Coumarin dyes containing low-band-gap chromophores for dye-sensitised solar cells

Article abstract of DOI:10.1016/j.dyepig.2011.01.009

A series of coumarin dyes containing a low-band-gap chromophore of ethylenedioxythiophene (EDOT), which comprises a coumarin moiety as the electron donor and a cyanoacrylic acid moiety as electron acceptor in D-π-A chromophores, were developed for use in

Full text of DOI:10.1016/j.dyepig.2011.01.009

Dyes and Pigments 90 (2011) 304e310
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Coumarin dyes containing low-band-gap chromophores
for dye-sensitised solar cells
Kang Deuk Seo a, Hae Min Song a, Myung Jun Lee a, Mariachiara Pastore b, Chiara Anselmi b,
,
Filippo De Angelis b, Mohammad K. Nazeeruddin c, Michael Gräetzel c, Hwan Kyu Kim a
*
a Department of Advanced Materials Chemistry & Centre for Advanced Photovoltaic Materials (ITRC), Korea University, 208 Seochang Jochiwon,
Jochiwon, Chungnam 339-700, Republic of Korea
b Istituto CNR di Scienze e Tecnologie Molecolari c/o Dipartimento di Chimica Università di Perugia, via Elce di Sotto 8, I-06123, Perugia, Italy
c Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of coumarin dyes containing a low-band-gap chromophore of ethylenedioxythiophene (EDOT),
which comprises a coumarin moiety as the electron donor and a cyanoacrylic acid moiety as electron
Received 17 November 2010
Received in revised form
18 January 2011
Accepted 19 January 2011
Available online 27 January 2011
acceptor in DepeA chromophores, were developed for use in dye-sensitised solar cells (DSSCs). These
coumarin dyes have been used to fabricate DSSCs using Iꢀ/I3ꢀ liquid electrolytes and their device
performances were compared with that of NKX-2677 as a standard dye. Even though HKK-CM2 and
HKK-CM3 have more extended aromatic units than HKK-CM1, the degree of p-conjugation in HKK-CM2
and HKK-CM3 is less efficient than that of HKK-CM1, due to the relatively larger torsion angle between
the plane of the donor and that of the acceptor. It is also in a good agreement with Density Functional
Theory (DFT) calculations. As a result, a solar cell based on HKK-CM1 sensitiser shows better photovoltaic
performance with JSC of 14.2 mA cmꢀ2, VOC of 0.60 V, and a fill factor of 0.70, corresponding to an overall
Keywords:
Coumarin
Dye-sensitised solar cell
Low-band-gap chromophore
Ethylenedioxythiophene
Molecular orbitals
conversion efficiency
h
of 6.07% under the standard AM 1.5 irradiation, than HKK-CM2 and HKK-CM3-
based solar cells.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
[28e31], and phthalocyanine [32] dyes, have been developed, and
solar-to-electric power-conversion efficiencies have been sharply
increasing reaching ca. 10% [3,33,34]. Although remarkable prog-
ress has been made in the organic dyes as sensitisers for DSSCs, it is
still needed to optimise their chemical structures for further
improvement in performances.
Among the metal-free organic dyes studied in DSSCs, coumarin-
based dyes are stuff for TiO2 kind of promising sensitisers because
of their good photoresponse in the visible region, good long-term
stability under one sun soaking [35], and appropriate lowest
unoccupied molecular orbital (LUMO) levels matching the
conduction band of TiO2. NKX-2677 dye has been used successfully
Dye-sensitised solar cells (DSSCs) based on mesoporous nano-
crystalline TiO2 films have attracted significant attention due to
their low cost and high sunlight-to-electric power-conversion
efficiencies of 10e11% [1e4]. In these cells, the sensitiser is one of
the key components for high power-conversion efficiency, and the
Ru(II) complex is the most efficient heterogeneous charge-transfer
sensitiser that is widely used in the nanocrystalline TiO2-based
DSSC [5e9]. However, the main drawback of the Ru(II) complex
sensitiser is expensive ruthenium metal and requires careful
synthesis and tricky purification steps [10].
In recent years, much interest in metal-free organic dyes as an
alternative to noble metal complexes has increased due to many
advantages, such as diversity of molecular structures, high molar
extinction coefficient, simple synthesis as well as low cost and
environmental issues. Metal-free organic dyes such as triarylamine
dyes [11e16], hemicyanine dyes [17,18], thiophene-based dyes [19],
indoline dyes [20e26], heteropolycyclic dyes [27], porphyrin dyes
as a photosensitiser in DSSC, and obtained maximum h value of up
to 7.4% [36]. Though low-band-gap chromophore-based polymers
[37e42] have been widely employed in photovoltaic applications,
coumarin dyes containing low-band-gap chromophore as struc-
tural motifs have not been exploited for DSSCs. Here, we report
coumarin dyes containing a low-band-gap chromophore, so-called,
3,4-ethylenedioxythiophene (EDOT) substituted coumarin deriva-
tives, in which an electron-rich EDOT unit is connected to a donor
group in order to destabilise the energy of the highest occupied
* Corresponding author. Tel.: þ82 41 860 1493; fax: þ82 41 867 5396.
E-mail address: hkk777@korea.ac.kr (H.K. Kim).
molecular orbital (HOMO) in DepeA chromophores [34,43].
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.01.009

K.D. Seo et al. / Dyes and Pigments 90 (2011) 304e310
305
2. Experimental
for 0.5 h. The mixture was extracted with chloroform and then
washed with aqueous saturated NaCl solution and then dried over
MgSO4. After filtration and evaporation of the solvent, the resulting
precipitate was recrystallised from DMF. Yield was 59%. 1H NMR
2.1. Materials
All reactions were carried out under a nitrogen atmosphere.
