–QA3 dyes.
24356 | J. Mater. Chem., 2012, 22, 24356–24365
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To some extent, the introduction of long alkyl chains into QA
rings can also effectively inhibit the charge recombination and
increase the electron lifetime.20 At present, quinacridone-based
molecular donors have been employed successfully in solution
processed bulk-heterojunction organic solar cells and showed
good performances.21 The triphenylamine can improve the hole
transporting ability of the materials, and thanks to the nonplanar
structure of triphenylamine, the formation of dye aggregates can
also be relieved. Lately, triphenylamine-based dyes have been
widely used in organic photovoltaic functional materials, espe-
cially in the field of solar cells.22–25 In addition, furan would be
more efficient for hole location and reinforcing the stability of the
dye-sensitizers owing to its smaller resonance energy (16 kcal
molꢀ1) in the spacer than the thiophene (29 kcal molꢀ1) and
benzene (36 kcal molꢀ1).26,27 Hence, there have been some reports
on incorporating a furan moiety into organic dyes as an alter-
native to thiophenes for photovoltaics, revealing that the optical
and charge carrier mobility can be quite comparable to thio-
phenes.28–34 Most promisingly, furan derivatives can be obtained
from a variety of natural products, and therefore dye-sensitizers
containing a furan moiety can be considered as renewable and
sustainable synthetic resources and the production of DSSCs on
a large scale can be allowed.35 To the best of our knowledge,
there is only one report on the application of small molecular
dyes containing quinacridone in DSSCs.36 However, the quina-
cridone-based dyes only showed a PCE of 3.86%, for which the
main reason may be the lack of efficient donors and too large a
dihedral angle between the quinacridone moiety and thiophene
or the benzene ring (19.8 and 33.6 degrees, respectively), resulting
in less efficient intramolecular charge transfer (ICT).36 Mean-
while, for QA-based sensitizers with different p-spacers (furan,
thiophene, and benzene ring, Scheme S1†), we carried out density
functional theory (DFT) at the B3LYP/6-31G* level in the gas
phase. The theoretical calculations show that the dihedral angle
between the quinacridone moiety and the furan ring is only
0.2 degrees, much smaller than that between the quinacridone
moiety and thiophene or the benzene ring (16.4 and 32.3 degrees,
respectively), indicating that the QA molecule bearing a furan
moiety shows excellent planarity, which is beneficial for intra-
molecular charge transfer (ICT). Based on the consideration
above, a new series of quinacridone-based dyes containing a
furan moiety (QA1–3) were designed and synthesized (Scheme 1)
as sensitizers for DSSCs. These dyes can be easily synthesized by
a concise route and an environmentally-friendly procedure.
without further purification. Transparent FTO conducting glass
(fluorine doped SnO2, transmission >90% in the visible, sheet
resistance 15 U per square) was obtained from Geao Science and
Educational Co. Ltd. of China. Commercial TiO2 (P25) was used
for the preparation of the nanocrystalline films.
Spectroscopic measurements
1H NMR spectra and 13C NMR spectra were obtained with a
€
Brucker AM 400 spectrometer (relative to TMS). Mass spectra
were recorded with a Waters LCT Premier XE spectrometer.
Melting point measurements were performed with a SGW X-4
micro melting point meter. Elemental analysis was obtained with
an Elementar Vario EL III spectrometer. The absorption spectra
of the dyes in solution and adsorbed on TiO2 films were
measured with a Varian Cary 500 spectrophotometer.
Preparation of the solar cells
The dye-sensitized TiO2 electrode was prepared by following the
procedure reported in the literature.38 A screen-printed double
layer of TiO2 particles was used as the photoelectrode. A 7 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
soaked in 0.04 M aqueous TiCl4 solution overnight in a closed
chamber. After being washed with deionized water and fully
rinsed with ethanol, the films were heated again at 450 ꢁC for
30 min followed by cooling to 80 ꢁC and dipping into a 3 ꢂ
10ꢀ4 M solution of dyes in THF for 12 h at room temperature.
For the co-adsorption, chenodeoxycholic acid was added with
different concentrations. To prepare the counter electrode, Pt
catalyst was deposited on cleaned FTO glass by coating with a
drop of H2PtCl6 solution (0.02 M in 2-propanol solution) with
ꢁ
heat treatment at 400 C for 15 min. A hole (0.8 mm diameter)
was drilled in the counter electrode by a drill-press. For the
assembly of DSSCs, the dye-contained 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 redox electrolyte was placed
in a drilled hole in the counter electrode by capillary force, and
was driven into the cell by means of vacuum backfilling. Two
electrolytes were used for device evaluation, in which one was
composed of 0.1 M lithium iodide, 0.6 M 1,2-dimethyl-3-pro-
pylimidazolium iodide (DMPII), 0.05 M I2, and 0.5 M 4-tert-
butylpyridine (4-TBP) in acetonitrile (AN) as the liquid electro-
lyte, and the other was composed of 0.1 M I2, 0.1 M lithium
iodide and 0.45 M benzimidazole (BI) in 1-propyl-3-methyl
imidazolium iodide (PMII) as the ionic liquid electrolyte. Finally,
the hole was sealed using a UV-melt gum and a cover glass
(0.1 mm thickness).
Experimental
Materials
Tetra-n-butylammonium hexafluorophosphate (TBAPF6), 4-
tert-butylpyridine (4-TBP), lithium iodide, and 2-cyanoacetic
ꢀ
acid were purchased from Fluka. THF was pre-dried over 4 A
molecular sieves and distilled under an argon atmosphere from
sodium benzophenone ketyl immediately prior to use. The
starting materials 4-(N,N-diphenylamino) phenyl-boronic acid,
Photovoltaic performance measurements
N,N-diphenyl-4-vinylaniline
and
4-(bis(4-methoxyphenyl)-
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).
amino)phenyl-boronic acid were synthesized according to the
corresponding literature methods.8,37 All other solvents and
chemicals were purchased from Aldrich and used as received
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 24356–24365 | 24357

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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 the
photocurrent were 10 mV and 40 ms, respectively. The photo-
current action spectra were measured with an 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 spectrometer.
Hz, 2H), 8.48 (s, 2H). 13C NMR (CDCl3, 100 MHz), d (TMS,
ppm): 14.1, 22.7, 27.0, 29.3, 29.4, 31.8, 46.5, 113.9, 114.1, 116.9,
122.1, 126.0, 130.2, 135.5, 137.2, 140.9, 176.6. HRMS (ESI, m/z):
[M + H]+ calcd for (C36H43N2O2Br2): 693.1691; found: 693.1693.
2-Bromo-9-(4-(diphenylamino)phenyl)-5,12-dioctylquinolino
[2,3-b]acridine-7,14-dione (3). Compound 2 (1.39 g, 2 mmol), 12.5
mL of 2.0 M K2CO3 aqueous solution and a catalytic amount of
Pd(PPh3)4 (116 mg, 5 mol%) in 25 mL THF were heated to reflux
for 0.5 h and then injected with 12.5 mL THF containing 4-(N,N-
diphenylamino)phenyl-boronic acid (578 mg, 2 mmol). After
refluxing for 12 h, the mixture was extracted with 100 mL of
CH2Cl2. The organic portion was combined and removed by
rotary evaporation. The residue was purified by column chro-
matography on silica using petroleum ether and methylene
chloride (1 : 2, v/v) as eluent to yield 700 mg of a red powder
(yield 40.7%). Melting point: 207 ꢁC. 1H NMR (CDCl3,
400 MHz), d (TMS, ppm): 0.87–0.93 (m, 6H), 1.28–1.42 (m,
12H), 1.42–1.52 (m, 4H), 1.57–1.66 (m, 4H), 1.90–2.01 (m, 4H),
4.42–4.51 (m, 4H), 7.03–7.09 (m, 2H), 7.15–7.20 (m, 6H), 7.25–
7.32 (m, 5H), 7.48 (d, J ¼ 9.1 Hz, 1H), 7.58 (d, J ¼ 8.6 Hz, 2H),
7.68 (dd, J1 ¼ 9.2 Hz, J2 ¼ 2.5 Hz, 1H), 7.91 (dd, J1 ¼ 9.0 Hz,
J2 ¼ 2.3 Hz, 1H), 8.54 (d, J ¼ 2.5 Hz, 1H), 8.60 (s, 1H), 8.64 (s,
1H), 8.67 (d, J ¼ 2.3 Hz, 1H). 13C NMR (CDCl3, 100 MHz), d
(TMS, ppm): 14.8, 23.3, 27.8, 30.1, 32.5, 47.3, 47.4, 114.3, 114.5,
116.0, 117.3, 122.0, 123.8, 124.6, 125.3, 125.5, 126.8, 127.0, 128.1,
130.0, 131.0, 133.8, 133.9, 136.1, 137.9, 141.4, 141.7, 147.9, 148.3,
177.9. HRMS (ESI, m/z): [M + H]+ calcd for (C54H57N3O2Br):
858.3634; found: 858.3632.
