Improving the photovoltaic performance by employing alkyl chains perpendicular to the π-conjugated plane of an organic dye in dye-sensitized solar cells

Article abstract of DOI:10.1039/c9tc01520e

We demonstrate that employing long alkyl chains perpendicular to the π-conjugated plane of the backbone is a promising strategy to construct efficient organic sensitizers for dye-sensitized solar cells (DSSCs) with efficient photovoltaic performance. The

Full text of DOI:10.1039/c9tc01520e

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PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Cite this: DOI: 10.1039/c0xx00000x
PAPER
Improving photovoltaic performance by installing alkyl chains
perpendicular to π-conjugated plane of organic dye for dye-sensitized
solar cells
Hongjin Chen1, Guangyu Lyu1, Youfeng Yue2, Tingwei Wang1, Dong-Ping Li3*, Heng Shi1, Jieni Xing1,
Junyan Shao1, Rui Zhang1*, and Jian Liu1*
5
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
We demonstrate that employing long alkyl chains perpendicular to π-conjugated backbone’s plane is a
10 promising strategy to construct efficient organic sensitizers for dye-sensitized solar cells (DSSCs) with
efficient photovoltaic performance. The perpendicular alkyl chains have been suggested as superior
alternatives to those parallel alkyl chains for preventing intermolecular interactions in D-π-A dyes,
therefore co-adsorption with deoxycholic acid (DCA) can be avoided. DSSCs employing iodine/iodide
electrolytes and an organic dye LJ-8 with long alkyl chains perpendicular to its π-conjugated plane
15 produced high photovoltage of 0.801 V and high conversion efficiency over 8%.
features, such as low costs, easier tunable absorptions, bandgaps
Introduction
and electrochemical properties through variation of the
35 molecular structure, which typically consists of a donor-π-
acceptor (D-π-A) system.5 Besides expanding the light
absorption edge of the organic dyes for efficient light harvesting
associated with short-circuit photocurrent,6 valid concept toward
high η of DSSCs based on them is minimizing the interficial
40 losses with respect to open-circuit photovoltage.7 The low
photovoltage is partially caused by dye aggregation resulting in
intermolecular charge transfer, and charge recombination
As a promising alternative to conventional photovoltaic devices,
dye-sensitized solar cells (DSSCs) have received considerable
attention and investigation with the aim of low cost and high
20 efficiency.1 During the past two decades, tremendous research
efforts have been devoted to develop efficient sensitizers which
play a crucial role in DSSCs.2 So far, there are mainly two kinds
of sensitizers, involving metal-complexes and metal-free organic
dyes.3 Metal-complexes such as ruthenium(II)–polypyridyl
25 complexes (N3, N719, and black dye) and metal porphyrins,
possess broad absorption spectra extending into the near-IR
region and produce solar-to-electrical energy conversion
efficiencies (η) of up to 13% under AM1.5 irradiation.4 However,
the shortage of noble metal resource and the environmental
30 issues would greatly limit their application prospect. In contrast,
metal-free organic dyes have been gradually considered as more
promising candidates for DSSCs due to their advantageous
-
between injected electrons and acceptors (dye cation and I3
ions).8 Coadsorption with bulk molecules, such as deoxycholic
45 acid (DCA), 1-decylphosphonic acid (DPA), cyclodextrins (CD)
has been frequently employed to supress dye-aggregation to
improve efficiency.9 However, the co-adsorption strategy
requires strict optimization of the adsorption conditions (e.g.
solvent, dye and additive concentrations), reduces the dye
50 coverage rate and is hard to drive the orderly assembling (one to
one match) on the TiO2 surface.10 As another solution, molecular
engineering of organic sensitizers by introduction of long alky
chains in the dye backbone has been intensively investigated.11
However, this strategy claimed the consideration of the chain
55 length and number, as well as the substitution position. For
instance, a famous dye MK-2 with four hexylthiophene groups
made a successful advance for retarding charge recombination by
molecular engineering and achieved efficient photovoltaic
performance,11a but its analogues MK-4, MK-5, MK-7, MK-8
60 can not perform efficiently.12a Moreover, alkyl chains can not
suppress the intermolecular interactions completely due to their
orientation, and in some case, the alkylation decreases the
efficiencies in comparison with the pristine dyes.12 Therefore, the
development of valid method for the suppression of dye
[] 1. College of Chemical Engineering, Nanjing Forestry University,
Jiangsu Key Lab of Biomass-based Green Fuels and Chemicals,
Nanjing 210037, P. R. China
2. Electronics and Photonics Research Institute, National Institute of
Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi
305-8565, Tsukuba, Japan
3. Department of Chemistry, Nanchang University, Nanchang
330031, PR China
E-mail: liu.jian@njfu.edu.cn, zhangrui@njfu.edu.cn,
nculdp@126.com
† Electronic supplementary information (ESI) available: Spectra
characterization, DFT calculations of dyes LJ-7, LJ-8 and LJ-9,
NMR spectra of key intermediates and desired dyes. See DOI:
10.1039/b000000x
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Journal of Materials Chemistry C
Page 2 of 10
aggregation is still a big challenge. Herein, we envisage that
substiuents being perpendicular to the π-conjugated backbone
may be a superior strategy to construct efficient dyes toward high
efficiency without additive engineering.
In order to confirm our assumption, we employ the oligothio-
phene as π-conjugated backbone, which was often used to
develop efficient sensitizers for DSSCs. On the other hand, it had
been investigated that the phenyl substituent is almost
perpendicular to the thiophene plane in 3-phenylthiophene due to
10 its inherent cross-conjugation properties.13 Therefore, it is
reasonable to install alky chains on the phenyl group in -
positions of thiophene moieties to be perpendicular to the π-
conjugated backbone. In this context, we design three new D-π-
A dyes employing substituted oligothiophene as π-spacer for
15 DSSCs, and investigated the relationship between substituents
and photovoltaic performance, particularly focus on the effect of
the orientation of the alky chains on the photovoltaic
performance.
