Synthesis and photovoltaic performance of long wavelength absorbing organic dyes for dye-sensitized solar cells

Article abstract of DOI:10.1080/15421406.2011.601189

We prepared novel organic photo-sensitizers based on a carbazole framework containing various acceptors with thiophene bridge units in the chromophore for the application to dye-sensitized solar cell (DSSC). Furthermore, organic dyes without bridge in the chromophore also synthesized to investigate the correlation between conjugation lengths and photon-to-current efficiency (PCE). Compared to non-bridged dyes, the DSSCs device containing multi-anchoring dyes with thiophene bridge exhibited much higher PCEs, resulting from efficient electron extraction pathways, higher molar extinction coefficients, and better light absorption in longer wavelengths.

Full text of DOI:10.1080/15421406.2011.601189

Mol. Cryst. Liq. Cryst., Vol. 551: pp. 283–294, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.601189
Synthesis and Photovoltaic Performance of Long
Wavelength Absorbing Organic Dyes for
Dye-Sensitized Solar Cells
SANG AH KIM,1 HYO JEONG JO,1 MI RAN JUNG,1
YOUNG CHEOL CHOI,2 DO KYUNG LEE,2,∗
MOONYONG LEE,1 AND JAE HONG KIM1,∗
1School of Display and Chemical Engineering, Yeungnam University,
Gyeongsan, Gyeongsangbuk-do 712-749, Republic of Korea
2R&D Affairs Department, Gumi Electronics and Information Technology
Research Institute, Gumi, Gyeongsangbuk-do 730-853, Republic of Korea
We prepared novel organic photo-sensitizers based on a carbazole framework containing
various acceptors with thiophene bridge units in the chromophore for the application
to dye-sensitized solar cell (DSSC). Furthermore, organic dyes without bridge in the
chromophore also synthesized to investigate the correlation between conjugation lengths
and photon-to-current efficiency (PCE). Compared to non-bridged dyes, the DSSCs
device containing multi-anchoring dyes with thiophene bridge exhibited much higher
PCEs, resulting from efficient electron extraction pathways, higher molar extinction
coefficients, and better light absorption in longer wavelengths.
Keywords Dye-sensitized solar cells; Organic Photosensitizers; Multi-anchoring dyes;
Carbazole chromophore; Photovoltaic performances
Introduction
The supply of renewable energy sources has been considered as the most significant research
field for human life during the last years. One of promising candidates for them is the solar
energy, since it is renewable, clean, and inexhaustible resource. The improvement of solar
energy-to-electricity conversion efficiency has continued to be an important research area
of dye-sensitized solar cell (DSSC).
A typical DSSC is constructed with dye-absorbed wide band gap oxide semiconductor
electrode such as TiO2 or ZnO, electrolyte containing I-/I3- redox couples, and Pt-coated
counter electrode [1–7]. The photo-sensitizing dye plays the important roles in capturing the
photons and generating the electron-hole pair, as well as transferring them to the interface
of the semiconductor.
Organic dye molecules have been employed recently as promising candidates for
the photo-sensitizer in DSSC, because of their great potential advantages (high molar
extinction coefficient, convenient, and customized molecular design) for the photophysical
∗Corresponding authors. E-mail: dklee@geri.re.kr, jaehkim@ynu.ac.kr
[603]/283