Electrochemical measurements
The oxidation potentials of the dyes adsorbed on TiO2 films were
measured in a normal three-electrode electrochemical cell. A
TiO2 film stained with sensitizer was used as the working elec-
trode, a platinum wire was the counter electrode, and a regular
calomel electrode in saturated KCl solution was the reference
electrode. The measurements were performed using a potentio-
stat/galvanostat model K0264 (Princeton Applied Research).
The supporting electrolyte was 0.1 M TBAPF6 (tetra-n-buty-
lammonium hexafluorophosphate) with THF as the solvent. The
electrochemical impedance spectroscopy (EIS) measurements of
all the DSSCs were performed using a Zahner IM6e Impedance
Analyzer (ZAHNER-Elektrik GmbH & CoKG, Kronach,
Germany). The frequency range is 0.05 Hz–100 kHz. The applied
voltage bias is ꢀ0.70 V. The magnitude of the alternating signal
is 10 mV.
Synthesis
5,12-Dioctylquinolino[2,3-b]acridine-7,14-dione (1). 5,12-Dihy-
droquino[2,3-b] acridine-7,14-dione (4.68 g, 15 mmol) and tet-
rabutylammonium bromide (966 mg, 3 mmol) were dissolved in
100 mL toluene, and while stirring vigorously, 50% NaOH
(20 mL) and n-octyl bromide (11.58 g, 60 mmol) were added
slowly. The resulting mixture was heated to reflux for 24 h. The
reaction was quenched with water (30 mL) and filtered. The
organic layer from the filtrate was separated, and the solvent was
removed under reduced pressure. The resulting crude product
was purified by column chromatography, eluting with petroleum
ether and methylene chloride (1 : 2, v/v) yielding the product as
red solids (2.00 g, 24.9%). 1H NMR (CDCl3, 400 MHz), d (TMS,
ppm): 0.88–0.93 (m, 6H), 1.29–1.36 (m, 12H), 1.42–1.51 (m,
4H), 1.58–1.67 (m, 4H), 1.95–2.04 (m, 4H), 4.50 (t, J ¼ 7.8 Hz,
4H), 7.22–7.30 (m, 2H), 7.51 (d, J ¼ 8.8 Hz, 2H), 7.70–7.79 (m,
2H), 8.56 (dd, J1 ¼ 8.0 Hz, J2 ¼ 1.5 Hz, 2H), 8.76 (s, 2H).
(E)-2-Bromo-9-(4-(diphenylamino)styryl)-5,12-dioctylquinolino
[2,3-b]acridine-7,14-dione (4). Compound 2 (1.39 g, 2 mmol),
N,N-diphenyl-4-vinylaniline (542 mg, 2 mmol), Pd(OAc)2 (6.5
mg, 0.03 mmol), tris(2-methylphenyl)phosphine (20 mg, 0.066
mmol) and triethylamine (0.5 mL) in 20 mL DMF under a
nitrogen atmosphere were heated to react at 85 ꢁC for 22 h. After
cooling to room temperature, the mixture was poured into water
and the precipitate was filtered. The residue was purified by
column chromatography on silica using CH2Cl2/petroleum ether
(2 : 1, v/v) to yield 180 mg of a red solid (yield: 10.2%). Melting
point: 208 ꢁC. 1H NMR (CDCl3, 400 MHz), d (TMS, ppm): 0.87–
0.93 (m, 6H), 1.28–1.41 (m, 12H), 1.41–1.51 (m, 4H), 1.57–1.66
(m, 4H), 1.89–1.99 (m, 4H), 4.39–4.48 (m, 4H), 7.02–7.11 (m,
6H), 7.12–7.16 (m, 4H), 7.25–7.31 (m, 5H), 7.36–7.44 (m, 3H),
7.70 (dd, J1 ¼ 9.2 Hz, J2 ¼ 2.5 Hz, 1H), 7.81 (dd, J1 ¼ 9.1 Hz,
J2 ¼ 2.1 Hz, 1H), 8.40 (d, J ¼ 2.0 Hz, 1H), 8.50 (d, J ¼ 2.5 Hz,
1H), 8.54 (s, 1H), 8.58 (s, 1H). 13C NMR (CDCl3, 100 MHz), d
(TMS, ppm): 14.1, 22.7, 25.9, 27.0, 29.3, 29.5, 31.8, 45.9, 115.1,
116.2, 123.0, 123.1, 123.2, 123.5, 124.6, 125.1, 125.7, 127.5, 128.0,
129.3, 130.3, 130.6, 131.2, 132.8, 135.1, 140.9, 141.1, 147.4, 147.5,
174.6. HRMS (ESI, m/z): [M + H]+ calcd for (C56H59N3O2Br):
884.3791; found: 884.3787.
2,9-Dibromo-5,12-dioctylquinolino[2,3-b]acridine-7,14-dione
(2). 5,12-Dioctyl-quinolino[2,3-b]acridine-7,14-dione 1 (1.07 g,
2 mmol) and NBS (1.07 g, 6 mmol) in 50 mL CCl4 were heated to
reflux for 10 h. The mixture was cooled to room temperature and
organic solution was removed by rotary evaporation. The
residue was purified by column chromatography with silica gel
using petroleum ether and methylene chloride (1 : 2, v/v) as
eluent to yield 800 mg of a red powder (yield 57.6%). Melting
point: 301 ꢁC. 1H NMR (CDCl3, 400 MHz), d (TMS, ppm): 0.87–
0.94 (m, 6H), 1.30–1.39 (m, 12H), 1.41–1.50 (m, 4H), 1.53–1.65
(m, 4H), 1.83–1.97 (m, 4H), 4.34–4.42 (m, 4H), 7.27 (d, J ¼ 9.2
Hz, 2H), 7.71 (dd, J1 ¼ 9.3 Hz, J2 ¼ 2.5 Hz, 2H), 8.45 (d, J ¼ 2.5
2-(4-(Bis(4-methoxyphenyl)amino)phenyl)-9-bromo-5,12-dio-
ctylquinolino[2,3-b]acridine-7,14-dione (5). The synthetic proce-
dure for compound 5 was followed using 2 (695 mg, 1 mmol),
2.0 M K2CO3 aqueous solution (8 mL) and a catalytic amount of
Pd(PPh3)4 (58 mg, 5 mol%) in 16 mL of THF which were heated
to reflux for 0.5 h and then injected with 8 mL THF containing
24358 | J. Mater. Chem., 2012, 22, 24356–24365
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4-(bis(4-methoxyphenyl)-amino)phenyl-boronic acid (349 mg, 1
mmol). The pure product was separated by a silica gel column
chromatography using CH2Cl2/petroleum ether (2 : 1, v/v) to
yield 320 mg of a red solid (yield 34.8%). Melting point: 204 ꢁC.
