Tuning the electrical and optical properties of diketopyrrolopyrrole complexes for panchromatic dye-sensitized solar cells

Article abstract of DOI:10.1002/asia.201200648

A series of metal-free organic dyes that were bridged by a diketopyrrolopyrrole moiety and were composed of indoline and triphenylamine as donor groups and furan and benzene as conjugated spacer groups were designed and synthesized for use in dye-sensitized solar cells (DSCs). The photophysical properties, electrochemical properties, and performance of the DSCs were related to the structure of their corresponding dyes. Their absorption spectra broadened upon the introduction of the indoline and heterocyclic furan moieties through fine-tuning of their molecular configuration. The overall conversion efficiencies of DSCs that were based on these dyes ranged from 5.14-6.53 %. Among the four dyes that were tested, indoline-based ID01 and ID02 showed higher efficiencies (6.35 % and 6.53 %) as a result of their improved light-harvesting efficiency and larger electron driving force. The ID01 dye, which contained an indoline moiety as an electron donor and a furan group as a π-conjugated linker, showed an excellent monochromatic incident-photon-to-current-conversion efficiency (IPCE) spectrum (350-650 nm) with a maximum value of 78 % in the high plateau region and an onset value close to 800 nm. Intensity-modulated photovoltage spectroscopy (IMVS) and impedance spectroscopy (IS) revealed that dyes that contained benzene conjugation spacers suppressed the charge-recombination rate more efficiently than dyes that contained furan spacers, thereby resulting in improved photovoltage. Panchromatic's labyrinth: A series of metal-free dyes that were bridged by a diketopyrrolopyrrole moiety were synthesized. DSCs based on ID01 showed a broader incident-photon-to- current-conversion efficiency (IPCE) spectrum over the whole visible range close to 800 nm. Intensity-modulated photovoltage spectroscopy (IMVS) and impedance spectroscopy (IS) of the relationship between benzene and furan moieties as spacers were also performed. Copyright

Full text of DOI:10.1002/asia.201200648

DOI: 10.1002/asia.201200648
Tuning the Electrical and Optical Properties of Diketopyrrolopyrrole
Complexes for Panchromatic Dye-Sensitized Solar Cells
Sanyin Qu,[a, b] Chuanjiang Qin,[b] Ashraful Islam,[b] Jianli Hua,[a] Han Chen,[c]
He Tian,*[a] and Liyuan Han*[b, d]
Abstract: A series of metal-free organ-
ic dyes that were bridged by a diketo-
pyrrolopyrrole moiety and were com-
posed of indoline and triphenylamine
as donor groups and furan and benzene
as conjugated spacer groups were de-
signed and synthesized for use in dye-
sensitized solar cells (DSCs). The pho-
tophysical properties, electrochemical
properties, and performance of the
DSCs were related to the structure of
their corresponding dyes. Their absorp-
tion spectra broadened upon the intro-
duction of the indoline and heterocy-
clic furan moieties through fine-tuning
of their molecular configuration. The
overall conversion efficiencies of DSCs
that were based on these dyes ranged
from 5.14–6.53%. Among the four
dyes that were tested, indoline-based
ID01 and ID02 showed higher efficien-
cies (6.35% and 6.53%) as a result of
their improved light-harvesting effi-
ciency and larger electron driving
force. The ID01 dye, which contained
an indoline moiety as an electron
donor and a furan group as a p-conju-
gated linker, showed an excellent mon-
ochromatic incident-photon-to-current-
conversion efficiency (IPCE) spectrum
(350–650 nm) with a maximum value
of 78% in the high plateau region and
an onset value close to 800 nm. Intensi-
ty-modulated photovoltage spectrosco-
py (IMVS) and impedance spectrosco-
py (IS) revealed that dyes that con-
tained benzene conjugation spacers
suppressed the charge-recombination
rate more efficiently than dyes that
contained furan spacers, thereby result-
ing in improved photovoltage.