284/[604]
S. A. Kim et al.
and photochemical properties and low-cost production [8–16]. Usually, organic dyes have
an intra-molecular donor-acceptor structure called as the push-pull architecture. Upon light
irradiation, intra-molecular charge transfer from the electron donor to the electron acceptor
is induced and a subsequent electron transfer to TiO2 wide band gap semi-conductors via
the anchoring group takes place.
We had reported that the organic dyes with double electron acceptors exhibited the
high power conversion efficiency in the DSSC due to the increase of electron extraction
paths from electron donor as compared with that of single electron acceptor type [10]. Also,
this earlier report had the important implications for the needs of further studies on the
organic dyes containing multi-acceptors with a viewpoint of photovoltaic performance.
In this work, we have studied on the synthesis and characterization of the organic
dyes containing different kinds of electron acceptor moieties in a molecule. In particular,
photovoltaic properties for the DSSC devices are mainly discussed.
Experimental
Synthesis
Materials. All commercially available starting materials and solvents were purchased from
Aldrich, TCI, and ACROS Co. and used without further purification unless otherwise stated.
HPLC grade toluene and tetrahydrofuran (THF) were purchased from Samchun chemical
and distilled from CaH2 immediately before use.
10-hexyl-10H-carbazole (1)
An oven dried 250 ml round bottom flask was charged with 100 ml DMF, 1-bromohexane
(11.8 g, 0.071 mol) and sodium hydride (1.72 g, 0.071 mol). Then, DMF solution of car-
bazole (10 g, 0.059 mol) was added into the mother solution and the reaction mixture was
kept for 6 h at room temperature. After completion of the reaction, the solution was neutral-
ized with dilute HCl aqueous solution. The mixture was extracted with chloroform/water.
The dried solution was concentrated. The resulting crude oily product was purified by
silica gel column chromatography (only hexane). Yield: 5.4 g (82%). 1H NMR (300 MHz,
CDCl3): δ 8.14(d, J = 7.5 Hz, 2H), 7.59(d, J = 8.1 Hz, 2H), 7.43(t, J = 7.3 Hz, 2H), 7.19(t,
J = 7.3 Hz, 2H), 4.38(t, J = 7.1 Hz, 2H), 1.76-0.80(m, 11H)
3,6-dibromo-9-hexyl-9H-carbazole (2)
Bromine (2 ml, 0.035 mol) was slowly added using a syringe to the solution of 10-hexyl-
10H-carbazole (4.0 g, 0.016 mol) in 40 mL of dichloromethane. After stirring the mixture
at room temperature for 12 h, the reaction was terminated by adding dilute aqueous sodium
hydroxide (0.1 M). The reaction mixture was extracted using dichloromethane and water.
The organic layer was separated and dried over anhydrous magnesium sulfate. The crude
product was purified using column chromatography with hexane as the eluent. The product
was colorless solid. Yield: 5.4 g (82%). 1H NMR (300 MHz, CDCl3): δ 8.05(d, J = 1.8 Hz,
2H), 7.48(dd, J1 = 8.1 Hz, J2 = 1.8H, 2H), 7.20(d, J = 8.1 Hz, 2H), 4.18(t, J = 7.1 Hz,
2H), 1.84 − 0.83(m, 11H).