Solvents were distilled from appropriate reagents. All reagents
were purchased from Aldrich. 9-(5-bromothiophen-2-yl)-
1,1,6,6-tetramethyl-2,3,5,6-tetrahydro-1H,4H,11-oxa-3a-azabenzo
[de]anthracen-10-one [36], 2-tri-butylstannyl-3,4-ethyl-enediox-
ythiophene [44], 2-tributylstanyl-5-dioxolanylthiophene [45] were
synthesised by following the same procedures as described
previously.
(300 MHz, CDCl3) d (TMS, ppm): 1.32 (6H, s), 1.58 (6H, s), 1.75e1.84
(4H, m), 3.25e3.33 (4H, m), 4.43 (4H, s), 7.16 (1H, s), 7.41 (1H, d),
7.60 (1H, d), 7.88 (1H, s), 9.89 (1H, s).
2.4.3. Preparation of 5-((1,1,6,6-tetramethyl-10-oxo-2,3,5,6-
tetrahydro-1H,4H,10H-11-oxa-3a-azabenzo[de]anthracen-9-yl)-
thiophene-2-yl-)-3,4-ethylenedioxythiophene-5-yl) acrylic acid
(HKK-CM1)
An acetonitrile solution (15 mL) of compound 4 (0.25 g,
0.46 mmol) and cyanoacetic acid (0.07 g, 0.82 mmol) was refluxed
at 120 ꢁC in the presence of piperidine (0.04 mL) for 6 h. After
cooling the solution, the product was purified by chromatography
on silica gel (CH2Cl2eMeOH 5:1). Yield was 69%. 1H NMR (DMSO-
2.2. Measurement
The 1H NMR spectra were recorded at room temperature with
Varian Oxford 300 spectrometers and chemical shifts were repor-
ted in ppm units with tetramethylsilane as an internal standard. FT-
IR spectra were measured as KBr pellets on a Perkin Elmer Spec-
trometer. UVevisible absorption spectra were obtained in THF on
a Shimadzu UV-2401PC spectrophotometer. Cyclic voltammetry
was carried out with a Versa STAT3 (AMETEK). A three-electrode
system was used and consisted of a reference electrode (Ag/AgCl),
a working electrode, and a platinum wire electrode. Redox potential
of dyes on TiO2 was measured in CH3CN with 0.1 M TBAPF6 with
d6)
3.27e3.33 (4H, m), 4.53 (4H, s), 7.40 (1H, s), 7.44 (1H, d), 7.66 (1H,
d), 8.07 (1H, s), 8.38 (1H, s). UVevis (THF, nm): lmax (log ) 532
d (TMS, ppm): 1.27 (6H, s), 1.48 (6H, s), 1.70e1.76 (4H, m),
3
(52,700). PL (THF, nm): lmax 666. FAB-mass: Calcd. for
C33H30N2O6S2, 614.15; found, 614.00.
2.4.4. Preparation of 5-((1,1,6,6-tetramethyl-10-oxo-2,3,5,6-
tetrahydro-1H,4H,10H-11-oxa-3a-azabenzo[de]anthracen-9-yl)-
thiophene-2-yl-)-5-bromo-3,4-ethylenedioxythiophene (5)
a scan rate between 50 mV sꢀ1
.
Compound 3 (1.74 g, 3.35 mmol) was dissolved in 20 mL of DMF
and then 13 mL of DMF solution including N-bromosuccinimide
(0.18 g, 3.43 mmol) was added to the solution, which was then kept
for 12 h. Ethanol (60 mL) and water (10 mL) were added and
a precipitate of compound 6 was formed. Yield was 56%. 1H NMR
2.3. Density functional theory (DFT)/time-dependent
DFT (TDDFT) calculations
The ground state geometries of the dyes have been optimised,
in gas phase, by DFT employing the B3LYP [46] functional and a 6-
31G* basis set; only the protonated species have been considered.
The vertical excitation energies in THF have been calculated by
TDDFT at MPW1K [47]/6-31G* level, including the solvation
effects by the Conductor-like Polarisable Continuum Model
(CPCM) [48]. All the calculations were carried out with GAUSSIAN
03 [49].
(300 MHz, CDCl3)
d (TMS, ppm): 1.31 (6H, s), 1.57 (6H, s), 1.74e1.83
(4H, m), 3.22e3.30 (4H, m), 4.25e4.34 (4H, m), 7.11 (1H, s), 7.13(1H,
d), 7.61 (1H, d), 7.81 (1H, s).
2.4.5. Preparation of 5-((1,1,6,6-tetramethyl-10-oxo-2,3,5,6-
tetrahydro-1H,4H,10H-11-oxa-3a-azabenzo[de]anthracen-9-yl)-
thiophene-2-yl-)-3,4-ethylenedioxythiopheneyl-5- benzaldehyde
(6)
2.4. Synthesis
Compound 5 (0.57 g, 0.94 mmol), 4-fomyl phenyl boronic acid
(0.17 g, 1.13 mmol), Pd (PPh3)4 (0.07 g, 0.06 mmol), Na2CO3 (0.40 g,
3.77 mmol) in H2O were dissolved in 40 mL of solvent (THF and
toluene, 1:1) and then kept at 80 ꢁC for 8 h under an inert N2
atmosphere. The mixture was extracted with chloroform and then
washed with aqueous saturated NaCl solution and then dried over
dehydrated MgSO4. After filtration and evaporation of the solvent,
the resulting precipitate was recrystallised from DMF. Yield was
2.4.1. Preparation of 5-((1,1,6,6-tetramethyl-10-oxo-2,3,5,6-
tetrahydro-1H,4H,10H-11-oxa-3a-azabenzo[de]anthracen-9-yl)-
thiophene-2-yl-)-3,4-ethylenedioxythiophene (3)
A mixture of compound 1 (4.00 g, 8.73 mmol), compound 2
(4.90 g,11.4 mmol) and Pd(PPh3)4 (0.80 g, 0.69 mmol) was heated in
dry THF 250 mL at 80 ꢁC for 20 h under an inert N2 atmosphere.
