Organic dyes incorporating a thiophene or furan moiety for efficient dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2013.12.025

Four novel carbazole organic dyes containing either a furan or a thiophene unit as the conjugated bridge have been designed, synthesized and characterized for nanocrystalline TiO₂ dye-sensitized solar cells. The modified carbazole containing linear or branched alkyl side chain acts as an electron donor, triphenylamine acts as an electron-donating group and cyanoarylic acid acts as an electron acceptor and anchoring unit. The absorption spectra, electrochemical, photovoltaic and sensitizing properties of the new dyes were fully characterized. Among these four dyes, the dye incorporating an octyl-substituted carbazole and disubstituted furan shows the best photovoltaic performance: a maximum monochromatic incident photon-to-current conversion efficiency of 92.4%, a short-circuit photocurrent density of 13.26 mA cm ^-2, an open-circuit photovoltage of 724 mV, and a fill factor of 0.66, corresponding to an overall conversion efficiency of 6.68% under standard global AM 1.5 solar light condition. The results indicate that the introduction of longer alkyl chain into the carbazole moiety will retard recombination and the introduction of furan as the conjugate bridge will increase the photocurrent response. For comparison, the ruthenium dye, designated as N719, sensitized TiO₂ solar cell showed an efficiency of 6.98% under the same experiment conditions.

Full text of DOI:10.1016/j.dyepig.2013.12.025

Dyes and Pigments 104 (2014) 75e82
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Organic dyes incorporating a thiophene or furan moiety for efficient
dye-sensitized solar cells
,
Jinxiang He a, Jianli Hua a, Guangxia Hu b, Xi Jiang Yin c, Hao Gong b, Chunxiang Li c
*
a Key Laboratory for Advanced Materials, Institute of Fine Chemicals and Department of Chemistry, East China University of Science and Technology,
130 Meilong Road, Shanghai 200237, PR China
b Department of Materials Science and Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore
c Advanced Materials Technology Centre, Singapore Polytechnic, 500 Dover Road, 139651, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
Four novel carbazole organic dyes containing either a furan or a thiophene unit as the conjugated bridge
have been designed, synthesized and characterized for nanocrystalline TiO2 dye-sensitized solar cells.
The modified carbazole containing linear or branched alkyl side chain acts as an electron donor, tri-
phenylamine acts as an electron-donating group and cyanoarylic acid acts as an electron acceptor and
anchoring unit. The absorption spectra, electrochemical, photovoltaic and sensitizing properties of the
new dyes were fully characterized. Among these four dyes, the dye incorporating an octyl-substituted
carbazole and disubstituted furan shows the best photovoltaic performance: a maximum mono-
chromatic incident photon-to-current conversion efficiency of 92.4%, a short-circuit photocurrent density
of 13.26 mA cmꢀ2, an open-circuit photovoltage of 724 mV, and a fill factor of 0.66, corresponding to an
overall conversion efficiency of 6.68% under standard global AM 1.5 solar light condition. The results
indicate that the introduction of longer alkyl chain into the carbazole moiety will retard recombination
and the introduction of furan as the conjugate bridge will increase the photocurrent response. For
comparison, the ruthenium dye, designated as N719, sensitized TiO2 solar cell showed an efficiency of
6.98% under the same experiment conditions.
Received 19 June 2013
Received in revised form
16 December 2013
Accepted 24 December 2013
Available online 7 January 2014
Keywords:
Alkyl functionalized carbazole
Thiophene
Furan
Organic dye
Dye-sensitized solar cell
Photovoltaic
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
conditions and also show favorable stabilities [4]. However, their
practical applications are restricted by the limited supply and high
Increasing demand for energy consumption has led to a greater
focus on renewable energy resources research all over the world
over the past couple of decades. Dye-sensitized solar cells (DSSCs),
considered as a promising alternative to conventional solar cells,
have attracted intense attention due to the relatively high solar
cost of ruthenium metal. In recent years, the interest in metal-free
organic dyes as substitutes for noble metal complexes is increasing
owing to their high molar extinction coefficient, simple synthesis,
low cost and facile molecular design. An enormous number of
studies have been focused on molecular engineering of chromo-
phores to attain a capability of panchromatic light-harvesting [5]. A
variety of organic dyes, such as diketopyrrolopyrrole dyes [6,7],
phenothiazine [8e10], cyanine [11e14], merocyanine [15,16],
coumarin Refs. [17e19], thiophene [20], indoline [21e26], hemi-
cyanine [27e29], phenoxazine [30], carbazole [7,31e34] and fluo-
rine based dyes [35e37] have been reported and found to exhibit
promising device characteristics. Impressive photovoltaic perfor-
mances have been obtained using organic dyes with efficiency
exceeding 10% [38e40].
power-to-electricity conversion efficiency (h) and potentially low
cost production since Grätzel and co-workers reported DSSCs high
efficiency in 1991 [1]. The photosensitizer is one of the key mate-
rials in DSSCs, which plays an important role in determining the
performance of the solar cells. Central to this device is a TiO2
nanoparticle film that provides a large surface area for the
adsorption of dye sensitizers, which absorbs light and injects
electrons into the conduction band of the TiO2 after light excitation
and converts the sunlight to photocurrent. To date, Ruepoly-
pyridine complexes have achieved power conversion efficiencies
over 11% [2,3] under standard global air mass 1.5 solar simulated
Organic dyes commonly consist of donor, linker, and acceptor
groups (i.e., a De
could be finely tuned by alternating independently or matching the
different groups of De eA dyes [41e43]. Generally organic dyes
peA molecular structure). Their various properties
p
used for efficient solar cells are required to possess broad and
intense spectral absorption in the visible light region. One strategy
* Corresponding author. Tel.: þ65 68708241; fax: þ65 68708083.
E-mail address: licx@sp.edu.sg (C. Li).
0143-7208/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.12.025

76
J. He et al. / Dyes and Pigments 104 (2014) 75e82
toward this end is to introduce more
tween the donor and acceptor, thereby forming De
p
-conjugation segments be-
eA struc-
resistance15 U/square) wasobtained from theNipponSheetGlassCo.,
Ltd. All otherchemicalsand solvents used inthis work were ofreagent
p
e
p
tures [44,45]. However, such rod-like molecules may facilitate the
recombination of electrons to the triodide and the formation of
aggregates between molecules [14]. Therefore, a further improve-
ment could be made by introducing auxiliary electron donor tri-
grade and used without further purification unless otherwise noted.