Keywords: biosensors
·
conjuga-
tion · diketopyrrolopyrroles · dyes/
pigments · solar cells
Introduction
costs.[1] The most-important component of DSCs is the sensi-
tizer, which absorbs the sunlight, thereby generating elec-
trons to inject into the conduction band of the semiconduc-
tor.[2] Until now, the most-promising sensitizers have been
Ru complexes, which have achieved a highest efficiency of
11.4% with a broad photon-to-current-conversion efficiency
spectrum of up to 900 nm.[3] At the same time, metal-free
dyes have also been developed over the past few years
owing to their high molar-extinction coefficients, flexible
structural modification, and low cost.[4–7] However, their ab-
sorption bands are much narrower compared to ruthenium-
based dyes, thus resulting in lower short-circuit current den-
sities (Jsc) and poorer efficiencies. Thus, it is desirable to de-
velop panchromatic sensitizers for DSCs that can absorb
visible light of all colors. Although a few examples of effi-
cient DSCs that use panchromatic dyes have been report-
ed,[8,9] it is still necessary to develop new dyes with broader
incident-photon-to-current-conversion efficiency (IPCE)
spectra to further improve the efficiency of the correspond-
ing DSCs.
To meet the increasing global demand for energy, various
photovoltaic devices that are able to directly convert solar
irradiation into electric power have been developed. Among
those devices, dye-sensitized solar cells (DSCs) have attract-
ed increasing attention owing to their low manufacturing
[a] S. Qu, Prof. J. Hua, Prof. H. Tian
Key Laboratory for Advanced Materials
Institute of Fine Chemicals and Department of Chemistry
East China University of Science and Technology
130 Meilong Road, Shanghai 200237 (China)
Fax : (+86)21-64252758
E-mail: tianhe@ecust.edu.cn
[b] S. Qu, Dr. C. Qin, Dr. A. Islam, Dr. L. Han
Photovoltaic Materials Unit
National Institute for Materials Science
Sengen 1-2-1, Tsukuba, Ibaraki (Japan)
Fax : (+81)29-859-2304
E-mail: Han.Liyuan@nims.go.jp
[c] Dr. H. Chen
One strategy to expand the absorption spectra of dye mol-
ecules to cover a broader region of solar irradiation involves
the introduction of more-conjugated bridges between the
donor and the acceptor groups. In a previous work, we ex-
tended the p-conjugation by introducing a diketopyrrolopyr-
role (DPP) moiety into the dye molecules and the absorp-
tion spectrum was bathochromically shifted.[10] However, the
onset of the IPCE spectrum was limited to 700 nm and
School of Materials Science and Engineering
Shanghai Jiao Tong University
800 Dong Chuan RD. Minhang District
Shanghai, 200240 (China)
[d] Dr. L. Han
State Key Laboratory of Metal Matrix Composites
Shanghai Jiao Tong University
800 Dong Chuan RD. Minhang District
Shanghai, 200240 (China)
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there remains much room for improvement. Recently, Tian
and co-workers reported a series of organic dyes that incor-
porated additional donor groups into the triphenylamine
framework, thereby resulting in a red-shifted absorption and
better efficiency.[11] Thus, we speculate that incorporating
more-powerful donor units into the sensitizer may be
a promising approach to further broaden the absorption
spectrum. The use of indoline derivatives in organic sensitiz-
ers has been reported to afford strong and intense absorp-
tion bands and to show more-powerful donor abilities com-
pared to triphenylamine.[12] The furan moiety, as the oxygen
analogue of thiophene, which has a higher oxidation poten-
tial, has been found to exhibit a more suitable energy gap.[13]
Moreover, furan can provide better conjugation and, owing
to its smaller resonance energy in space, it can also lower
the energy of the charge-transfer transition, thereby allow-
ing an efficient broadening of the absorption spectra.[14]
Based on these studies, four metal-free organic dyes (TD01,
TD02, ID01, and ID02) with a diketopyrrolopyrrole group
at their core were designed and synthesized with the aim of
systematically studying the relationship between their struc-
tures and their photophysical properties. The molecular
structures of these dyes are presented in Scheme 1. ID01,
which contained an indoline donor and a furan spacer,
showed an IPCE spectrum with an onset of close to 800 nm.
Moreover, calculation studies and electron-lifetime meas-
urements were also performed to investigate the effect of
twisting the phenyl group on the photovoltage compared to
the furan-linker-containing sensitizers.[15] Their absorption
spectra, as well as their electrochemical and photovoltaic
properties, have been investigated in detail.
