A new type of donor-acceptor dye bridged by the bidentate moiety; Metal ion complexation enhancing DSSC performance

Article abstract of DOI:10.1039/c0jm04032k

The bidentate 2,2′-bithiazole moiety was strategically designed as the bridging unit for the new donor-acceptor dye, Alko1. Dye sensitized solar cells (DSSCs) made by Alkol bound with various metal ions have been examined. Upon complexing with ZnI₂

Full text of DOI:10.1039/c0jm04032k

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COMMUNICATION
A new type of donor–acceptor dye bridged by the bidentate moiety; metal ion
complexation enhancing DSSC performance†
*
Bo-So Chen, Yi-Ju Chen and Pi-Tai Chou
Received 22nd November 2010, Accepted 24th January 2011
DOI: 10.1039/c0jm04032k
The bidentate 2,20-bithiazole moiety was strategically designed as
the bridging unit for the new donor–acceptor dye, Alko1. Dye
sensitized solar cells (DSSCs) made by Alkol bound with various
metal ions have been examined. Upon complexing with ZnI2, the
Zn(II)–Alko1 complex on TiO2 achieved both higher molar
absorptivity and a red-shifted absorption peak, increasing the
performance of DSSCs by ꢀ23% (cf. metal ion free Alko1).
organic DSSC dye 3-(5’-(4-(bis(4-tert-butylphenyl)amino)phenyl)-
2,2’-bithiazol-5-yl)-2-cyanoacrylic acid, Alko1, was strategically
designed and synthesized. The synthetic routes, characterization,
corresponding metal ion response, and hence cell performance are
elaborated as follows.
Results and discussion
The synthetic route of Alko1 is illustrated in Scheme 2, in which 1a is
synthesized according to the literature report.30 The BTA moiety is
then brominated by N-bromosuccinimide (NBS) to give the disub-
stituted 5,50-dibromo-2,20-bithiazole (1b).31 Monolithiation of 1b
using n-butyllithium and subsequent reaction with N-for-
mylmorpholine render the corresponding aldehyde 1c in 56% yield.
The Ullmann coupling of 4,40-di-tert-butyl-diphenylamine with
iodobenzene in the presence of CuI as a catalyst in toluene under
refluxing conditions produces 2a in 84% yield. Bromination of 2a
with NBS in chloroform gives 2b in 92% yield. The stannylation of 2b
with n-butyllithium and subsequent reaction with tributylstannyl
chloride yield 2c. Stille coupling between 1c and 2c is then carried out
using Pd(PPh3)4 as a catalyst in toluene under refluxing conditions to
produce 3 in 72% yield. Compound 3 is then treated with cyanoacetic
acid in the presence of piperidine to yield Alko1 in 62% yield (see ESI†
for the detailed synthetic procedure and characterization).
Introduction
Owing to their prospect of high energy conversion efficiency and low
production costs, dye-sensitized solar cells (DSSCs) have been
attracting considerable attention as a promising photovoltaic tech-
nology during the past two decades.1–3 Typical DSSCs consist of
a dye adsorbed semiconductor TiO2 photoelectrode, a Pt counter
electrode, and redox electrolytes. It has been recognized that the
nature of the optical responses of sensitizers, independent of their
being ruthenium based4–9 or pure organic types,10–21 plays a pivotal
role in determining cell efficiencies. Accordingly, progress in optimi-
zation of dye components has been made through increasing the
optical absorption cross-section as well as expanding the light
absorption capability with an aim to improve the short-circuit current
Jsc. Numerous efforts have been made to extend p-conjugation of the
sensitizers to enhance molar absorptivity and/or to cover a broad,
visible-to-near-infrared range. However, a trade-off of extending
p-conjugation commonly lies in its lack of planarity or lessening of
the bulky sensitizer uptake amount onto the spatially limited nano-
crystalline TiO2 surface.
