Dye-sensitized solar cells based on 1,3-dithiol-2-ylidene derivatives: Substituent and π-spacer effects on the efficiency

Article abstract of DOI:10.1246/cl.130931

Four novel 1,3-dithiol-2-ylidene dyes BDY-1, BDY-2, BDY-3, and BDY-4 were developed for dye-sensitized solar cells. Among these dyes, BDY-4 containing a 9,9-dibutyl-9H-fluorene spacer and a phenyl group at the vinyl position showed the highest efficiency (3.26%), indicating the importance of the phenyl substituent and the fluorene unit in improving the efficiency.

Full text of DOI:10.1246/cl.130931

CL-130931
Received: October 4, 2013 | Accepted: November 9, 2013 | Web Released: November 14, 2013
Dye-sensitized Solar Cells Based on 1,3-Dithiol-2-ylidene Derivatives:
Substituent and π-Spacer Effects on the Efficiency
Atsushi Wada,1 Jun-ichi Nishida,1 Masato M. Maitani,2 Yuji Wada,2 and Yoshiro Yamashita*1
1Department of Electronic Chemistry, Tokyo Institute of Technology,
G1-8, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8502
2Department of Chemistry, Tokyo Institute of Technology,
E4-3, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552
(E-mail: yoshiro@echem.titech.ac.jp)
NC
Four novel 1,3-dithiol-2-ylidene dyes BDY-1, BDY-2, BDY-
COOH
3, and BDY-4 were developed for dye-sensitized solar cells.
Among these dyes, BDY-4 containing a 9,9-dibutyl-9H-fluorene
spacer and a phenyl group at the vinyl position showed the
highest efficiency (3.26%), indicating the importance of the
phenyl substituent and the fluorene unit in improving the
efficiency.
S
S
BDY-1
NC
NC
COOH
COOH
S
S
n-Bu n-Bu
S
S
BDY-2
BDY-3
Dye-sensitized solar cells (DSSCs) using a nanocrystalline
TiO2 electrode have attracted much attention due to their
possibilities for providing electricity with low cost and simple
fabrication.1,2 Dyes strongly affect the power-conversion effi-
ciency (PCE) through their important roles in light harvesting
and electron injection. So far, various kinds of dyes, including
metal complexes such as ruthenium complexes and zinc
porphyrins, have been developed and applied to the DSSCs.3,4
The highest conversion efficiency of 12.3% has been achieved
using a porphyrin dye.5 However, metal complexes are difficult
to synthesize and they are also expensive. On the other hand,
organic dyes without metals have received increasing attention
due to the large molecular extinction coefficients and easy
modification, in addition to their lower costs. Donor-(π-spacer)-
acceptor (D-π-A)-type organic dyes have been designed to
extend the absorption region, in which various arylamines,
including triarylamines, carbazoles, phenothiazines, and indo-
lines, have been used as electron-donating moieties.6 However,
the number of dyes that do not use arylamine as the donor is
limited. Therefore, it is still important to explore new materials
for making progress in this field. 1,3-Dithiol-2-ylidene has a
strong electron-donating property and has been used as a donor
unit in donor-acceptor systems.7 Although D-π-A dyes con-
taining a 1,3-dithiol-2-ylidene unit for DSSCs have been also
reported, they have not been fully developed.8 We have now
prepared novel dyes containing these units and investigated the
relationship between the structures of dyes and the DSSC
performances.
NC
COOH
S
n-Bu n-Bu
S
BDY-4
Figure 1. Benzo-1,3-dithiol-2-ylidene dyes, BDY-1, BDY-2,
BDY-3, and BDY-4.
biphenyl unit. The alkyl groups of the fluorene can suppress the
aggregation of dyes.9 BDY-4 has both the phenyl group and the
fluorene π-spacer unit.
The syntheses of the 1,3-dithiol-2-ylidene dyes were
achieved by four steps using Wittig, Friedel-Crafts, Suzuki-
Miyaura, and Knoevenagel reactions. The synthetic procedures
are outlined in Supporting Information.10 The products were
purified by silica gel chromatography and were characterized
with NMR and MS spectra.
DFT calculations based on the B3LYP method with
6-31G(d) level were performed to gain an insight into the
structure and electronic states of dyes. Figure S1 shows the
HOMO and LUMO orbitals of BDY dyes.10 The calculations
show that the HOMOs are mainly located at the benzo-1,3-
dithiol-2-ylidene part, while the LUMOs are located at the
cyanoacrylic acid unit. The HOMO part is far from the TiO2
surface and the LUMO part is close to the TiO2, which is
favorable for electron injection from the dyes and protecting the
back electron transfer from TiO2 to the dyes. The HOMO is
delocalized to the additional phenyl group at the vinyl position.
The UV-vis absorption spectra of 1,3-dithiol-2-ylidene dyes
in solution are shown in Figure 2 (dot-line; in THF). The
spectroscopic date are summarized in Table S1.10 All these dyes
have two absorption peaks in THF. The absorption in a shorter-
wavelength region (300-350 nm) corresponds to the π-π*
electronic transition. BDY-3 with a fluorene unit shows the
absorption (326 nm, 3.10 © 104 M¹1 cm¹1) at a longer wave-
length than BDY-1 with a biphenyl unit (315 nm, 2.43 ©
104 M¹1 cm¹1). Similarly, the absorption of BDY-4 with a
We have developed four novel 1,3-dithiol-2-ylidene dyes:
BDY-1, BDY-2, BDY-3, and BDY-4. Their structures are
depicted in Figure 1. BDY-1 is a donor-acceptor compound
composed of benzo-1,3-dithiol-2-ylidene, a biphenyl π-spacer,
and an electron-accepting cyanoacrylic acid unit. A phenyl
group is substituted at the vinyl position in BDY-2. Introducing
the phenyl group was expected to prevent the dimerization of the
cation radical and sterically protect the aggregation of the 1,3-
dithiole moiety. BDY-3 has a 9,9-dibutyl-9H-fluorene unit as
a π-spacer. The unit was expected to enhance the conjugation
between the donor and acceptor groups compared to the twisted
296 | Chem. Lett. 2014, 43, 296–298 | doi:10.1246/cl.130931
© 2014 The Chemical Society of Japan