After concentration, the residue was dissolved in CH2Cl2. The
organic phase was washed twice with a saturated solution of
NaHCO3 and then with water. After drying over MgSO4 and evap-
orating the solvent, the product was purified by chromatography on
silica gel (CH2Cl2ehexane 2:1) to give compound 3 as an orange
88%. 1H NMR (300 MHz, CDCl3)
d (TMS, ppm): 1.32 (6H, s), 1.58 (6H,
s), 1.77e1.82 (4H, m), 3.24e3.31 (4H, m), 4.43 (4H, s), 7.15 (1H, s),
7.26 (1H, d), 7.61 (1H, d), 7.84e7.90 (5H, m), 9.96 (1H, s).
2.4.6. Preparation of 5-(((1,1,6,6-tetramethyl-10-oxo-2,3,5,6-
tetrahydro-1H,4H,10H-11-oxa-3a-azabenzo[de]anthracen-9-yl)-
thiophene-2-yl-)-3,4-ethylenedioxythiophene-5-yl)-benzyl-4-yl)
acrylic acid (HKK-CM2)
solid. Yield was 59%. 1H NMR (300 MHz, CDCl3)
(6H, s),1.56 (6H, s),1.76e1.83 (4H, m), 3.22e3.33 (4H, m), 4.25e4.34
(4H, m), 6.22 (1H, s), 7.14 (1H, s), 7.19 (1H, d), 7.61 (1H, d), 7.81
(1H, s).
d (TMS, ppm): 1.31
An acetonitrile solution (15 mL) of compound 6 (0.51 g,
0.82 mmol) and cyanoacetic acid (0.11 g,1.29 mmol) was refluxed at
120 ꢁC in the presence of piperidine (0.07 mL) for 6 h. After cooling
the solution, the product was purified by chromatography on silica
2.4.2. Preparation of 5-((1,1,6,6-tetramethyl-10-oxo-2,3,5,6-
tetrahydro-1H,4H,10H-11-oxa-3a-azabenzo[de]anthracen-9-yl)-
thiophene-2-yl-)-3,4-ethylenedioxythiophene carbaldehyde (4)
Compound 3 (0.42 g, 0.81 mmol) was dissolved in 10 mL of DMF
and then phosphorus oxychloride (0.21 g, 1.37 mmol) was added
into the solution, which was then kept at room temperature for
1.5 h. The resulting solution was added to water. After neutralisa-
tion with 25% (wt/wt) NaOH solution, the solution was kept at 50 ꢁC
gel (CH2Cl2eMeOH 5:1). Yield was 69%. 1H NMR (DMSO-d6)
d (TMS,
ppm): 1.27 (6H, s), 1.58 (6H, s), 1.70e1.78 (4H, m), 3.26e3.33 (4H,
m), 4.43 (4H, s), 7.33 (1H, d), 7.42 (1H, s), 7.64 (1H, d), 7.83 (2H, d),
8.03 (2H, d), 8.22 (1H, s), 8.32 (1H, s). UVevis (THF, nm): lmax (log 3)
509 (67,200). PL (THF, nm): lmax 597. FAB-mass: Calcd. for
C39H34N2O6S2, 690.19; found, 690.00.

306
K.D. Seo et al. / Dyes and Pigments 90 (2011) 304e310
2.4.7. Preparation of 5-((1,1,6,6-tetramethyl-10-oxo-2,3,5,6-
tetrahydro-1H,4H,10H-11-oxa-3a-azabenzo[de]anthracen-9-yl)-
thiophene-2-yl-)-3,4-ethylenedioxythiopheneyl-5-
cell (PV Measurement Inc.). The applied potential and measured
cell current were measured using a Keithley model 2400 digital
source metre. The currentevoltage characteristics of the cell under
these conditions were determined by biasing the cell externally and
measuring the generated photocurrent. This process was fully
automated using Wavemetrics software.
thiophenylaldehyde (8)
A mixture of compound 5 (0.54 g, 0.91 mmol), compound 7
(0.60 g, 1.35 mmol) and Pd(PPh3)4 (0.08 g, 0.07 mmol) was heated
in dry THF 100 mL at 80 ꢁC for 24 h under an inert N2 atmosphere.
After concentration, the residue was dissolved in CH2Cl2. The
organic phase was washed twice with a saturated solution of
NaHCO3 and then with water. After drying over MgSO4 and evap-
orating the solvent, the product was suspended in glacial acetic acid
(25 mL) and heated at 50 ꢁC for 5 h. The mixture was cooled and
added to ice water. The resulting orange precipitate formed was
filtered and washed with water and methanol. The compound 10
was recrystallised from DMF. Yield was 52%. 1H NMR (300 MHz,
3. Results and discussion
The synthetic procedures of the coumarin dyes containing EDOT
are depicted in Scheme 1. Coumarin aldehyde derivatives were
synthesized by either a Suzuki or Stille coupling reaction. These
aldehydes, on reaction with cyanoacetic acid in the presence of
piperidine catalyst, produced a series of coumarin dyes of HKK-
CM1-3, presented here.