Fig. 1 depicts the absorption spectrum of Alko1 in tetrahydrofuran
(THF) solution. The Alko1 dye shows an intense absorption band
around 450 nm (28 100 Mꢁ1 cmꢁ1, Table 1), which is reasonably
assigned to the intramolecular charge transfer from the 4-tert-butyl-
N-(4-tert-butylphenyl)-N-phenylaniline donor to the acceptor cya-
noacrylic acid moiety. Fig. 1 also reveals the titration spectrum of
Alko1 by ZnI2 in THF solution. Throughout careful titration, no
obvious isosbestic point could be resolved. The absence of an
isosbestic point may indicate the establishment of equilibrium among
more than two species. This proposal is reasonable, for free Alko1 in
To minimize the above inferior factors, unconventionally, we herein
propose the exploitation of a bidentate 2,20-bithiazole (BTA) moiety
(see Scheme 1) as the bridging unit. In comparison to 2,20-bithio-
phene,22–25 which is generally applied as a p-bridged linker for organic
DSSCs, it is anticipated that the 2,20-bithiazole moiety should allow
complexation with metal ions directly through the bidentate nitrogen
lone pairs,26–29 resulting in enhancement of the planarity and hence the
p-conjugation. Various types of metal ion binding may thus be
examined to optimize the spectral response. On this basis, a new
Department of Chemistry, National Taiwan University, Taipei, 10617,
Taiwan. E-mail: chop@ntu.edu.tw
† Electronic supplementary information (ESI) available. Supplementary
information. See DOI: 10.1039/c0jm04032k
Scheme 1
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two factors: (1) the incorporation of Zn(II), which enhances the
electron withdrawing strength, and thereby the charge transfer effi-
ciency and (2) the increase of the planarity in the BTA moiety, and
hence the facilitation of the p-conjugation. The net effect is also
reflected by lowering of the energy gap as well as the increase of
absorptivity shown in Fig. 2.
Prior to the fabrication of the solar devices, cyclic voltammetry was
also performed to ensure that the work function of Alko1 was suit-
able for injecting electrons into the conduction band of TiO2, as well
as to determine whether their HOMO matched the energy level of the
redox electrolyte. Accordingly, the Eox of Alko1 on a 6 mm thick TiO2
nanocrystalline film onto FTO glass was measured, and the results
ꢁ
are listed in Table 1. Table 1 reveals that the redox potential of Iꢁ/I3
(ca. 0.4 V vs. NHE)32 is more negative than the HOMO for Alko1,
and is able to regenerate the dye from electron donation. Their
LUMO levels are also sufficiently more negative than the conduction
band edge of the TiO2 electrode (ca. ꢁ0.5 V vs. NHE at pH 7).32
Photovoltaic experiments for Alko1 sensitizer were then performed
using various compositions of acetonitrile-based electrolytes. The
electrolyte compositions were as follows: device (A): 0.6 M 1-butyl-3-
methylimidazolium iodide (BMII), 0.1 M LiI, 0.05 M I2, and 0.5 M
4-tert-butylpyridine (TBP). Devices (B–D): 0.6 M BMII, 0.1 M LiI,
0.05 M I2, 0.1 M guanidinium thiocyanate (GNCS) were containing
various concentrations of ZnI2: device (B), 0.05 M ZnI2; device (C),
0.10 M ZnI2; and device (D), 0.15 M ZnI2. Fig. 3 shows the incident
photon-to-current conversion efficiencies (IPCEs) of the Alko1 dye
obtained with a sandwich cell using various compositions of elec-
trolytes. For device (A), ZnI2–free Alko1 dye exhibited a moderate
plateau at 62%, and the onset of IPCE data was close to ꢀ680 nm. In
comparison, increasing the ZnI2 concentrations from (B) to (D)
gradually shifted the onset values from ꢀ680 nm to ꢀ730 nm,
consistent with that observed from the UV-visible absorption.
Fig. 4 shows the photocurrent–voltage curves of Alko1 dye
employing various electrolytes under AM 1.5G simulated irradiation
sunlight at a light intensity of 100 mW cmꢁ2, and pertinent perfor-
mance data are summarized in Table 2. The short-circuit current
density (Jsc), open-circuit voltage (Voc), fill factor (FF), and overall
conversion efficiency (h) of Alko1 for device (A) were 9.40 mA cmꢁ2,
663 mV, 0.73 and 4.54%, respectively. In comparison, device (B)
showed slightly higher current density, 11.67 mA cmꢁ2, which
correlates well with the incident photon-to-current conversion effi-
ciency spectra. With maximum ZnI2 (0.15 M) added, device (D) gave
Jsc ¼ 13.13 mA cmꢁ2, Voc ¼ 587 mV and FF ¼ 0.68, corresponding
to an overall h ¼ 5.24%. Upon increase of the ZnI2 concentrations,
the Jsc values progressively increased from 9.40 mA cmꢁ2 (device with
metal ion free Alkol) to around a saturated 13.20 mA cmꢁ2 (device C).