5
O
O
O
O
O
O
S
O
S
S
S
S
N
OH
N
OH
N
S
OH
S
O
S
O
S
N
R
N
N
O
O
LJ-9
LJ-7
LJ-8
O
O
O
R
R
N
HO
R
O
OH
R
O
R
B
O
S
CHO
S
S
S
Br
Br
NBS
DMF
S
K2CO3, Pd(PPh3)4
THF, H2O, 50 oC
S
Br
S
O
K2CO3, Pd(PPh3)4
1,4-dioxane, H2O, 90 o
S
+
+
O
R
O
R
R
B
C
O
CHO
O
O
O
O
R
R
R
2a (R = methyl)
2b (R = n-butyl)
1a (R = methyl)
1b (R = n-butyl)
3a (R = methyl)
3b (R = n-butyl)
O
O
R
R
O
R
O
S
R
O
S
CHO
S
S
N
N
OH
S
O
S
CNCH2CO2H,
R
O
R
N
piperidine
CHCl3, reflux
O
O
R
R
4a (R = methyl)
4b (R = n-butyl)
LJ-7 (R = methyl)
LJ-8 (R = n-butyl)
OH
B
S
HO
O
CHO
CHO
N
B
S
CHO
S
S
CNCH2CO2H,
N
Br
S
S
O
Br
S
S
Br
S
LJ-9
piperidine
CHCl3, reflux
K2CO3, Pd(PPh3)4,
THF, H2O, 25oC
K2CO3, Pd(PPh3)4,
1,4-dioxane, H2O, 80 o
C
7
5
6
20
Scheme 1 Molecular structures and synthetic routes of circle chain embracing D-π-A dyes LJ-7, LJ-8 and LJ-9.
1
sensitizers were confirmed by H and 13C NMR spectroscopy as
well as mass spectrometry.
Results and discussion
40
The synthesis of three D-π-A dyes LJ-7, LJ-8 and LJ-9 was
achieved via bromination, Suzuki coupling and Knoevenagel
condensation reactions in a moderated yield by rational routes
35
LJ-7
30
LJ-8
LJ-9
25 shown in Scheme 1. Two substituted bithiophene derivates 1a
and 1b were prepared according to the reported method.14
Compound 5 was used as received from chemical companies
without further purification. Bromination of the substituted
bithiophene 1a and 1b by NBS produced the desired compound
30 2a and 2b,15 respectively. Compound 2a, 2b and 5 were reacted
with 5-formylthiophene-2-boronic acid to synthesize aldehyde 3a,
3b and 6,16 respectively. Suzuki coupling between the resulted
aldehydes (3a, 3b and 6) and 9-ethyl-9H-carbazol-3-ylboronic
acid obtain dye precusors 4a, 4b and 7, which were converted to
35 dyes LJ-7, LJ-8 and LJ-9 as black solids via Knoevenagel
condensation, respectively. The structures of desired organic
25
20
15
10

5
0
400
500
600
700
800
Wavelength (nm)
40 Fig. 1 Absorption spectra of dyes LJ-7, LJ-8 and LJ-9 in CH2Cl2 (2 ×
10-5 M).
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Journal of Materials Chemistry C
skeleton, which will be further disscussed in the DFT calculation
details (vide infra). All these three new D-π-A dyes LJ-7, LJ-8
and LJ-9 exhibit similar emission in CH2Cl2 solution but
different stokes shifts (Fig. S1, ESI†). Among these three new
30 dyes, dye LJ-9 exhibits the largest stokes shift, which indicates
significant structural reorganization of dye molecule upon
photoexcitation and the large degree of rotational freedom in dye
LJ-9.17 On the contrary, dye LJ-8 exhibits the smallest stokes
shift probably due to its relative rigid configuration of dye
35 molecule.
LJ-7
LJ-7-DCA
LJ-8
LJ-8-DCA
LJ-9
LJ-9-DCA
The UV-vis absorption spectra of dyes LJ-7, LJ-8 and LJ-9
adsorbed on TiO2 films with or without DCA are shown in Fig. 2.
The λmax for the dye-loaded TiO2 films are also listed in Table 1.
After being adsorbed on TiO2 film, The maximum absorption of
40 these dyes anchored on TiO2 film exhibits approximately 40 nm
hypochromatic shift in comparison with that in solution,
respectively. Such blue shift would be assigned to the solvent
effect and/or deprotonation of the carboxylic acid in the
anchoring process.18 The influence of the co-adsorption of DCA
45 on the TiO2 surface on the absorption spectra has been
investigated (Fig. 2). After coadsorption with DCA, the intensity
of the ICT band for these three new dyes reduces, which would
must be attributed to the decrease of the dye-loading amount
during the DCA coadsorption.8b Both dipping in the solution
50 with and without DCA, dialkoxyphenyl substituted dyes LJ-7
and LJ-8 perform fine absorption peaks similar to those in
solution, suggesting that the steric hindrance of dialkoxyphenyl
groups are effective to supress the intermolecular interactions
effectively. Whereas, the situation of dye LJ-9 employing two
55 hexyl chains are slightly different. Without co-adsorption, the
absorption spectrum of LJ-9 is broadened. The results suggests
that the hexyl groups on the conjugated oligothiophene backbone
of dye LJ-9 are not effective enough to supress the
intermolecular interaction and/or dye aggregation on the TiO2
60 surface, which will be prevented by means of coadsorption with
DCA. The probable reason is further disscussed by molecular
structure analysis (vide infra).
400
500
600
700
Wavelength (nm)
Fig. 2 Absorption spectra of LJ-7, LJ-8 and LJ-9 anchored on a
transparent TiO2 film with or without DCA.
The absorption spectra of three new D-π-A dyes LJ-7, LJ-8
and LJ-9 in dichloromethane are shown in Fig. 1, and the data
are summarized in Table 1. These three new dyes LJ-7, LJ-8
and LJ-9 exhibit two distinct absorption bands: the longer
wavelength absorption band in the visible region (440-500 nm)
that can be assigned to an intramolecular charge transfer (ICT)
5
10 from the carbazole donating group to the cyanoacrylic acid
anchoring moiety; the shorter wavelength absorption band in the
UV region (360~400 nm) is originated from the π-π* electron
transitions of the conjugated molecules as well as the overlapped
n-π* electron transitions. There was no obvious difference
15 between the absorption spectra of LJ-7 and LJ-8. Owing to the
longer alky chains in the electron-donating aromatic substituents
on conjugated oligothiophene backbone, dye LJ-8 exhibts a
slight bathochromic shift as well as a little higher molar
extinction coefficient (ε) in comparison with its analogue LJ-7.
