Organic dyes containing thienylfluorene conjugation for solar cells

Article abstract of DOI:10.1039/b506732d

New organic dyes that contain variable lengths of conjugation featuring alternating thiophene and fluorene segments have been synthesized and efficient nano-crystalline TiO₂ based dye-sensitized solar cells were fabricated using these molecules

Full text of DOI:10.1039/b506732d

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COMMUNICATION
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Organic dyes containing thienylfluorene conjugation for solar cells{
K. R. Justin Thomas,a Jiann T. Lin,*ab Ying-Chan Hsuc and Kuo-Chuan Ho*c
Received (in Cambridge, UK) 12th May 2005, Accepted 23rd June 2005
First published as an Advance Article on the web 13th July 2005
DOI: 10.1039/b506732d
we show that by increasing the population of the linking segments,
the optical density can be dramatically raised. To the best of our
knowledge fluorene-based dyes have not been exploited for the
fabrication of DSSCs.
New organic dyes that contain variable lengths of conjugation
featuring alternating thiophene and fluorene segments have
been synthesized and efficient nano-crystalline TiO2 based dye-
sensitized solar cells were fabricated using these molecules as
light-harvesting sensitizers.
The dyes were constructed by the stepwise synthetic protocol
illustrated in Scheme 1.{ From the known compound, 2-(7-bromo-
9,9-diethyl-9H-fluoren-2-yl)thiophene (2), diarylamine substituted
fluorenylthiophene derivatives (3–5) were obtained by C–N
coupling reactions involving Hartwig’s catalyst10 and the corre-
sponding diarylamine. These thiophene derivatives were converted
into their corresponding thiophenealdehydes (6–8) by lithiation
with n-butyl lithium and subsequent quenching with dimethylfor-
mamide. Later the aldehydes 6–8 were converted to the dyes 9–11
by reaction with cyanoacetic acid in refluxing acetic acid in the
presence of ammonium acetate. In another step, the stannylene
precursor of 3 prepared by treating the lithium derivative of 3 with
tributyltin chloride in thf was used in the Stille reaction with 2 to
produce the bis(thienylfluorene) conjugated analog of 3. This was
then converted to the aldehyde which produced the dye 12 on
treatment with cyanoacetic acid. One more similar stepwise
sequence starting with the bis(thienylfluorene) conjugated analog
of 3 was executed to obtain the third generation dye 13. The dyes
are red colored and soluble in thf.
Electronic devices such as light-emitting diodes,1 field-effect
transistors,2 and solar cells3 fabricated using organic materials
continue to attract wide attention owing to their ease in
manufacturing and structural flexibility towards modification.
Although silicon based photovoltaics have received commercial
success they are devoid of possibilities for improvement by
structural alternation.4 Recent progress with nanocrystalline TiO2
based dye-sensitized solar cells (DSSCs) has triggered the demand
for the search for organic and organometallic dyes suitable for
application in such modules.5 The organic dyes that were efficient
in photocurrent generation all contained a donor and acceptor
bridged by p-conjugation. In most designs, amines served as
donors while a 2-cyanoacrylic acid or rhodamine unit rendered the
acceptor functionality. The presence of a carboxylic acid moiety in
the acceptor end assisted the anchoring of the dye molecules on
the TiO2 surface. Donor–acceptor architectured coumarin-,6
indoline-,7 cyanine-8 and hemicyanine-based9 dyes were found to
perform efficiently in DSSCs. A dye possessing broad and intense
spectral features that is capable of absorbing the entire solar
spectrum is an ideal choice. In this communication we report new
dyes that contain a conjugation bridge composed of alternating
thiophene and fluorene units, diphenylamine donor and
2-cyanoacrylic acid as acceptor (Fig. 1). In such an architecture
The absorption and emission spectra of the dyes recorded in thf
solution are displayed in Fig. 2 and the data are collected in
Table 1. All the dyes exhibit a single prominent band probably
representing the superposition of p–p* and charge transfer
transitions. Though the absorption maximum remains nearly the
same for all the compounds, the optical density of this band
increases rapidly on extending the conjugation by introducing the
additional thienylfluorene segments (Fig. 2). This can be under-
stood from molecular modeling (SPARTAN, PM3) studies on 9
and 12. The ground state structure of 12 possesses a nearly 90u
twist between the thienyl and the fluorenyl segments, while there is
only a slight twist in 9. These compounds are the first examples of
DSSC active dyes to date to display larger absorption coefficients.
Interestingly the interaction between the donor and acceptor is
