A highly efficient organic sensitizer for dye-sensitized solar cells

Article abstract of DOI:10.1039/b709859f

We have synthesized a highly efficient organic dye for a dye-sensitized solar cell; the overall solar-to-energy conversion efficiency was 9.1% at AM 1.5 illumination (100 mW cm^-2): short-circuit current density (J _sc) = 18.1 mA cm^-2, open circuit photovoltage (V _oc) = 743 mV and fill factor (ff) = 0.675. The Royal Society of Chemistry.

Full text of DOI:10.1039/b709859f

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A highly efficient organic sensitizer for dye-sensitized solar cells{
Suyoung Hwang,a Jung Ho Lee,a Chanmoo Park,a Hoinglae Lee,a Chaekyu Kim,a Chiyoung Park,a
Mi-Hyeon Lee,b Wanin Lee,b Jihee Park,c Kyungkon Kim,c Nam-Gyu Park*c and Chulhee Kim*a
Received (in Cambridge, UK) 28th June 2007, Accepted 22nd August 2007
First published as an Advance Article on the web 14th September 2007
DOI: 10.1039/b709859f
The HOMO level of the dye was estimated from its oxidation
potential by using cyclic voltammetry, as shown in Fig. 3. The
measurement was carried out using a Pt electrode in acetonitrile
solution containing 0.1 M tetrabutylammonium tetrafluoroborate
as an electrolyte. A standard ferrocene/ferrocenium (Fc/Fc+)
system was used to calibrate the redox peak. The HOMO level
was determined from the oxidation potential value with respect to
ferrocene using an Ag/AgCl reference electrode. The HOMO level
of TA-St-CA was 25.2 eV. The band gap of TA-St-CA was 2.4 eV,
which was estimated from the absorption edge of the absorption
spectrum. The LUMO of TA-St-CA, calculated from the HOMO
and optical band gap, was 22.8 eV, which provides a favorable
energy matching with the conduction level of TiO2 particles for the
electron transfer from the LUMO of the dye to the conduction
band of TiO2. The HOMO, LUMO and band gap energy results
are summarized in Table 1.8
We have synthesized a highly efficient organic dye for a dye-
sensitized solar cell; the overall solar-to-energy conversion
efficiency was 9.1% at AM 1.5 illumination (100 mW cm22):
short-circuit current density (Jsc) = 18.1 mA cm22, open circuit
photovoltage (Voc) = 743 mV and fill factor (ff) = 0.675.
Dye-sensitized solar cells (DSSCs) have been a target of intensive
research in recent years due to their capability to convert solar light
into electricity with low materials and production costs.1,2 One of
the key material issues in DSSCs is the photosensitizer, which can
exhibit high efficiencies in the conversion of solar energy into
electricity. Ru complex photosensitizers such N3, N719 and Black
dyes exhibit efficiencies higher than 10%.1,2 The performance of
DSSCs based on organic dyes have also recently been remarkably
improved. Therefore, it is expected that organic dyes could
compete with Ru complexes in the near future.3
Organic dyes have many advantages as photosensitizers, such as
large molar extinction coefficient, control of absorption wave-
length, facile design and synthesis, and lower cost than Ru
complexes.4 Efficient organic dyes such as coumarin, indoline,
phthalocyanine and conjugated oligo-ene dyes have been
reported.3–7 However, as yet, the use of organic dyes for DSSCs
has a limitation due to their relatively low efficiency compared Ru
complexes. Herein, we report a highly efficient dye which exhibits
9.1% overall solar-to-electric energy conversion efficiency.
Fig. 4 shows the optimized molecular structure and frontier
molecular orbitals of TA-St-CA. At the HOMO of the dye,
electrons are homogeneously distributed in the p-conjugated oligo-
phenylenevinylene unit and the triphenylamine electron donor
As shown in Fig. 1, we have prepared the organic dye TA-St-
CA, which contains a p-conjugated oligo-phenylenevinylene unit
with an electron donor–acceptor moiety for intramolecular charge
transfer and a carboxyl group as an anchoring unit for the
attachment of the dye onto TiO2 nanoparticles.{
The UV-vis spectrum of TA-St-CA in Fig. 2 shows an
absorption maximum at 386 nm in ethanol (e = 31600 M21 cm21).
Fig. 2 Absorption spectrum of TA-St-CA in ethanol.
Fig. 1 Molecular structure of TA-St-CA dye.
aDepartment of Polymer Science and Engineering, Inha University,
Incheon 402-751, Korea. E-mail: chk@inha.ac.kr; Fax: +82 32-865-5178;
Tel: +82 32-860-7487
bDepartment of Chemistry, Inha University, Incheon 402-751, Korea
cCenter for Energy Materials, Korea Institute of Science and
Technology (KIST), Seoul 136-791, Korea. E-mail: npark@kist.re.kr;
Fax: +82 2-958-5309; Tel: +82 2-958-5365
{ Electronic supplementary information (ESI) available: Detailed synthetic
procedures and spectral characterization. See DOI: 10.1039/b709859f
Fig. 3 Cyclic voltammogram of TA-St-CA measured at a sweep rate of
20 mV sec21
.
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Chem. Commun., 2007, 4887–4889 | 4887