20 Whereas the spectra of LJ-9 shows an ~50 nm bathochromic
shift and a lower ε value relative to those of LJ-7 and LJ-8.
Without regard to the solvent effect, we suggest the different
kinds of substituted group on conjugated oligothiophene
backbone in dye LJ-9 and the former two dyes (LJ-7 and LJ-8)
25 lead to significant diversity of the configuration of the dye
Table 1 Optical and Redox Parameters of Dyes LJ-7, LJ-8, and LJ-9
Dye
LJ-7
aλmax/nm (ε/ L mol-1 cm-1)
492(31800), 360(20950)
bλmax/nm
451c
eS+/0/V (vs NHE)
0.97
fS+/*/eV (vs NHE)
-1.09
gE0-0/V
2.06
451d
LJ-8
LJ-9
498(32880), 361(22420)
440(27110), 332(19780)
453c
0.93
1.01
-1.12
-1.14
2.05
2.15
453d
416c
412d
a) The absorption spectra were measured in CH2Cl2 solution (2 × 10-5 M). b) measured on TiO2 film. c) TiO2 films were dipping in a 0.3 mM solution of
65 dye in CH3CN/n-BuOH (1/1, v/v). d) TiO2 films were dipping in a mixture of dye (0.3 mM) and DCA (20 mM) in CH3CN/n-BuOH (1/1, v/v). e) The S+/0
corresponding to the ground-state oxidation potential (vs NHE) in CH2Cl2 internally calibrated with ferrocene. f) S+/* = S+/0 – E0-0, where E0-0 is the zero-
zero transition energy. g) E0-0 values were estimated from the intersection between the normalized absorption and emission spectra in CH2Cl2 (Fig. S1,
ESI†).
70
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Page 4 of 10
dye LJ-9, they can not prevent the intermolecular π-π interaction
and/or dye aggregation completely eventhough two thiophene
units are twisted, which was suggested by the broaden absorption
50 spectra on TiO2 film in the presence of coadsorption with DCA.
However, four butyl chains are located on two sides of thiophene
planes and perpendicular to the π-conjugated backbone in dye
LJ-8, which are effective to supress the intermolecular
interaction and/or charge transfer. Moreover, DFT calculations
55 of dyes LJ-7, LJ-8 and LJ-9 are carried out at the B3LYP/6-
31G** level to gain further insight into their frontier molecular
orbitals. As shown in Fig. S4 (ESI†), LUMOs show localized
electron distributions through the cyanoacrylic acid and its
adjacent π-spacer, whereas HOMOs are mainly delocalized from
60 the donor parts to the oligothiophene π-spacer. Hence, both
orbitals provide sufficient overlap between donor and acceptor to
guarantee a fast charge transfer transition.
LJ-7
LJ-8
LJ-9
10
5
0
-5
-10
-1
0
1
2
Voltage (V) vs Ag/AgNO
3
Fig. 3 Cyclic voltammograms of dyes LJ-7, LJ-8, and LJ-9 in
CH2Cl2/TBAHFP (0.1 M), [c] = 1 × 10−4 mol·L−1, 293 K, scan rate = 100
mV·s−1.
The device fabrication and characterization were performed
according to our previous method,15,16 which was detailed in
65 supporting information (ESI†). Photovoltaic characteristics of
the DSSCs based on dyes LJ-7, LJ-8 and LJ-9 and the
influences of DCA co-adsorption on their photovoltaic
performances have been investigated,19 employing a solution of
0.6 M 1-propyl-2,3-dimethylimidazolium iodide, 0.05 M I2, 0.1
70 M LiI and 0.5 M tert-butylpyridine (TBP) in acetonitrile as
electrolyte. The device performance statistics of them were
provided in Fig. S5, which suggested that the reproductivity of
devices based on pristine dyes was slightly better than those with
DCA coadsorption.
5
Cyclic voltammetry (CV) was carried out in CH2Cl2
containing 0.1 M tetra-n-butylammonium hexafluorophosphate
(TBAHFP) as
a supporting electrolyte to investigate the
electrochemical properties of three new D-π-A dyes LJ-7, LJ-8
and LJ-9. As shown in Fig. 3, all these dyes exhibit two
10 reversible oxidative waves. Due to these three new employing
same donor part and similar D-π-A skeleton, their HOMO levels
corresponding to the first oxidation potentials exhibit little
difference, increase in the order of LJ-8 (0.93 V) < LJ-7 (0.97 V)
< LJ-9 (1.01 V), which was associated with the electron-
15 donoating ability of the corresponding substituent on the
conjugated oligothiophene backbone in these three dyes. The
energy-level diagram of dyes, the electrolyte and TiO2 was
shown in Fig. S2. The HOMO levels of these three dyes are all
more positive than the Nernst potential of I-/I3- redox couple (0.4
20 V vs NHE), ensuring regeneration of the oxidized dyes by I- after
electron injection, with the driving force of 0.53 ~ 0.61 V,
respectively. The LUMO levels corresponding to the excited
state redox potentials of these three dyes (-1.09 to -1.14 V vs
NHE) are sufficiently more negative than the conduction band of
25 TiO2 (-0.5 V vs NHE), ensuring sufficient driving force (0.59 ~
0.64 V) for electron injection from the excited dyes to the
conduction band of TiO2. These results suggested the potential
application of these three new sensitizers in dye-sensitized solar
cells.5e
100
80
60
LJ-7
LJ-7-DCA
LJ-8
40
LJ-8-DCA
LJ-9
LJ-9-DCA
20
0
300
400
500
600
700
800
Wavelength (nm)
75
Fig. 4 IPCE spectra of DSSCs based on dyes LJ-7, LJ-8 and LJ-9
with/without DCA coadsorption.