shown in Scheme 2. The starting material, 3,6-bis-(4-bromo-
phenyl)-2,5-di-n-butyl-pyrrolo[3,4-c]pyrrole-1,4-dione, and
the indoline derivatives were synthesized according to litera-
ture procedures.[10,16] Compound TD01 was obtained as re-
ported previously.[17] The asymmetrical Suzuki coupling re-
AHCTUNGTRENNUNG
action of compound
3 with 7-boronic acid-4-p-tolyl-
1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole gave an orange/
red compound (4), as well as disubstituted side-products;
fortunately, the monocapped compound could be easily sep-
arated by column chromatography. In the next step, this
bromo-exposed intermediate (4) reacted with 4-formylphe-
nylboronic acid or 2-formylfuran-5-boronic acid, thereby af-
fording red-colored monoaldehyde-substituted precursors (5
and 6, respectively). Following the same Suzuki-reaction
procedure, compound 2 was obtained from the reaction be-
tween the starting material (1) and 4-formylphenylboronic
acid. Finally, a typical Knoevenagel reaction between 2-cya-
noacetic acid and compounds 2, 5, and 6 was employed to
afford the target dyes (TD02, ID01, and ID02, respectively).
All of these intermediates and target dyes were character-
ized by using standard spectroscopic methods. The indoline-
containing intermediate and final products became photo-
stable after attaching a diketopyrrolopyrrole unit, because
the electron-withdrawing properties of the latter moiety fa-
vorably distributed the electrons of the donor section.
Theoretical Investigation
To explain their experimental spectra and to gain an insight
into the structural properties of the chromophores, density
functional theory (DFT) calculations were performed at the
B3LYP/6-31G* level of theory with Gaussian 03. The elec-
tron distributions of the HOMOs and LUMOs of the dyes
are shown in Figure 1. In the ground state, the electrons are
mainly localized on the triphenylamine or indoline donor
system and extend into the diketopyrrolopyrrole core for all
of the dyes. On the other hand, the excited state (LUMO) is
mainly localized on the cyanoacetic-acid unit and also partly
Results and Discussion
Synthesis
The synthetic route to four dyes (TD01, TD02, ID01, and
ID02) that contained a diketopyrrolopyrrole moiety is
Scheme 1. Chemical structures of the diketopyrrolopyrrole dyes.
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Diketopyrrolopyrrole Complexes for Panchromatic DSCs
Scheme 2. Synthesis of the diketopyrrolopyrrole dyes.
on the DPP core through the spacers. The calculation results
suggest that photo-induced electron transfer from the donor
to the cyanoacetic-acid unit can be sufficiently induced by
light irradiation.
The optimized structures of the four dyes are shown in
Figure 2. The dihedral angles between DPP and the neigh-
boring furan or benzene rings in TD01, TD02, ID01, and
Figure 1. Calculated HOMOs and LUMOs of the diketopyrrolopyrrole
dyes.
Figure 2. Optimized structures of the diketopyrrolopyrrole dyes.
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Table 1. Optical and electrochemical properties of the diketopyrrolopyr-
role dyes.
ID02 are 0.4, 34.6, 0.4, and 34.48, respectively. The lower tor-
sion angle between DPP and the furan moiety ensures good
molecular co-planarity, which is reflected in the larger red-
shifted absorption band in the optical spectra of TD01 and
ID01.
[a]
[b]
[d]
Dye
lmax [nm]
lmax
HOMO[c]
[V]
E0–0
LUMO[e]
[V]
(eꢁ10À4 mÀ1 cmÀ1
)
TiO2
[nm]
[eV]
(versus
NHE)
(versus
NHE)
TD01 514 (4.12),
397 (3.80)
TD02 500 (2.67),
333 (5.88)
ID01 529 (4.36),
396 (3.98),
ID02 517 (2.76),
339 (4.48)
546
505
536
533
1.03
1.04
0.78
0.79
1.94
1.99
1.89
1.95
À0.91
À0.95
À1.11
À1.16
Photophysical and Electrochemical Properties
The UV/Vis absorption spectra of the four dyes, both in
CH2Cl2 and on TiO2 film, are shown in Figure 3. Their cor-
responding photophysical data are listed in Table 1. In
[a] Absorption maximum in CH2Cl2 (2.5ꢁ10À5 m). [b] Absorption maxi-
mum on a TiO2 film (without DCA). [c] The HOMO was measured in
CH2Cl2 with 0.1m tetrabutylammonium hexafluorophosphate (TBAPF6)
as an electrolyte (working electrode: FTO/TiO2/dye; reference electrode:
SCE; calibrated with reference to the ferrocene/ferrocenium (Fc/Fc+)
couple as an external reference; counter electrode: Pt). [d] E0–0 was esti-
mated from the absorption thresholds in the absorption spectra of dyes
that were adsorbed onto the TiO2 film. [e] The LUMO was estimated by
subtracting E0–0 from the HOMO.