Long Wavelength Absorbing Organic Dyes
9-hexyl-3,6-di(thiophen-2-yl)-9H-carbazole (3)
[605]/285
An oven dried 250 ml round bottom flask was charged with 100 ml dry THF (50 ml),
3,6-dibromo-9-hexyl-9H-carbazole (5 g, 0.012 mol). The solution was tributyl(thiophen-
2-yl)stannane (10.48 ml, 0.033 mol). Then, Pd(PPh3)4 (0.43 g, 0.61 mmol) was added
into the mother solution and the reaction mixture was kept for 10 h at room temperature.
The mixture was stirred at 65◦C for overnight under nitrogen atmosphere. After THF was
removed under vacuum, the reaction mixture was poured into water and extracted with
dichloromethane and water. The solvent was removed, and the product was purified using
column chromatography on a silica gel with dichloromethane/hexane (volume ratio 1:3).
The product was obtained as yellow solid. Yield: 3.5 g (69%). 1H NMR (300 MHz, CDCl3):
δ 8.35(d, J = 1.5 Hz, 2H), 7.75(dd, J1 = 8.4 Hz, J2 = 1.5 2H), 7.40(d, J = 8.4 Hz, 2H),
7.37(d, J = 3.3 Hz, 2H), 7.28(d, J = 3.4 Hz, 2H), 7.12(t, J = 3.4 Hz, 2H), 4.31(t, J = 7.2
Hz, 2H), 1.91−0.84(m, 11H).
5,5ꢀ-(9-hexyl-9H-carbazole-3,6-diyl)dithiophene-2-carbaldehyde (4)
9-Hexyl-3,6-di(thiophen-2-yl)-9H-carbazole (2.5 g, 6 mmol) was dissolved in 1,2-
dichloroethane and DMF (1.16 ml, 0.015 mol), and (1.4 ml, 0.015 mol) of phosphorus
oxychloride was carefully added through a dropping funnel, keeping the reaction tempera-
ture below 0◦C. After the complete addition of POCl3, the reaction solution had a red color
and was stirred for 8 h at reflux. Then the mixture was poured into water. The solution
was neutralized with a sodium hydroxide solution and extracted with dichloromethane. The
formed precipitate was filtered off, dried over MgSO4, and purified using column chro-
matography on a silica gel with hexane/dichloromethane (volume ratio 1:9). The product
1
was obtained as a yellow powder. Yield: 2.2 g (77%). H NMR (300 MHz, CDCl3): δ
9.95(s, 2H), 8.43(s, 2H), 7.82(d, J = 8.4 Hz, 2H), 7.79(d, J = 3.9 Hz, 2H), 7.49(d, J = 3.9
Hz, 2H), 7.46(d, J = 8.7 Hz, 2H), 4.33(t, J = 7.2 Hz, 2H), 1.92–0.84 (m, 11H).
3,3ꢀ-(5,5ꢀ-(9-hexyl-9H-carbazole-3,6-diyl)bis(thiophene-5,2-diyl))bis(2-cyanoacrylic
acid) (LOFM1)
In a dried 100 ml, round bottom flask, 5,5ꢀ-(9-hexyl-9H-carbazole-3,6-diyl)dithiophene-2-
carbaldehyde (0.5 g, 1.06 mmol), cyano acetic acid (0.225 g, 2.65 mmol) ammonium acetate
(0.204 g, 2.65 mmol) were dissolved in Acetic acid (20 ml). After heating the solution at
110◦C for 12 h, the solvent was evaporated to obtain a dark solid. It was poured into 100 ml
of water, and solution was extracted with chloroform. The resulting product was purified
by silica gel column chromatography chloroform/Methanol (volume ratio 9:1). Yield:
1
0.5 g (77%). H NMR (300 MHz, DMSO): δ 8.81(s, 2H), 8.47(s, 2H), 8.04(d, J = 3.6
Hz, 2H), 7.93(d, J = 7.5 Hz, 2H), 7.84(d, J = 3.9 Hz, 2H), 7.77(d, J = 8.4 Hz, 2H), 4.45
(t, J = 7.2 Hz, 2H), 1.91−0.81 (m, 11H).
3,3ꢀ-(5,5ꢀ-(9-hexyl-9H-carbazole-3,6-diyl)bis(thiophene-5,2-diyl))bis(2-cyanoacrylic
acid) (LOFM2)
In a dried 100 ml, round bottom flask, 5,5ꢀ-(9-hexyl-9H-carbazole-3,6-diyl)dithiophene-2-
carbaldehyde (0.3 g, 0.636 mmol), rhodanine acetic acid (0.2 g, 1.59 mmol) ammonium
acetate (0.122 g, 1.59 mmol) were dissolved in Acetic acid (40 ml) under argon. After
heating the solution at 110◦C for 7 h, the solvent was evaporated to obtain a dark solid.
The resulting product was purified by silica gel column chromatography Ethyl Acetate