30
To explain the different absorption properties between
dialkyoxyphenyl substituted dye (LJ-7 or LJ-8) and hexyl
substituted dye LJ-9, we further investigate the optimized
configuration of dyes LJ-8 and LJ-9 by density functional
theory (DFT) caculation. The optimized ground-state geometries
The onset wavelength of IPCE spectra shift toward the red
region in the order LJ-9 < LJ-7 < LJ-8 cells (Fig. 4), in accord
80 with the absorption spectra observed for the dye-loaded TiO2
films. DSSCs based on dialkoxyphenyl substituted dyes LJ-7
and LJ-8 without coadsorption perform similar maximum IPCE
value observed in the range of 400~600 nm in comparison with
those under coadsorption with DCA. However, in the case of
85 hexyl substituted dye LJ-9, the IPCE value was increased after
the coadsorption with DCA. These results implied that alkyl
chains perpendicular to π-conjugated plane of dyes LJ-7 and LJ-
35 and the dihedral angles between the π-conjugated units indicated
that the configurations of the bithiophene units connecting the
donor moieties were different in dyes LJ-8 and LJ-9 (Figure S3).
Two flexible hexyl β-substituents in LJ-9 caused large dihedral
twisting of the bithiophene units (torsion angle, 62.5°). In
40 contrast, in LJ-8, the two dibutoxyphenyl groups and the
bithiophene backbone seem to be restrained each other and
hamper the rotation around the thiophene-thiophene bond, so that
the thiophene units were relative rigid and twist would be
restricted, which facilitated ICT, as suggested by the absorption
45 properties described above. As shown in Fig. S3, the hexyl
chains are horizontal to their corresponding thiophene planes in
8
are efficient to retard the dye-aggregation and/or
intermolecular interaction for efficient electron injection,
90 whereas that in LJ-9 should be achieved by coadorption with
DCA. This phenomena was similar with that reported
previously.8b As shown in Fig. S6, integrated photocurrent values
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Journal of Materials Chemistry C
calculated from IPCE of DSSCs based on LJ-7, LJ-7-DCA, LJ-
5
circuit current density (JSC) of their corresponding devices
measured by J-V characterizations at the 100 mW cm-2,
simulated AM1.5G conditions (Table 2, Fig. 5).
8, LJ-8-DCA, LJ-9 and LJ-9-DCA were 12.85 mA·cm-2, 12.54
mA·cm-2, 13.1 mA·cm-2, 12.28 mA·cm-2, 10.27 mA·cm-2 and
10.88 mA·cm-2, respectively. The result closed to the short-
Table 2 Photovoltaic performance for DSSCs based on dyes LJ-7, LJ-8 and LJ-9a
Dyes
DCA[mM]
JSC [mA·cm-2]
VOC [V]
FF
η [%]
Dye-loading amount
[10-7 mol·cm- 2]
LJ-7
LJ-7
LJ-8
LJ-8
LJ-9
LJ-9
0
20
0
20
0
12.94
12.78
13.05
12.51
9.96
0.738
0.747
0.801
0.751
0.715
0.751
0.734
0.758
0.766
0.753
0.711
0.727
7.01
7.24
8.01
7.07
5.06
5.77
2.01
1.89
1.92
1.73
1.94
1.82
20
10.57
10 a) Measurements were performed under AM 1.5 irradiation on the DSSC devices with 0.2304 cm2 active surface. JSC, short-circuit current density; VOC
,
open-circuit voltage; FF, fill factor; η, conversion efficiency.
1
14
12
10
8
LJ-7
LJ-7-DCA
LJ-8
LJ-8-DCA
LJ-9
0.1
LJ-7
LJ-9-DCA
LJ-7-DCA
LJ-8
6
LJ-8-DCA
LJ-9
LJ-9-DCA
4
2
0.01
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Voltage (V)
0.55
0.60
0.65
0.70
0.75
0.80
Fig. 5 J-V curves of DSSCs based on LJ-7, LJ-8 and LJ-9 with/without
Voltage (V)
DCA coadsorption.
Fig. 6 Electron lifetime (τe) as a function of VOC for DSSCs based on
dyes LJ-7, LJ-8 and LJ-9 with and without DCA coadsorption.
15
The open-circuit voltages (VOC) of DSSCs based on the
pristine dyes LJ-7, LJ-8 and LJ-9 are 0.738 V, 0.801 V and
0.715 V, respectively. After coadsorption with DCA, VOC of
DSSCs based on LJ-7 and LJ-9 increase to 0.747 V and 0.751 V,
respectively, whereas that of LJ-8 based device decrease to
35
As shown in Fig. 7, we proposed the possible mechanisms of
the influence of DCA on the dye adsorption on the TiO2 surface
and the photovoltaic properties. For dyes LJ-7 and LJ-8,
intermolecular energy transfer may be prevented by the steric
hindrance being perpendicular to π-conjugated plane of organic
20 0.751. The different influence of DCA coadsorption on the VOC
of DSSCs based on these three dyes implied that various effect
of coadsorption on charge recombination rates, which were
further investigated by the measurement of electron lifetimes (τe)
in the conduction band of TiO2 by means of intensity-modulated
25 photovoltage spectroscopy (IMVS). As shown in Fig. 6, the τe
values of the DSSCs based on pristine dyes vary in the order of
LJ-8 ＞ LJ-7 ＞ LJ-9, suggesting that the long alkyl chains
perpendicular to π-conjugated plane of organic dyes (LJ-7 and
LJ-8) are superior to those in parallel mode (dye LJ-9) for
40 dyes even without DCA, leading to a high IPCE value. On the
other hand, upon coadsorption with DCA during the dipping
process, the photocurrent decreased slightly. This is probably
attributed to the decrease in dye loading amount on the TiO2
surface because the DCA molecules occupied a part of the TiO2
45 surface area. In contrast, dye LJ-9 with two hexyl groups
attaching on the dye backbone can not retard the intermolecular
interactions and/or dye aggregation on the TiO2 surface
completely without DCA, which resulted in the photocurrent and
photovoltage loss due to the intermolecular energy transfer and
50 excited-state quenching of the dye, as well as serious charge
recombination.20 After co-adsorption with DCA, it is expected
that the intermolecular energy-transfer between dye LJ-9
molecules was efficiently suppressed, and the photocurrent was
therefore improved eventhough dye loading amount decrease.