1H NMR (CDCl3, 400 MHz), d (TMS, ppm): 0.87–0.92 (m, 6H),
1.28–1.40 (m, 12H), 1.44–1.50 (m, 4H), 1.58–1.65 (m, 4H), 1.89–
2.00 (m, 4H), 3.82 (s, 6H), 4.40–4.49 (m, 4H), 6.85–6.89 (m, 4H),
7.03 (d, J ¼ 8.7 Hz, 2H), 7.11–7.14 (m, 4H), 7.28 (d, J ¼ 9.3 Hz,
1H), 7.44 (d, J ¼ 9.2 Hz, 1H), 7.52 (d, J ¼ 8.7 Hz, 2H), 7.66 (dd,
J1 ¼ 9.2 Hz, J2 ¼ 2.5 Hz, 1H), 7.89 (dd, J1 ¼ 9.0 Hz, J2 ¼ 2.3 Hz,
1H), 8.53 (d, J ¼ 2.5 Hz, 1H), 8.57 (s, 1H), 8.63 (s, 1H), 8.64
(d, J ¼ 2.4 Hz, 1H). 13C NMR (CDCl3, 100 MHz), d (TMS,
ppm): 14.1, 22.7, 27.0, 27.2, 29.3, 29.4, 29.6, 31.8, 55.5, 113.6,
114.8, 116.9, 120.6, 121.2, 124.3, 126.7, 127.2, 131.1, 133.0, 133.5,
133.8, 135.1, 135.5, 137.3, 140.8, 148.0, 156.0, 170.2. HRMS
(ESI, m/z): [M + H]+ calcd for (C56H61N3O4Br): 918.3845; found:
918.3850.
26.1, 27.2, 29.2, 30.6, 45.9, 106.6, 112.5, 113.2, 114.0, 114.2,
117.8, 118.5, 121.8, 124.0, 124.2, 124.5, 125.8, 126.2, 126.5, 126.8,
127.3, 130.4, 131.4, 133.1, 136.0, 138.1, 141.2, 145.6, 150.9, 156.5,
174.3, 175.2. HRMS (ESI, m/z): [M
+
H]+ calcd for
(C61H62N3O4): 900.4740; found: 900.4737.
5-(9-(4-(Bis(4-methoxyphenyl)amino)phenyl)-5,12-dioctyl-7,
14-dioxo-5,7,12,14-tetrahydroquinolino[2,3-b]acridin-2-yl)furan-
2-carbaldehyde (8). The synthesis method resembles that of
compound 6 and the compound was purified by column chro-
matography on silica (CH2Cl2 as eluent) to yield 148 mg of a
1
dark red solid (yield 86.8%). Melting point: 212 ꢁC. H NMR
(CDCl3, 400 MHz), d (TMS, ppm): 0.87–0.94 (m, 6H), 1.30–1.43
(m, 12H), 1.43–1.53 (m, 4H), 1.60–1.69 (m, 4H), 1.95–2.05 (m,
4H), 3.82 (s, 6H), 4.47–4.56 (m, 4H), 6.84–6.91 (m, 5H), 6.99–
7.05 (m, 2H), 7.06–7.15 (m, 4H), 7.33–7.37 (m, 1H), 7.48–7.55
(m, 4H), 7.89 (d, J ¼ 8.0 Hz, 1H), 8.11 (d, J ¼ 8.8 Hz, 1H), 8.64
(s, 1H), 8.67 (s, 1H), 8.71 (s, 1H), 8.80 (s, 1H), 9.64 (s, 1H). 13C
NMR (CDCl3, 100 MHz), d (TMS, ppm): 14.0, 22.7, 26.1, 30.4,
31.6, 32.4, 46.6, 48.8, 105.1, 110.5, 112.9, 113.4, 116.8, 117.5,
117.7, 120.2, 123.4, 123.9, 125.5, 125.7, 125.9, 126.2, 126.9, 129.0,
129.3, 131.8, 132.0, 133.2, 134.6, 140.4, 144.2, 146.9, 148.5, 156.9,
5-(9-(4-(Diphenylamino)phenyl)-5,12-dioctyl-7,14-dioxo-5,7, 12,
14-tetrahydroquinolino[2,3-b]acridin-2-yl)furan-2-carbaldehyde (6).
Compound 3 (257 mg, 0.3 mmol), 2.0 M K2CO3 aqueous solu-
tion (12.5 mL) and a catalytic amount of Pd(PPh3)4 (17 mg, 5 mol
%) in 25 mL THF were heated to reflux for 0.5 h and then
injected with 12.5 mL THF containing 5-formyl-2-furanylbor-
onic acid (84 mg, 0.6 mmol), and the mixture was reacted at
reflux for 12 h. After cooling to room temperature, the mixture
was extracted with 100 mL of CH2Cl2. The organic portion was
combined and removed by rotary evaporation. The residue was
purified by column chromatography on silica (methylene chlo-
ride as eluent) to yield 250 mg of a red solid (yield 95.4%).
175.3, 175.7. HRMS (ESI, m/z): [M
+
H]+ calcd for
(C61H64N3O6): 934.4795; found: 934.4789.
2-Cyano-3-(5-(9-(4-(diphenylamino)phenyl)-5,12-dioctyl-7,14-
dioxo-5,7,12,14-tetrahydroquinolino[2,3-b]acridin-2-yl)furan-2-yl)
acrylic acid (QA1). Compound 6 (140 mg, 0.16 mmol), 2-cya-
noacetic acid (85 mg, 1 mmol) and ammonium acetate (3 mg,
0.039 mmol) in 20 mL acetic acid were heated to reflux under a
nitrogen atmosphere for 12 h. After cooling to room tempera-
ture, the precipitate was filtered and recrystallized with THF/
EtOH to yield 130 mg of a red solid (yield 86.3%). Melting point:
306 ꢁC. 1H NMR (THF-d8, 400 MHz), d (TMS, ppm): 0.80–0.85
(m, 6H), 1.24–1.38 (m, 12H), 1.40–1.50 (m, 4H), 1.51–1.61 (m,
4H), 1.85–1.95 (m, 4H), 4.40–4.50 (m, 4H), 6.88–6.94 (m, 2H),
6.99–7.05 (m, 7H), 7.13–7.18 (m, 4H), 7.24–7.27 (m, 1H), 7.42–
7.46 (m, 2H), 7.50 (d, J ¼ 9.2 Hz, 1H), 7.60 (d, J ¼ 9.2 Hz, 1H),
7.72 (dd, J1 ¼ 9.0 Hz, J2 ¼ 2.1 Hz, 1H), 7.76 (s, 1H), 8.01 (dd, J1
¼ 9.0 Hz, J2 ¼ 1.9 Hz, 1H), 8.36 (s, 1H), 8.40–8.43 (m, 1H), 8.45–
8.48 (m, 2H). 13C NMR (THF-d8, 100 MHz), d (TMS, ppm):
13.5, 22.6, 26.8, 29.4, 31.8, 46.1, 54.0, 61.4, 77.3, 86.8, 91.6, 93.0,
93.9, 103.0, 104.8, 108.7, 113.5, 114.2, 114.6, 120.8, 122.8, 123.9,
124.2, 124.4, 124.7, 125.8, 127.2, 128.3, 132.3, 133.7, 138.5, 142.0,
142.5, 146.1, 147.2, 147.8, 149.6, 158.8, 163.3, 181.2, 182.6, 183.2.
HRMS (ESI, m/z): [M + H]+ calcd for (C62H61N4O5): 941.4642;
found: 941.4644. Anal. calcd for (C62H60N4O5): C 79.12, H 6.43,
N 5.95; found: C 79.01, H 6.34, N 5.88%.