2.2. Characterization
arylamine antennas into the molecule to form the DeDe
peA
1H NMR and 13C NMR spectra were recorded on Bruker AM
400 MHz instruments and spectra were referenced against tetra-
methylsilane as the internal standard. Mass spectra were performed
usinga ThermoFinnigan LCQquadrupoleiontrapmass spectrometer.
The absorption spectra of the organic dyes either in solution or the
adsorbed TiO2 films were measured with a UV-1800 spectropho-
tometer (Shimadzu Pte. Ltd.). Cyclic Voltammetry measurements
were determined with Autolab potentiostat galvanostat PGSTAT30
differential electrometer amplifier using a three-electrode cell with a
platinum (Pt) working electrode, a Pt wire counter electrode, and a
saturated calomel reference electrode (SCE) in saturated KCl solution,
0.1 M TBAPF6 was used as the supporting electrolyte in dichloro-
methane (DCM). The ferrocenium/ferrocene (Fc/Fcþ) redox couple
was used as an internal potential reference. The E1/2 of the Fc/Fcþ
redox couple was found to be 0.50 V versus the SCE reference elec-
trode or 0.74 V versus the normal hydrogen electrode (NHE).
structure, which avoids the charge recombination processes of
injected electrons with triiodide in the electrolyte and leads to
increased open circuit voltage (Voc) [46e50] reported by some
groups. These studies suggested that superior performance of
organic dyes based on DeDepeA structure over the simple DepeA
configuration can be achieved by molecular design. Carbazole is a
potential hole transport semiconductor in organic devices, pre-
senting unique electronic and optical properties. Therefore, carba-
zole based organic dyes have attracted broad interest in organic
light-emitting diodes (OLEDs) and DSSCs [33,34,51e53]. Triaryl-
amine has been widely used in opto- and electro-active materials
for its good electron donating and transporting capability, as well as
its special propeller starburst molecular structure [54]. Organic
photovoltaic functional materials with such triarylamine as elec-
tron donor have aroused great interest and become the focus of
intensive research in the field of solar cells. These materials have
significantly reinforced the conversion efficiency of the next-
generation solar cells, especially for dye sensitized solar cells [54].
The combination of carbazole and triarylamine will provide more
effective electron density from carbazole to the triarylamine donor
centre [55]. When carbazole carrying lateral alkyl chains, it was
found that such groups help to form a blocking layer to keep Iꢀ3 ions
away from the TiO2 electrode surface, hence increasing the electron
lifetime and open circuit voltage (Voc) [56]. Also, the thiophene and
furan moiety are expected to provide effective conjugation and
lower the energy of the charge transfer transition due to their small
resonance energies (thiophene, 29 kcal/mol; furan, 16 kcal/mol)
[57]. It is therefore interesting to incorporate carbazole, triaryl-
amine and a furan or thiophene moiety. In this study, four new
organic dyes K1eK4 have been designed and synthesized through
2.3. Synthesis of dyes
2.3.1. 3,6-Diiodo-9-octyl-9H-carbazole (1)
A mixture of 3,6-diiodo-9H-carbazole (2.00 g, 4.77 mmol), 1-
bromooctane (1.10 g, 5.70 mmol) and potassium 2-methylpropan-
2-olate (0.64 g, 5.70 mmol) in dimethylformamide (DMF) (15 ml)
were stirred under room temperature for 12 h. The mixture was
poured into water and extracted with CH2Cl2 (3 ꢁ 10 ml). The
combined organic layers were dried with anhydrous MgSO4. The
solvent was evaporated, and the residue was purified by column
chromatography on silica gel (hexane) to give a colorless solid
(yield 88%). 1HNMR(CDCl3, 400 MHz),
d
8.32 (s, 2H), 7.71 (d, J ¼ 8.0
Hz, 2H), 7.17 (D, J ¼ 8.0 Hz, 2H), 4.21 (t, J ¼ 8.0 Hz, 2H), 1.84e1.77 (m,
2H), 1.30e1.23 (m, 10H), 0.86 (t, J ¼ 8.0 Hz, 3H). 13CNMR(CDCl3,
the modification of simple carbazole-p-spacer dyes with auxiliary
100 MHz), d 139.49, 134.48, 129.34, 123.96, 110.88, 81.61, 43.23,
electron donor and linear or branched long alkyl groups (Fig. 1).
These four new sensitizers have been applied successfully to
sensitization of nanocrystalline TiO2-based solar cells and their
corresponding photovoltaic properties, electronic and optical
properties were investigated in detail.
31.72, 29.09, 27.19, 22.55, 14.03. MS (ESI, m/z) calcd for C20H24I2N
[M þ H]þ: 532.00, Found: 531.90.
2.3.2. 3,6-Diiodo-9-hexyl-9H-carbazole (2)
The synthesis method resembles that of compound 1 but with
1-bromohexane instead (0.78 g, 4.73 mmol) of 1-bromooctane, and
the compound was purified by column chromatography on silica
gel (hexane) to yield 3.4 g of colorless solid (yield 86%). 1H NMR
2. Experimental section
(CDCl3, 400 MHz),
d
8.34 (s, 2H), 7.74 (d, J ¼ 8.8 Hz, 2H), 7.19 (d,
2.1. Materials and reagents
J ¼ 8.8 Hz, 2H), 4.23 (t, J ¼ 7.2 Hz, 2H), 1.88e1.79 (m, 2H), 1.37e1.29
(m, 6H), 0.88 (t, J ¼ 7.0 Hz, 3H). MS (ESI, m/z) calcd for C18H20 I2N
[M þ H]þ: 503.97, Found: 503.68.