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Table 1 Electrochemical properties and photovoltaic performance of TA-St-CA
Dye
EHOMO/eVa
Ebandgap/eVb
ELUMO/eVc
Jsc/mA cm22
Voc/mV
ff
g (%)
TA-St-CA
N719
25.2
25.6d
2.4
2.6d
22.8
23.0d
18.1
19.9
743
769
0.675
0.657
9.1
10.1
a
b
Estimated from the oxidation potential, determined by cyclic voltammetry. Estimated from the edge of absorption spectrum. Calculated
c
d
from the band gap and HOMO value. See ref. 8.
moiety. However, at the LUMO level, with electronic excitation,
intramolecular charge transfer induced electron movement from
the donor site to the acceptor moiety of the dye. Therefore, excited
electrons can be injected into the conduction band of TiO2 via the
carboxyl group adjacent to the cyano group.
A TA-St-CA DSSC was prepared, where a TiO2 film was
composed of a double layer, with a y5 mm thick 400 nm particle
film over-coated onto a 10 mm thick 20 nm particle film on FTO
glass (Pilkington TEC8, y8 V/sq). The annealed TiO2 film was
further treated with 0.2 M TiCl4 for 22 h at ambient temperature.
Dye adsorption was performed with a 0.5 mM TA-St-CA solution
Fig. 5 Photocurrent density–photovoltage curve of TA-St-CA under
AM 1.5 (0.7 N-methyl-N-butyl
radiation (100 mW cm22
in ethanol. For comparison, a N719 DSSC was prepared by the
same procedure. Photocurrent and voltage were measured using a
solar simulator equipped with a 1000 W ozone-free Xenon lamp
and AM 1.5 G filter (Oriel), where the light intensity was adjusted
with an NREL-calibrated Si solar cell with a KG-5 filter to
1 sunlight intensity (100 mW cm22).
G
)
M
imidazolium iodide, 0.2 M LiI, 50 mM iodine, 0.5 M tert-butylpyridine
in acetonitrile/3-methoxypropionitrile (50 : 50 V/V)).
The photocurrent density–photovoltage (I–V) curve is shown in
Fig. 5. The short circuit current density (Jsc) and open circuit
voltage (Voc) of TA-St-CA were 18.1 mA cm22 and 743 mV,
respectively, with a fill factor (ff) of 0.675. The overall conversion
efficiency (g) for TA-St-CA was 9.1%, under which conditions the
efficiency of N719 was 10.1%.
Fig.
6 shows the incident photon-to-current conversion
efficiency (IPCE) data for TA-St-CA, where the IPCE data was
collected at a low chopping speed of 10 Hz under a monochro-
matic beam generated by a 75 W Xenon light source. The IPCE
value in the range 400 to 550 nm reached around 80%, which is
higher than that for N719. The red-shift in the IPCE of TA-St-CA
in Fig. 6, compared to the absorption spectrum of TA-St-CA in
Fig. 1, can be ascribed to aggregation of the dye on titania.
Fig. 6 IPCE spectrum for a DSSC based on TA-St-CA.
We are currently investigating the solar cell performance of
p-conjugated donor–acceptor sensitizers having strong absorptions
with wavelengths longer than 600 nm to achieve higher efficiencies.
In addition, a tandem cell with TA-St-CA and N719 is under
investigation because TA-St-CA and N719 showed good perfor-
mance over different wavelength ranges. The detailed cell structure
and photovoltaic characteristics will be published elsewhere.
This work was supported by a KIST internal project. We thank
Dr Jae Woong Yu for help with the experiments.
Notes and references
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