CDCl3)
d
(TMS, ppm): 1.32 (6H, s), 1.58 (6H, s), 1.77e1.82 (4H, m),
Fig. 1 shows the absorption and emission spectra of coumarin
dyes measured in THF solution and their photophysical properties
are summarised in Table 1. The calculated TDDFT absorption
wavelengths, together with the corresponding oscillator strengths,
are reported in Table 2. The absorption spectrum of the HKK-CM1
dye with the incorporation of an EDOT unit into a bithiophene-
containing coumarin dye of NKX-2677 to up-lift its HOMO level and
down-shift its LUMO level, shows the maximum absorption of
3.24e3.32 (4H, m), 4.43 (4H, s), 7.15 (1H, s), 7.23e7.28 (2H, m),
7.60e7.67 (2H, m), 7.84 (1H, s), 9.84 (1H, s).
2.4.8. Preparation of 5-(((1,1,6,6-tetramethyl-10-oxo-2,3,5,6-
tetrahydro-1H,4H,10H-11-oxa-3a-azabenzo[de]anthracen-9-yl)-
thiophene-2-yl-)-3,4-ethylenedioxythiophene-5-yl)-thiophene-2-
yl) acrylic acid (HKK-CM3)
An acetonitrile solution (20 mL) of compound 8 (0.30 g,
0.48 mmol) and cyanoacetic acid (0.07 g, 0.76 mmol) was refluxed
at 120 ꢁC in the presence of piperidine (0.05 mL) for 6 h. After
cooling the solution, the product was purified by chromatography
on silica gel (CH2Cl2eMeOH 5:1). Yield was 76%. 1H NMR (DMSO-
532 nm (
3
¼ 52,700 Mꢀ1 cmꢀ1), which is 18 nm red-shifted in
contrast to NKX-2677. In excellent agreement with experiment,
a small red shift is also predicted by TDDFT with lmax of 519 (HKK-
CM1) and 513 (NKX-2677) nm. Under a similar condition, the HKK-
CM3 dye exhibits the maximum absorption peak at 527 nm
d6)
d
(TMS, ppm): 1.26 (6H, s), 1.49 (6H, s), 1.70e1.77 (4H, m),
(
3
¼ 60,400 Mꢀ1 cmꢀ1). Obviously, with the introduction of the
3.26e3.32 (4H, m), 4.54 (4H, s), 7.33 (1H, s), 7.37 (1H, d), 7.40 (1H, s),
7.69 (1H, d), 7.88 (1H, d), 8.31 (1H, s), 8.37 (1H, s), 7.33 (1H, d), 7.42
(1H, s), 7.64 (1H, d), 7.83 (2H, d), 8.03 (2H, d), 8.22 (1H, s), 8.32 (1H,
EDOT, the charge-transfer transition absorption is not only red-
shifted but also its absorption molar extinction coefficient is
enhanced, due to extended p-conjugation. As shown in Table 2, for
s). UVevis (THF, nm): lmax (log
620. FAB-mass: Calcd. for C37H32N2O6S3, 696.14; found,696.00.
3
) 527 (60,400). PL (THF, nm): lmax
HKK-CM3, the first absorption maximum is computed at 543 nm
with a larger oscillator strength (2.202) with respect to those of
HKK-CM1 (1.839) and NKX-2677 (1.857). But, the HKK-CM2 dye
exhibits the maximum absorption peak at 509 nm
2.5. Fabrication and testing of DSSC
(
3
¼ 67,200 Mꢀ1 cmꢀ1), which is 5 nm blue-shifted in contrast to
FTO glass plates (Pilkington) were cleaned in a detergent solution
using an ultrasonic bath for 1 h, rinsed with water and ethanol. The
FTO glass plates were immersed in an aqueous solution of 40 mM
TiCl4 at 70 ꢁC for 30 min and washed with water and ethanol. The
NKX-2677; a comparable shift to shorter wavelengths is also
computed by TDDFT, which locates the first absorption band at
511 nm with an oscillator strength of 2.268. This increase in the
excitation energy of HKK-CM2 can be ascribed to the relatively
larger torsion angle (ca. 9ꢁ at B3LYP/6-31G* level) between the
plane of the donor and that of the acceptor, induced by the presence
of the phenyl fragment, ensuring less efficient electronic commu-
nication between the donor and the acceptor. In contrast, the
absorption spectra of coumarin dyes adsorbed on the TiO2 surface
are remarkably broadened relative to the absorption spectra of the
solution state. This might be due to dyeedye and/or dyeeTiO2
interactions [50]. In addition, it should be addressed that a series of
coumarin dyes of HKK-CM1-3 adsorbed on the TiO2 films has
a broader and better absorption band than that of NKX-2677 as
a standard dye.
The electrochemical properties were investigated by cyclic vol-
tammetry (CV) to obtain HOMO and LUMO levels of the present
dyes. The cyclic voltammogram curves were obtained from a three-
electrode cell in 0.1 M TBAPF6 in CH3CN at the scan rate of 50 mV/s,
using a dye-coated TiO2 electrode as a working electrode and Pt
wire counter electrode and Ag/AgCl (saturated KCl) reference
electrode (þ0.197 V vs NHE) and calibrated with ferrocene. All of
the measured potentials were converted to the NHE scale. The band
gap was estimated from the absorption edges of UVevis spectra
and LUMO energy levels were derived from HOMO energy levels
and the band gap. HOMO values (0.93e1.02 V vs NHE) are more
positive than the Iꢀ/I3ꢀ redox couple (0.4 V vs NHE). The electron
injection from the excited sensitisers to the conduction band of
first TiO2 layer of 8 mm thickness was prepared by screen printing
TiO2 paste (Solaronix, 13 nm anatase), and the second scattering
layer containing 400 nm sized anatase particles was deposited by
screen printing. The TiO2 electrodes were immersed into the dye
solution (0.3 mM in ethanol/THF (1:1) with DCA (10 mM)) and kept
at room temperature overnight. Counter electrodes were prepared
by coating with a drop of H2PtCl6 solution (2 mg of Pt in 1 mL of
ethanol) on an FTO plate. The dye-adsorbed TiO2 electrode and Pt-
counter electrode were assembled into a sealed sandwich-type cell.