286/[606]
S. A. Kim et al.
/Methanol (volume ratio 9:1). Yield: 0.45 g (86%). 1H NMR (300 MHz, DMSO): δ 8.79(s,
2H), 8.13(s, 2H), 7.93(d, J = 9 Hz, 2H), 7.83(m, 4H), 7.70(d, J = 8.4 Hz, 2H), 4.69(s, 4H),
4.41(t, J = 7.2 Hz, 2H), 1.72−1.25(m, 11H).
4,4ꢀ-(5,5ꢀ-(9-hexyl-9H-carbazole-3,6-diyl)bis(thiophene-5,2-diyl))bis(methan-
1-yl-1-ylidene)bis(5-oxo-1-phenyl-4,5-dihydro-1H-pyrazole-3-carboxylic acid)
(LOFM3)
In a dried 100 ml, round bottom flask, 5,5ꢀ-(9-hexyl-9H-carbazole-3,6-diyl)dithiophene-2-
carbaldehyde (0.2 g, 0.42 mmol), 5-oxo-1-phenyl-4,5-dihydro-1H-pyrazole-3-carboxylic
acid (0.21 g, 1.06 mmol) ammonium acetate (0.081 g, 1.06 mmol) were dissolved in Acetic
acid (40 ml) under argon. After heating the solution at 110◦C for 4 h, the solvent was
evaporated to obtain a dark solid. The resulting product was purified by silica gel column
chromatography Ethyl Acetate/Methanol (volume ratio 8:2). Yield: 0.28 g (80%). 1H NMR
(300 MHz, DMSO): δ 8.93(s, 2H), 8.84(s, 2H), 8.33(d, J = 3.9 Hz, 2H), 8.04(d, J =
7.2 Hz, 2H), 7.97(m, 4H), 7.88(d, J = 3.5 Hz, 2H), 7.79(d, J = 8.4 Hz, 2H), 7.49(t, J = 7.8
Hz, 4H), 7.28(t, J = 7.5 Hz, 2H), 4.47(t, J = 7.2 Hz, 2H), 1.82−0.80 (m, 11H).
9-hexyl-9H-carbazole-3,6-dicarbaldehyde (5)
10-Hexyl-10H-carbazole (5.05 g, 0.02 mol) was dissolved in 1,2-dichloroethane and DMF
(3.42 ml, 0.05 mol), and (7.6 g, 0.05 mol) of phosphorus oxychloride was carefully added
through a dropping funnel, keeping the reaction temperature below 0◦C. After the complete
addition of POCl3, the reaction solution had a red color and was stirred for 14 h at
reflux. Then the mixture was poured into water. The solution was neutralized with a sodium
hydroxide solution and extracted with dichloromethane. The formed precipitate was filtered
off, dried over MgSO4, and purified using column chromatography on a silica gel with
Hexane/Ethyl Acetate (volume ratio 4:1). The product was obtained as a white powder.
1
Yield: 4 g (65%). H NMR (300 MHz, CDCl3): δ 10.14(s, 2H), 8.68(s, 2H), 8.10(d, J =
8.7 Hz, 2H), 7.56(d, J = 8.7 Hz, 2H), 4.40(t, J = 7.2 Hz, 2H), 1.94–0.84 (m, 11H).
3,3ꢀ-(9-hexyl-9H-carbazole-3,6-diyl)bis(2-cyanoacrylic acid) (LOFM4)
In a dried 100 ml, round bottom flask, 9-hexyl-9H-carbazole-3,6-dicarbaldehyde (0.15 g,
0.48 mmol), cyano acetic acid (0.108 g, 0.63 mmol) ammonium acetate (0.054 g, 0.63
mmol) were dissolved in Acetic acid (20 ml). After heating the solution at 110◦C for 12 h,
the solvent was evaporated to obtain a yellow solid. It was poured in to 100 ml of water, and
solution was extracted with chloroform. The resulting product was purified by silica gel
column chromatography Chloroform. Yield: 0.18 g (83%). 1H NMR (300 MHz, DMSO):
δ 8.88(s, 2H), 8.52(s, 2H), 8.34(d, J = 8.7 Hz, 2H), 7.95(d, J = 8.7 Hz, 2H), 4.51(t, J = 7.2
Hz, 2H), 1.80–0.80(m, 11H).
2,2ꢀ-(5Z,5ꢀZ)-5,5ꢀ-(9-hexyl-9H-carbazole-3,6-diyl)bis(methan-1-yl-1-ylidene)
bis(4-oxo-2-thioxothiazolidin-3-yl-5-ylidene)diacetic acid (LOFM5)
In a dried 100 ml, round bottom flask, 9-hexyl-9H-carbazole-3,6-dicarbaldehyde (0.15
g, 0.48 mmol), rhodanine acetic acid (0.12 g, 0.63 mmol) ammonium acetate (0.054 g,
0.63 mmol) were dissolved in Acetic acid (30 ml). After heating the solution at 110◦C
for 10 h, the solvent was evaporated to obtain a orange solid. The resulting product was