55 The enhancement of VOC of devices based LJ-7 and LJ-9 after
DCA coadsorption was mainly attributed to the increasing
blocking effect. In the case of LJ-8, two long alkyl chains being
pendecular to the dye backbone would assemble a dense
blocking layer via crosslinking effect, and was sufficient to
60 retard the charge recombination between I3- and injected electron
-
30 suppression of charge recombination between I3 and injected
electron in TiO2.
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Journal of Materials Chemistry C
Page 6 of 10
in TiO2, which resulted in a high VOC over 0.8 V. However, when
loading amount and deteriorated blocking effect.
DCA molecules were coadsorbed on the TiO2 surface with LJ-8
lead to the decrease of VOC, probably owing to the reducing dye-
5
Fig. 7 Schematic drawings of LJ-7, LJ-8 and LJ-9 adsorbed on the TiO2 surface (a) without and (b) with DCA (ET: energy transfer).
30
Preparation of Compond 2a. To a stirred solution of
compound 1a (200 mg, 0.46 mmol) in AcOH/CHCl3 (10 mL/20
mL) was added NBS (171 mg, 0.96 mmol) portionwise at 0 °C.
The reaction mixture was stirred for 2 hours and then treated
with water (50 mL), extracted twice with CH2Cl2 (40 mL × 2).
Conclusion
10 In summary, we have successfully developed three new D-π-A
dyes LJ-7, LJ-8 and LJ-9 bearing substituted oligothiophene as
π-spacer, cyanoacrylic acid as the acceptor moieties for high-
efficiency DSSCs. Due to the preferable orientation for blocking
effect, the long alkyl chains being pendicular to the dye
15 backbone seems to be superior to those attaching on backbone in
parallel for suppressing charge recombination, which resulted in
high photovoltage. Moreover, the long alkyl chains in pendicular
was also superior to those attaching dye backbone directly for
reducing the twist effect between conjugated planes, which gain
20 braoder light harvesting and resulting high photocurrent. As a
result, a high VOC over 0.8V and high power conversion
efficiency over 8% were achieved in LJ-8 based devices through
molecular egineering by the present strategy. The strategy
described here should give a significant enlightenment for
25 molecular design of organic semiconductors in photoelectronic
field, particularly involving light harvesting and intermolecular
interaction.
35 The combined organic layers were washed twice with water and
once with brine, dried over anhydrous sodium sulfate. After
evaporation of the solvent under reduced pressure, The residue
was treated with MeOH (15 mL), the solid was collected by
filtration, and dried under vacuum to yield the desired product 2a
40 as a white powder (246 mg, 91%). 1H NMR (CDCl3, 400 MHz):
δ 7.14 (t, J = 8.4 Hz, 2H), 6.79 (s, 2H), 6.38 (d, J = 8.4 Hz, 4H),
3.56 (s, 12H). 13C NMR (CDCl3, 400 MHz): δ 157.45, 134.74,
133.76, 130.88, 128.35, 112.51, 109.64, 103.82, 55.25. ESI (m/z):
Calcd for C24H20Br2O4S2, 593.92 (M)+; found, 893.92.
Preparation of Compond 2b. To a stirred solution of
compound 1b (303 mg, 0.5 mmol) in AcOH/CHCl3 (15 mL/30
mL) was added NBS (178 mg, 1.0 mmol) portionwise at 0 °C.
The reaction mixture was stirred for 2 hours and then treated
with ice water (40 mL), extracted twice with CH2Cl2 (50 mL × 2).
45
50 The combined organic layers were washed twice with water and
once with brine, dried over anhydrous sodium sulfate. After
evaporation of the solvent under reduced pressure, The residue
was treated with MeOH (10 mL), and the solid was collected by
filtration, and dried under vacumm to yield compund 2b as a
Experimental
Synthesis and characterization
1
55 white powder (340 mg, 89%). H NMR (CDCl3, 400 MHz): δ
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Journal of Materials Chemistry C
7.03 (t, J = 8.4 Hz, 2H), 6.73 (s, 2H), 6.29 (d, J = 8.4 Hz, 4H),
extracted twice with CH2Cl2 (40 mL × 2). The combined organic
60 layers were washed twice with water and once with brine, dried
over anhydrous sodium sulfate. After removing the solvent under
reduced pressure, the residue was purified by chromatography
with PE/EA (5/1, v/v) as an eluent to yiled compound 6 as an
orange solid (85 mg, 72%). 1H NMR (CDCl3, 400 MHz), δ: 9.83
65 (s, 1H), 8.26 (d, J = 1.6 Hz, 1H), 8.10 (d, J = 7.6 Hz, 1H), 7.67
(dd, J1= 8.4 Hz, J2= 2.0 Hz, 1H), 7.64 (d, J = 4.0 Hz, 1H), 7.46-
7.49 (m, 1H), 7.41 (d, J = 8.0 Hz, 1H), 7.37 (d, J = 8.8 Hz, 1H),
7.13-7.24 (m, 6H), 6.42 (d, J = 1.6 Hz, 2H), 6.40 (d, J = 1.6 Hz,
2H), 4.37 (q, J = 7.2 Hz, 2H), 3.59 (s, 6H), 3.58 (s, 6H), 1.44 (t, J
70 = 7.2 Hz, 3H). 13C NMR (CDCl3, 400 MHz), δ: 182.51, 157.86,
148.37, 143.00, 140.86, 140.46, 139.46, 137.63, 137.33, 132.22,
132.09, 131.70, 131.28, 131.16, 128.83, 128.69, 126.88, 126.01,
125.80, 124.03, 123.40, 122.98, 120.65, 119.09, 117.51, 113.99,
113.31, 108.70, 104.09, 55.45, 37.74, 13.93. ESI (m/z): Calcd for
75 C43H35NO5S3, 741.17 (M)+; found, 741.17.
Preparation of Compond 4b. To a stirred solution of
compound 3b (70 mg, 0.088 mmol) in 1,4-dioxane/H2O (20
mL/5 mL) was added 9-ethyl-3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9H-carbazole (43 mg, 0.132 mmol),
80 potassium carbonate (37 mg, 0.264 mmol), followed by
Pd(PPh3)4 (12 mg, 0.01 mmol) as a catalyst at room temperature.