to TD02 and ID02 is derived from the higher planarity of
the optimized structures that contain the furan spacer
(TD01 and ID01), in agreement with the calculated struc-
ture data (Figure 2). As shown in Figure 3b, when the dyes
were attached onto TiO2 films, the absorption spectra for
TD01, TD02, ID01, and ID02 were broadened and red-shift-
ed by 32, 5, 7, and 16 nm, respectively. This result could be
ascribed to aggregation of the dyes on the surface of TiO2;
such phenomenon has also been observed in other DPP-
containing dyes.[10,18] The red-shift of TD01 was the largest
among the four dyes, thereby suggesting the existence of
more-severe aggregation for this dye. In the cases of TD02
and ID02, the red-shifts were much lower than those of
TD01 and ID01 because of the twist between the DPP and
benzene moieties; this trend was also observed when the
dyes were dissolved in CH2Cl2. Notably, of the four dyes,
ID01 showed the best light-harvesting efficiency, with an ab-
sorption threshold of longer than 700 nm on the TiO2 film.
The electrochemical properties and energy diagrams of
the four dyes are listed in Table 1 and shown in Figure 4, re-
spectively. The first oxidation potential (Eox), which corre-
sponded to the HOMO of these dyes, was obtained by cyclic
voltammetry (CV) with 0.1m tetrabutylammonium hexa-
fluorophosphate in CH2Cl2. The Eox values of TD01, TD02,
ID01, and ID02 dyes were 1.03, 1.04, 0.78, and 0.79 V
(versus NHE), respectively. These potentials were sufficient-
Figure 3. a) Absorption spectra of the dyes in CH2Cl2 (2.5ꢁ10À5 m) and
b) absorption spectra of the dyes after adsorption onto a transparent
TiO2 film.
CH2Cl2, TD01 and ID01 exhibited two major absorption
bands at 380–420 and 450–650 nm, whereas TD02 and ID02
only showed one absorption band at 450–550 nm. The ab-
sorption at around 400 nm for TD01 and ID01 was attribut-
ed to conjugation between the DPP and furan moieties. The
absorption at longer wavelength (around 500 nm) was attrib-
uted to intramolecular charge transfer between the donor
and the cyanoacetic-acid acceptor. Compared to TD01 and
TD02, the absorption bands of ID01 and ID02 were red-
shifted by about 15 and 17 nm, respectively. These results
reveal that the indoline donor is beneficial for extending the
visible-light-absorption spectrum compared to triphenyla-
mine. The significant red-shift of TD01 and ID01 compared
À
ly more positive than the I3 /IÀ couple, thus indicating that
the oxidized dyes that formed after electron injection into
the conduction band of TiO2 could thermodynamically
accept electrons from IÀ ions in the electrolyte. The HOMO
levels of ID01 and ID02 were negatively shifted compared
to TD01 and TD02, which was ascribed to an increase in
electron-donating ability upon the introduction of the indo-
line unit. On the other hand, the values of the HOMO for
TD01 and ID01 were similar to those of TD02 and ID02,
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Diketopyrrolopyrrole Complexes for Panchromatic DSCs
Figure 4. Energy diagram of the diketopyrrolopyrrole dyes.
Figure 5. J–V curves of the diketopyrrolopyrrole dyes with 0.5m TBP.
which implied that the different benzene and furan spacers
did not influence the HOMO levels. The band-gap energies
(E0–0) of the four dyes (Figure 4) indicate that the introduc-
tion of a furan moiety into the p spacer narrows the energy
gap between the LUMO and the HOMO. The LUMO
levels of TD01, TD02, ID01 and ID02, as calculated from
EoxÀE0–0, were À0.91, À0.95, À1.11, and À1.16 V, respective-
ly. These levels were much more negative than that of the
conduction band (ECB) of TiO2 (À0.5 V versus NHE), there-
by suggesting that all of the photoexcited dyes could effi-
ciently complete the process of electron injection into the
conduction band of TiO2. The LUMO levels of ID01 and
ID02 were much higher than those of TD01 and TD02,
which suggests that the dyes that contain the indoline donor
would have a larger electron driving force than those that
contain triphenylamine donor moieties.
Figure 6. IPCE spectra of DSCs that were based on diketopyrrolopyrrole
dyes.
ance and to a stronger driving force of the excited electrons.