minimized on increasing the conjugation length of the bridge. This
is reflected in the emission band position of the dyes 9, 12 and 13.
Consequently this slightly decreases the magnitude of charge
separation for elongated derivatives 12 and 13.
Fig. 1 Structure of the dyes.
aInstitute of Chemistry, Academia Sinica, 115 Nankang, Taipei, Taiwan.
E-mail: jtlin@chem.sinica.edu.tw; Fax: 886-2-27831341;
Tel: 886-2-27898522
The dyes can be reversibly oxidized at moderately high
oxidation potentials. The oxidation potentials are more positive
2
than the ferrocene/ferrocenium and I2/I3 redox couples.11 An
bDepartment of Chemistry, National Central University, 320 Chungli,
Taiwan
oxidation potential higher than that of the iodine couple is
necessary to forbid the backward electron transfer to electrolyte
solution and ensure forward electron injection into the TiO2 layer
in the DSSC setup. The reversibility of the oxidation couple will be
beneficial for the stability of the DSSCs in long run.
cDepartment of Chemical Engineering, National Taiwan University,
Taipei 106, Taiwan. E-mail: kcho@ntu.edu.tw
{ Electronic supplementary information (ESI) available: Characterisation
data of the new compounds described in this communication. See http://
dx.doi.org/10.1039/b506732d
4098 | Chem. Commun., 2005, 4098–4100
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Scheme 1 Synthesis of the first generation dyes.
LiI–0.5 M tert-butylpyridine in acetonitrile solution showed
impressive photon-to-electron conversion. The device performance
statistics under AM 1.5 illumination are listed in Table 1.
Particularly the performance of the device manufactured with
dye 9 is close to that of the standard ruthenium dye N3. The action
and absorption spectra of the dye 9 adsorbed on TiO2 are
presented in Fig. 3. We believe that the more efficient charge
separation in 9 compared to 12 and 13 (vide supra) accounts for the
better performance of DSSCs using 9, although 12 and 13 have
significantly higher absorption coefficients. Another factor that is
crucial in affecting the performance of the dyes in DSSCs is the
oxidation potential of the dyes. Incidentally the oxidation potential
of 9 is slightly higher than that of other dyes. This will increase the
driving force for the reduction of the oxidized dye and ensure the
retardation of back electron transfer from TiO2 to dye.
In summary, we have synthesized a new class of dyes
featuring thienylfluorene conjugation, diarylamine donors and
2-cyanoacrylic acid acceptors. Despite the lower wavelength
absorption, the fact that the DSSC performances achieved for
the dyes are nearing that of N3 is intriguing. We are currently
extending this strategy to synthesize spirobifluorene based dyes.
Fig. 2 Absorption and emission spectra of the dyes (9, 12 and 13)
recorded in thf.
The DSSCs fabricated, with an effective area of 0.25 cm2, using
these dyes as light harvesting sensitizers, nanocrystalline anatase
TiO2 particles, and the electrolyte composed of 0.05 M I2–0.5 M
Table 1 Optical, redox and DSSC performance parameters of the dyesa,b
Dye
l
abs/nm (e/M21 cm21
)
lem/nm (WF (%))
Eox (DEp)/mV
VOC/V
JSC/mA cm22
ff
g (%)
9
10
11
12
13
N3
a
421 (52 900)
421 (46 300)
425 (54 500)
423 (95 500)
423 (159 200)
538 (0.28)
536 (0.19)
537 (0.33)
512 (0.26)
472, 504 (0.37)
509 (82)
451 (109)
462 (66)
447 (59), 613 (61)
437 (65), 569 (70)
0.65
0.57
0.60
0.61
0.61
0.62
12.47
7.59
8.38
9.83
9.81
13.98
0.65
0.67
0.67
0.65
0.64
0.63
5.23
2.86
3.35
3.89
3.80
5.50
Absorption and electrochemical data were recorded in tetrahydrofuran solutions. Scan rate: 100 mV s21, electrolyte: (n-C4H9)4NPF6; DEp is
the separation between the anodic and cathodic peaks. Potentials are quoted with reference to the internal ferrocene standard (E1/2 5 +265 mV
vs. Ag/AgNO3). Experiments were conducted using TiO2 photoelectrodes with approximately 14-mm thickness and 0.25 cm2 working area on
b
the FTO (7 V square21) substrates.
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Fig. 3 Absorption spectrum of the dye 9 adsorbed on nanocrystalline
TiO2 and the action spectrum for the DSSC constructed using 9.
We thank CTCI Foundation, Taipei, Research Centre for
Applied Sciences, Academia Sinica (ASCAS) and National Science
Council, Taiwan for financial support.
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Notes and references
1
{ All the dyes described here were thoroughly characterized by H NMR
and mass spectral methods and elemental analyses. Full synthesis and
characterization details are furnished in the supporting information.
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11 The I2/I3 couple is located at 0.2 V vs. SCE (or 20.18 V vs. Fc).
2
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4100 | Chem. Commun., 2005, 4098–4100
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