The resulting mixture was stirred at 90 °C under argon
atmosphere overnight. Evaporation of the solvent under reduced
pressure and the residue was treated with water (40 ml),
85 extracted twice with CH2Cl2 (40 mL × 2). The combined organic
layers were washed twice with water and once with brine, dried
over anhydrous sodium sulfate. After removing the solvent under
reduced pressure, the residue was purified by chromatography
with PE/EA (5/1, v/v) as an eluent to yield compound 6 as an
90 orange solid (61 mg, 76%). 1H NMR (CDCl3, 400 MHz), δ: 9.83
(s, 1H), 8.26 (d, J = 1.6 Hz, 1H), 8.09 (d, J = 7.6 Hz, 1H), 7.65-
7.68 (m, 2H), 7.46-7.48 (m, 1H), 7.37-7.42 (m, 2H), 7.22-7.24
(m, 1H), 7.19 (s, 1H), 7.16 (d, J = 4.0 Hz, 1H), 7.14 (s, 1H),
7.00-7.05 (m, 2H), 6.30 (d, J = 1.2 Hz, 2H), 6.28 (d, J = 1.2 Hz,
95 2H), 4.38 (q, J = 7.2 Hz, 2H), 3.65-3.76 (m, 8H), 1.51-1.62 (m,
8H), 1.47 (t, J = 7.2 Hz, 3H), 1.24-1.37 (m, 8H), 0.80-0.84 (m,
12H). 13C NMR (CDCl3, 150 MHz), δ: 182.51, 157.26, 148.76,
142.22, 140.62, 140.45, 139.38, 137.81, 137.86, 132.33, 132.02,
131.44, 128.30, 128.12, 127.60, 126.05, 125.95, 123.84, 122.96,
100 120.52, 119.07, 117.28, 114.37, 113.55, 108.67, 104.41, 104.33,
67.57, 37.75, 31.31, 31.28, 19.27, 13.90. ESI (m/z): Calcd for
C43H35NO5S3, 909.36 (M)+; found, 909.36.
3.65-3.72 (m, 8H), 1.50-1.57 (m, 8H), 1.23-1.35 (m, 8H), 0.89 (t,
J = 7.2 Hz, 12H). 13C NMR (CDCl3, 400 MHz): δ 157.42, 134.80,
134.33, 131.60, 128.65, 113.16, 108.98, 104.43, 67.68, 31.19,
19.19, 13.90. ESI (m/z): Calcd for C36H44Br2O4S2, 762.10 (M)+;
found, 762.10.
Preparation of Compond 3a. To a stirred solution of
compound 2a (240 mg, 0.404 mmol) in THF/H2O (40 mL/10 mL)
was added 5-Formyl-2-thiopheneboronic acid (63 mg, 0.404
10 mmol), potassium carbonate (167 mg, 1.21 mmol), followed by
Pd(PPh3)4 (46 mg, 0.04 mmol) as a catalyst at room temperature.
The reaction mixture was stirred at 40 °C under argon
atmosphere for 4 h. Evaporation of the solvent under reduced
pressure and the residue was treated with water (50 mL),
15 extracted twice with CH2Cl2 (50 mL × 2). The combined organic
layers were washed twice with water and once with brine, dried
over anhydrous sodium sulfate. After removing the solvent under
reduced pressure, the residue was purified by chromatography
using hexane/EA (10/1, v/v) as an eluent to yield compound 3a
20 as an orange solid (116 mg, 46%). 1H NMR (CDCl3, 400 MHz):
δ 9.84 (s, 1H), 7.63 (d, J = 3.6 Hz, 1H), 7.14-7.18 (m, 4H), 6.85
(s, 1H), 6.40-6.43 (m, 4H), 3.59 (s, 6H), 3.58 (s, 6H); 13C NMR
(CDCl3, 400 MHz): δ 182.47, 158.17, 147.84, 141.27, 137.44,
135.51, 134.79, 133.96, 132.82, 131.87, 131.46, 130.53, 129.24,
25 123.78, 112.68, 110.28, 103.96, 55.61. ESI (m/z): Calcd for
C29H23BrO5S3, 625.99 (M+); found, 648.98 [M+Na]+.
5
Preparation of Compond 3b. To a stirred solution of
compound 2b (320 mg, 0.42 mmol) in THF/H2O (40 mL/10 mL)
was added 5-Formyl-2-thiopheneboronic acid (66 mg, 0.42
30 mmol), potassium carbonate (174 mg, 1.26 mmol), followed by
Pd(PPh3)4 (48 mg, 0.042 mmol) as a catalyst at room temperature.
The reaction mixture was stirred at 40 °C under argon
atmosphere for 4 h. Evaporation of the solvent under reduced
pressure and the residue was treated with water (40 mL),
35 extracted twice with CH2Cl2 (50 mL × 2). The combined organic
layers were washed twice with water and once with brine, dried
over anhydrous sodium sulfate. After removing the solvent under
reduced pressure, the residue was purified by chromatography
with hexane/EA (10/1, v/v) as an eluent to yield compound 3b as
1
40 an orange solid (160 mg, 48%). H NMR (CDCl3, 400 MHz): δ
9.81 (s, 1H), 7.62 (d, J = 4.2 Hz, 1H), 7.11-7.12 (m, 2H), 7.05 (t,
J = 8.4 Hz, 1H), 7.03 (t, J = 8.4 Hz, 1H), 6.78 (s, 1H), 6.30 (d, J
= 8.4 Hz, 2H), 6.28 (d, J = 8.4 Hz, 2H), 3.65-3.75 (m, 8H), 1.48-
1.54 (m, 8H), 1.22-1.32 (m, 8H), 0.87 (t, J = 7.2 Hz, 6H), 0.82 (t,
45 J = 7.2 Hz, 6H); 13C NMR (CDCl3, 400 MHz): δ 182.40, 157.30,
148.15, 140.90, 137.56, 135.71, 134.83, 134.57, 132.49, 132.12,
132.05, 131.80, 131.43, 130.89, 129.22, 123.15, 113.11, 109.47,
104.40, 67.63, 31.16, 19.13, 13.81. ESI (m/z): Calcd for
C41H47BrO5S3, 794.18 (M+); found, 817.17 [M+Na]+.