3,6-diiodo-9H-carbazole and 1-Propyl-3-methylimidazolium io-
dide (PMII) were prepared according to the published procedures
[58,59]. Methoxypropionitrile (MPN), tetra-n-butylammonium hex-
afluorophosphate (TBAPF6), 4-tert-butylpyridine (TBP), lithium iodide
and iodinewere purchased from Aldrich. Transparent FTO conducting
glass (fluorine doped SnO2, transmission > 90% in the visible, sheet
2.3.3. 3,6-Diiodo-9-(4-methylpentyl)-9H-carbazole (3)
The synthesis method resembles that of compound 1 but with 1-
bromo-4-methylpentane (0.78 g, 4.73 mmol) instead of 1-
bromooctane, and the compound was purified by column chroma-
tography on silica gel (hexane) to yield 2.7 g of colorless solid (yield
82%). 1H NMR (CDCl3, 400 MHz),
d
8.32 (s, 2H), 7.72 (d, J ¼ 8.4 Hz,
2H), 7.16 (d, J ¼ 8.4 Hz, 2H), 4.18 (t, J ¼ 7.4 Hz, 2H), 1.86e1.78 (m, 2H),
1.61e1.54 (m,1H),1.30e1.22 (m, 2H), 0.88 (d, J ¼ 6.4 Hz, 6H). MS (ESI,
m/z) calcd for C18H20 I2N [M þ H]þ: 503.97, Found: 503.68.
2.3.4. 5-(6-Iodo-9-octyl-9H-carbazol-3-yl)thiophene-2-
carbaldehyde (4)
A mixture of 1 (100 mg, 0.19 mmol), Pd(PPh3)4 (20 mg,
0.017 mmol), and K2CO3 (1.02 g, 0.01 mol) in THF (8 ml) and water
Fig. 1. Molecular structures of carbazole dyes (K1eK4).

J. He et al. / Dyes and Pigments 104 (2014) 75e82
77
(5 ml) were heated to 45 ꢂC under an nitrogen atmosphere for
30 min. 5-formylthiophen-2-ylboronic acid (30 mg, 0.19 mmol) in
THF (3 ml) was added, and the mixture was heated under refluxed
for a further 18 h. After cooling to room temperature, THF was
evaporated and the mixture was extracted with CH2Cl2 (3 ꢁ 10 ml).
The combined organic layers were dried with anhydrous MgSO4.
The solvent was evaporated, and the residue was purified by col-
umn chromatography on silica gel (hexane/DCM ¼ 2/1, V/V) to give
400 MHz),
d
9.89 (s, 1H), 8.44 (s, 1H), 8.32 (s, 1H), 7.90 (d, J ¼ 8.0 Hz,
1H), 7.76 (d, J ¼ 4.0 Hz,1H), 7.74 (d, J ¼ 8.0 Hz,1H), 7.61 (d, J ¼ 8.0 Hz,
2H), 7.48e7.43 (m, 3H), 7.29 (t, J ¼ 8.0 Hz, 4H), 7.21e7.16 (m, 6H),
7.04 (t, J ¼ 8.0 Hz, 2H), 4.33 (t, J ¼ 8.0 Hz, 2H), 1.94e1.87 (m, 2H),
1.39e1.25 (m, 10H), 0.86 (t, J ¼ 8.0 Hz, 3H). MS (ESI, m/z) calcd for
C
43H41N2OS [M þ H]þ: 633.29, Found: 633.15.
2.3.9. 5-(6-(4-(Diphenylamino)phenyl)-9-octyl-9H-carbazol-3-yl)
furan-2-carbaldehyde (9)
a yellow solid (yield 25%). 1H NMR (CDCl3, 400 MHz),
d 9.89 (s, 1H),
8.44 (s, 1H), 8.33 (s, 1H), 7.79 (d, J ¼ 8.0 Hz, 1H), 7.76 (d, J ¼ 4.0 Hz,
1H), 7.74 (d, J ¼ 8.0 Hz, 1H), 7.44 (d, J ¼ 4.0 Hz, 1H), 7.42 (d,
J ¼ 8.0 Hz, 1H), 7.21 (d, J ¼ 8.0 Hz, 1H), 4.28 (t, J ¼ 8.0 Hz, 2H), 1.88e
1.82 (m, 2H), 1.33e1.23 (m, 10H), 0.86 (t, J ¼ 8.0 Hz, 3H). MS (ESI, m/
z) calcd for C25H27INOS [M þ H]þ: 516.09, Found: 516.10.
The synthesis method resembles that of compound 8 but 5
instead of 4, and the compound was purified by column chroma-
tography on silica gel (hexane/DCM ¼ 2/1, V/V) to give 160 mg of
orange solid in 81% yield. 1H NMR (CDCl3, 400 MHz),
d 9.66 (s, 1H),
8.66 (s, 1H), 8.39 (s, 1H), 7.94 (d, J ¼ 6.8 Hz, 1H), 7.76 (d, J ¼ 6.8 Hz,
1H), 7.64 (d, J ¼ 6.8 Hz, 2H), 7.48e7.44 (m, 2H), 7.40 (s, 1H), 7.32 (t,
J ¼ 6.2 Hz, 4H), 7.24e7.19 (m, 6H), 7.07 (t, J ¼ 5.8 Hz, 2H), 6.88 (s,
1H), 4.35 (t, J ¼ 8.0 Hz, 2H), 1.97e1.88 (m, 2H), 1.46e1.28 (m, 10H),
0.89 (t, J ¼ 6.0 Hz, 3H). MS (ESI, m/z) calcd for C43H41N2O2 [M þ H]þ:
617.32, Found: 617.35.
2.3.5. 5-(6-Iodo-9-octyl-9H-carbazol-3-yl)furan-2-carbaldehyde
(5)
The synthesis method resembles that of compound 4 but 5-
formylfuran-2-ylboronic acid (26 mg, 0.18 mmol) instead of 5-
formylthiophen-2-ylboronic acid, and the compound was purified
by column chromatography on silica gel (hexane/DCM ¼ 2/1, V/V)
to yield 154 mg of yellow solid (yield 30%). 1H NMR (CDCl3,
2.3.10. 5-(6-(4-(Diphenylamino)phenyl)-9-hexyl-9H-carbazol-3-
yl)furan-2-carbaldehyde (10)
400 MHz),
d
9.64 (s, 1H), 8.50 (s, 1H), 8.46 (s, 1H), 7.93 (d, J ¼ 8.0 Hz,
The synthesis method resembles that of compound 8 but 6
instead of 4, and the compound was purified by column chroma-
tography on silica gel (hexane/DCM ¼ 2/1, V/V) to give 120 mg of
1H), 7.74 (d, J ¼ 8.0 Hz, 1H), 7.42 (d, J ¼ 8.0 Hz, 1H), 7.36 (d,
J ¼ 4.0 Hz,1H), 7.20 (d, J ¼ 8.0 Hz,1H), 6.84 (d, J ¼ 4.0 Hz,1H), 4.27 (t,
J ¼ 8.0 Hz, 2H), 1.89e1.82 (m, 2H), 1.33e1.23 (m, 10H), 0.86 (t,
J ¼ 8.0 Hz, 3H). MS (ESI, m/z) calcd for C25H27INO2 [M þ H]þ: 500.11,
Found: 500.20.