A drop of electrolyte was then introduced into the cell, which was
composed of 0.6 M 1,2-dimetyl-3-propyl imidazolium iodide, 0.05 M
iodine, 0.1 M LiI, and 0.5 M tert-butylpyridine in acetonitrile. It was
introduced into the inter-electrode space from the counter electrode
side through predrilled holes. The drilled holes were sealed with
a microscope cover slide and Surlyn to avoid leakage of the elec-
trolyte solution.
2.6. Photoelectrochemical measurements of DSSC
Photoelectrochemical data were measured using a 1000 W
xenon light source (Oriel, 91193) that was focused to
give1000 W m2, the equivalent of one sun at Air Mass (AM) 1.5, at
the surface of the test cell. The light intensity was adjusted with a Si
solar cell that was double-checked with an NREL-calibrated Si solar

K.D. Seo et al. / Dyes and Pigments 90 (2011) 304e310
307
O
O
Br
O
O
S
Pd(PPh3)4
THF, 80oC
S
S
+
Sn
N
N
O
O
3
O
O
S
1
2
O
O
O
O
CN
CO2H
POCl3 / DMF
r.t., 12h
NCCH2COOH
AN
S
CHO
S
S
S
N
O
O
N
O
O
HKK-CM1
4
O
S
O
Br
NBS
DMF, r.t.
S
(HO)2B
CHO
3
+
N
O
O
O
6
O
O
O
CHO
Pd(PPh3)4
Na2CO3, Toluene
NCCH2COOH
AN
S
S
S
COOH
S
NC
N
O
O
N
O
O
HKK-CM2
7
O
S
O
O
6
CHO
S
+
S
S
Pd(PPh3)4
THF, 80oC
Sn
O
N
O
O
9
10
O
O
CN
CO2H
NCCH2COOH
AN
S
S
S
S
S
COOH
NC
N
N
O
O
O
O
NKX-2677
HKK-CM3
Scheme 1. Chemical structures and synthesis of coumarin dyes containing EDOT.
TiO2 should be energetically favourable because of the more
negative LUMO values (ꢀ1.13 to ꢀ1.24 vs NHE) compared to the
conduction band edge energy level of the TiO2 electrode [51]. As
shown in Fig. 3, HKK-CM1 only exhibits the lifted HOMO level,
since it is well known that an EDOT electron-rich EDOT unit is
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2.0
1.5
1.0
0.5
0.0
HKK-CM1
HKK-CM2
HKK-CM3
NKX-2677
HKK-CM1
HKK-CM2
HKK-CM3
NKX-2677
Table 1
400
500
600
700
800
Wavelength (nm)
Optical properties and DSSC performance parameters of coumarin dyes.
Dye
labsa/nm
/Mꢀ1 cmꢀ1
Eoxb/V E0e0c/eV ELUMOd/V Jsc
(mA cmꢀ2) (eV)
Voc FF
h
(
3
)
(%)
HKK-CM1 532 (52,700) 0.93 2.06
HKK-CM2 509 (67,200) 0.99 2.23
HKK-CM3 527 (60,400) 1.02 2.15
NKX-2677 514 (59,900) 0.95 2.17
ꢀ1.13
ꢀ1.24
ꢀ1.13
ꢀ1.18
14.2
10.5
13.3
16.1
0.60 0.70 6.07
0.62 0.57 3.73
0.57 0.56 4.37
0.64 0.70 7.19
a
Absorption and emission spectra were measured in THF.
Oxidation potentials of dyes on TiO2 were measured in CH3CN with 0.1 M
300
400
500
600
700
800
b
TBAPF6 with a scan rate of 50 mV sꢀ1 (vs NHE).
Wavelength (nm)
c
E0e0 was determined from the intersection of absorption and emission spectra
Fig. 1. Absorption and emission spectra of 2 ꢂ 10ꢀ5 M coumarin dyes in THF (inset:
dyes adsorbed on TiO2 films).
in THF.
d
LUMO was calculated by EoxꢀE0e0
.

308
K.D. Seo et al. / Dyes and Pigments 90 (2011) 304e310
Table 2
Computed and experimental maximum absorption wavelengths (lmax) and oscil-
lator strengths in THF for HKK-CM1, HKK-CM2, HKK-CM3 and NKX-2677.
Dye
lmax
lmax (nm)
(Theor.)
3
/Mꢀ1 cmꢀ1
Osc. Strength
(Theor.)
(nm) (Exp.)
(Exp.)
HKK-CM1
HKK-CM2
HKK-CM3
NKX-2677
532
509
527
514
519
511
543
513
52,700
67,200
60,400
59,900
1.839
2.268
2.202
1.857
connected to donor group in DepeA chromophores and destabi-
lised the energy of the HOMO to be lifted [34,43]. In contrast, it was
surprising that HKK-CM2 and HKK-CM2 exhibit the lowered
HOMO level. To gain more insight of their photophysical behaviour
adsorbed on TiO2 films, theoretical analysis (DFT, B3LYP/6-31G*
level) on the molecular orbitals involved in the transitions was
carried out, and the resulting frontier orbitals are depicted in
further density functional B3LYP/3-21G* calculations performed for
three compounds. Fig. 2 shows the dominant electron density
change for the HOMO-LUMO transition is that the electrons
Fig. 3. Overview of the evaluated energy levels of coumarin dyes determined by cyclic
voltammetry.
transfer from the coumarin unit to acrylic acid moiety in DepeA
chromophores.