ꢁ
1
Melting point: 213 C. H NMR (CDCl3, 400 MHz), d (TMS,
ppm): 0.90 (t, J ¼ 6.7 Hz, 6H), 1.30–1.44 (m, 12H), 1.45–1.55 (m,
4H), 1.57–1.70 (m, 4H), 1.95–2.06 (m, 4H), 4.48–4.56 (m, 4H),
6.88 (d, J ¼ 3.7 Hz, 1H), 7.06 (t, J ¼ 7.3 Hz, 2H), 7.14–7.19 (m,
6H), 7.27–7.32 (m, 4H), 7.34 (d, J ¼ 3.7 Hz, 1H), 7.50 (t, J ¼ 7.9
Hz, 2H), 7.54–7.57 (m, 2H), 7.88 (dd, J1 ¼ 9.1 Hz, J2 ¼ 2.3 Hz,
1H), 8.08 (dd, J1 ¼ 9.0 Hz, J2 ¼ 2.0 Hz, 1H), 8.62 (s, 1H), 8.67 (d,
J ¼ 2.2 Hz, 1H), 8.68 (s, 1H), 8.77 (d, J ¼ 2.1 Hz, 1H), 9.64 (s,
1H). 13C NMR (CDCl3, 100 MHz), d (TMS, ppm): 14.1, 22.7,
27.1, 29.4, 31.8, 46.9, 107.6, 113.5, 115.2, 115.4, 118.2, 118.3,
121.2, 123.0, 123.8, 124.5, 124.6, 124.7, 126.0, 126.2, 127.3, 129.4,
130.8, 132.9, 135.0, 135.6, 142.3, 147.6, 151.9, 158.5, 177.3, 177.9.
HRMS (ESI, m/z): [M + H]+ calcd for (C59H60N3O4): 874.4584;
found: 874.4579.
(E)-5-(9-(4-(Diphenylamino)styryl)-5,12-dioctyl-7,14-dioxo-5,
7,12,14-tetrahydroquinolino[2,3-b]acridin-2-yl)furan-2-carbalde-
hyde (7). The synthesis method resembles that of compound 6
and the compound was purified by column chromatography on
silica (CH2Cl2 as eluent) to yield 120 mg of a dark red solid (yield
2-Cyano-3-(5-(9-(4-(diphenylamino)styryl)-5,12-dioctyl-7,14-
dioxo-5,7,12,14-tetrahydroquinolino[2,3-b]acridin-2-yl)furan-2-yl)
acrylic acid (QA2). The synthesis method resembles that of
compound QA1 and the compound was recrystallized with THF/
EtOH to yield 99 mg of a dark red solid (yield: 83.8%). Melting
ꢁ
1
86.9%). Melting point: 216 C. H NMR (CDCl3, 400 MHz), d
(TMS, ppm): 0.89–0.95 (m, 6H), 1.30–1.45 (m, 12H), 1.45–1.55
(m, 4H), 1.61–1.70 (m, 4H), 1.91–2.04 (m, 4H), 4.40–4.50 (m,
4H), 6.79–6.81 (m, 1H), 6.94 (d, J ¼ 7.2 Hz, 2H), 7.02–7.08
(m, 4H), 7.14 (d, J ¼ 7.5 Hz, 4H), 7.25–7.32 (m, 5H), 7.32–7.38
(m, 3H), 7.41–7.45 (m, 1H), 7.70 (dd, J1 ¼ 9.1 Hz, J2 ¼ 2.0 Hz,
1H), 7.96 (dd, J1 ¼ 9.0 Hz, J2 ¼ 2.1 Hz, 1H), 8.25 (d, J ¼ 2.0 Hz,
1H), 8.42 (s, 1H), 8.49 (s, 1H), 8.56 (d, J ¼ 2.2 Hz, 1H), 9.59
(s, 1H). 13C NMR (CDCl3, 100 MHz), d (TMS, ppm): 14.0, 22.5,
1
point: 310 ꢁC. H NMR (THF-d8, 400 MHz), d (TMS, ppm):
0.80–0.86 (m, 6H), 1.23–1.37 (m, 12H), 1.39–1.50 (m, 4H), 1.51–
1.59 (m, 4H), 1.86–1.98 (m, 4H), 4.43–4.52 (m, 4H), 6.88–6.94
(m, 4H), 6.96–7.04 (m, 6H), 7.06–7.09 (m, 1H), 7.12–7.17 (m,
4H), 7.28–7.31 (m, 1H), 7.34–7.39 (m, 2H), 7.49 (d, J ¼ 9.0 Hz,
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1H), 7.66 (d, J ¼ 9.1 Hz, 1H), 7.78 (d, J ¼ 9.1 Hz, 1H), 7.83 (s,
1H), 8.12 (d, J ¼ 9.0 Hz, 1H), 8.27 (s, 1H), 8.46 (s, 1H), 8.53
(s, 1H), 8.59 (s, 1H). 13C NMR (THF-d8, 100 MHz), d (TMS,
ppm): 11.6, 20.7, 27.6, 30.0, 54.1, 76.1, 76.4, 76.7, 79.9, 81.7, 82.1,
88.4, 100.5, 101.0, 103.5, 104.5, 105.8, 106.7, 110.5, 111.4, 112.2,
113.2, 117.8, 119.9, 121.7, 122.5, 125.4, 126.8, 127.2, 127.5, 128.2,
128.7, 129.5, 130.2, 132.1, 135.8, 140.5, 145.9, 147.8, 150.9, 153.2,
156.9, 161.4, 161.9, 182.8, 186.7, 187.6. HRMS (ESI, m/z): [M +
H]+ calcd for (C64H63N4O5): 967.4798; found: 967.4805. Anal.
calcd for (C64H62N4O5): C 79.48, H 6.46, N 5.79; found: C 79.30,
H 6.32, N 5.61%.
3-(5-(9-(4-(Bis(4-methoxyphenyl)amino)phenyl)-5,12-dioctyl-
7,14-dioxo-5,7,12,14-tetrahydroquinolino[2,3-b]acridin-2-yl)furan-
2-yl)-2-cyanoacrylic acid (QA3). The synthesis method resembles
that of compound QA1 and the compound was recrystallized
with THF/EtOH to yield 115 mg of a dark red solid (yield:
80.7%). Melting point: 299 ꢁC. 1H NMR (THF-d8, 400 MHz), d
(TMS, ppm): 0.89–0.94 (m, 6H), 1.34–1.48 (m, 12H), 1.49–1.59
(m, 4H), 1.75–1.82 (m, 4H), 1.97–2.06 (m, 4H), 3.77 (s, 6H), 4.56–
4.65 (m, 4H), 6.86 (d, J ¼ 8.8 Hz, 4H), 6.98 (d, J ¼ 8.6 Hz, 2H),
7.07 (d, J ¼ 8.8 Hz, 4H), 7.20 (d, J ¼ 3.7 Hz, 1H), 7.41 (d, J ¼ 3.8
Hz, 1H), 7.51 (d, J ¼ 8.6 Hz, 2H), 7.67 (d, J ¼ 9.1 Hz, 1H), 7.81
(d, J ¼ 9.3 Hz, 1H), 7.92 (d, J ¼ 8.8 Hz, 1H), 7.94 (s, 1H), 8.27 (d,
J ¼ 8.7 Hz, 1H) 8.59 (d, J ¼ 2.1 Hz, 1H), 8.63 (s, 1H), 8.71 (s,
1H), 8.77 (d, J ¼ 2.0 Hz, 1H). 13C NMR (THF-d8, 100 MHz), d
(TMS, ppm): 13.5, 22.6, 29.5, 29.4, 31.9, 54.8, 68.9, 72.4, 74.3,
78.7, 97.8, 100.1, 111.8, 114.4, 115.6, 120.6, 121.5, 126.5, 126.9,
130.5, 131.6, 135.7, 137.1, 137.3, 138.5, 138.7, 143.3, 151.1, 151.3,
156.3, 157.4, 158.8, 163.9, 172.6, 174.7, 178.8. HRMS (ESI, m/z):
[M + H]+ calcd for (C64H65N4O7): 1001.4853; found: 1001.4856.
Anal. calcd for (C64H64N4O7): C 76.78, H 6.44, N 5.60; found: C
76.58, H 6.36, N 5.46%.
Results and discussion
Scheme 2 The synthetic procedure of the dyes QA1–3.
