Simple organic molecules bearing a 3,4-ethylenedioxythiophene linker for efficient dye-sensitized solar cells

Article abstract of DOI:10.1039/b808535h

3,4-Ethylenedioxythiophene and bis[2-(2-methoxyethoxy)ethoxy]thiophene bridged donor-acceptor molecules for dye-sensitized solar cells have been synthesized, one of which achieved a solar-to-energy conversion efficiency of 7.3%, compared to 7.7% optimized for N719 dye. The Royal Society of Chemistry.

Full text of DOI:10.1039/b808535h

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Simple organic molecules bearing a 3,4-ethylenedioxythiophene linker
for efficient dye-sensitized solar cellsw
Wei-Hsin Liu,a I-Che Wu,a Chin-Hung Lai,a Cheng-Hsuan Lai,a Pi-Tai Chou,*a
Yi-Tsung Li,b Chao-Ling Chen,b Yu-Yen Hsub and Yun Chi*b
Received (in Cambridge, UK) 20th May 2008, Accepted 29th August 2008
First published as an Advance Article on the web 24th September 2008
DOI: 10.1039/b808535h
3,4-Ethylenedioxythiophene and bis[2-(2-methoxyethoxy)ethoxy]-
thiophene bridged donor–acceptor molecules for dye-sensitized
solar cells have been synthesized, one of which achieved a solar-
to-energy conversion efficiency of 7.3%, compared to 7.7%
optimized for N719 dye.
Owing to their versatility and low cost,1 dye-sensitized solar
cells (DSSCs) have attracted much attention since the break-
through in conversion efficiency that Gratzel and co-workers
¨
made with Ru-based photosensitizers. Accordingly, their
breakthrough initiated intensive investigation, among which
the most common sensitizers should be ascribed to cis-dithio-
cyanato bis(4,40-dicarboxy-2,20-bipyridine) ruthenium(II) and
trithiocyanato-4,40,400-tricarboxy-2,20:60,200-terpyridine ruthe-
nium(II), known as N3 and black dye,2,3 respectively. A new
series of these dyes are widely used and show efficiencies of up
to 10% under simulated AM 1.5 irradiation (100 mW cmꢀ2).4
However, the high cost of Ru metal may hamper further
developments. Alternatively, numerous organic DSSC dyes
have been developed.5 Their practical advantages include: (1)
larger molar extinction coefficients resulting from the allowed
pp* transitions, (2) simple synthesis as well as facile structural
modification, and (3) much less concern regarding their avail-
ability.6
Scheme 1 Schematic structure of TPA dyes mentioned in this article.
poly-3,4-ethylenedioxythiophene (PEDOT) has been applied
in the fabrication of polymer based photovoltaics.9 Thus, if
successful, the high product yield, low cost, and versatility of
chemical modification may provide greatly superior DSSCs.
The synthetic protocols leading to the isolation of L1 and
LJ1–LJ4 are depicted in the ESI.w They consist of treatment of
TPA substituted boronic acid with 5-formyl-2-bromothiophene
or the corresponding EDOT derivative under conditions for
Suzuki coupling, followed by condensation with cyanoacrylic
acid or rhodanine-3-acetic acid in the presence of ammonium
acetate.10 Note that L1, which serves as a control unit for
other dyes, has already been documented, including its
performance properties.7
Herein, we report the design and syntheses of a series of
simple organic dyes (Scheme 1) that contain common triphe-
nylamine (TPA) as donor and cyanoacrylic acid or rhodanine-
3-acetic acid as electron acceptor (anchoring groups). These
two moieties are bridged by thiophene and its derivatives, such
as 3,4-ethylenedioxythiophene (EDOT) or 3,4-bis[2-(2-methoxy-
ethoxy)ethoxy]thiophene (BMEET), to form organic dyes,
namely: L1,7 LJ1–LJ3, and LJ4. In contrast to many recent
approaches aimed at increasing the conjugation to extend the
absorption cross section and/or conducting the bathochromic
shift,8 we attempted instead to simplify the D–A structure such
that planarity can be reached easily and hence the efficiency of
charge transfer can be enhanced to compensate for its possible
Fig. 1 shows the UV-Vis spectra for L1 and LJ1–LJ4 in
tert-butanol–acetonitrile (1 : 1) solutions. The strong absorp-
tion band can apparently be attributed to the intramolecular
charge transfer between the TPA donor to cyanoacrylic acid
or rhodanine-3-acetic acid. LJ2 and LJ3 dyes bearing rhoda-
nine-3-acetic acid as the acceptor show a significant red shift in
inferiority of a relatively large energy gap. Especially,
the introduction of EDOT has its own niche in that
a Department of Chemistry, National Taiwan University, Taipei
10617, Taiwan. E-mail: chop@ntu.edu.tw
b Department of Chemistry, National Tsing Hua University, Hsinchu
30013, Taiwan. E-mail: ychi@mx.nthu.edu.tw
w Electronic supplementary information (ESI) available: Syntheses,
characterization, devices and measurements. See DOI: 10.1039/
b808535h
Fig. 1 Absorption spectra of TPA derivatives in tert-butanol–
acetonitrile (1 : 1) solution.