The trend in photocurrent was in good agreement with the
IPCE measurements (Figure 6). The slightly higher Jsc value
of ID01 compared to that of ID02 could be explained by its
better absorption properties and IPCE response. However,
the Jsc value of TD01 was lower than that of TD02, which
was probably due to severe aggregation of TD01 on the
TiO2 film, thereby leading to dissipative intermolecular
charge transfer and, finally, to a decrease in the photocur-
rent density. Moreover, an enhancement of 40 mV in the Voc
of the cell that was sensitized with ID02 (benzene spacer)
compared to that with ID01 (furan spacer) was observed;
this trend was also observed in the case of the TD01- and
TD02-sensitized solar cells.
Performance of the Photovoltaic Devices
The photovoltaic performance of the DSCs was measured at
100 mWcmÀ2 under simulated AM 1.5 G solar light. The
open-circuit photovoltages (Voc), short-circuit photocurrent
densities (Jsc), fill factors (FF), and photovoltaic-conversion
efficiencies (h) of devices that were based on the studied
dyes are summarized in Table 2. The photocurrent-density–
voltage (J–V) characteristics and the IPCEs of these devices
are shown in Figure 5 and Figure 6, respectively. The short-
circuit currents of ID01 and ID02 were higher than those of
TD01 and TD02 (Figure 5), which was attributed to both
a much broader spectra that matched with the solar irradi-
As shown in Figure 6, the four dyes maintained a high
plateau in the visible-light region between 350–600 nm with
a maximum value of above 70%. Compared to dyes TD01
and TD02, ID01 and ID02 exhibited a broader IPCE spec-
trum, which was ascribed to the better absorption properties
of the indoline donor dyes, as well as to a more-efficient
electron-injection process. Significantly, DSCs that were
based on ID01 showed the broadest IPCE spectrum (300–
780 nm), with a maximum value of 78% in the high-plateau
region. The broader IPCE spectra of these dyes compared
to those that were obtained either in solution or after ad-
Table 2. Photovoltaic performance of the DSCs that were based on the
diketopyrrolopyrrole dyes.[a]
Dye
Jsc [mAcmÀ2
]
Voc [V]
FF
h [%]
TD01
TD02
ID01
ID02
11.05
12.11
13.66
12.97
0.69
0.73
0.71
0.74
0.68
0.69
0.66
0.68
5.18
6.10
6.35
6.53
[a] Light intensity: 100 mWcmÀ2, simulated AM 1.5 G solar light; active
area: 0.25 cm2; thickness of the TiO2 film: 16 mm; dye bath: 0.3 mm in
CH3CN/tBuOH (1:1, v/v); sensitized time: 24 h; electrolyte: 0.6m MPImI,
0.05m I2, LiI 0.1m, and 0.5m TBP in CH3CN.
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sorption onto TiO2 films, was due to the scattering layer of
larger TiO2 particles increasing the optical-path length in
the low-energy region.[19]
(IMVS). At a certain voltage (0.68 V), the electron lifetime
was 5.8, 25, 4.2, and 18 ms for TD01, TD02, ID01, and ID02.
The lifetimes in cells that were sensitized with TD02 and
ID02 were larger than those that were sensitized with TD01
and ID01, which contained furan spacers. This result sug-
gests that TD02 and ID02, which contained benzene spacers,
formed a compact layer on the TiO2 surface that could ef-
fectively suppress the recombination of electrons in the
Electron Lifetimes and EIS Measurements
To gain insight into the reason for the difference between
the Voc values of the dyes with benzene and furan spacers,
electron-lifetime measurements were performed on the
DSCs that were based on these dyes. The Voc value of
a DSC is defined as the difference in voltage between the
redox potential of the electrolyte (Eredox/q) and the quasi-
Fermi potential of electrons (EF,n/q) in the TiO2 semiconduc-
tor, where Eredox is the HOMO level of the redox couples, q
is the unit charge, and n is the number of electrons in TiO2.
Assuming that the redox potential of the electrolyte is con-
stant, the Voc value will be determined by the position of the
edge of the conduction band (ECB) and its electron density
in the TiO2 film.[20] As shown in Figure 7a, the ECB value,
which was measured by using the charge-extraction method
(CEM), was almost the same for all four dyes. Therefore,
the difference between the Voc values might be attributed to
the electron density of the conduction band in the TiO2
film, which is influenced by the electron lifetime and is re-
lated to the charge-recombination process. In Figure 7b, the
electron lifetime was derived as a function of electron densi-
ty by using intensity-modulated photovoltage spectroscopy
À
TiO2 film with I3 ions, thereby resulting in an improved Voc
value.