Synthesis
The synthetic route to the three dyes QA1–3 containing quina-
cridone moiety is depicted in Scheme 2. The octyl attached to
the quinacridone group can improve the solubility, forming a
tightly packed insulating monolayer blocking the I3ꢀ or cations
from approaching the TiO2 and enhancing the open circuit
voltage. The synthesis of QA1–3 was started from a commer-
cially available quinacridone precursor. The N-alkylation of
quinacridone gave 1, in which the bromination of 1 with N-
bromosuccinimide (NBS) provided compound 2 in good yield.
Asymmetrical Suzuki coupling reaction of compound 2 with
4-(diphenylamino)phenyl-boronic acid and 4-(bis(4-methox-
yphenyl)-amino)phenyl-boronic acid gave compounds 3 and 5,
respectively. Heck reaction of 2 with N,N-diphenyl-4-vinylani-
line provided compound 4. In the next step, Suzuki coupling of
3–5 with 5-formyl-2-furanylboronic acid afforded mono-
aldehyde substituted precursors 6–8, respectively. Finally, the
target products (QA1–3) were obtained via Knoevenagel
condensation reaction of the respective aldehydes with 2-cya-
noacetic acid in the presence of ammonium acetate. All the
intermediates and target dyes were confirmed by standard
spectroscopic methods.
Spectroscopic properties of QA1–3 in solution and adsorbed on
TiO2 films
The UV-Vis absorption spectra of the three dyes QA1–3 in THF
solutions and on TiO2 films are shown in Fig. 1, and their
absorption data are listed in Table 1. In the UV-Vis spectra in
Fig. 1a, these dyes exhibit three major prominent bands,
appearing at 300–390 nm, 400–500 nm, and 520–570 nm,
respectively. The former shows typical triarylamine absorption
Fig. 1 UV-Vis absorption spectra of QA1–3 in THF solutions (a) and
on TiO2 films (7 mm) (b).
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Table 1 Photophysical and electrochemical properties of dyes
HOMOc/Vꢀ1 (vs. NHE)
E0–0 /Vꢀ1
LUMOe/Vꢀ1 (vs. NHE)
a
b
d
Dye
lmax /nmꢀ1 (3 ꢂ 10ꢀ4 M cm)
lmax /nmꢀ1
QA1
QA2
QA3
534 (1.38), 446 (5.12), 350 (5.88)
540 (0.93), 447 (4.34), 388 (4.92)
539 (0.80), 447 (3.28), 352 (3.69)
416
447
432
1.06
0.99
0.83
2.03
1.99
2.01
ꢀ0.97
ꢀ1.00
ꢀ1.18
a
b
c
Absorption maximum in THF solution at room temperature. Absorption maximum on TiO2 film (without CDCA). HOMOs were measured in
THF with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte (working electrode: FTO/TiO2/dye; reference electrode: SCE;
calibrated with ferrocene/ferrocenium (Fc/Fc+) as an external reference and converted to NHE by addition of 0.07 V. Counter electrode: Pt).
d
e
E0–0 was estimated from the absorption thresholds from the absorption spectra of dyes adsorbed on the TiO2 film. LUMO is estimated by
subtracting E0–0 from HOMO.
peaks at 350/388/352 nm in THF,19 respectively, in which the
peak of QA2 at 388 nm shows about a 40 nm red-shift because of
the conjugation between triphenylamine and vinyl. The middle
band of QA1–3 is ascribed to a localized aromatic p–p* transi-
tion of quinacridone with furan and the terminal cyanoacrylic
acid group, in which QA1–3 show an absorption peak at around
446 nm in THF. It is demonstrated by the measurement of the
UV-Vis absorption of the intermediates 1,3,5-(9-bromo-5,12-
dioctyl-7,14-dioxo-5,7,12,14-tetrahydroquinolino[2,3-b]acridin-
2-yl)furan-2-carbaldehyde (a), 3-(5-(9-bromodioctyl-7,14-dioxo-
5,7,12,14-tetrahydroquinolino-[2,3,5,12-b]acridin-2-yl)furan-2-
yl)-2-cyanoacrylic acid (b) and QA1 in THF solutions (Fig. S1†).
Meanwhile, time-dependent density functional theory (TDDFT)
calculations were carried out with the solvent effects of THF
taken into account by the polarizable continuum model (PCM).
The PBE0 functional was used together with the 6-31G* basis set
(Table S1 and S2†). The largest oscillator strengths (f) around
450 nm correspond to excitation from HOMOꢀ1 to LUMO. The
molecular orbital image shows that the transition is consistent
with our experimental attribution.
Generally, due to the strong attractive force between the mole-
cules, they have a strong tendency to aggregate in solution or at
the solid–liquid interface when the sensitizers are adsorbed onto
a nanocrystalline TiO2 surface. Dye aggregates usually have
three forms: red-shifted J-aggregates, blue-shifted H-aggregates
and both red- and blue-shifted herring-bone aggregates.40 The
experimental results indicate that QA1–3 used in this work form
H-aggregates on the TiO2 nanocrystal surface. On the other
hand, after the sensitizer molecule adsorbs on TiO2 film, the
decreased electron-withdrawing ability of the acceptor (cyn-
aoacrylic acid group) can also cause the blue-shifted absorption
spectra. Therefore, the blue-shifted absorption spectra of the
dyes on TiO2 are attributed to the H-aggregates and deproto-
nation of the carboxylic acid. In addition, it is noteworthy that
the absorption spectra of those dyes anchored onto transparent
TiO2 show an obviously broad profile, which is beneficial for
light-harvesting.
Electrochemical properties of QA1–3
The bands at around 520–570 nm can be attributed to the
intramolecular charge transfer (ICT) between the triarylamine
donor and the acceptor. For the charge-transfer band, the
absorption maxima for QA1–3 in THF are at 534 nm, 540 nm
and 539 nm, respectively. Compared with QA1 and QA3, QA2
has a vinyl group between triphenylamine and the QA moiety,
which enhanced the extent of electron delocalization over the
whole molecule, so its maximum absorption peak was red shif-
ted. At the same time the absorption band of QA3 shows a slight
red-shift compared with QA1 due to the effect of the two
methoxy groups attached to the triphenylamine on the electron
donor side. It is worth noting that the corresponding maximum
extinction coefficients of the three dyes at around 446 nm are
higher than that of the band at around 540 nm, in which they are
5.12 ꢂ 104, 4.34 ꢂ 104 and 3.28 ꢂ 104 Mꢀ1 cmꢀ1, respectively. In
comparison with conventional ruthenium complexes (for
example, 1.39 ꢂ 104 Mꢀ1 cmꢀ1 at 541 nm for N3),39 the present
dye molecules show about three times higher absorption coeffi-
cients. A corresponding thinner nanocrystalline film is allowed
owing to the greater maximum absorption coefficients of the
organic dyes, which could be beneficial for DSSCs with an ionic
liquid electrolyte for better conversion efficiencies.4
To evaluate the possibility of electron transfer from the excited
dye to the conduction band (CB) of TiO2, the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) energy levels are usually employed by means of
cyclic voltammetry (CV) measurements of these dyes. The CV
measurements of QA1–3 were performed in tetrahydrofuran
solution,
using
0.1
M
tetrabutylammonium
hexa-
fluorophosphate as the supporting electrolyte, Pt as the counter
electrode and a saturated calomel electrode (SCE) as the refer-
ence electrode. Two sets of measurement were carried out for
comparison, one employing TiO2 films stained with sensitizers as
the working electrode and the other with the sensitizer dissolved
in THF solution. 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 0.56 V versus the SCE reference electrode. The
potentials versus NHE were calibrated by the addition of 0.63 V
to the potentials versus Fc/Fc+.7 Therefore, the potentials
measured vs. SCE were converted to normal hydrogen electrode
(NHE) by the addition of 0.07 V. The voltammetric results are
presented in Fig. S2†, and the data on the films are collected in
Table 1. The first half-wave potentials of QA1–3 on the films are
0.99 V, 0.92 V and 0.76 V (vs. SCE). Therefore, the HOMO levels
of QA1–3 corresponding to their first redox potential are 1.06 V,
0.99 V and 0.83 V vs. NHE, respectively, which are much more
positive than the iodine/iodide redox potential value (0.4 V vs.