The absorption edges of BDY-3 and BDY-4 with a fluorene
unit are red-shifted compared with those of BDY-1 and BDY-2
with a biphenyl unit. The red shift of the absorption edges
suggests a more effective intramolecular charge-transfer inter-
action in the planar fluorene system. The UV-vis absorption
spectra of BDY dyes on TiO2 are shown in Figure 2 (solid line;
on TiO2) and Figure S2, where the TiO2 film was immersed in
the THF solution of dyes (0.5 mM) for 12 h.10 The absorption
spectra of BDY-1 and BDY-3 are blue-shifted compared to those
in solution. Considering the planar structures of BDY-1 and
BDY-3, the formation of H-aggregates on the TiO2 surface may
be responsible for the formation of blue shifts.10 In contrast,
BDY-2 and BDY-4 do not exhibit such blue shifts. This
result suggests that introducing the phenyl group at the vinyl
position is useful for suppressing the aggregation of dyes on
TiO2.
The orbital energy levels of the dyes were deduced from the
cyclic voltammograms (CVs) in DMF containing TBAPF6 as a
supporting electrolyte and a Pt working electrode (Table S1).10
All of the dyes exhibited irreversible oxidation waves. The
HOMO levels were obtained from the first oxidation potentials
(Eox) calculated as Epa ¹ 0.03 in the CVs. The Eox of BDY-3 and
BDY-4 having a fluorene unit are slightly lower than those of
BDY-1 and BDY-2. Their values are more positive than the
iodine/iodide redox potential (0.4 V vs. NHE), indicating that
these dyes can be regenerated with the iodine/iodide redox
system. The excitation transition energies (E0-0) were deter-
mined by the edge of absorption. The LUMO levels of BDY
dyes calculated as Eox ¹ E0-0 are sufficiently negative relative to
the conduction-band edge of TiO2 (¹0.5 V vs. NHE). This fact
indicates that carrier separation takes place smoothly from the
excited dyes to TiO2.
300
300
300
300
350
350
350
350
400
450
500
550
550
550
550
600
600
600
600
Wavelength/nm
400
450
500
Wavelength/nm
400
450
500
Wavelength/nm
400
450
500
The photovoltaic characteristics of BDY dyes for DSSCs
were evaluated with sandwich cells. The film thickness of TiO2
is 6.5 ¯m. The redox electrolyte consists of 0.6 M DMPImI,
0.1 M LiI, 0.05 M I2, and 0.5 M tert-butylpyridine in acetonitrile.
Figure 3 shows the current density-voltage curve of DSSCs
under AM 1.5G solar irradiation. The photovoltaic data,
including short-circuit current (Jsc), open-circuit voltage (Voc),
fill factor (FF), and photon-to-current efficiency (PCE), are
listed in Table 1. The JSC and VOC values of BDY-2 with the
additional phenyl group were higher than those of BDY-1. A
similar effect of the phenyl group can be seen in a comparison of
BDY-3 with BDY-4. The high JSC value can be explained by
considering that the phenyl group disturbs the aggregation of the
dyes, as suggested in the absorption spectra (Figure S2).10 The
Wavelength/nm
Figure 2. UV-vis absorption spectra of BDY dyes in THF and
on TiO2. Absorption spectra of BDY-1, BDY-2, BDY-3, and
BDY-4 are shown in red, blue, green, and black lines. The dotted
lines are spectra in THF and the solid lines are specta on TiO2.
fluorene unit (341 nm, 2.92 © 104 M¹1 cm¹1) is red-shifted
compared with BDY-2 with a biphenyl unit (326 nm, 3.46 ©
104 M¹1 cm¹1). This fact indicates that the conjugation between
the donor and acceptor parts is more favorable in the fluorene
unit than in the biphenyl one due to the more planar geometry
of the fluorene. Due to the extension of π-conjugation, the
absorption of BDY-2 with the phenyl group is red-shifted
compared with that of BDY-1. The absorption in a longer-
wavelength region (400-500 nm) is attributed to the intra-
molecular charge transfer. The absorption in BDY-3 with a
¹
phenyl group also protects the approach of I3 to the TiO2
electrode, which is useful for disturbing the back electron
¹
11
transfer from the TiO2 to I3 , resulting in an increase in VOC
.
BDY-3 and BDY-4 with a fluorene unit showed higher JSC
values than the corresponding dyes with a biphenyl unit. This
is attributed to the better conjugation between the donor and
acceptor parts in the fluorene-containing dyes. DSSCs based on
BDY-4 containing both the phenyl group and fluorene unit
showed the highest efficiency (3.26%) among these BDY dyes.
The incident photon-to-current efficiency (IPCE) spectra for
DSSCs of BDY-1, BDY-2, and BDY-4 are shown in Figure S5.
The IPCE values of 2 and 4 (ca. 80%) are significantly higher
than that of 1 (ca. 50%), confirming the effect of the phenyl
group on the conversion efficiency.10
fluorene unit (425 nm, 5.78 © 104 M¹1 cm¹1) is observed at a
longer wavelength than BDY-1 (406 nm, 2.99 © 104 M¹1 cm
)
¹1
with a biphenyl unit. On the other hand, the spectra of BDY-2
(395 nm, 1.58 © 104 M¹1 cm¹1) and BDY-4 (396 nm, 2.00 © 104
M
¹1 cm¹1) show that the introduction of the phenyl substituent
brings about the blue shifts of the absorption band and weaker
intensities compared with those of BDY-1 and BDY-3. This
result may be explained by considering that the phenyl group
disturbs the conjugation between the 1,3-dithiole unit and the
spacer parts.
Chem. Lett. 2014, 43, 296–298 | doi:10.1246/cl.130931
© 2014 The Chemical Society of Japan | 297