orange solid in 88% yield. 1H NMR (CDCl3, 400 MHz),
d 9.66 (s, 1H),
8.65 (s, 1H), 8.39 (s, 1H), 7.93 (d, J ¼ 7.2 Hz, 1H), 7.76 (d, J ¼ 7.2 Hz,
1H), 7.64 (d, J ¼ 6.8 Hz, 2H), 7.48 (d, J ¼ 6.8 Hz, 2H), 7.46 (d,
J ¼ 6.8 Hz, 2H), 7.38 (d, J ¼ 2.8 Hz, 1H), 7.32 (t, J ¼ 6.2 Hz, 4H), 7.25e
7.20 (m, 6H), 7.07 (t, J ¼ 6.0 Hz, 2H), 6.88 (d, J ¼ 2.8 Hz, 1H), 4.34 (t,
J ¼ 5.6 Hz, 2H), 1.96e1.90 (m, 2H), 1.46e1.30 (m, 6H), 0.91 (t,
J ¼ 5.6 Hz, 3H). MS (ESI, m/z) calcd for C41H37N2O2 [M þ H]þ:
589.29, Found: 589.35.
2.3.6. 5-(9-Hexyl-6-iodo-9H-carbazol-3-yl)furan-2-carbaldehyde
(6)
The synthesis method resembles that of compound 4 but 2
instead of 1, and the compound was purified by column chroma-
tography on silica gel (hexane/DCM ¼ 2/1, V/V) to yield 170 mg of
yellow solid (yield 37%). 1H NMR (CDCl3, 400 MHz),
d
9.65 (s, 1H),
2.3.11. 5-(6-(4-(Diphenylamino)phenyl)-9-(4-methylpentyl)-9H-
carbazol-3-yl)furan-2-carbaldehyde (11)
8.48 (d, J ¼ 8.4 Hz, 2H), 7.92 (d, J ¼ 6.8 Hz, 1H), 7.74 (d, J ¼ 6.8 Hz,
1H), 7.42 (d, J ¼ 6.8 Hz, 1H), 7.38 (d, J ¼ 2.8 Hz, 1H), 7.20 (d,
J ¼ 6.8 Hz, 1H), 6.86 (d, J ¼ 3.2 Hz, 1H), 4.26 (t, J ¼ 5.8 Hz, 2H), 1.91e
1.83 (m, 2H),1.38e1.29 (m, 6H), 0.88 (t, J ¼ 5.6 Hz, 3H). MS (ESI, m/z)
calcd for C23H23INO2 [M þ H]þ: 472.08, Found: 472.10.
The synthesis method resembles that of compound 8 but 7
instead of 4, and the compound was purified by column chroma-
tography on silica gel (hexane/DCM ¼ 2/1, V/V) to yield 180 mg of
orange solid (yield 79%). 1H NMR (CDCl3, 400 MHz),
d 9.66 (s, 1H),
8.64 (s, 1H), 8.40 (s, 1H), 7.94 (d, J ¼ 6.8 Hz, 1H), 7.76 (d, J ¼ 6.8 Hz,
1H), 7.65 (d, J ¼ 6.8 Hz, 2H), 7.47 (d, J ¼ 6.8 Hz, 1H), 7.45 (d,
J ¼ 7.2 Hz, 1H), 7.38 (d, J ¼ 2.8 Hz, 1H), 7.32 (t, J ¼ 6.2 Hz, 4H), 7.25e
7.20 (m, 6H), 7.08(t, J ¼ 5.8 Hz, 2H), 6.87 (d, J ¼ 2.8 Hz, 1H), 4.31 (t,
J ¼ 5.8 Hz, 2H), 1.96e1.90 (m, 2H), 1.66e1.60 (m, 1H), 1.36e1.30 (m,
2H), 0.92 (d, J ¼ 5.2 Hz, 6H). MS (ESI, m/z) calcd for C41H37N2O2
[M þ H]þ: 589.29, Found: 589.35.
2.3.7. 5-(6-Iodo-9-(4-methylpentyl)-9H-carbazol-3-yl)furan-2-
carbaldehyde (7)
The synthesis method resembles that of compound 6 but 3
instead of 1, and the compound was purified by column chroma-
tography on silica gel (hexane/DCM ¼ 2/1, V/V) to yield 160 mg of
yellow solid (yield 21%). 1H NMR (CDCl3, 400 MHz),
d 9.65 (s, 1H),
8.46 (d, J ¼ 8.4 Hz, 2H), 7.92 (d, J ¼ 6.8 Hz, 1H), 7.74 (d, J ¼ 6.8 Hz,
1H), 7.40 (d, J ¼ 6.8 Hz,1H), 7.37 (d, J ¼ 2.8 Hz,1H), 7.19 (d, J ¼ 6.8 Hz,
1H), 6.84 (d, J ¼ 2.8 Hz, 1H), 4.23 (t, J ¼ 5.8 Hz, 2H), 1.89e1.83 (m,
2H), 1.64e1.54 (m, 1H), 1.29e1.25 (m, 2H), 0.88 (d, J ¼ 2.8 Hz, 6H).
MS (ESI, m/z) calcd for C23H23INO2 [M þ H]þ: 472.08, Found: 472.10.