Fig. 1b shows the absorption spectra of QA1–3 on 7 mm
transparent TiO2 films after 10 h adsorption. Compared to the
spectrum in THF solution, the absorption peaks for QA1–3 on
TiO2 films are blue-shifted by 30, 3 and 15 nm, respectively.
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NHE), ensuring that there is enough driving force for the effi-
cient dye regeneration through the recapture of the injected
electrons from Iꢀ by the dye cationic radical. The excitation
transition energy (E0–0) of QA1–3 is estimated from their
absorption thresholds of dye-sensitized TiO2 films to be 2.03 V,
1.99 V and 2.01 V, respectively. The estimated excited state
potentials corresponding to the LUMO levels of QA1–3, calcu-
lated from EHOMO ꢀ E0–0, are ꢀ0.97 V, ꢀ1.00 V and ꢀ1.18 V vs.
NHE, respectively. Thanks to the more negative LUMO values
of the three dyes compared to the bottom of the conduction band
of TiO2 (ꢀ0.5 V vs. NHE), the electron injection process from the
excited dye molecule to the TiO2 conduction band is energetically
permitted.
large particles in the film and the reflective effect of the Pt counter
electrode can be considered as reasonable interpretations.13 The
IPCE of QA1 is higher than those of QA2 and QA3 in the region
from 400 nm to 605 nm, though the IPCE values of QA1
compared to QA2 and QA3 are slightly lower in the region of
610–680 nm, indicating that the QA1 sensitized TiO2 electrode
would generate a higher conversion yield among the three dyes.
Fig. 2b shows the current–voltage characteristics of the DSSCs
fabricated with QA1–3 as sensitizers under standard global AM
1.5 solar light conditions. 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. Generally, Jsc is related to the molar extinction
coefficient of the dye molecule, in which a higher molar extinc-
tion coefficient has good light-harvesting ability and yields a
higher short circuit, correspondingly. QA1 dye has the highest
light harvesting efficiency and consequently an improved Jsc due
to the largest molar extinction coefficient. It is noteworthy that
all of the three dyes have higher open circuit voltages (Voc) (773–
794 mV) than N719 (726 mV) measured under the same condi-
tions. The reason is that the introduction of two hydrophobic
alkyl chains into the quinacridone rings can act as barriers pre-
To scrutinize the electronic properties of these dyes further, the
optimized structures and electron distribution of the HOMOs
and LUMOs of QA1–3 performed by density functional theory
(DFT) at the B3LYP/6-31G* level in the gas phase are shown in
Table S3 and S4†, respectively. The theoretical calculations show
that the dihedral angle between the quinacridone ring and the
furan ring is less than 1 degree, which is beneficial for intra-
molecular charge transfer. As shown in Table S4†, the HOMOs
in QA1–3 are primarily located on the p-framework of the donor
part (triphenylamine), while the electron density of the LUMOs
is delocalized over the furan group and anchoring group. Thus,
the electron distribution from the donor unit to the anchoring
moiety can be moved by excitation from the HOMO to the
LUMO orbital to realize electron injection into the conduction
band of TiO2.
ꢀ
venting hydrophilic I3 from approaching the TiO2 surface,
inhibiting charge recombination and thus improving Voc. QA2-
based DSSCs show a lower Voc (773 mV) than QA1 and QA3-
based DSSCs. One plausible explanation is that the extended
conjugation and good planarity can enhance the charge recom-
bination and result in a lower Voc,41,42 The dark current–voltage
characteristics also agree well with the Voc values, in which the
quinacridone-bridged dyes’ dark current onset potential shifted
to a larger value than N719. DSSCs based on QA1 exhibit the
best overall light to electricity conversion efficiency of 6.80%
(Jsc ¼ 12.42 mA cmꢀ2, Voc ¼ 776 mV, FF ¼ 0.71) under AM
1.5 irradiation (100 mW cmꢀ2), which reached 87% with respect
to that of an N719-based device fabricated under similar
conditions.
Photovoltaic performances of DSSCs based on QA1–3
The DSSCs were prepared by using a dye-absorbed TiO2 elec-
trode, Pt-coated glass as the counter electrode, and an acetoni-
trile solution with iodine (0.05 M), lithium iodide (0.1 M),
1,2-dimethyl-3-propylimidazolium iodide (0.6 M), and 4-tert-
butylpyridine (0.5 M) as the electrolyte. Fig. 2a shows the
incident photon-to-electron conversion efficiency (IPCE) as a
function of incident wavelength for DSSCs based on these dyes.
The solar cells based on QA1–3 show high IPCEs above 60% in
the range of 450–605 nm and with the highest values of 83% at
570 nm for QA1, 77% at 550 nm for QA2 and 75% at 535 nm for
QA3, respectively. Meanwhile the IPCE values for QA1–3 are
obviously decreased at 610 to 680 nm, which is in accordance
with their absorption spectra on the TiO2 film. However,
compared to the absorption spectra of sensitized films, the IPCE
spectra are broadened significantly. The scattering effect of the
Due to strong intermolecular interactions, p–p stacking of
organic dye molecules usually occurs, causing inefficient electron
injection and low power conversion efficiency.43 It has been
demonstrated that co-adsorption of co-adsorbents with dye
Table 2 Photovoltaic performance of untreated DSSCs based on QA1,
QA2, QA3 and N719 and CDCA with different concentrations of 10 mM,
20 mM and 30 mM co-adsorbed DSSCs based on QA1a
Concentration of
CDCA/mMꢀ1
Dye
Jsc/mAꢀ1 cmꢀ2 Voc/mVꢀ1 FF
h (%)
QA1b
QA2b
QA3b
0
0
0
12.42
12.06
11.78
12.82
13.25
12.30
16.33
776
773
794
792
804
813
726
0.71 6.80
0.69 6.39
0.69 6.46
0.73 7.41
0.73 7.70
0.72 7.23
0.66 7.86
QA1b 10
QA1b 20
QA1b 30
N719c
0
a
Measured under irradiation of AM 1.5 simulated solar light (100 mW
cmꢀ2) at room temperature, 14 mm film thickness, 0.28 cm2 working
area, electrolyte containing: 0.1 M LiI + 0.05 M I2 + 0.6 M DMPII +
b
0.5 M TBP in acetonitrile. The concentration of QA dyes is 3 ꢂ 10ꢀ4
Fig. 2 (a) IPCE spectra for DSSCs based on dyes QA1–3. (b) I–V curves
for DSSCs based on dyes QA1–3 and N719 under light (100 mW cmꢀ2
simulated AM 1.5 solar light) and dark.
M in THF. c The concentration of N719 is 3 ꢂ 10ꢀ4 M in ethanol.
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molecules which suffer from aggregation can improve solar cell
performance significantly.10 Since QA1 exhibits the best perfor-
mance among the three dyes studied in this work, we focus on the
optimization of QA1-based DSSCs using a co-adsorption
strategy, and the data are summarized in Table 2 (the I–V curves
are shown in Fig. S3†). As shown in Fig. S3†, the short-circuit
photocurrent (Jsc) of QA1-based DSSC increases from 12.42 to
13.25 mA cmꢀ2 with 10 and 20 mM CDCA, respectively, and
then slightly decreases with further increasing CDCA to 30 mM.