In addition, electrochemical-impedance spectroscopy (IS)
was also employed to analyze DSCs that were based on
these four dyes (applied voltage bias: À0.68 V, under dark
conditions) to study the dye-coating behavior on the TiO2
film. In the dark, the forward-bias electrons are injected in
the conduction band of the nanoparticles and their motion
À
is coupled to that of IÀ/I3 ions in the electrolyte.[21] The dye
molecules that are adsorbed onto the surface of the TiO2
semiconductor decrease the area that is active for recombi-
nation, thereby resulting in an increase in the recombination
resistance.[22] As shown in the Nyquist plots (Figure 8), the
Figure 8. Impedance spectra of DSCs that were based on diketopyrrolo-
pyrrole dyes; the spectra were recorded at À0.68 V in the dark.
radius of the larger semicircle is related to the recombina-
tion resistance at the TiO2/electrolyte interface, which in-
creases in the order TD01ꢀID01<TD02ꢀID02, thus indi-
cating that the coating layer of TD02 and ID02 (with the
furan spacer) is more compact than that of TD01 and ID01
(with benzene spacer), which results in better suppression of
À
the recombination of electrons in the TiO2 film with I3
ions.[23]
Conclusions
We have successfully designed and synthesized four metal-
free dyes (TD01, TD02, ID01, and ID02) that contained a di-
ketopyrrolopyrrole core by incorporating triphenylamine
and indoline donor groups, furan and benzene spacer
groups, and a carboxylic-acid acceptor group. The introduc-
tion of the indoline unit shifted the absorption spectra
Figure 7. Charge-extraction (a) and electron lifetimes (b) of DSCs that
were based on diketopyrrolopyrrole dyes.
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Diketopyrrolopyrrole Complexes for Panchromatic DSCs
Japan, UXL-500D-O), which was attenuated (if necessary) with a neu-
tral-density filter. An acoustic optical modulator (Isomet Co., Springfield,
VA, 1205 C-1) was used to produce sinusoidal modulation of a laser
beam. Electrochemical-impedance spectroscopy (EIS) measurements of
all of the DSCs were performed on a Zahner IM6e Impedance Analyzer
(ZAHNER-Elektrik GmbH &CoKG, Kronach, Germany). The frequen-
cy range was 0.1 Hz–100 kHz. The magnitude of the alternative signal
was 10 mV. The applied voltage bias was À0.68 V.
toward longer wavelength and improved the Jsc value. The
dyes with furan as a p-spacer showed a broader absorption
compared to those that contained benzene, which was attrib-
uted to the higher planarity of the optimized structure. As
a result, the ID01 dye exhibited an excellent IPCE spectrum
from 350–650 nm, with a maximum value of 78% in the
high plateau region and an onset value of close to 800 nm.
This result suggests that diketopyrrolopyrrole dyes with in-
doline donors and furan spacers are promising sensitizers
for panchromatic absorption in DSCs. At the same time, the
more-conjugated structure of the TD01 and ID01 dyes with
furan spacer groups showed enhanced electron-recombina-
tion compared to TD02 and ID02, which utilized benzene
spacers, and, hence, the Voc value decreased. Future research
will focus on this trade-off between broadening the absorp-
tion spectrum and the drop in Voc value. Possible solutions
that involve the introduction of bulky groups to prevent
charge-recombination and further broadening the absorp-
tion spectra into the infrared region will be considered.
Fabrication of Dye-Sensitized Solar Cells
The DSC devices were fabricated according to a literature procedure.[24]
A main transparent layer (12 mm) with titania particles (about 20 nm)
and a scattering layer (4 mm) with titania particles (about 400 nm) were
screen-printed onto the fluorine-doped tin oxide (FTO)-conducting glass
substrate. The films were then sintered at 5008C for 1 h. The thickness of
the films was measured on a Surfcom 1400A surface profiler (Tokyo Sei-
mitsu Co. Ltd.). The films were treated with a 0.1m aqueous solution of
HCl before examination. Coating of the titania films was carried out by
immersion in a 3ꢁ10À4 m solution of the sensitizers in MeCN/tBuOH
(1:1, v/v) for 24 h. Deoxycholic acid (DCA, 20 mmol) was added to the
solution of the dye as a co-adsorbant. The dye-covered TiO2 electrode
and the Pt counter electrode were assembled into a sandwich-type cell
and sealed with a hot-melt gasket (thickness: 25 mm) that was made of
the ionomer Surlyn1702 (DuPont). Finally, the electrolyte, which consist-
ed of 0.6m 1-methyl-3-propylimidazolium iodide (MPImI), 0.1m LiI,
0.05m I2, and 0.5m tert-butylpyridine in MeCN, was injected into the cell
and sealed with a cover glass.