Long Wavelength Absorbing Organic Dyes
[607]/287
purified by silica gel column chromatography Chloroform. Yield: 0.25 g (78%). 1H NMR
(300 MHz, DMSO): δ 8.62(s, 2H), 8.08(s, 2H), 7.89(d, J = 8.7 Hz, 2H), 7.81(d, J = 8.7
Hz, 2H), 4.77(s, 4H), 4.48(t, J = 7.2 Hz, 2H), 1.80–0.78(m, 11H).
4,4ꢀ-(9-hexyl-9H-carbazole-3,6-diyl)bis(methan-1-yl-1-ylidene)bis(5-oxo-1-phenyl-
4,5-dihydro-1H-pyrazole-3-carboxylic acid) (LOFM6)
In a dried 100 ml, round bottom flask, 9-hexyl-9H-carbazole-3,6-dicarbaldehyde (0.15 g,
0.48 mmol), 5-oxo-1-phenyl-4,5-dihydro-1H-pyrazole-3-carboxylic acid (0.13 g, 0.63
mmol) ammonium acetate (0.054 g, 0.63 mmol) were dissolved in Acetic acid (40 ml).
After heating the solution at 110◦C for 15 h, the solvent was evaporated to obtain a red
solid. The resulting product was purified by silica gel column chromatography Chloroform.
Yield: 0.25 g (75%). 1H NMR (300 MHz, DMSO): δ 9.49(s, 2H), 8.99(d, J = 9.0 Hz, 2H),
8.89(s, 2H), 7.95(d, J = 7.5 Hz, 4H), 7.89(d, J = 9.0 Hz, 2H), 7.50(t, J = 7.9 Hz, 4H),
7.31(t, J = 7.3 Hz, 2H), 4.52(t, J = 7.2 Hz, 2H), 1.83−0.79(m, 11H).
Instrumental Analysis
1H NMR spectra were recorded on Bruker Advance NMR 300 Hz spectrometer using
CDCl3 and DMSO-d6 purchased from Cambridge Isotope Laboratories, Inc. The redox
properties of six dyes were examined using cyclic voltammetry (Model:CV-BAS-Epsilon).
The electrolyte solution employed was 0.10 M tetrabutylammonium hexafluorophosphate
(TBAPF6) in freshly dried acetonitrile. The Ag/AgCl and Pt wire (0.5 mm in diameter)
electrodes were utilized as reference and counter electrodes, respectively. The scan rate
was fixed at 100 mV/s.
Assembly and Characterization of DSSCs
The conducting glass substrate (FTO; TEC8, Pilkington, 8 ꢀ/cm2, Thickness of 2.3 mm)
was cleaned in ethanol by ultrasonication. TiO2 pastes were prepared using ethyl cellulose
(Aldrich), Lauric acid (Fluka) and Terpineol (Aldrich). The size of TiO2 particles is ca.
20–30 nm. The prepared TiO2 paste was coated on the pre-cleaned glass substrate using
doctor-blade, and sintered at 500◦C for 30 min. The thickness of the sintered TiO2 layer
was measured with Alpha-step IQ surface profiler (KLA Tencor). The other TiO2 paste was
re-coated over the sintered layer using ca. 250 nm size of TiO2 particle as a scattering layer,
and then sintered again at 500◦C for 30 min. The prepared TiO2 film was dipped in 0.04 M of
TiCl4 aqueous solution at 70◦C for 30 min. For dye adsorption, the annealed TiO2 electrodes
were immersed in dye solution (0.5 mM of dye in DMF; Triphenylamine series, N3/N719)
at 50◦C for 3 hours. Pt counter electrodes were prepared by thermal reduction of thin film
formed from 7 mM of H2PtCl6 in 2-propanol at 400◦C for 20 min. The dye-adsorbed TiO2
electrode and Pt counter electrode were assembled using 60 µm-thick Surlyn (Dupont
1702) as a bonding agent. A liquid electrolyte was introduced through a pre-punctured hole
on the counter electrode. The electrolyte was composed of 3-propyl-1-methyl-imidazolium
iodide (PMII, 0.7 M), lithium iodide (LiI, 0.2 M), iodine (I2, 0.05 M), t-butylpyridine (TBP,
0.5 M) in acetonitrile/valeronitrile (85:15). The active areas of dye-adsorbed TiO2 films
were estimated by a digital microscope camera with an image-analysis-software (Moticam
1000).

288/[608]
S. A. Kim et al.
Results and Discussion
Synthesis and Electo-optical Properties of Organic Dyes
The intra-molecular electron donor-acceptor type of organic dyes, which are containing
carbazole as an electron donor with various acceptor moieties and thiophene bridge units in
the chromophore, were designed to investigate the correlations with the kinds of acceptors,
conjugation lengths and photon-to-current efficiency, as shown in Scheme 1. We synthe-
sized intermediates with mono- and bis-aldehyde containing thiophene bridges, which were
separated successively by using the column chromatography. The organic photo-sensitizers
containing multi-electron acceptor with various anchoring groups and thiophene bridge
units, LOFM1 ∼ LOFM3, and without thiophene bridge LOFM4 ∼ LOFM6 were synthe-
sized successfully.
Scheme 1. Synthesis of organic dyes.