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Table 1 Photophysical, electrochemical and photovoltaic performance data of organic dyes
Potentials and energy levels
Photovoltaic performance datad
Absorption
labsa/nm (e/Mꢀ1 cmꢀ1
Emission
lema/nm
)
Eoxb/V
E0–0c/V
Eox–E0–0/V
Jsc/mA cmꢀ2
Voc/mV
FF
Z (%)
Dye
L1
410 (25 800)
426 (24 500)
477 (29 000)
501 (26 100)
420 (24 000)
549
548
624
625
548
1.06
0.98
1.00
0.93
1.05
2.40
2.53
2.24
2.17
2.53
ꢀ1.34
ꢀ1.55
ꢀ1.24
ꢀ1.24
ꢀ1.48
12.8
15.5
7.8
8.0
10.7
b
620
690
570
580
750
0.66
5.20
7.30
3.04
3.18
5.36
LJ1
LJ2
LJ3
LJ4
a
0.683
0.684
0.685
0.668
Absorption and emission spectra were measured in tert-butanol–acetonitrile (1 : 1) solution. The oxidation potentials of dyes on TiO2 were
measured in CH3CN with 0.1 M tetrabutylammonium hexafluorophosphate (TBAP) with a scan rate of 50 mV sꢀ1 (vs. NHE). E0–0 was
determined from the intersection of absorption and emission spectra. The concentration was maintained at 3 ꢂ 10ꢀ4 M in tert-butanol–
c
d
acetonitrile (1 : 1) solution, with 1 mM deoxycholic acid (DCA) as a coadsorbate, and 0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 0.1 M LiI,
0.05 M I2, 0.5 M 4-tert-butylpyridine (TBP) in dry acetonitrile as electrolyte. Performances of DSSCs were measured with a 0.25 cm2 working area.
the S0–S1 absorption band compared with that of the L1 and
LJ1 dyes. Moreover, the LJ4 dye with BMEET, and the LJ1
and LJ3 dyes with the EDOT linker, also show a bathochro-
mic shift compared with L1 and LJ2, respectively. This
red-shift may favor light harvesting and hence photocurrent
generation in DSSCs (vide infra).
relatively lower Jsc and Voc, leading to a lower Z value of 5.2%.
The device based on L1 with a thinner TiO2 film showed a
lowered Z value of 2.75%.7 Moreover, the related TPA dye
without the bridging thiophene showed an even lower Z value
of 2.47%.13 In another approach, DSSCs based on LJ2 and
LJ3 showed greatly inferior efficiencies of 3.04% and 3.18%,
respectively, even though they had much broader spectral
response and higher extinction coefficients. For a fair compar-
ison, the N719-sensitized TiO2 solar cell showed an efficiency
of 7.7%, with a Jsc of 15.6 mA cmꢀ2, a Voc of 750 mV, and a
fill factor (FF) of 0.66.
The Eox of the dyes adsorbed on a 6 mm thick TiO2
nanocrystalline film on transparent conducting oxide (TCO)
glass were measured using cyclic voltammetry (see Table 1).
The results reveal that the redox potential of I/I3ꢀ (ca. 0.4 V vs.
NHE) is more negative than the HOMO and is able to
regenerate the dyes from electron donation. The LUMO levels
of these dyes are also sufficiently more negative than the
conduction band edge of the TiO2 electrode (ꢀ0.5 V vs.
NHE at pH 7).11 The large energy gap, calculated from the
LUMO of the dye and the Ecb of the TiO2 electrode, provides
a favorable energy to inject electrons into the TiO2 electrode,
while incorporation of 4-tert-butylpyridine also decreased the
dark current and improved device efficiency.12 As a result, the
open-circuit voltage and fill factor are improved, leading to an
increase in overall conversion efficiencies.