Fig. 4 shows the incident monochromatic photon-to-current
conversion efficiency (IPCE) of the coumarin dyes sensitised. The
onset wavelengths of the IPCE spectra for DSSCs based on HKK-
CM2 and HKK-CM3 were >800 nm. IPCE values of higher than 70%
were observed in the range of 410e605 nm with a maximum value
of 78% at 500 nm for the DSSC based on HKK-CM1. The maximum
IPCE value of the DSSC based on HKK-CM2 and HKK-CM3 are
slightly lower than the efficiencies of the DSSCs based on HKK-CM1.
But, all maximum IPCE values of the DSSC based on the presented
coumarin dyes are lower than that of the DSSCs based on NKX-
2677, even though a series of coumarin dyes of HKK-CM1-3
adsorbed on the TiO2 films has a broader and better absorption
band than that of NKX-2677 as a standard dye (see Fig. 1). The
lower IPCE values of the DSSC based on HKK-CM2 and HKK-CM3
are probably due to extended
p-conjugation elongation, which
might lead to decreased electron-injection yield relative to that of
HKK-CM1 dye [36].
The photovoltaic performances of the coumarin DSSCs were
summarised in Table 1. Under the standard global AM 1.5 solar
condition, the HKK-CM1 sensitised cell gave a short circuit photo-
current density (JSC) of 14.2 mA cmꢀ2, open circuit voltage (VOC) of
0.60 V, and a fill factor (FF) of 0.70, corresponding to an overall
conversion efficiency 6.07%. The HKK-CM2 sensitised cell gave a JSC
of 10.5 mA cmꢀ2, VOC of 0.62 V, and FF of 0.57, corresponding to an
overall conversion efficiency
h of 3.73% and the HKK-CM3 sensitised
cell gave a JSC of 13.3 mA cmꢀ2, VOC of 0.57 V, and FF of 0.56, cor-
responding to an overall conversion efficiency
h
of 4.37%. Fig. 5
100
90
80
70
60
50
40
30
20
10
0
HKK-CM1
HKK-CM2
HKK-CM3
NKX-2677
300
400
500
600
700
800
λ/nm
Fig. 2. Plots of the isodensity surfaces (MPW1K/6-31G* in THF) of HOMOs and LUMOs
Fig. 4. Typical action spectra of incident photon-to-current conversion efficiencies
of HKK-CM1, HKK-CM2 and HKK-CM3.
(IPCE) obtained for nanocrystalline TiO2 solar cells sensitized by coumarin dyes.

K.D. Seo et al. / Dyes and Pigments 90 (2011) 304e310
309
photoelectrochemical cell ruthenium sensitizers. J Am Chem Soc 2005;127:
16835e47.
20
18
16
14
12
10
8
HKK-CM1
HKK-CM2
HKK-CM3
NKX-2677
[2] Nazeeruddin MK, Pechy P, Renouard T, Zakeeruddin SM, Humphry-Baker R,
Grätzel M, et al. Engineering of efficient panchromatic sensitizers for nano-
crystalline TiO2-based solar cells. J Am Chem Soc 2001;123:1613e24.
[3] Mishra A, Fischer MKR, Bäuerle P. Metal-free organic dyes for dye-sensitized
solar cells: from structure: property relationships to design rules. Angew
Chem Int Ed 2009;48:2474e99.
[4] Ning Z, Fu Y, Tian H. Improvement of dye-sensitized solar cells: what we know
and what we need to know. Energy Environ Sci 2010;3:1170e81.
[5] Asbury JB, Ellingson RJ, Gosh HN, Ferrere S, Notzig AJ, Lian T. Femtosecond IR
study of excited-state relaxation and electron-injection dynamics of Ru
(dcbpy)2(NCS)2 in solution and on nanocrystalline TiO2 and Al2O3 thin films.
J Phys Chem B 1999;103:3110e9.
[6] Park NG, Kang MG, Kim KM, Ryu KS, Kim DK, Frank AJ, et al. Morphological
and photoelectrochemical characterization of core-shell nanoparticle films for
dye-sensitized solar cells: ZneO type shell on SnO2 and TiO2 cores. Langmuir
2004;20:4246e53.
[7] Saito Y, Fukuri N, Senadeera R, Kitamura T, Wada Y, Yanagida S. Solid state dye
sensitized solar cells using in situ polymerized PEDOTs as hole conductor.
Electrochem Commun 2004;6:71e4.
6
4
2
0
0.0
0.1
0.2
0.3
Voltage [V]
0.4
0.5
0.6
0.7
[8] Fabregat-Santiago F, Garcia-Cañadas J, Palomares E, Clifford JN, Durrant JR,
Bisquert J, et al. The origin of slow electron recombination processes in
dye-sensitized solar cells with alumina barrier coatings. J Appl Phys 2004;96:
6903e7.
Fig. 5. Photocurrentevoltage characteristics of representative TiO2 electrodes sensi-
[9] Furube A, Katoh R, Hara K, Murata S, Arakawa H, Tachiya M, et al. Ultrafast
direct and indirect electron-injection processes in a photoexcited dye-sensi-
tized nanocrystalline zinc oxide film: the importance of exciplex intermedi-
ates at the surface. J Phys Chem B 2004;108:12583e92.
[10] Tian H, Meng F. Organic photovoltaics: mechanisms, materials, and devices.
London: CRC; 2005.
[11] Wu W, Yang J, Hua J, Tang J, Zhang L, Tian H, et al. Efficient and stable dye-
sensitized solar cells based on phenothiazine sensitizers with thiophene units.
J Mater Chem 2010;20:1772e9.