Preparation of Compond 4a. To a stirred solution of
compound 3a (100 mg, 0.159 mmol) in 1,4-dioxane/H2O (20
mL/5 mL) was added 9-ethyl-3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9H-carbazole (61 mg, 0.191 mmol),
potassium carbonate (66 mg, 0.477 mmol), followed by
Preparation of Compond 6. To a stirred solution of
compound 5 (300 mg, 0.612 mmol) in THF /H2O (40 mL/10 mL)
105 was added 5-Formyl-2-thiopheneboronic acid (95 mg, 0.612
mmol), potassium carbonate (253 mg, 1.837 mmol), followed by
Pd(PPh3)4 (69 mg, 0.06 mmol) as a catalyst at room temperature.
The reaction mixture was stirred at 50 °C under argon
atmosphere for 2 h. Evaporation of the solvent under reduced
110 pressure and the residue was treated with water (40 mL),
extracted with CH2Cl2 (50 mL × 2). The combined organic layers
were washed twice with water and once with brine, dried over
anhydrous sodium sulfate. After removing the solvent under
reduced pressure, the residue was purified by chromatography
115 with hexane/EA (10/1, v/v) as an eluent to yield compound 3a as
50
55 Pd(PPh3)4 (18 mg, 0.016 mmol) as a catalyst at room temperature.
The reaction mixture was stirred at 90 °C under argon
atmosphere overnight. Evaporation of the solvent under reduced
pressure and the residue was treated with water (40 ml),
1
an orange solid (140 mg, 44%). H NMR (CDCl3, 600 MHz): δ
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Journal of Materials Chemistry C
Page 8 of 10
9.86 (s, 1H), 7.66 (d, J = 4.2 Hz, 1H), 7.22 (d, J = 3.6 Hz, 1H),
7.21 (s, 1H), 6.94 (s, 1H), 2.47-2.50 (m, 4H), 1.51-1.57 (m, 4H),
1.23-1.30 (m, 12H), 0.84-0.88 (m, 6H); 13C NMR (CDCl3, 400
MHz): δ 182.44, 146.90, 144.43, 143.76, 141.78, 137.23, 135.87,
131.58, 129.57, 129.04, 127.55, 124.15, 112.58, 31.57, 30.54,
30.50, 29.03, 28.98, 28.88, 22.54, 14.03. ESI (m/z): Calcd for
C25H31BrOS3, 522.07 (M+); found, 522.07.
Preparation of dye LJ-8. A solution of the compound 4b (55
60 mg, 0.06 mmol) in anhydrous CHCl3 (15 mL) was added
cyanoacetic acid (15 mg, 0.18 mmol), followed by piperidine (30
mg, 0.36 mmol) as a base at room temperature. The resultinig
mixture was stirred at 70 °C for 10 h. After being cooled to room
temperature, the reaction mixture was treated with water (30 mL)
65 and acidified with 1 M aqueous hydrochloric acid (10 mL),
extracted twice with CHCl3 (30 mL × 2). The combined organic
layers were washed twice with water and once with brine, dried
over anhydrous sodium sulfate. After removing the solvent under
reduced pressure, the residue was purified by chromatography
70 with CH2Cl2/CH3OH (15/1, v/v) as an eluent to yield the desired
5
Preparation of Compond 7. To a stirred solution of
compound 6 (130 mg, 0.249 mmol) in 1,4-dioxane/H2O (25
10 mL/5 mL) was added 9-ethyl-3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9H-carbazole (96 mg, 0.3 mmol), potassium
carbonate (103 mg, 0.747 mmol), followed by Pd(PPh3)4 (24 mg,
0.02 mmol) as a catalyst at room temperature. The reaction
mixture was stirred at 90 °C under argon atmosphere overnight.
15 Evaporation of the solvent under reduced pressure and the
residue was treated with water (30 ml), extracted twice with
CH2Cl2 (30 mL × 2). The combined organic layers were washed
twice with water and once with brine, dried over anhydrous
sodium sulfate. After removing the solvent under reduced
20 pressure, the residue was loaded onto silica gel column and
purified by chromatography with PE/EA (5/1, v/v) as an eluent
to yield compound 7 as an orange solid (114 mg, 72%). 1H NMR
(CDCl3, 600 MHz), δ: 9.87 (s, 1H), 8.32 (d, J = 1.2 Hz, 1H), 8.14
(d, J = 7.8 Hz, 1H), 7.72 (dd, J1 = 8.4, J2 = 1.8 Hz, 1H), 7.67 (d,
25 J = 4.2 Hz, 1H), 7.49 (t, J = 7.8 Hz, 1H), 7.42 (t, J = 8.4 Hz, 2H),
7.24-7.27 (m, 3H), 4.39 (q, J = 7.2 Hz, 2H), 2.60 (q, J = 8.4 Hz,
4H), 1.62-1.67 (m, 4H), 1.46 (t, J = 7.2 Hz, 3H), 1.26-1.36 (m,
12H), 0.86-0.92 (m, 6H). 13C NMR (CDCl3, 400 MHz), δ:
182.51, 147.43, 145.78, 143.82, 143.75, 141.46, 140.45, 139.65,
30 137.43, 135.16, 131.59, 127.78, 126.06, 125.64, 125.29, 123.99,
123.93, 123.76, 123.42, 122.90, 120.61, 119.13, 117.67, 108.77,
108.71, 37.71, 31.69, 31.63, 30.76, 30.64, 29.29, 29.20, 29.11,
29.02, 22.62, 22.59, 14.11, 14.09, 13.86. ESI (m/z): Calcd for
C39H43NOS3, 637.25 (M)+; found, 637.25.