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Figure 3. Photocurrent-voltage curves of the DSSCs based on
BDY dyes. BDY-1, BDY-2, BDY-3, and BDY-4 are shown in
red, blue, green, and black lines.
Table 1. Photovoltaic performance of DSSCs based on BDY
dyes
¹2
JSC/mA cm
V
OC/V
FF
PCE/%
BDY-1
BDY-2
BDY-3
BDY-4
N719
4.55
6.49
6.98
7.91
14.69
0.54
0.6
0.58
0.65
0.68
0.65
0.66
0.63
0.64
0.56
1.61
2.56
2.54
3.26
5.53
7
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In summary, we have developed a series of benzo-1,3-
dithiol-2-ylidene dyes by a facile synthetic method. A 9,9-
dibutyl-9H-fluorene π-spacer is effective in improving the JSC
value by protecting the aggregation of dyes and increasing
the efficiency of intramolecular charge transfer. The additional
phenyl group at the vinyl position is effective in increasing the
VOC value. Further development of novel dyes based on such
donor-acceptor systems containing a 1,3-dithiol-2-ylidene unit
is in progress.
9
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10 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chemlett/
index.html.
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298 | Chem. Lett. 2014, 43, 296–298 | doi:10.1246/cl.130931
© 2014 The Chemical Society of Japan