2.3.12. 2-Cyano-3-(5-(6-(4-(diphenylamino)phenyl)-9-octyl-9H-
carbazol-3-yl)thiophen-2-yl)acrylic acid (K1)
A mixture of 8 (60 mg, 0.095 mmol), 2-cyanoacetic acid (40 mg,
0.45 mmol), ammonium acetate (50 mg) and acetic acid (8 ml) were
heated at 120 ꢂC under an argon atmosphere for 12 h. After cooling
to room temperature, the mixture was added to water. The pre-
cipitate was filtered and washed with water. The residue was pu-
rified by column chromatography on silica gel (DCM/EtOH ¼ 20/1,
v/v) to give a red solid (yield 87%). 1H NMR(400 MHz, THF-d8),
2.3.8. 5-(6-(4-(Diphenylamino)phenyl)-9-octyl-9H-carbazol-3-yl)
thiophene-2-carbaldehyde (8)
A mixture of 4 (100 mg, 0.194 mmol), Pd(PPh3)4 (10 mg,
0.009 mmol), 4-(diphenylamino)phenylboronic acid (56 mg, 0.
0.194 mmol) and toluene (5 ml) were stirred under an nitrogen
atmosphere. K2CO3 (1.02 g, 0.01 mol) in H2O (5 ml) was injected to
the solution and the mixture was heated under reflux for a further
12 h. After cooling to room temperature, the mixture was extracted
with CH2Cl2 (3 ꢁ 10 ml). The combined organic layers were dried
with anhydrous MgSO4. The solvent was evaporated, and the res-
idue was purified by column chromatography on silica gel (hexane/
DCM ¼ 2/1, v/v) to give a yellow solid in 86% yield. 1H NMR (CDCl3,
d
8.49 (s, 1H), 8.43 (s, 1H), 8.34 (d, J ¼ 8.4 Hz, 1H), 7.74 (t, J ¼ 7.8 Hz,
3H), 7.66 (d, J ¼ 7.2 Hz, 2H), 7.49 (s, 2H), 7.26 (t, J ¼ 7.2 Hz, 4H), 7.18e
7.12 (m, 7H), 7.01 (t, J ¼ 7.0 Hz, 2H), 4.28 (t, J ¼ 7.2 Hz, 2H), 1.90e1.80
(m, 2H), 1.33e1.22 (m, 10H), 0.87 (t, J ¼ 6.8 Hz, 3H). 13C NMR (THF-
d8, 100 MHz),
d 161.3, 148.2, 147.9, 146.4, 141.1, 140.1, 136.4, 134.8,
129.0, 127.6, 124.5, 124.4, 124.0, 123.3, 122.5, 118.5, 117.9, 109.1, 42.7,
31.8, 29.4, 29.2, 28.9, 27.1, 22.5, 13.4. HRMS (ESI, m/z) calcd for
C
46H41N3O2S [MeH]ꢀ: 699.2919, Found: 699.2895.

78
J. He et al. / Dyes and Pigments 104 (2014) 75e82
2.3.13. 2-Cyano-3-(5-(6-(4-(diphenylamino)phenyl)-9-octyl-9H-
carbazol-3-yl)furan-2-yl)acrylic acid (K2)
drill-press. The perforated sheet was cleaned with ultrasound in an
ethanol bath for 10 min. For the assembly of DSSCs, the dye-covered
TiO2 electrode and Pt-counter electrode were assembled into a
A procedure similar to that for the dye K1 but with compound 9
(120 mg, 0.17 mmol) instead of compound 8 giving the dye K2 as a
sandwich type cell and sealed with a hot-melt gasket of 25 mm
dark red solid (yield 67%). 1H NMR (400 MHz, THF-d8),
d
8.82 (s,1H),
thickness made of the ionomer Surlyn 1702 (DuPont). The elec-
trolyte was introduced into the cell via backfilling from a hole in the
back of the counter electrode. Finally, the hole was sealed using a
self-adhesive aluminium film. In this work, two liquid electrolytes
were employed: electrolyte A, 0.1 M LiI, 0.03 M I2, 0.6 M PMII, 0.1 M
Guanidine thiocyanate (GuSCN), and 0.5 M TBP in a mixture of
acetonitrile and valeronitrile (volume ratio ¼ 7:3); electrolyte B,
0.1 M LiI, 0.05 M I2, 0.6 M PMII, and 0.5 M TBP in a mixture of
acetonitrile and MPN (volume ratio ¼ 7:3).
8.45 (s, 1H), 8.12 (d, J ¼ 8.4 Hz, 1H), 8.04 (s, 1H), 7.78 (d, J ¼ 8.4 Hz,
1H), 7.70e7.59 (m, 4H), 7.47 (s, 1H), 7.29 (t, J ¼ 8.0 Hz, 4H), 7.21e7.14
(m, 7H), 7.03 (t, J ¼ 7.2 Hz, 2H), 4.44 (t, J ¼ 7.2 Hz, 2H), 1.96e1.93 (m,
2H), 1.46e1.30 (m, 10H), 0.90 (d, J ¼ 7.0 Hz, 3H). 13C NMR (THF-d8,
100 MHz), d 161.1, 148.0, 147.7, 146.6, 141.6, 140.4, 136.4, 132.5, 129.0,
127.6, 125.1, 124.4, 124.0, 123.3, 122.5, 120.2, 118.2, 117.8, 109.5,
109.4, 42.9, 31.8, 29.4, 29.2, 29.0, 27.1, 22.5, 13.4. HRMS (ESI, m/z)
calcd for C46H41N3O3 [MeH]ꢀ: 683.3148, Found: 683.3092.
2.3.14. 2-Cyano-3-(5-(6-(4-(diphenylamino)phenyl)-9-hexyl-9H-
carbazol-3-yl)furan-2-yl)acrylic acid (K3)
2.5. Photovoltaic performance measurements
A procedure similar to that for the dye K1 but with compound
10 (120 mg, 0.17 mmol) instead of compound 8 giving the dye K3 as
Photocurrentevoltage (JeV) measurements were performed
using an AM 1.5 solar simulator (Newport, USA). The solar simu-
lator was calibrated by using a standard silicon cell (Newport, USA).
a red solid (yield 58%). 1H NMR (400 MHz, THF-d8),
d 8.67 (s, 1H),
8.30 (s, 1H), 7.97(d, J ¼ 7.6 Hz, 1H), 7.91 (s, 1H), 7.64 (d, J ¼ 8.4 Hz,
1H), 7.55 (d, J ¼ 8.4 Hz, 2H), 7.45 (d, J ¼ 8.4 Hz, 2H), 7.32 (s, 1H), 7.15
(t, J ¼ 7.8 Hz, 4H), 7.07e7.01 (m, 7H), 6.90 (t, J ¼ 7.2 Hz, 2H), 4.28 (t,
J ¼ 7.2 Hz, 2H), 1.82e1.78 (m, 2H), 1.32e1.20 (m, 6H), 0.76 (d,
Currentevoltage curves were measured using a computer-
controlled IV tracer (VS-6810, Industrial Vision Technology (S) Pte
Ltd., Singapore). A black mask with an aperture area of 0.1589 cm2
was used to test cell active area. The photocurrent action spectra
were measured with a monochromatic Incident Photon-to-electron
Conversion Efficiency (IPCE) test system consisting of a Model
SR830 DSP Lock-In Amplifier, a Model SR540 Optical Chopper
(Stanford Research Corporation, USA), a 7IL/PX150 xenon lamp
with power supply, and a 7ISW301 Spectrometer.