[12] Qu S, Wu W, Hua J, Kong C, Long Y, Tian H. New diketopyrrolopyrrole
(DPP) dyes for efficient dye-sensitized solar cells. J Phys Chem C 2010;114:
1343e9.
tized with coumarin dyes.
shows the JeV curves of the coumarin dyes. Among the three dye-
based DSSCs, the HKK-CM2-based device gives the lowest efficiency
although the dye gave the broadest light harvesting range. The
phenylene unit in HKK-CM2 may also lead to a twisted intra-
molecular charge-transfer geometry of the dye excited state, which
would decrease the excited-state energy and cause a lower effi-
ciency of electron injection as reported previously [52].
[13] Ning Z, Tian H. Triarylamine: a promising core unit for efficient photovoltaic
materials. Chem Commun; 2009:5483e95.
[14] Xu W, Peng B, Chen J, Liang M, Cai F. New triphenylamine-based dyes for dye-
sensitized solar cells. J Phys Chem C 2008;112:874e80.
4. Conclusions
[15] Ning Z, Zhang Q, Wu W, Pei H, Liu B, Tian H. Starburst triarylamine based dyes
for efficient dye-sensitized solar cells. J Org Chem 2008;73:3791e7.
[16] Li G, Jiang KJ, Li YF, Li SL, Yang LM. Efficient structural modification of tri-
phenylamine-based organic dyes for dye-sensitized solar cells. J Phys Chem C
2008;112:11591e9.
Coumarin dyes containing low-band-gap chromophore have
been designed and synthesized. Surprisingly, even though HKK-
CM2 and HKK-CM3 have more extended aromatic units than HKK-
CM1, the degree of
p-conjugation in HKK-CM2 and HKK-CM3 is
[17] Wang ZS, Li FY, Huang CH, Wang L, Wei M, Jin LP, et al. Photoelectric
conversion properties of nanocrystalline TiO2 electrodes sensitized with
hemicyanine derivatives. J Phys Chem B 2000;104:9676e82.
[18] Chen YS, Li C, Zeng ZH, Wang WB, Wang XS, Zhang BW. Efficient electron
injection due to a special adsorbing group’s combination of carboxyl and
hydroxyl: dye-sensitized solar cells based on new hemicyanine dyes. J Mater
Chem 2005;15:1654e61.
[19] Tanaka K, Takimiya K, Otsubo T, Kawabuchi K, Kajihara S, Harima Y. Devel-
opment and photovoltaic performance of oligothiophene-sensitized TiO2 solar
cells. Chem Lett 2006;35:592e3.
[20] Horiuchi T, Miura H, Uchida S. Highly efficient metal-free organic dyes for
dye-sensitized solar cells. J Photochem Photobiol Chem 2004;164:29e32.
[21] Horiuchi T, Miura H, Sumioka K, Uchida S. High efficiency of dye-sensitized
solar cells based on metal-free indoline dyes. J Am Chem Soc 2004;126:
12218e9.
less efficient than that of HKK-CM1, due to the relatively larger
torsion angle between the plane of the donor and that of the
acceptor. As a result, a solar cell based on HKK-CM1 sensitiser
shows better photovoltaic performance with JSC of 14.2 mA cmꢀ2
,
VOC of 0.60 V, and FF of 0.70, corresponding to an overall conversion
efficiency of 6.07% under the standard AM 1.5 irradiation, than
h
HKK-CM2 and HKK-CM3-based solar cells. The lower IPCE values of
the DSSCs based on HKK-CM2 and HKK-CM3 are probably due to
extended p-conjugation elongation, which might lead to decreased
electron-injection yield relative to that of HKK-CM1 dye.
Acknowledgements
[22] Schmidt-Mende L, Bach U, Humphry-Baker R, Horiuchi T, Miura H, Ito S, et al.
Organic dye for highly efficient solid-state dye-sensitized solar cells. Adv
Mater 2005;17:813e5.
This research was supported by MKE (The Ministry of Knowl-
edge Economy), Korea, under the ITRC support program supervised
by the IITA (Institute for Information Technology Advancement)
(IITA-2008-C1090-0804-0013), WCU (the Ministry of Education
and Science) program (R31-2008-000-10035-0) and Converging
Research Center Program through the Ministry of Education,
Science and Technology (2010K000973).
[23] Ito S, Zakeeruddin SM, Humphry-Baker R, Liska P, Charvët R, Grätzel M, et al.
High-efficiency organic-dye-sensitized solar cells controlled by nanocrystal-
line-TiO2 electrode thickness. Adv Mater 2006;18:1202e5.
[24] Howie WH, Claeyssens F, Miura H, Peter ML. Characterization of solid-state
dye-sensitized solar cells utilizing high absorption coefficient metal-free
organic dyes. J Am Chem Soc 2008;130:1367e75.
[25] Kuang D, Uchida S, Humphry-Baker R, Zakeeruddin SK, Grätzel M. Organic
dye-sensitized ionic liquid based solar cells: remarkable enhancement in
performance through molecular design of indoline sensitizers. Angew Chem
Int Ed 2008;47:1923e7.
[26] Ito S, Miura H, Uchida S, Takata M, Sumioka K, Liska P, et al. High-conversion-
efficiency organic dye-sensitized solar cells with a novel indoline dye. Chem
Commun; 2008:5194e6.
Appendix. Supplementary material
Supplementary data related to this article can be found online at
doi:10.1016/j.dyepig.2011.01.009.
[27] Ooyama Y, Shimada Y, Kagawa Y, Imae I, Harima Y. Photovoltaic performance
of dye-sensitized solar cells based on donoreacceptor
furo[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes with
p
-conjugated benzo-
carboxyl
a
group at different positions of the chromophore skeleton. Org Biomol Chem
2007;5:2046e54.
References
[28] Imahori H, Umeyama T, Ito S. Large
p-aromatic molecules as potential
sensitizers for highly efficient dye-sensitized solar cells. Acc Chem Res
2009;42:1809e18.