One explanation is that the amount of adsorbed dye on the TiO2
surface is reduced by the co-adsorbents of CDCA (as shown in
Fig. S4†), resulting in a loss in light harvesting and thus a
decreased short-circuit photocurrent. The open-circuit voltage in
the DSSCs with QA1 is improved, indicating that the charge
recombination process is effectively prohibited in the presence of
CDCA. Resulting from the co-grafting of the two molecules, the
formation of a mixed mono-layer should be more tightly packed
than when the sensitizer is absorbed alone, providing an insu-
lating barrier against back electron transfer more effectively from
the TiO2 conduction band to the electrolyte.43,44 As shown in
Table 2, the highest power conversion efficiency could be
obtained at 20 mM CDCA, in which QA1 achieved an overall
conversion efficiency of 7.70% (Jsc ¼ 13.25 mA cmꢀ2, Voc ¼ 804
mV, FF ¼ 0.73).
larger semicircle after co-sensitization, which is reflected in the
improvement shown in the Voc.
Fig. 3b shows the Bode plots of the DSSCs based on QA1–3.
All EIS Bode plots exhibit two peak features for the frequency
investigated. The one at higher frequency corresponds to charge
transfer at the Pt/electrolyte interface and meanwhile the other
one at lower frequency corresponds to thecharge transfer at the
TiO2/dye/electrolyte interface, which is related to the charge
recombination rate and whose reciprocal is associated with the
electron lifetime.47 In Fig. 3b, the low frequency peak of QA3
shows a lower frequency than those of QA1 and QA2, indicating
that QA3-based DSSCs have longer electron lifetimes, which
leads to a lower rate of charge recombination. The electron
lifetime values of QA1–3 are 54.9 ms, 43.0 ms and 66.3 ms,
respectively, likewise supporting the observed shift in the
Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) analysis is
considered as a powerful tool to elucidate the electronic and ionic
transport processes in DSSCs. Fig. 3 shows the EIS for DSSCs
based on dyes QA1–3 and 20 mM CDCA co-adsorbed DSSCs
based on dye QA1 under a forward bias of ꢀ0.70 V from 0.1 Hz
to 100 KHz in the dark. As shown in Fig. 3a, the Nyquist
diagram shows two semicircles in the measured frequency range,
in which the larger semicircle located in the low-frequency region
is assigned to the dark reaction impedance caused by charge
transportation at the TiO2–dye–electrolyte interface, and the
other smaller semicircle located in the high-frequency region is
attributed to the charge-transfer at the counter electrode.45,46 The
radius of the larger semicircle increases in the order QA2 < QA1
< QA3, indicating that the electron recombination resistance
increases from QA2, QA1, QA3. This result is in agreement with
the observed shift in the Voc value under standard global AM 1.5
illumination and in the dark. With an optimized dye QA1 to be
co-adsorbed with 20 mM CDCA, the QA1-based DSSCs show a
Fig. 3 Impedance spectra of untreated DSSCs based on dyes QA1–3
and 20 mM CDCA co-adsorbed DSSCs based on dye QA1 measured
at ꢀ0.70 V bias in the dark. (a) Nyquist plots; (b) Bode phase plots.
Fig. 4 Variations of photovoltaic parameters (Jsc, Voc, FF, and h) with
aging time for the DSSC device based on QA1–3 with ionic-liquid elec-
trolyte under visible-light soaking.
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Voc value under standard global AM 1.5 illumination and in the
dark. Due to the slight blocking effect of the methoxy group,
QA3 based DSSCs show a higher Voc than QA1 and QA2 based
DSSCs, as a result of the enhanced electron lifetime. After co-
sensitization with CDCA, the low frequency peak of QA1 shows
a lower frequency than before, and this indicates that the 20 mM
CDCA co-sensitized QA1-based cell has a longer electron life-
time (79.6 ms), which leads to a lower rate of charge recombi-
nation and thus an improved Voc.
Acknowledgements
This work was supported by NSFC/China (2116110444 and
21172073), National Basic Research 973 Program
(2011CB808400), the Fundamental Research Funds for the
Central Universities (WJ0913001), and Ph.D. Programs Foun-
dation of Ministry of Education of China (20090074110004).
Notes and references
€
1 B. O’Regan and M. Gratzel, Nature, 1991, 353, 737.
2 F. Gao, Y. Wang, J. Zhang, D. Shi, M. Wang, R. Humphry-Baker,
€
P. Wang, S. M. Zakeeruddin and M. Gratzel, Chem. Commun.,
2008, 2635.
3 (a) J. H. Yum, I. Jung, C. Baik, J. Ko, M. K. Nazeeruddin and
Long-term stability measurements of ionic liquid electrolyte
based DSSCs
Long-term stability is considered as an important parameter for
sustained cell operation for applications in the future. Hence, due
to low volatility and high thermal stability, ionic-liquid based
electrolytes are a better choice.48 The DSSCs were prepared using
0.1 M iodine, 0.1 M lithium iodide, 0.45 M benzimidazole in 1-
propyl-3-methyl imidazolium iodide as the redox electrolyte. The
photovoltaic performances of QA1–3 sensitized TiO2 film elec-
trodes with a solvent-free ionic liquid electrolyte under standard
global AM 1.5 solar light conditions (100 mW cmꢀ2) are shown in
Fig. S5† and summarized in Table S5†. In accordance with
DSSCs based on liquid electrolytes, the photovoltaic parameters
of the QA1 based DSSCs with ionic-liquid electrolytes show best
performance of 4.28% (Jsc ¼ 10.60 mA cmꢀ2, Voc ¼ 640 mV, FF
¼ 0.63). The Jsc, Voc and FF show little change over 1000 h in
Fig. 4. Moreover, Fig. 4 reveals that the DSSCs based on QA1–3
with an ionic-liquid electrolyte remained at 96%, 97%, 98% of the
initial overall efficiency value after 1000 h of visible-light soak-
ing, respectively, which could demonstrate the good stability of
DSSCs based on quinacridone sensitizers.
€
M. Gratzel, Energy Environ. Sci., 2009, 2, 100; (b) L. Y. Han,
A. Islam, H. Chen, C. Malapaka, B. Chiranjeevi, S. Zhang,
X. Yang and M. Yanagida, Energy Environ. Sci., 2012, 5, 6057.
4 A. Mishra, M. K. Fischer and P. Bauerle, Angew. Chem., Int. Ed.,
2009, 48, 2474.
5 H. N. Tian, X. C. Yang, J. Y. Cong, R. K. Chen, J. Liu, Y. Hao,
A. Hagfeldt and L. C. Sun, Chem. Commun., 2009, 6288.
6 H. N. Tian, X. C. Yang, R. K. Chen, A. Hagfeldt and L. C. Sun,
Energy Environ. Sci., 2009, 2, 674.
7 H. N. Tian, X. C. Yang, R. K. Chen, Y. Z. Pan, L. Li, A. Hagfeldt and
L. C. Sun, Chem. Commun., 2007, 3741.
8 W. J. Wu, J. B. Yang, J. L. Hua, J. Tang, L. Zhang, Y. T. Long and
H. Tian, J. Mater. Chem., 2010, 20, 1772.
9 D. R. Cao, J. A. Peng, Y. P. Hong, X. M. Fang, L. Y. Wang and
H. Meier, Org. Lett., 2011, 13, 1610.
10 S. Y. Qu, W. J. Wu, J. L. Hua, C. Kong, Y. T. Long and H. Tian,
J. Phys. Chem. C, 2010, 114, 1343.
11 F. L. Guo, S. Y. Qu, W. J. Wu, J. Li, W. J. Ying and J. L. Hua, Synth.
Met., 2010, 160, 1767.
12 T. Horiuchi, H. Miura, K. Sumioka and S. Uchida, J. Am. Chem.
Soc., 2004, 126, 12218.
13 W. H. Zhu, Y. Z. Wu, S. T. Wang, W. Q. Li, J. Chen, Z. S. Wang and
H. Tian, Adv. Funct. Mater., 2011, 21, 756.
14 W. Q. Li, Y. Z. Wu, X. Li, Y. S. Xie and W. H. Zhu, Energy Environ.
Sci., 2011, 4, 1830.
15 W. Q. Li, Y. Z. Wu, Q. Zhang, H. Tian and W. H. Zhu, ACS Appl.
Mater. Interfaces, 2012, 4, 1822.
