Indoline dyes with benzothiazole unit for dye-sensitized solar cells

Article abstract of DOI:10.1246/cl.160084

We report a new series of indoline dyes with a donoraromatic-acceptor (D-π-A) structure. D-π-A metal-free organic dyes with indoline-benzothiazole-rhodanine units were synthesized and their photovoltaic performances were evaluated. The photoelectric conversion efficiency (η) of the indoline-benzothiazole-rhodanine dye is 3.7%, while that of the indolinethiophene-rhodanine dye is 0.9% under the same conditions. The incident photon-to-current conversion efficiencies (IPCEs) of these dyes are 60% and 25%, respectively, at 500 nm. To understand their electronic structures, the geometries of the dyes were optimized by density functional theory (DFT) calculations at the 6-31G(d) level using a B3LYP exchange-correlation functional. As a result, the localized highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the indoline?benzothiazole?rhodanine dye were obtained and were compared with those of the indoline-thiophene-rhodanine dye.

Full text of DOI:10.1246/cl.160084

Received: January 26, 2016 | Accepted: February 15, 2016 | Web Released: February 20, 2016
CL-160084
Indoline Dyes with Benzothiazole Unit for Dye-sensitized Solar Cells
Tamotsu Horiuchi,*1 Tohru Yashiro,1 Ryo Kawamura,2 Satoshi Uchida,3 and Hiroshi Segawa4
1System Device Development Department, Ricoh Institute of Future Technology, Ricoh Company Limited,
16-1 Shinei-cho, Tsuzuki-ku, Yokohama, Kanagawa 224-0035
2Research & Development Group, intellim Corporation, 3-4-5 Umeda, Kita-ku, Osaka 530-0001
3Komaba Organization for Educational Excellence College of Arts and Science (KOMEX), The University of Tokyo,
3-8-1 Komaba, Meguro-ku, Tokyo 153-8902
4Research Center for Advanced Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904
(E-mail: tamotsu.horiuchi@nts.ricoh.co.jp)
We report a new series of indoline dyes with a donor-
aromatic-acceptor (D-π-A) structure. D-π-A metal-free organic
dyes with indoline-benzothiazole-rhodanine units were synthe-
sized and their photovoltaic performances were evaluated. The
photoelectric conversion efficiency (©) of the indoline-benzo-
thiazole-rhodanine dye is 3.7%, while that of the indoline-
thiophene-rhodanine dye is 0.9% under the same conditions.
The incident photon-to-current conversion efficiencies (IPCEs)
of these dyes are 60% and 25%, respectively, at 500 nm. To
understand their electronic structures, the geometries of the dyes
were optimized by density functional theory (DFT) calculations
at the 6-31G(d) level using a B3LYP exchange-correlation
functional. As a result, the localized highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) of the indoline-benzothiazole-rhodanine dye were
obtained and were compared with those of the indoline-
thiophene-rhodanine dye.
derivative should be introduced between the donor and acceptor
units.11-13 The development of the NKX-2195 dye, with a
rhodanine ring as the acceptor and a coumarin ring as the donor,
has been reported.14 This dye has oxidation potential values
(EOX) minus the 0-0 energy (E0-0) that are more negative than
approximately ¹0.7 V vs. NHE, and show high incident photon-
to-current conversion efficiencies (IPCEs). For EOX = ¹0.63 V,
the IPCE value of NKX-2195 at -max was 40%. In donor-
aromatic-acceptor (D-π-A)-type dyes with a rhodanine ring, the
absorption band of LMc with a 4-methine chain as the π unit
was red-shifted compared to that of Mc with a 2-methine chain
as the π unit, and the absorption thresholds in ethanol solution
were 700 and 570 nm, respectively.15 However, the photoelectric
conversion efficiency (©) and short-circuit photocurrent density
¹2
(JSC) of the LMc were very low (©: 0.4%, JSC: 2.62 mA cm
)
compared to those of the Mc (©: 4.5%, JSC: 11.4 mA cm¹2).16
The low excitation energy, occurrence of cis-trans isomer-
ization, and disorder of the alignment of chromophores in the
J-like aggregate were used to explain these observations.
Molecular design of the dyes is thus necessary to prevent
cis-trans isomerization and to obtain good energy matching to
TiO2 and I2. We examined π units that would be effective for
D-π-A type dyes with a rhodanine ring as the acceptor, and
found that the benzothiazole ring has a molecular structure that
would not allow cis-trans isomerization. Here, we report on a
new type of organic dye, based on indoline with a benzothiazole
unit.
Keywords: Dye-sensitized solar cell (DSSC)
Metal-free organic dye Indoline
|
|
Dye-sensitized solar cells have attracted considerable
attention as inexpensive next-generation power-generating de-
vices.1-3 These are the only solar cells capable of combining
colorful designs, transparency, and flexibility. By exploiting
these advantages, even from the standpoint of photovoltaic
devices, the use of dye-sensitized solar cells will become
widespread, and an industry that can actively support future
energy levels will grow. However, the conversion efficiency
and durability of dye-sensitized solar cells need to be further
improved to suit more practical applications and to establish
the industry. Thus, continuous development of new materials is
essential. In particular, the refinement of dyes plays a crucial
role in fulfilling this task. These dyes are classified into two
groups: “metal complex dyes,” such as ruthenium complexes,
and “metal-free organic dyes,” which do not contain metal
ions. Metal-free dyes have unique features like high molecular
absorption coefficients, vivid colors, and a wide variety of
molecular designs. In particular, good photovoltaic performance
has been reported for dyes containing indoline as the donor
unit.4-6 Perovskite solar cells have attracted considerable
attention in recent years due to their excellent photovoltaic
performances.7-10 Although the energy conversion efficiencies
of dye-sensitized solar cells are lower than perovskite solar cells,
it has little load for environment.
The chemical structure of the indoline dyes (BT-1 and BT-2)
are shown in Figure 1. T-1 and T-2, both of which have a
thiophene unit, were synthesized for reference. Details of the
conditions and route for the synthesis of BT-2, T-1, and T-2 are
presented in the Supporting Information. Only the synthesis of
BT-1 is described below.
Et
S
N
O
S
BT-2
S
BT-1
S
S
N
COOH
CN
N
N
N
N
COOH
O
Et
N
S
O
T-2
T-1
S
CN
COOH
S
COOH
N
N
N
S
S
O
It has recently been shown that to fabricate sensitized dyes
with large wavelengths, an aromatic ring such as the thiophene
Figure 1. Chemical structure of indoline dyes.
Chem. Lett. 2016, 45, 517–519 | doi:10.1246/cl.160084
© 2016 The Chemical Society of Japan | 517