Long Wavelength Absorbing Organic Dyes
[609]/289
Figure 1. (A) Absorption spectra of the organic dyes in the DMF solution. (a = LOFM1, b =
LOFM2, c = LOFM3, d = LOFM4, e = LOFM5, f = LOFM6) (B) Normalized absorption spectra
of the organic dyes that were adsorbed onto the TiO2 films. (a = LOFM1, b = LOFM2, c = LOFM3,
d = LOFM4, e = LOFM5, f = LOFM6).
Figure 1 shows the UV-Vis absorption spectra of the synthesized dyes in DMF solution
(a) and on the TiO2 film state (b). The optical and electrochemical properties of the organic
dyes are summarized in Table 1. The maximum absorption wavelength (λmax) of the organic
dye corresponding to the π–π∗ transitions in the conjugated molecules appeared from
400 nm to 507 nm with molar extinction coefficient (εmax) of 2.4 × 104 M−1 cm−1 to 6.4 ×
104 M−1 cm−1, as seen in Table 1. The organic dyes with strong electron acceptor moieties
in a chromophore, LOFM3 and LOFM6, exhibited strong bathochromic shift in the λmax
,

290/[610]
S. A. Kim et al.
Table 1. Electrochemical parameters of the organic dyes.
a
d
e
εmax
λmax nm E0–0 (eV)c
Eox
Eox–E0–0 HOMO LUMO
(eV)
b
Dye
/M−1 cm−1 Soln. TiO2
(abs)
(V vs NHE) (V vs NHE) (eV)
LOFM1
LOFM2
LOFM3
LOFM4
LOFM5
LOFM6
43243
64658
64009
30580
62309
23569
406 411
474 454
507 493
389 400
458 462
464 468
2.403
2.183
2.033
3.024
2.489
2.275
1.324
1.268
1.296
1.533
1.474
1.440
−1.079 −5.718 −3.315
−0.915 −5.662 −3.479
−0.737 −5.690 −3.657
−1.491 −5.929 −2.905
−1.015 −5.870 −3.381
−0.835 −5.830 −3.555
aMaximum absorption and extinction coefficient at the maximum absorption of the dyes in the
b
c
DMF solution. Maximum absorption of the dyes that were adsorbed onto the TiO2 films. E0–0
(band gap) was determined from the intersection of the absorption and emission spectra in the DMF
solution. dThe oxidation potential (EHOMO) of the dye was measured using the cyclic voltammogram
in the DMF solution. eEox−E0–0 = ELUMO
.
compared to LOFM1 and LOFM4, due to the low LUMO values. It should be noted that
the λmax and εmax of organic dyes containing thiophene bridge units are enhanced, owing
to its longer conjugated π-chromophore, which can improve the light harvesting efficiency
(or photogenerated electron-hole pairs) of organic dye. The blue-shift of absorption peak
may be caused by the formation of the H-aggregate of organic dyes on the TiO2 surface
[17].
To ensure regeneration of the dye molecules, the highest occupied molecular orbital
(HOMO) level of the dye molecules should be more positive than the redox potential of
I3−/I− in the electrolyte, and the energy of the lowest unoccupied molecular orbital (LUMO)
must be more negative than that of the conduction band edge in TiO2. The HOMO and
LUMO energy levels of the dye molecules were estimated by cyclic voltammetry, as shown
in Table 1.
Photovoltaic Performances of the DSSCs with Organic Dyes
Figure 2 shows photocurrent-voltage (I–V) curves (A) and the incident photon-to-current
efficiency (IPCE) spectra (B) of the DSSCs containing different organic dyes under illu-
mination of simulated solar light (AM 1.5, 100 mW/cm2). The corresponding results are
indicated in Table 2. The higher efficiencies were achieved in the DSSC with LOFM1 ∼
LOFM3 rather than LOFM4 ∼ LOFM6, mainly owing to the higher short-circuit current
(Jsc) value (see Table 2). This is originated in their improved light harvesting efficiency,
which is resulted from the longer conjugated π-chromophore. Among the organic dyes,
LOFM1 containing bis-anchors/acceptors in carbazole chromophore with thiophene bridge
unit exhibited highest energy conversion efficiency, which is 3.0%, as revealed in Table 2.
The IPCEs of DSSCs with organic dyes were also measured, and the result is represented
in Figure 2(B). The integrated IPCE values were consistent with the Jsc values of organic
dyes.
The DSSCs with the LOFM1 ∼ LOFM3 exhibited slightly lower open-circuit voltage
(Voc) than those with LOFM4 ∼ LOFM6. The properties of charge transport in DSSCs
have been evaluated by measuring their impedance spectra (IS) under light illumination
condition. Under illumination, a DSSC is biased by the photovoltage, and thus photocurrent