Conclusion
In conclusion, three new metal-free quinacridone-based organic
dyes with a furan unit (QA1–3) were conveniently prepared and
characterized. It was found that the introduction of the triphe-
nylamine group as the electron-donor brought about improved
photovoltaic performance compared with methoxytriphenyl-
amine and N,N-diphenyl-4-vinylbenzenamine for quinacridone
dyes. Co-adsorption of CDCA can break up the dye aggregates,
leading to an increase of the photocurrent density, and sup-
pressing charge recombination, leading to increasing electron
lifetimes and thus photovoltage. A maximum solar energy to
16 A. Yella, H. W. Lee, H. N. Tsao, C. Y. Yi, A. K. Chandiran,
M. K. Nazeeruddin, E. W. G. Diau, C. Y. Yeh, S. M. Zakeeruddin
€
and M. Gratzel, Science, 2011, 334, 629.
17 S. S. Labana and L. L. Labana, Chem. Rev., 1967, 67, 1.
18 T. Beyerlein, B. Tieke, S. Forero-Lenger and W. Brutting, Synth.
Met., 2002, 130, 115.
19 A. R. Rabindranath, Y. Zhu, I. Heim and B. Tieke, Macromolecules,
2006, 39, 8250.
20 N. Koumura, Z. S. Wang, M. Miyashita, Y. Uemura, H. Sekiguchi,
Y. Cui, A. Mori, S. Mori and K. Hara, J. Mater. Chem., 2009, 19,
4829.
21 J. J. Chen, T. L. Chen, B. S. Kim, D. A. Poulsen, J. L. Mynar,
J. M. Frechet and B. Ma, ACS Appl. Mater. Interfaces, 2010, 2, 2679.
22 Z. J. Ning and H. Tian, Chem. Commun., 2009, 5483.
23 J. X. He, W. J. Wu, J. L. Hua, Y. H. Jiang, S. Y. Qu, J. Li, Y. T. Long
and H. Tian, J. Mater. Chem., 2011, 21, 6054.
electricity conversion efficiency of 7.70% (Jsc ¼ 13.25 mA cmꢀ2
,
Voc ¼ 804 mV, FF ¼ 0.73) under simulated AM 1.5 solar light
irradiation (100 mW cmꢀ2) with a 20 mM CDCA co-adsorbed
DSSC based on QA1 was obtained. Most importantly, the long-
term stability of the quinacridone DSSCs with ionic-liquid elec-
trolytes under 1000 h light-soaking was demonstrated. The qui-
nacridone moiety and the furan ring have excellent planarity
from the DFT calculations. In addition, as an alternative to
thiophene, the furan ring can be advantageously incorporated
into dye-sensitizers as a p-spacer and can improve the cell effi-
ciency and stability. Our findings demonstrate that quinacri-
done-based organic sensitizers with a furan moiety are promising
for the further improvement of the conversion efficiency of
DSSCs owing to the original and versatile molecular design.
24 Z. J. Ning, Y. Fu and H. Tian, Energy Environ. Sci., 2010, 3, 1170.
25 J. Tang, J. L. Hua, W. J. Wu, J. Li, Z. G. Jin, Y. T. Long and H. Tian,
Energy Environ. Sci., 2010, 3, 1736.
26 R. Katoh, A. Furube, S. Mori, M. Miyashita, K. Sunahara,
N. Koumuraa and K. Hara, Energy Environ. Sci., 2009, 2, 542.
27 U. F. Bunz, Angew. Chem., Int. Ed., 2010, 49, 5037.
28 B. Walker, A. B. Tamayo, X. D. Dang, P. Zalar, J. H. Seo, A. Garcia,
M. Tantiwiwat and T. Q. Nguyen, Adv. Funct. Mater., 2009, 19, 3063.
29 T. Yamamoto, Z.-H. Zhou, T. Kanbara, M. Shimura, K. Kizu,
T. Maruyama, Y. Nakamura, T. Fukuda, B.-L. Lee, N. Ooba,
S. Tomaru, T. Kurihara, T. Kaino, K. Kubota and S. Sasaki,
J. Am. Chem. Soc., 1996, 118, 10389.
30 Y. Araki, T. Wada, O. Yoshikawa, T. Sagawa, S. Yoshikawa and
H. Imahor, J. Phys. Chem. C, 2009, 113, 10798.
24364 | J. Mater. Chem., 2012, 22, 24356–24365
This journal is ª The Royal Society of Chemistry 2012

View Online
31 J. T. Lin, P.-C. Chen, Y.-S. Yen, Y.-C. Hsu, H.-H. Chou and
M.-C. P. Yeh, Org. Lett., 2009, 11, 97.
32 R. Li, X. J. Lv, D. Shi, D. F. Zhou, Y. M. Cheng, G. L. Zhang and
P. Wang, J. Phys. Chem. C, 2009, 113, 7469.
33 J. Y. Mao, F. L. Guo, W. J. Ying, W. J. Wu, J. Li and J. L. Hua,
Chem.–Asian J., 2012, 7, 982.
41 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.
42 K. M. Karlsson, X. Jiang, S. K. Eriksson, E. Gabrielsson,
H. Rensmo, A. Hagfeldt and L. C. Sun, Chem.–Eur. J., 2011, 17,
6415.
34 (a) S. Y. Qu and H. Tian, Chem. Commun., 2012, 48, 3039; (b)
S. Y. Qu, B. Wang, F. L. Guo, J. Li, W. J. Wu, C. Kong,
Y. T. Long and J. L. Hua, Dyes Pigm., 2012, 92, 1384; (c) S. Y. Qu,
J. L. Hua and H. Tian, Sci. China: Chem., 2012, 55, 677.
35 C. H. Woo, P. M. Beaujuge, T. W. Holcombe, O. P. Lee and
J. J. Frechet, J. Am. Chem. Soc., 2010, 132, 15547.
36 C. Sakong, S. H. Kim, S. B. Yuk, J. Y. Kim, S. W. Park, M. J. Ko and
J. P. Kim, Bull. Korean Chem. Soc., 2011, 32, 2553.
37 H. Im, S. Kim, C. Park, S. Jang, C. Kim, K. Kim, N. Park and
C. Kim, Chem. Commun., 2010, 46, 1335.
38 M. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker and
43 Z. S. Wang, Y. Cui, Y. Dan-oh, C. Kasada, A. Shinpo and K. Hara,
J. Phys. Chem. C, 2007, 111, 7224.
44 J. He, G. Benko, F. Korodi, T. Polivka, R. Lomoth, B. Akermark,
L. C. Sun, A. Hagfeldt and V. Sundstrm, J. Am. Chem. Soc., 2002,
124, 4922.
45 L. Y. Han, N. Koide, Y. Chiba, A. Islam, R. Komiya, N. Fuke,
A. Fukui and R. Yamanaka, Appl. Phys. Lett., 2005, 86, 213501.
€
46 Q. Wang, J. Moser and M. Gratzel, J. Phys. Chem. B, 2005, 109,
14945.
47 L. Y. Lin, C. H. Tsai, K. T. Wong, T. W. Huang, L. Hsieh, S. H. Liu,
H. W. Lin, C. C. Wu, S. H. Chou, S. H. Chen and A. I. Tsai, J. Org.
Chem., 2010, 75, 4778.
€
M. Gratzel, J. Am. Chem. Soc., 1993, 115, 6382.
39 T. Horiuchi, H. Miura and S. Uchida, Chem. Commun., 2003, 3036.
40 M. Guo, P. Diao, Y. J. Ren, F. Meng, H. Tian and S. M. Cai, Sol.
Energy Mater. Sol. Cells, 2005, 88, 23.
48 H. Qin, S. Wenger, M. F. Xu, F. F. Gao, X. Y. Jing, P. Wang,
€
S. M. Zakeeruddin and M. Gratzel, J. Am. Chem. Soc., 2008, 130,
9202.
This journal is ª The Royal Society of Chemistry 2012
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