Table 1. Photovoltaic performance of indoline dyes
BT-1 was produced by heating a mixture of 2-{1-[4-(2,2-
diphenylvinyl)phenyl]indolin-5-yl}benzo[d]thiazole-5-carbalde-
hyde (0.50 g, 0.935 mmol), cyanoacetic acid (0.16 g, 1.88 mmol),
piperidine (0.35 g, 4.11 mmol), and acetonitrile (20.0 mL) and
chlorobenzene (3 mL) to 80 °C in a N2 atmosphere. The reaction
finished after 1.5 h, and the mixture was cooled to room
temperature. It was then poured into water, and ethyl acetate and
toluene were used to extract the reaction products. The organic
layer was washed with water and separated, dried over
magnesium sulfate, and was then subjected to vacuum concen-
tration. The residue was purified using silica gel chromatography
with a mixture of toluene and ethyl acetate as the solvent, and
recrystallized from a mixture of toluene and hexane. The yield of
the target product obtained thus was 0.53 g.
VOC
/V
JSC
/mA cm
©
/%
Dye
T-1
CDCA tBP
ff
¹2
0.504
0.459
0.525
0.462
0.591
0.543
0.572
0.529
0.442
0.407
0.403
0.373
0.508
0.462
0.51
12.72
15.21
12.28
15.7
12.72
12.02
13.51
13.87
5.42
8.01
3.56
6.32
9.88
0.614
0.375
0.676
0.527
0.525
0.457
0.651
0.556
0.591
0.402
0.633
0.608
0.58
3.94
2.62
4.36
3.82
4.02
2.98
5.03
4.09
1.42
1.31
0.91
1.43
2.91
2.01
3.71
3.31
®
®
®
®
®
®
®
®
®
®
BT-1
T-2
®
®
®
®
The experimental section is presented in the Supporting
Information.
BT-2
Recently, solid-state dye-sensitized solar cells have been
reported.17-19 These solid-state dye-sensitized solar cells are
necessary for 2-μm TiO2 thin films due to the use of high-
resistance hole-transport materials. Then, a sensitizing dye with
a high absorbance coefficient is required for effective light
absorption. The absorption spectra of T-1 and BT-1 in dichloro-
methane are presented in the Supporting Information. The
absorbance coefficients measured for BT-1 and BT-2 in
8.69
10.89
14.69
0.502
0.668
0.504
®
®
0.447
¹1
dichloromethane were, respectively, 42700 M¹1 cm at 410 nm
¹1
and 41300 M¹1 cm at 391 nm. In contrast, T-1 and T-2 show
similar absorption maxima to that of BT-1, at 361 and 332 nm,
respectively. The highest occupied molecular orbital (HOMO)
was measured with a photoelectron spectrometer (AC-2, Riken
Keiki Co., Ltd.); the band gap was determined from the edge of
the UV-vis spectra; and the lowest unoccupied molecular orbital
(LUMO) was calculated. The HOMOs and LUMOs of these
dyes are presented in the Supporting Information. The band gap
of the indoline dyes with the benzothiazole unit was larger
than that of the thiophene unit. In terms of the red shift of the
absorption spectra, the benzothiazole unit is less favorable than
the thiophene unit.
Figure 2. Absorption spectra of BT-2 and T-2.
The photovoltaic performances of the solar cells constructed
from these dyes were measured. The measured open-circuit
photovoltage (VOC), short-circuit photocurrent density (JSC),
fill factor (ff), and solar-to-electric conversion efficiency (©) of
these indoline dyes are listed in Table 1. Figure 2 shows the
IPCE spectra of BT-2 and T-2, respectively. The IPCE spectra
of T-1, BT-1, and these reference dyes are presented in the
Supporting Information.