J ¼ 6.6 Hz, 3H). 13C NMR (THF-d8, 100 MHz),
d 159.4, 146.1, 145.7,
144.7, 139.8, 138.5, 135.2, 134.5, 130.6, 127.2, 125.7, 123.2, 122.5,
122.1, 121.5, 121.4, 120.7, 118.4, 116.4, 115.9, 107.6, 107.5, 105.3, 41.0,
29.7, 27.1, 24.9, 20.6,11.5. HRMS (ESI, m/z) calcd for C44H37N3O3 [Me
H]ꢀ: 655.2835, Found: 655.2806.
2.3.15. 2-Cyano-3-(5-(6-(4-(diphenylamino)phenyl)-9-(4-
methylpentyl)-9H-carbazol-3-yl)furan-2-yl)acrylic acid (K4)
A procedure similar to that for the dye K1 but with compound 11
(120 mg, 0.17 mmol) instead of compound 8 giving the dye K4 as a
3. Results and discussion
3.1. Synthesis and structural characterization
red solid (yield 56%). 1H NMR(400 MHz, THF-d8),
d
8.81 (s, 1H), 8.44
The synthetic route employed to access four dyes (K1eK4) is
shown in Scheme 1. The dyes were prepared from 3,6-diiodo-9H-
carbazole, which was synthesized according to a published proce-
dure [58]. Notably, the alkyl chain on the carbazole group can
improve the solubility and form a tightly packed insulating
monolayer blocking the Iꢀ3 anions or cations from approaching the
TiO2 surface, thereby retarding the charge recombination processes
of injected electrons with Iꢀ3 ions in the electrolyte, which is the
main decreasing factor for Voc [33]. Accordingly, we incorporated
the carbazole unit with different alkyl groups on the 9-position as
an electron donor, cyanoacrylic acid as an electron acceptor, furan
or thiophene as a linker and triphenylamine as an auxiliary electron
(s, 1H), 8.11 (d, J ¼ 7.6 Hz, 1H), 8.05 (s, 1H), 7.78 (d, J ¼ 8.4 Hz, 1H),
7.68 (d, J ¼ 8.4 Hz, 2H), 7.59 (d, J ¼ 8.4 Hz, 2H), 7.45 (s, 1H), 7.29 (t,
J ¼ 7.8 Hz, 4H), 7.21e7.14 (m, 7H), 7.03 (t, J ¼ 7.4 Hz, 2H), 4.42 (t,
J ¼ 7.2 Hz, 2H), 1.96e1.93 (m, 2H), 1.66e1.59 (m, 1H), 1.38e1.33 (m,
2H), 0.91 (d, J ¼ 6.4 Hz, 6H). 13C NMR (THF-d8, 100 MHz),
d 159.0,
148.0,147.6,146.5,141.7,140.3,136.4,132.4,129.0,127.6,125.1,124.4,
124.0, 123.5, 123.4, 123.2, 120.2, 118.2, 117.8, 109.4, 109.3, 43.1, 36.1,
27.9, 26.8, 21.9. HRMS (ESI, m/z) calcd for C44H37N3O3 [MeH]ꢀ:
655.2835, Found: 655.2786.
2.4. Fabrication of DSSCs
donor into the donoredonorepeacceptor (DeDepeA) system. As
The TiO2 films were fabricated with a screen printing method
illustrated in Scheme 1, the target dyes K1eK4 were conveniently
prepared by a conventional Suzuki cross-coupling and Knoevenagel
condensation reactions. Suzuki coupling between the intermediate
according to the published method [60]. The thickness is about
15 m
m. The TiO2 electrodes were heated under an air flow at 450 ꢂC
for 15 min and 500 ꢂC for 15 min. The sintered films were further
treated with 40 mM TiCl4 aqueous solution at room temperature for
12 h and washed with water and ethanol, a process which can
significantly increase the short-circuit photocurrent. Because TiCl4
(aqueous) had the effect to remove the iron contamination source,
which can quench the dye-sensitized photocurrent (Jsc) [61]. Then
the films were annealed at 450 ꢂC for 30 min. After the TiCl4 pre-
treated films were cooled down at around 50 ꢂC, the films were
immersed into a 5 ꢁ 10ꢀ4 M dye bath in CH2Cl2 solutions and
maintained under dark for 48 h at room temperature. The electrode
was then rinsed with CH2Cl2 and ethanol and dried. The size of the
TiO2 electrodes used was 0.28 cm2. To prepare the counter elec-
trode, the Pt catalyst was deposited on the cleaned FTO glass by
spinning with a drop of H2PtCl6 solution (20 mM 2-propanol so-
lution) with the heat treatment at 400 ꢂC for 15 min. Two holes
(0.8 mm diameter) were drilled on the counter electrode using a
1e3 and the corresponding boronic acid of the p-conjunction
bridge was carried out using Pd(PPh3)4 as a catalyst in a mixture of
2 M aqueous K2CO3 and THF under reflux conditions to give the
corresponding intermediate 4e7. These intermediates were reacted
with (4-(diphenylamino)phenyl)boronic acid through a Suzuki
coupling reaction using toluene as solvent to provide the corre-
sponding intermediate 8e11. Finally, the target dyes (K1eK4) were
synthesized via Knoevenagel condensation reaction of aldehydes
8e11 with cyanoacetic acid in the presence of acetic acid and
ammonium acetate. All the intermediates and target dyes were
confirmed by standard spectroscopic methods.