[1] Nazeeruddin MK, De Angelis F, Fantacci S, Selloni A, Viscardi G, Grätzel M,
et al. Combined experimental and DFT-TDDFT computational study of

310
K.D. Seo et al. / Dyes and Pigments 90 (2011) 304e310
[29] Lu HP, Tsai CY, Yen WN, Hsieh CP, Yeh CY, Diau EWG, et al. Control of dye
aggregation and electron injection for highly efficient porphyrin sensitizers
adsorbed on semiconductor films with varying ratios of coadsorbate. J Phys
Chem C 2009;113:20990e7.
[30] Imahori H, Matsubara Y, Iijima H, Umeyama T, Matano Y, Ito S, et al. Effects of
meso-diarylamino group of porphyrins as sensitizers in dye-sensitized solar
cells on optical, electrochemical, and photovoltaic properties. J Phys Chem C
2010;114:10656e65.
conjugated copolymers for light-emitting diodes and photovoltaic devices.
Macromolecules 2005;38:244e53.
[41] Zhang FL, Gadisa A, Inganäs O, Svensson M, Andersso MR. Influence of buffer
layers on the performance of polymer solar cells. Appl Phys Lett
2004;84:3906e8.
[42] Kim J, Kim SH, Jung IH, Jeong E, Xia Y, Woo HY, et al. Synthesis and charac-
terization of indeno[1,2-b]fluorene-based low bandgap copolymers for
photovoltaic cells. J Mater Chem 2010;20:1577e86.
[31] Mai CL, Huang WK, Lu HP, Lee CW, Diau EWG, Yeh CY, et al. Synthesis and
characterization of diporphyrin sensitizers for dye-sensitized solar cells. Chem
Commun 2010;46:809e11.
[43] Zhang G, Bala H, Cheng Y, Shi D, Lv X, Wang P, et al. High efficiency and stable
dye-sensitized solar cells with an organic chromophore featuring a binary p-
conjugated spacer. Chem Commun; 2009:2198e200.
[32] Eu S, Katoh T, Umeyama T, Matano Y, Imahori H. Synthesis of sterically
hindered phthalocyanines and their applications to dye-sensitized solar cells.
Dalton Trans; 2008:5476e83.
[44] Hergué N, Leriche P, Blanchard P, Allain M, Gallego-Planas N, Frère P, et al.
Evidence for the contribution of sulfurebromine intramolecular interactions
to the self-rigidification of thiophene-based
Chem 2008;32:932e6.
p-conjugated systems. New J
[33] Ooyama Y, Harima H. Molecular designs and syntheses of organic dyes for
dye-sensitized solar cells. European J Org Chem; 2009:2903e34.
[34] Zeng W, Cao Y, Bai Y, Wang Y, Shi Y, Wang P. Efficient dye-sensitized solar
cells with an organic photosensitizer featuring orderly conjugated ethyl-
enedioxythiophene and dithienosilole blocks. Chem Mater 2010;22:1915e25.
[35] Wang ZS, Cui Y, Hara K, Dan-oh Y, Kasada C, Shinpo A. A high-light-har-
vesting-efficiency coumarin dye for stable dye-sensitized solar cells. Adv
Mater 2007;19:1138e41.
[45] Prim D, Auffrant A, Plyta ZF, Tranchier JP, Munch FR, Rose E. Bimetallic
-conjugated complexes modulated by a carbonyl spacer: synthesis of are-
p
netricarbonylchromiumeferrocene derivatives. J Organomet Chem 2001;624:
124e30.
[46] Becke AD. A new mixing of Hartree-Fock and local density-functional theories.
J Chem Phys 1993;98:1372e7.
[47] Lynch BJ, Fast PL, Harris M, Truhlar DG. Adiabatic connection for kinetics.
J Phys Chem A 2000;104:4811e5.
[48] Cossi M, Baron V. Time-dependent density functional theory for molecules in
liquid solutions. J Chem Phys 2001;115:4708e17.
[36] Hara K, Wang ZS, Sato T, Furube A, Katoh R, Sugihara H, et al. Oligothiophene-
containing coumarin dyes for efficient dye-sensitized solar cells. J Phys Chem
B 2005;109:15476e82.
[37] Ehret A, Stuhl L, Spitler MT. Spectral sensitization of TiO2 nanocrystalline
electrodes with aggregated cyanine dyes. J Phys Chem B 2001;105:9960e5.
[38] Yao QH, Meng FS, Li FY, Tian H, Huang CH. Photoelectric conversion properties
of four novel carboxylated hemicyanine dyes on TiO2 electrode. J Mater Chem
2003;13:1048e53.
[49] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al.
Gaussian 03. Revision B05. Pittsburgh PA: Gaussian Inc.; 2003.
[50] Hara K, Kurashige M, Dan-oh Y, Kasada C, Shinpo A, Arakaw H, et al. Design of
new coumarin dyes having thiophene moieties for highly efficient organic-
dye-sensitized solar cells. New J Chem 2003;27:783e5.
[39] Wang ZS, Li FY, Huang CH. Highly efficient sensitization of nanocrystalline
TiO2 films with styryl benzothiazolium propylsulfonate. Chem Commun;
2000:2063e4.
[40] Yang R, Tian R, Yan J, Zhang Y, Yang J, Cao Y, et al. Deep-red electrolumi-
nescent polymers: synthesis and characterization of new low-band-gap
[51] Hagfeldt A, Grätzel M. Light-induced redox reactions in nanocrystalline
systems. Chem Rev 1995;95:49e68.
[52] Hagberg DP, Marinado T, Karlsson KM, Nonomura K, Hagfeldt A, Sun L, et al.
Tuning the HOMO and LUMO energy levels of organic chromophores for dye
sensitized solar cells. J Org Chem 2007;72:9550e6.