large absorption peak at 410 nm of T-1 was not observed in the
IPCE.
Table 1 summarizes the photovoltaic performances of the
indoline dyes with a double-rhodanine ring as the adsorption
unit. The © value of BT-2 is slightly higher than that of T-2. This
differed from the behavior observed when cyanoacetic acid was
the adsorption unit, and T-2 showed an increase in JSC when
CDCA was used. However, BT-2 showed a decrease in JSC
.
The photovoltaic performances of the indoline dyes con-
taining cyanoacetic acid were compared. The © values of BT-1
and T-1 were almost the same. When CDCA was added as a
coadsorbent to BT-1 and T-1 solutions, the efficiency of these
dyes decreased because of the weak molecular interaction
between the dye molecules on TiO2. A decrease in the IPCE
value was evident in the spectra of both BT-1 and T-1 with the
coaddition of CDCA. On the other hand, the photovoltaic
performance slightly increased with the addition of 0.05-mM
tBP to the electrolyte. When the thiophene unit was changed to
the benzothiazole unit, the IPCE increased slightly in the 400-
500-nm region. However, a shift to shorter wavelength was
obtained. In the dye with cyanoacetic acid as the acceptor, the
edges of the absorption and IPCE spectra showed the same
behavior, and T-1 was red-shifted with respect to BT-1. The
Figure 2 shows that the IPCE spectrum of BT-2 was higher than
that of T-2. There is no example of a high IPCE value for a D-π-
A-type sensitized dye containing a double-rhodanine unit. We
found that the benzothiazole unit was useful for sensitized dyes.
For the dye with a rhodanine ring as the acceptor, the edges of
absorption and IPCE spectra also showed the same behavior, and
T-2 was red-shifted in comparison to BT-2. The IPCE of BT-2
displayed a flat shape, and the IPCE of T-2 had a similar shape
to that of the absorption spectrum. The energy levels of BT-2
and T-2 were almost equivalent. This result may arise from a
difference in the efficiency of electronic injection from the dye
cation to TiO2. The thermogravimetry (TG) of the dyes with
cyanoacetic acid, BT-1 and T-1, were 406 and 378 °C,
respectively. The TG of the dye with double rhodanine ring,
BT-2 and T-2 were 284 and 276 °C, respectively. It is apparent
518 | Chem. Lett. 2016, 45, 517–519 | doi:10.1246/cl.160084
© 2016 The Chemical Society of Japan

DFT calculations showed that dyes with benzothiazole units
have localized HOMO and LUMO. Therefore, we conclude that
the benzothiazole unit allows effective electron injection from
the dye cation to the TiO2 surface.
BT-2
N
O
O
T-2
S
CH2COOH
CH2COOH
N
N
N
N
S
N
S
S
S
S
O
O
S
N
S
H3CH2C
H3CH2C
This research is supported by the Japan Society for the
Promotion of Science (JSPS) through its “Funding Program for
World-Leading Innovative R&D on Science and Technology
(FIRST Program).” “Development of Organic Photovoltaics
toward a Low-Carbon Society” from the Cabinet Office of Japan.
Supporting Information is available on http://dx.doi.org/
10.1246/cl.160084.
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Chem. Lett. 2016, 45, 517–519 | doi:10.1246/cl.160084
© 2016 The Chemical Society of Japan | 519