3.2. Absorption properties in solutions and on TiO2 film
The UVevis absorption spectra of the dyes K1eK4 in DCM and
adsorbed on 7 mm transparent TiO2 films are displayed in Fig. 2. The

J. He et al. / Dyes and Pigments 104 (2014) 75e82
79
I
(HO)2B
I
X
I
CHO
RBr, KOtBu
DMF
CHO
I
X
I
Pd(PPh3)4, K2CO3
THF, H2O
N
H
N
N
R
R
1-3
4-7
CHO
(4-(diphenylamino)phenyl)
boronic acid
N
NC
COOH
X
Pd(PPh3)4, K2CO3, Toulene, H2O
AcONH4, AcOH
N
R
8-11
CN
COOH
K1: X=S, R=
K2: X=O, R=
K3: X=O, R=
K4: X=O, R=
N
X
N
R
K1-K4
Scheme 1. Synthetic routes of carbazole dyes (K1eK4).
related photophysical data are summarized in Table 1. The four dyes
K1eK4 in DCM exhibit two strong absorption peaks, one at 300e
380 nm and the other at around 380e550 nm. The first adsorption
between the adjacent molecules on the TiO2 surfaces [64]. The
blue-shift in the absorption spectra on the TiO2 film of K2 and K4
might arise by the interaction of the anchoring groups with the
TiO2 surface because of the introduction of alkyl substituted
carbazole, which is beneficial to the inhibition of dye aggregation.
band might be ascribed to a localized aromatic
pep* electron
transition of typical triarylamine [48,62]. The second absorption
band may be assigned to the intramolecular charge transition (ICT)
between the central carbazole (electron donor) and the cyanoacetic
acid (electron acceptor and anchoring moiety) [9]. Compared to
that of K1, the adsorption spectra of K2eK4 are red-shifted by
26 nm together with an increase in molar extinction coefficient,
indicating that the furan unit can exhibit a better absorption per-
formance compared to the thiophene unit. This may be caused by
the different aromatic ability of the two five-membered hetero-
aromatic bridges. The aromatic ability of furan is larger than that of
thiophene due to the stronger electronegative of oxygen element
than of sulfur [63]. As summarized in Table 1, the molar extinction
coefficient of K2eK4 are determined to be 36,700, 36,400 and
33,500 Mꢀ1 cmꢀ1, respectively, which are about 1.22, 1.21 and 1.12
times the corresponding value of K1. This may also demonstrate the
superiority of a furan conjunction bridge in light harvesting in the
visible region. The dye K2 gives a maximum absorption similar to
the dye K3 and K4 but its molar extinction coefficient
This TiO2 surface interaction directly reduces the energy of the p*
level and the formation of J-aggregates on the TiO2 surface [65].
3.3. Electrochemical properties
To investigate the ability of electron transfer from the excited
dye to the conduction band of TiO2, cyclic voltammetric analysis of
the four dyes was performed. Fig. 3 depicts the typical cyclic vol-
tammogram of K1eK4, the highest occupied molecular orbitals
(HOMOs) of K1eK4 corresponding to their first redox potential are
0.95e0.97 V vs. NHE. As estimated from the absorption threshold of
these dyes adsorbed onto TiO2 films, the resulting E0e0 is 2.34, 2.31,
2.31 and 2.31 eV for K1, K2, K3 and K4, respectively. The estimated
excited state potential corresponding to the lowest unoccupied
molecular orbital (LUMO) levels, calculated from EHOMO e E0e0
,
are ꢀ1.38, ꢀ1.36, ꢀ1.34 and ꢀ1.36 V, respectively. The examined
HOMO and LUMO levels are listed in Table 1. From these data, it is
observed that the HOMO levels of K1eK4 are sufficiently more
positive than the iodine/iodide redox potential value (0.4 V vs.
NHE), indicating that the oxidized dyes formed after electron in-
jection into the conduction band of TiO2 could thermodynamically
accept electrons from Iꢀ ions. The LUMO levels of these dyes 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 dye into the conduction band of
TiO2 is energetically permitted [66].
(
(
3
¼ 3.67 ꢁ 104 Mꢀ1 cmꢀ1) is higher than those of the dye K3
3
¼ 3.647 ꢁ 104 Mꢀ1 cmꢀ1) and K4 (
3
¼ 3.35 ꢁ 104 Mꢀ1 cmꢀ1),
which indicates that the absorption capacity of K2 can be enhanced
by increasing length of alkyl chain. Similar studies with results from
increasing alkyl chain have been reported [9,64]. The absorption
spectra of K1eK4 adsorbed on the surface of 7 mm transparent
TiO2 films are shown in Fig. 2(b). All dyes show broadened ab-
sorption bands as compared to those in solutions, which is bene-
ficial to light-harvesting and thus increase the photocurrent
response, leading to an increasing Jsc [9]. The maximum absorption
peaks for K1eK4 on the TiO2 film are at 463, 457, 488 and 445 nm,
respectively. The red shifts of the lmax of K1 and K3 are possible due
to the J-aggregation of the dyes on the TiO2 surface. Such J-aggre-
3.4. Photovoltaic device performance
Fig. 4 shows the action spectra of incident photon-to-current
gation formation may indicate
a
head-to-tail configuration
conversion efficiency (IPCE) for DSSCs based on the four dyes

80
J. He et al. / Dyes and Pigments 104 (2014) 75e82
Fig. 3. Cyclic voltammetry plots of K1eK4 attached to a nanocrystalline TiO2 film
deposited on conducting FTO glass.
conversion yield among four dyes; a feather which is supported by
the device results. The highest IPCE value of K2 may be attributed to
its higher adsorption and enhanced molar extinction coefficient.
This is in good agreement with the absorbance spectra of K2 in
solution.
The currentevoltage characteristics of DSSCs fabricated with
these dyes (K1eK4) as sensitizers were measured 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 summa-
rized in Table 2. Two kinds of electrolyte were employed in the
DSSCs application. When electrolyte B is used, the DSSC based on
K2 produces the highest
h
of 6.68% (Jsc ¼ 13.26 mA cmꢀ2
,
Voc ¼ 0.724 V, FF ¼ 0.66), which should be attributed to its best IPCE
performance. As seen in the results, the Jsc increases from
12.14 mA cmꢀ2 to 13.26 mA cmꢀ2 by increasing chain length from
methylpentyl (K4) to octyl (K2) for K2eK4. This behavior of dyes
may be due to alkyl chains self-assembling on TiO2 film and a
longer distance between the TiO2 film and electrolyte by length-
ening of the alkyl chains, which block the recapture of the photo-
Fig. 2. Normalized UVeVis absorption spectra of carbazole dyes (K1eK4) in CH2Cl2
solution (a) and on TiO2 film (b).