In contrast to the trend of the Jsc, however, Voc values gradually
decreased from 663 mV to 587 mV. This result is perhaps due to the
fact that Zn(II) acted as a Lewis acid that penetrated into the TiO2
framework. The net result was to shift the flat band potential of TiO2
more positively.33 An alternative possibility is that 4-tert-butylpyr-
idine (TBP) was not used for device (B)–device (D). The employment
of TBP as the additive should shift the Ecb of the TiO2 negatively and
hence improve the open-circuit voltage.34 Unfortunately, upon
addition of TBP to devices (B–D), it was found that the color of the
TiO2 films changed back to that of the ZnI2-free device (A). Due to its
Lewis base, TBP plausibly competes with the bithiazole bidentate of
Alkol via its complexation with Zn(II). Overall, device (C)
represents the best condition among this series of electrolytes, giving
Scheme 2
solution possesses dual binding sites. In addition to the 2,20-bithiazole
moiety, which allows the complexation with Zn(II), the carboxylic
acid group of Alko1 in tetrahydrofuran solution is also capable of
binding Zn(II). At the end point of titration, the absorption spectrum
exhibits a peak wavelength around 465 nm, which is red shifted from
Zn(II)-free Alko1 by ꢀ15 nm. The complexation with Zn(II) through
the carboxylic acid site can be avoided once Alko1 is adsorbed onto
the TiO2 surface. Fig. 2 reveals the absorption spectrum of a film
prepared by adsorbing Alko1 on a 6 mm thick layer of TiO2. In
comparison to that in solution, the peak wavelength of Alko1 was red
shifted to 465 nm. After this Alko1 anchored TiO2 film was dipped
into an acetonitrile solution containing ZnI2, (10ꢁ2 M) for 10 seconds,
the film absorption showed further red shift of the peak wavelength
by ꢀ10 nm, accompanied by an increase in the absorptivity
(see Fig. 2). Such a change toward a darker color can be promptly
detected by the naked eye. The results unambiguously conclude the
formation of Zn(II)–Alko1 complex via binding with the 2,20-bithia-
zole moiety (see Scheme 1). Spectral changes are plausibly caused by
Fig. 1 Absorption spectra of 10ꢁ5 (M) Alko1 dye in THF titrated by
different concentrations of ZnI2 in THF.
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Table 1 Photophysical and electrochemical data of Alko1 dye
Dye
labs/nm, 3a/Mꢁ1 cmꢁ1
labs,film/nm
lema/nm
Eoxb/V
E0–0c/V
Eox ꢁ E0ꢁ0/V
Alko1
Zn(II)–Alko1
450, 28 100
—
465
475
631
—
1.02
1.06
2.10
2.01
ꢁ1.08
ꢁ0.95
Absorption and emission spectra were measured in THF. b Oxidation potential of dyes on TiO2 was measured in CH3CN with 0.1 M TBAP with a scan
a
rate of 50 mV sꢁ1 (vs. NHE). c E0–0 was determined based on lonset from film absorption spectra.
Fig. 2 (A) (___) The film absorption of Alko1 adsorbed on 6 mm thick
Fig. 4 The photocurrent–voltage curves of Alko1 dye employing various
TiO2 layer. (B) (—C—) After the as-prepared Alko1 anchored TiO2 film
electrolytes under AM 1.5G simulated sunlight at a light intensity of 100
was dipped into an acetonitrile solution containing ZnI2, (10ꢁ2 M) for 10
mW cmꢁ2
.
seconds.
Table 2 Photovoltaic data of Alko1 fabricated using various electro-
lytesa
Jsc ¼ 13.20 mA cmꢁ2, Voc ¼ 612 mV, and FF ¼ 0.69, corresponding
to an overall h ¼ 5.56%, which is 22.4% greater than that of the
reference solar cell fabricated in (A).
Device
Jsc/mA cmꢁ2
Voc/mV
FF
h (%)
The performance of various metal ions bound to DSSCs has also
been examined. In this approach, other d10 metal ions, such as CdI2
and HgI2, were employed to form Cd2+ (or Hg2+)–Alkol complex.