1
dye LJ-8 as a black solid (50 mg, 84%). H NMR (DMSO-d6,
400 MHz), δ: 8.34 (d, J = 1.6 Hz, 1H), 8.24 (s, 1H), 8.16 (d, J =
7.6 Hz, 1H), 7.78 (d, J = 3.6 Hz, 1H), 7.58-7.65 (m, 3H), 7.45 (m,
1H), 7.32 (d, J = 4.0 Hz, 1H), 7.22 (t, J = 7.6 Hz, 1H), 7.18 (s,
75 1H), 7.15 (s, 1H), 7.05-7.12 (m, 2H), 6.42 (d, J = 0.8 Hz, 2H),
6.39 (d, J = 0.8 Hz, 2H), 4.45 (q, J = 7.2 Hz, 2H), 3.67-3.78 (m,
8H), 1.45-1.55 (m, 8H), 1.17-1.34 (m, 11H), 0.73-0.77 (m, 12H).
13C NMR (DMSO-d6, 400 MHz), δ: 157.22, 141.85, 140.55,
139.50, 132.99, 132.04, 131.23, 129.38, 129.20, 127.93, 126.67,
80 125.41, 124.18, 123.57, 123.15, 122.58, 120.97, 119.57, 117.07,
113.82, 113.12, 110.20, 109.90, 108.79, 105.00, 70.50, 67.49,
60.77, 37.62, 31.27, 19.19, 14.29, 14.13. ESI (m/z): Calcd for
C58H60N2O6S3, 976.36 (M+); found, 976.35.
Preparation of LJ-9. A solution of the aldehyde 7 (90 mg,
85 0.141 mmol) in anhydrous CHCl3 (25 mL) was added
cyanoacetic acid (24 mg, 0.282 mmol), followed by piperidine
(48 mg, 0.564 mmol) as a base at room temperature. The
resulting mixture was stirred at 70 °C for 10 h. After being
cooled to room temperature, the reaction mixture was treated
90 with water (30 mL) and acidified with 1 M aqueous hydrochloric
acid (10 mL), extracted twice with CHCl3 (30 mL × 2). The
combined organic layers were washed twice with water and once
with brine, dried over anhydrous sodium sulfate. After removing
the solvent under reduced pressure, the residue was purified by
95 chromatography with CH2Cl2/CH3OH (15/1, v/v) as an eluent to
35
Preparation of dye LJ-7. A solution of compound 4a (74 mg,
0.1 mmol) in anhydrous CHCl3 (15 mL) was added cyanoacetic
acid (26 mg, 0.3 mmol), followed by piperidine (51 mg, 0.6
mmol) as a base at room temperature. The resulting solution was
stirred at 70 °C for 8 h. After being cooled to room temperature,
1
yield the desired dye LJ-9 as a brown solid (82 mg, 83%). H
NMR (DMSO-d6, 600 MHz), δ: 8.48 (d, J = 1.8 Hz, 1H), 8.33 (s,
1H), 8.23 (d, J = 7.8 Hz, 1H), 7.85 (d, J = 3.6 Hz, 1H), 7.73 (dd,
J1 = 8.4, J2 = 1.2 Hz, 1H), 7.60-7.63 (m, 2H), 7.48-7.52 (m, 3H),
100 7.47 (t, J = 7.8 Hz, 1H), 7.22 (t, J = 7.8 Hz, 1H), 4.44 (q, J = 7.2
Hz, 2H), 2.53-2.57 (m, 4H), 1.55-1.63 (m, 4H), 1.32 (t, J = 7.2
Hz, 3H), 1.20-1.24 (m, 12H), 0.77-0.81 (m, 6H). 13C NMR
(DMSO-d6, 600 MHz), δ: 163.78, 145.62, 144.01, 140.59,
139.78, 135.36, 135.24, 130.50, 128.50, 126.62, 125.27, 125.23,
105 124.89, 124.65, 124.05, 123.24, 122.65, 121.13, 119.51, 117.64,
110.09, 109.79, 37.60, 31.43, 31.38, 30.37, 30.28, 29.16, 28.92,
28.86, 22.51, 22.43, 14.30, 14.15. ESI (m/z): Calcd for
C42H44N2O2S3, 704.26 (M+); found, 704.25.
40 the reaction mixture was treated with water (30 mL) and
acidified with 1 M aqueous hydrochloric acid (10 mL), extracted
twice with CHCl3 (20 mL × 2). The combined organic layers
were washed twice with water and once with brine, dried over
anhydrous sodium sulfate. After removing the solvent under
45 reduced pressure, the residue was purified by chromatography
with CH2Cl2/CH3OH (15/1, v/v) as an eluent to yield the desired
1
dye LJ-7 as a black solid (62 mg, 76%). H NMR (DMSO-d6,
400 MHz), δ: 8.36 (d, J = 2.0 Hz, 1H), 8.33 (s, 1H), 8.18 (d, J =
8.0 Hz, 1H), 7.85 (d, J = 4.0 Hz, 1H), 7.62 (s, 1H), 7.60 (s, 1H),
50 7.45-7.52 (m, 2H), 7.32-7.37 (m, 3H), 7.19-7.25 (m, 3H), 6.67 (d,
J = 1.6 Hz, 2H), 6.65 (d, J = 1.6 Hz, 2H), 4.44 (q, J = 7.2 Hz,
2H), 3.63 (s, 6H), 3.62 (s, 6H), 1.32 (t, J = 7.2 Hz, 3H). 13C
NMR (DMSO-d6, 400 MHz), δ: 158.52, 142.72, 140.57, 139.52,
135.97, 133.38, 132.45, 132.01, 130.85, 130.60, 130.50, 130.34,
55 126.77, 126.69, 124.93, 124.71, 123.75, 123.21, 121.20, 119.55,
117.35, 113.59, 112.99, 110.20, 104.86, 60.29, 56.03, 55.97,
37.62, 14.62, 14.25. ESI (m/z): Calcd for C46H36N2O6S3, 808.17
(M+); found, 808.16.
Acknowledgements
110 There are no conflicts to declare.
Acknowledgements
This work was supported in part by the National Natural Science
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Journal of Materials Chemistry C
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Foundation of China (201502088, 21631006), the Jiangsu
Province Science Foundation for Youths (BK20150569) and the
High Level Talent Project of Nanjing Forestry University
(GXL2018003).
70
75
80
5
Notes and references
8
9
1
2
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Journal of Materials Chemistry C
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DOI: 10.1039/C9TC01520E
We demonstrate that installing alkyl chains perpendicular to π-conjugated plane is a promising
strategy to construct efficient sensitizers for DSSCs.