Experimental Section
Materials and Reagents
Synthesis
Optically transparent FTO-conducting glass electrodes (Nippon Sheet
Glass Co., Japan) with a sheet resistance of 8–10 ohmÀ2 per square and
an optical transmission of greater than 80% in the visible-light range
were used and cleaned by using a standard procedure. A main transpar-
ent layer (12 mm) with titania particles (about 20 nm) and a scattering
layer (4 mm) with titania particles (about 400 nm) were screen-printed
onto the fluorine-doped tin oxide (FTO)-conducting glass substrate.
Tetra-n-butylammonium hexafluorophosphate (TBAPF6), 4-tert-butylpyr-
idine (4-TBP), and lithium iodide were bought from Fluka and iodide
(99.999% purity) was purchased from Alfa Aesar. THF was pre-dried
over molecular sieves and distilled under an argon atmosphere from
sodium benzophenoneketyl immediately prior to use. CH2Cl2 was dis-
tilled under normal atmospheric pressure and dried over calcium hydrox-
ide. All other chemicals were purchased from Aldrich and used as re-
ceived.
Compound 2: A mixture of compound 1 (100 mg, 0.14 mmol), [Pd-
AHCTNUGERTN(GNNU PPh3)4] (25 mg, 0.022 mmol), THF (10 mL), and K2CO3 (1.00 g,
7.25 mmol) in water (5 mL) was heated at 458C under a nitrogen atmos-
phere for 30 min. A solution of 4-formylphenylboronic acid (42 mg,
0.28 mmol) in THF (3 mL) was added slowly and the mixture was heated
at reflux for a further 15 h. After cooling to RT, the solvent was evapo-
rated under vacuum, the mixture was resolved in CH2Cl2 and dried over
MgSO4, and the volatile components were removed by rotary evapora-
tion. The residue was purified by column chromatography on silica gel
(CH2Cl2/petroleum ether, 3:1 v/v) to give a bright-orange solid (80 mg).
Yield: 76.2%; 1H NMR (CDCl3, 500 MHz): d=10.08 (s, 1H), 7.96 (m,
6H), 7.82 (m, 4H), 7.74 (d, J=8.44 Hz, 2H), 7.54 (d, J=8.66 Hz, 2H),
7.29 (dd, J=7.24 Hz, 14,83 Hz, 4H), 7.16 (dd, J=1,80 Hz, 8.24 Hz, 6H),
7.07 (t, J=7.34 Hz, 7.34 Hx, 2H), 3.83 (t, J=4.0 Hz, 4.0 Hz, 4H), 1.38–
1.77 (m, 4H), 1.26–1.39 (m, 4H), 0.88 ppm (t, J=7.35 Hz, 7.35 Hz, 6H);
HRMS (ESI): m/z calcd for C51H44N3O3: 746.3383 [MÀH]À; found:
746.3385.
Instruments and Characterization
NMR spectroscopy was performed on a Brucker AM 500 spectrometer
(relative to TMS). MS was performed on a HP5989 mass spectrometer.
Absorption spectra of the dyes, both in solution and adsorbed onto TiO2
films, were measured on a Varian Cary 500 spectrophotometer. Cyclic
voltammograms of the dyes were obtained on a Versastat II electrochem-
ical workstation (Princeton applied research) by using a normal three-
electrode cell with a Pt working electrode, a Pt wire counter electrode,
and a regular calomel reference electrode in saturated KCl solution.