Long Wavelength Absorbing Organic Dyes
[611]/291
Figure 2. (A) Current density-voltage characteristics for DSSCs containing organic dyes under
illumination of simulated solar light (AM 1.5, 100 mW/cm2). (B) IPCE curves for DSSCs based on
organic dyes. ∗Sample condition : Blocking layer, TiO2 paste : B32, Organic Dye 0.5 mM − PMII(0.7
M) + LiI (0.2 M) + I2 (0.05 M) + TBP (0.5 M) in ACN/VN = 85:15 - Pt electrode solution (7 mM),
60 µm surlyn, Dr. blade(2T).
flow depends on the photoelectric performance of the cell. Figure 3 shows the measured IS at
forward bias applied condition under illumination. There are two well defined semicircles
in high frequency region and middle frequency region. Compared with the results of
LOFM4 ∼ LOFM6 dyes, the semicircle size in the middle frequency region of LOFM1 ∼

292/[612]
S. A. Kim et al.
LOFM3 reduces significantly, indicating acceleration of electron transport process in the
photoanode. The Bode-phases for DSSCs are also shown in Figure 3(B).
The frequency peaks of DSSC composed of LOFM4 ∼ LOFM6 appeared in lower
frequency region than those of DSSC with LOFM1 ∼ LOFM3, due to relatively more rapid
electron transfer process occurred in the LOFM1 ∼ LOFM3. Since the frequency is related
with the inverse of electron life time (τ) in TiO2 electrodes, the higher electron life time
were obtained in LOFM4 ∼ LOFM6, compared to LOFM1 ∼ LOFM3. Thus, in the case
of LOFM1 ∼ LOFM3, the lower electron life times reduce the Voc values in the DSSC,
as shown in Table 3. The performances of DSSCs containing respective N3 and organic
dyes and the electron transport properties in their photoanodes from the IS analysis are
summarized in Table 3.
Conclusions
The organic photo-sensitizers based on carbazole framework with multi-anchors/acceptors
and thiophene bridge units were synthesized and applied to DSSCs. The multi-anchoring or-
ganic dyes with thiophene bridge provided efficient electron extraction pathways from elec-
tron donor, higher molar extinction coefficients, and better optical absorption in the longer
Table 2. Photovoltaic Performance of DSSCs. (Scattering layer; 250 nm, 4 µm + TiCl4).
Jsc
Voc
/V
η
Active
Layer
thickn. /µm
dyes
/mAcm−2
FF
/%
area /cm2
N3
12.8
7.7
4.9
7.2
4.1
2.9
2.1
0.70
0.58
0.54
0.58
0.62
0.53
0.56
0.62
0.66
0.67
0.67
0.68
0.70
0.69
5.6
3.0
1.8
2.8
1.7
1.1
0.8
0.242
0.247
0.239
0.241
0.245
0.247
0.244
23.4
24.2
26.2
24.1
25.2
23.9
24.7
LOFM1
LOFM2
LOFM3
LOFM4
LOFM5
LOFM6
Table 3. Performances of the N3 and Organic dyes-based DSSCs and the electron transport
properties in their photoanodes determined by impedance analysis. The cell areas are
0.24 cm2.
f
dyes
R1(Ω)a
R2(Ω)b
C1c
R3(Ω)d
C2e
τ (ms)
N3
17.1
17.7
14.8
19.0
17.3
14.9
21.0
6.2
9.3
9.0
9.3
6.8
1.4 × 10−5
1.3 × 10−5
1.7 × 10−5
1.4 × 10−5
1.7 × 10−5
1.8 × 10−5
3.2 × 10−5
18.7
18.2
33.1
21.0
49.0
50.3
82.2
1.2 × 10−3
2.6 × 10−4
8.1 × 10−5
2.8 × 10−4
7.6 × 10−4
8.4 × 10−5
3.5 × 10−4
16.0
3.1
1.7
4.3
22.0
2.2
LOFM1
LOFM2
LOFM3
LOFM4
LOFM5
LOFM6
13.4
23.6
16.0
aR1 is FTO Interface resistance. bR2 is due to the resistance at the interface between the counter
electrode and the electrolyte. dR3 is possibly originated from the backward charge transfer from TiO2
f
to the electrolyte and the electron conduction in porous TiO2 film.. τ is life time of an electron in
DSSC.