The incident photon-to-current conversion efficiencies
(IPCEs) of these DSSC dyes are shown in Fig 3. The onset
of the IPCE spectrum based on LJ1 is B660 nm, and high
IPCE performance (480%) was observed from 400 to 570 nm,
with the highest value, 92%, at 450 nm. In contrast, despite the
onset of the IPCE spectra of DSSCs of B750 nm for both LJ2
and LJ3, they exhibited lower IPCEs with maxima of 40% at
B500 nm. The rather low IPCE values for LJ2 and LJ3 dyes
reflect lower photocurrent and hence inferior photovoltaic
performance. The results may indicate that rhodanine-3-acetic
acid is a poor anchor in comparison to cyanoacrylic acid.14
We then estimated the amounts of dye adsorbed on the TiO2
films by desorbing the dye with basic solution. The concentra-
tions were then determined to be 1.8 ꢂ 10ꢀ7, 1.7 ꢂ 10ꢀ7 and
1.4 ꢂ 10ꢀ7 M cmꢀ2 for L1, LJ1 and LJ4, respectively. The side
chain of LJ4 leads to less dye-uptake. It is thus reasonable to
conclude that broader spectral response, higher molar
All essential properties of these DSSC dyes are listed in
Table 1, and the respective J–V curves are shown in Fig. 2.
Under the standard AM 1.5 G irradiation, the maximum
efficiency (Z) for the LJ1-sensitized solar cell with an active
cell area of 0.25 cm2 was calculated to be 7.3%, with a short-
circuit current (Jsc) of 15.5 mA cmꢀ2 and an open-circuit
voltage (Voc) of 690 mV, while the DSSCs based on L1 showed
Fig. 2 Photocurrent density vs. voltage curves for DSSCs based
on L1, LJ1–LJ4 and N719 under AM 1.5 G simulated solar light
(100 mW cmꢀ2).
Fig. 3 The incident photon-to-current conversion efficiencies spectra
for DSSCs based on L1, LJ1–LJ4 and N719.
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successful prospects of simple organic DSSC photosensitizers
such as LJ1 and its future derived analogues.
Notes and references
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Fig. 4 The calculated frontier orbitals of LJ1 (left) and LJ3 (right).
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absorption coefficient and higher amounts of dye adsorbed on
the TiO2 films account for the higher conversion efficiency for
LJ1. To gain more insight, theoretical analysis (density func-
tional theory (DFT), B3LYP/6-31G(d) level) on the molecular
orbitals involved in the transitions was carried out, and the
resulting frontier orbitals are depicted in Fig. 4. Clearly,
the lowest transition was dominated by charge transfer from
the TPA to the cyanoacrylic acid or rhodanine-3-acetic acid
moiety. It is noteworthy that the LUMO electron density of
L1 and LJ1 is located mainly on the cyanoacrylic acid, such
that the excited electron can be injected into the TiO2 electrode
effectively. However, LJ2 and LJ3 each have a methylene
group that disrupts the p* conjugation between rhodanine
and the carboxylic acid, hence decreasing the electron injection
efficiency in a dynamic manner. The electrochemical impe-
dance experiments showed the electron lifetime being shor-
tened in device LJ3 more than in LJ1 (see Fig. S3 in ESIw).
This explains the superiority of L1 and LJ1 over LJ2 and LJ3,
respectively.
In yet another approach, we synthesized the LJ4 dye with
the BMEET linker. The resulting J–V and IPCE plots are also
included in Fig. 2 and 3. In comparison to that of LJ1, the
slightly lower Z value of 5.36 directly reflects its lower spectral
response. However, the higher Voc value of 750 mV was
obtained due to the suppression of dark current from the free
TiO2 conduction-band to the counter electrolyte.15 Further
suppression has been achieved via replacement of lithium
iodide by cuprous iodide, as supported by the resulting Voc
of 800 mV. Unfortunately, the conversion efficiency was not
accordingly gained, due to a lower Jsc.16
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Ho, Org. Lett., 2005, 7, 1899; (b) K. R. J. Thomas, Y.-C. Hsu, J. T.
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11 C. Klein, M. K. Nazeeruddin, D. D. Censo, P. Liska and M.
Gratzel, Inorg. Chem., 2004, 43, 4216.
In summary, simple donor–acceptor designs bearing EDOT
or BMEET linkers were synthesized with high yields. The
introduction of the EDOT group in LJ1 increases the spectral
response and perhaps renders a better degree of charge
separation, resulting in a leap in the photovoltaic performance
in comparison to its parent compound L1, and exhibits a
conversion efficiency Z as high as 7.3%. The lower IPCEs
obtained for the LJ2 and LJ3 dyes could be the result of the
LUMO being located in a rhodanine framework rather than at
the carboxylic acid group; the result effectively reduces the
electron injection efficiency. Our results strongly support the
¨
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¨
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5154 | Chem. Commun., 2008, 5152–5154