(K1eK4). The dye-coated TiO2 film was used as the working elec-
trode, platinized FTO glass as the counter electrode and 0.6 M PMII,
0.05 M I2, 0.10 M LiI and 0.5 M TBP in acetonitrile and MPN (volume
ratio, 7: 3) mixture solution as the redox electrolyte. The solar cells
based on K1eK4 exhibit high IPCE values above 60% in the range of
340 nm to around 540 nm and with the highest value of 76.7% in
the range of 480 nm for K1, 92.4% in the range of 480 nm for K2,
82.4% at 430 nm for K3 and 79.6% at 420 nm for K4, respectively.
Maximum IPCE values of the dyes are in the order of
K2 > K3 > K4 > K1. In general, higher IPCE means higher electron
transfer yield [67]. Therefore, introduction of a furan moiety and a
longer side chain into dyes has a positive effect on IPCE. Thus, we
can infer that the DSSC based on K2 would generate the highest
Table 1
Optical and electrochemical properties of carbazole dyes.
Dye
lmax/nm
lonset
HOMO/V
(vs. NHE)
E0e0 (eV)
LUMO/V
(vs. NHE)
(
3
ꢁ 104 Mꢀ1 cmꢀ1
)
(nm)
K1
K2
K3
K4
442 (3.00)
468 (3.67)
468 (3.64)
468 (3.35)
530
537
537
537
0.96
0.95
0.97
0.95
2.34
2.31
2.31
2.31
ꢀ1.38
ꢀ1.36
ꢀ1.34
ꢀ1.36
Fig. 4. Photocurrent action spectra of the TiO2 electrodes sensitized by K1eK4.

J. He et al. / Dyes and Pigments 104 (2014) 75e82
81
Table 2
conditions, which indicates that the introduction of the alkyl chains
into the carbazole moiety can inhibit the charge recombination and
Photovoltaic performance of DSSCs based on the carbazole dyes K1eK4 and N719
with liquid electrolyte A and B.a
improve Voc
Conclusions
In summary, we have designed and synthesized four new
.
Dye
Electrolyte
Jsc (mA cmꢀ2
)
Voc (mV)
FF
h (%)
K1
K1
K2
K2
K3
K3
K4
K4
A
B
A
B
A
B
A
B
A
B
9.54
11.12
11.00
13.26
10.85
12.31
10.18
12.14
15.58
14.29
815
741
786
724
797
737
799
722
783
703
0.673
0.678
0.661
0.661
0.644
0.666
0.634
0.655
0.635
0.673
5.23
5.59
6.02
6.68
5.86
6.35
5.43
6.04
7.74
6.98
carbazole-based metal-free dyes, in which linear or branched alkyl
side chains were introduced into to the conjugated system to form a
blocking layer. The electron-donating groups of triphenylamine in-
crease the electron density of the donor moiety and enhance the
molarextinction coefficientof the dyes. Thefuranorthiophenelinker
was introduced to enhance the molar extinction coefficient of dyes.
The results based on photovoltaic experiments show that the four
dyes exhibit a high open circuit voltage (0.72e0.83 V). The photo-
voltaic performance of K2 in DSSCs demonstrates the best overall
conversion efficiency of 6.68%, 86.3% of the standard cell from N719
(7.74%) under the same testing conditions. The results indicate that
the introduction of a longer alkyl chain into the carbazole moiety will
retard recombination and the introduction of furan as the conjugate
bridge will increase the photocurrent response; features which lead
to enhanced Jsc. Therefore, we conclude that the introduction of both
octyl-substituted carbazole and a furan moiety into DSSC can
improve the cell efficiency significantly. It was expected that, by
adjusting the molecular structure of this kind of carbazole-based
metal-free dyes, higher efficiency could be achieved.
N719
N719
a
Illumination: 100 mW cmꢀ2 simulated AM 1.5 G solar light; electrolyte con-
taining: A: 0.1 M LiI, 0.6 M PMII, 0.03 M I2, 0.1 M GuSCN, 0.5 M TBP, in acetonitrile
and valeronitrile (7:3); B: 0.1 M LiI, 0.05 M I2, 0.6 M PMII, 0.5 M TBP in the mixed
solvent of acetonitrile and MPN (7:3, v/v).
injected electrons by triodide ions [9]. Compared to K2, K3 and K4,
K1 has a lower Jsc value, indicating that furan moiety may have
better charge carrier mobility than the thiophene moiety [68]. This
result may be due to the low resonance energy of the furan moiety
[57,69]_ENREF_71. This trend of Jsc (K2 > K3 > K4 > K1) is in good
accordance with the order of IPCE values and absorbance maxima
of the dyes in solution. A similar trend was also observed in Jsc
when electrolyte A was used. But the values of Jsc of K1eK4 using
electrolyte B are better than those based on electrolyte A. This
result is different in complex ruthenium dye N719 (Fig. 5), indi-
cating that electrolyte B benefits the IPCE value for organic dyes.
However, the Voc values of the DSSCs based on electrolyte B are
decreased compared to that of electrolyte A. It was proposed that
the dissociated positive Ti4þ ions and protons in electrolyte B can be
adsorbed onto the surface of TiO2 and hence the conduction band of
TiO2 is possible to undergo a positive shift. Similar phenomena have
already been reported in the case of adding additives in the elec-
trolyte, such as lithium ions, alkali ions and TBP, which are gener-
ally believed to affect the surface properties and the
electrochemical potentials of TiO2 [67]. It is also noteworthy that all
of carbazole dyes have higher open circuit voltages than N719
measured with two electrolytes under the same measurement
Acknowledgment
We would like to thank Ministry of Education, Singapore (tier 1
research grant MOE2008-IF-1-016), Singapore Polytechnic (Idea
Fund R733 and R734) for the financial support and Everlight
Chemical Industrial Co. for gifts of materials.
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