Long Wavelength Absorbing Organic Dyes
[613]/293
Figure 3. (A) Measured impedance spectra of DSSC at forward bias applied condition under light
illumination. Fitted curve calculated using circuit shown in the inset. (B) Displays of Bode-phase for
DSSC cell.
wavelengths, leading to improved short-circuit currents and overall energy-conversion effi-
ciencies. We believe that practically useful organic dye photo-sensitizers can be produced
by exploiting electron donor/acceptor system with proper length of π-conjugation in a
chromophore to control the absorption wavelength and enhance the photovoltaic perfor-
mance. The research on the spectral absorption optimization and the control of energy level
of HOMO and LUMO for various organic dyes are now in progress.
Acknowledgment
This research was supported by the 2008 Yeungnam University Research Grant.

294/[614]
S. A. Kim et al.
References
O’Regan, B., & Gra¨tzel, M. (1991). Nature, 353, 737.
Gra¨tzel, M. (2001). Nature, 338, 414.
Nazeeruddin, M. K., De Angelis, F., Fantacci, S., Selloni, A., Viscardi, G., Liska, P., Ito, S., Takeru,
B., & Gratzel, M. (2005). J. Am. Chem. Soc., 127, 16835.
Baxter, J. B., & Aydil, E. S. (2005). Appl. Phys. Lett., 86, 053114.
Nazeeruddin, M. K., Kay, A., Rodicio, I., Humphry-Baker, R., Mueller, E., Liska, P., Valchopoulos,
N., & Gratzel, M. (1993). J. Am. Chem. Soc., 115, 6382.
Nazeeruddin, M. K., Zakeeruddin, S. M., Humphry-Baker, R., Jirousek, M., Liska, P., Vlachopoulos,
N., Shklover, V., Fischer, C.-H., & Gratzel, M. (1999). Inorg. Chem., 38, 6298.
Gratzel, M. (2005). Inorg. Chem., 44, 6841.
Horiuchi, T., Miura, H., Sumioka, K., & Uchida, S. (2004). J. Am. Chem. Soc., 126, 12218.
Wang, Z.-S., Cui, Y., Hara, K., Dan-oh, Y., Kasada, C., & Shinpo, A. (2007). Adv. Mater., 19, 1138.
Park, S. -S., Won, Y. -S., Choi, Y. -C., & Kim, J. -H. (2009). Energy & Fuels, 23, 3732.
Tian, H., Yang, X., Cong, J., Chen, R., Liu, J., Hao, Y., Hagfeldt, A., & Sun, L. (2009). Chem. Comm.,
6288.
Hara, K., Sayama, K., Ohga, Y., Shinpo, A., Suga, S., & Arakawa, H. (2001). Chem. Comm., 569.
Hagberg, D. P., Edvinsson, T., Marinado, T., Boschloo, G., Hagfeldt, A., & Sun, L. (2006). Chem.
Comm., 2245.
Thomas, K. R. J., Lin, J. T., Hsu, Y.-C., & Ho, K.-C. (2005). Chem. Comm., 4098.
Velusamy, M., Thomas, K. R. J., Lin, J. T., Hsu, Y.-C., & Ho, K.-C. (2005). Org. Lett., 7, 1899.
Khazraji, A. C., Hotchandani, S., Das, S., & Kamat, P. V. (1997). J. Phys. Chem. B., 103, 4693.
William, F. M., Patti, E. B., John, C. R., Silvia, B. C., Lee, G. P., & David, G. W. (1984). J. Am.
Chem. Soc., 106, 5659.

Copyright of Molecular Crystals & Liquid Crystals is the property of Taylor & Francis Ltd and its content may
not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written
permission. However, users may print, download, or email articles for individual use.