Fig. 5 shows the film absorption spectra of Cd(II)–Alkol and Hg(II)–
Alkol on 6 mm TiO2. For comparison, the film absorption spectra of
metal ion-free Alkol and Zn(II)–Alkol complex adsorbed on TiO2 are
A
B
C
D
9.40
11.67
13.20
13.13
663
630
612
587
0.73
0.72
0.69
0.68
4.54
5.28
5.56
5.24
a
The concentration of the dye was maintained at 3 ꢂ 10ꢁ4 M in
tert-butanol/THF (1/1) solution with 1 mM deoxycholic acid (DCA) as
a coadsorbate. Performances of DSSCs were measured with 0.16 cm2
working area. Device A: metal ion free Alkol, see text for devices B–D.
also included in Fig. 5. Obviously, the formation of Hg(II)–Alkol or
Cd(II)–Alkol complex causes even further red shift of the absorption
(cf. Zn(II)–Alkol). However, the complexation also renders an adverse
effect, a slight decrease in the absorptivity at the peak wavelength.
The electrolyte compositions were as follows: device (E): 0.6 M BMII,
0.1 M LiI, 0.05 M I2, 0.1 M GNCS and 0.1 M CdI2. Device (F): 0.6
M BMII, 0.1 M LiI, 0.05 M I2, 0.1 M GNCS and 0.1 M HgI2. As
a result, the optimized device (E) gives Jsc ¼ 11.94 mA cmꢁ2, Voc
¼
643 mV and FF ¼ 0.65, corresponding to an overall h ¼ 4.99%
(Table 3). For device (F), Jsc ,Voc, FF and h are 12.25 mA cmꢁ2,
627 mV, 0.67 and 5.15%, respectively. The corresponding photo-
current–voltage curves of device (E) and device (F) are shown in
Fig. S1† of the ESI. The relative incident photon-to-current conver-
sion efficiencies (IPCEs) are also shown in Fig. S2†.
Fig. 3 The incident photon-to-current conversion efficiencies (IPCEs) of
Alko1 dye obtained with various compositions of electrolytes (see text for
the detailed description).
We have also made attempts to prepare and isolate the Zn(II)–
Alko1 complex for a comparative study. In this approach, Alko1 was
4092 | J. Mater. Chem., 2011, 21, 4090–4094
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comparison to the traditional methods of extending p-conjugation of
the sensitizers toward lower energy gap with high absorptivity, the
concept of metal ion complexation in situ turns out to be facile, with
successful DSSC enhancement. In addition to the energy conversion,
the metal-binding concept may be extended and exploited in metal
ion recognition. Accordingly, a DSSC dye can be designed to possess
a recognition unit, such that the current output of the DSSC can be
used as a signal readout.35 Relevant applications are thus quite
promising due to the versatility and feasibility in design and synthesis,
respectively. Further intensive work on this issue is currently in
progress.
Notes and references
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mixed with saturated ZnI2 in methanol solvent. After stirring for
2 hours at room temperature, the reaction mixture was then poured
into water to obtain dark-brown solid Zn(II)–Alko1 (see Fig. S3† for
1H-NMR measurement). We then tested this as-prepared Zn(II)–
Alko1 for DSSC performance. Under similar condition, i.e.
employing 0.6 M BMII, 0.1 M LiI, 0.05 M I2, 0.1 M guanidinium
thiocyanate (GNCS) as electrolytes, this sequential metal-binding
method gives lower current density compared with the above in situ
complexation by the ZnI2 method. We believe that less uptake of
Zn(II)–Alko1 in this sequential metal-binding method may play
a major role. This is due to the fact that free Alko1 in solution
possesses dual binding sites for the metal ion. In addition to the
2,20-bithiazole moiety allowing the complexation with Zn(II), the
carboxylic acid group is also capable of binding Zn(II). Since
the carboxylic acid acts as a main functional group for the TiO2
binding, the reduction of free carboxylic acid sites hence decreases the
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In summary, the bidentate 2,20-bithiazole moiety was strategically
applied as a bridging unit. The resulting new donor–acceptor dye,
Alko1, was then applied in an attempt to prove the concept of metal
complexation enhancing the performance of DSSCs. Upon com-
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absorptivity, accompanied by a red-shifted absorption peak,
increasing the performance of DSSCs by ꢀ23% (cf. Alko1). In
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