TD02: A mixture of compound 2 (0.08 g, 0.11 mmol), 2-cyanoacetic acid
(0.094 g, 1.10 mmol), and piperidine (0.5 mL) in THF (20 mL) was
heated at reflux under a nitrogen atmosphere for 9 h. After cooling to
RT, the precipitate was filtered. The residue was purified by column
chromatography on silica gel (CH2Cl2/EtOAc/EtOH, 10:1:1 v/v/v) and
then washed with solvent (CH2Cl2/EtOH, 1:100 v/v) to give a red solid
(yield: 48.0%). 1H NMR (CDCl3, 500 MHz): d=7.96 (m, 6H), 7.82 (m,
4H), 7.74 (d, J=8.44 Hz, 2H), 7.63 (s, 1H), 7.54 (d, J=8.66 Hz, 2H),
7.29 (dd, J=7.24 Hz, 14,83 Hz, 4H), 7.16 (dd, J=1,80 Hz, 8.24 Hz, 6H),
7.07 (t, J=7.34 Hz, 7.34 Hz, 2H), 3.83 (t, J=4.0 Hz, 4.0 Hz, 4H), 1.38–
1.77 (m, 4H), 1.26–1.39 (m, 4H), 0.88 ppm (t, J=7.35 Hz, 7.35 Hz, 6H);
13C NMR (125 MHz, CDCl3): d=191.7, 163.0, 162.7, 148.8, 148.1, 147.5,
147.0, 146.0, 143.4, 142.2, 135.7, 133.2, 129.4, 127.8, 126.8, 126.3, 124.8,
123.4, 123.3, 110.4, 109.7, 41.9, 31.7, 29.7, 20.0, 13.7 ppm; HRMS (ESI):
m/z calcd for C54H45N4O4: 813.3441 [MÀH]À; found: 813.3445.
The photocurrent–voltage (J–V) curves of the DSCs were measured by
using a digital source meter (2400, Keithley) under standard air mass
(AM 1.5) with simulated solar illumination at 100 mWcmÀ2 (WXS-90S-
L2, Wacom). Monochromatic incident-photon-to-current-conversion effi-
ciency (IPCE) spectra were measured with monochromatic incident light
of 1ꢁ1016 photonscmÀ2 in DC mode (CEP-2000 BX, Bunko-Keiki).
During the J–V and IPCE measurements, a black mask and edge were
used with an aperture area of 0.25 cmÀ2. Charge-extraction from TiO2
was calculated by integration of the transient current that was detected
when the laser illumination was turned off and the DSC was simultane-
ously stepped from open-circuit to short-circuit. Intensity-modulated pho-
tovoltage spectroscopy of the open circuit was carried out by using a com-
bination of sinusoidal low-intensity-modulated illumination from a green
diode laser (Cobolt Co., Stockholm, Sweden, Samba, 532 nm, 50 mW)
and constant-bias light illumination from a Xe lamp (Ushio, Tokyo,
Compound 4: A mixture of compound 3 (1.05 g, 1.90 mmol), [PdACHTUNGTRENNUNG(PPh3)4]
(100 mg, 0.087 mmol), THF (30 mL), and K2CO3 (1.65 g, 12 mmol) in
water (5 mL) was heated at 458C under a nitrogen atmosphere for
30 min. A solution of 7-boronic acid-4-p-tolyl-1,2,3,3a,4,8b-hexahydrocy-
clopenta[b]indole (42 mg, 0.28 mmol) in THF (3 mL) was added slowly
and the mixture was heated at reflux for a further 15 h. After cooling to
room temperature, the solvent was evaporated in vacuum, the mixture
Chem. Asian J. 2012, 00, 0 – 0
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He Tian, Liyuan Han et al.
FULL PAPER
was resolved in CH2Cl2 and dried over MgSO4, and the volatile compo-
nents were removed by rotary evaporation. The residue was purified by
column chromatography on silica gel (CH2Cl2/petroleum ether, 1:1 v/v)
to give an orange/red solid (430 mg). Yield: 31.4%; 1H NMR (CDCl3,
500 MHz): d=7.89 (d, J=8.46 Hz, 2H), 7.70 (m, 6H), 7.44 (s, 1H), 7.37
(dd, J=1.81 Hz, 8.34 Hz, 1H), 7.20 (q, J=8.54 Hz, 8.54 Hz, 8.51 Hz, 4H),
6.96 (d, J=8.33 Hz, 1H), 4.85 (m, 1H), 3.89 (t, J=7.69 Hz, 7,69 Hz, 1H),
3.78 (m, 4H), 2.03–2.15 (m, 2H), 1.93 (s, 3H), 1.75–1.87 (m, 2H), 1.52–
1.73 (m, 4H), 1.19–1.38 (m, 6H), 0.81–0.94 ppm (m, 6H); HRMS (ESI):
m/z calcd for C44H48N3O2: 724.2539 [MÀH]À; found: 724.2535.
Compound 5: The synthesis resembled that of compound 2, but with
compound 4 as the starting material and 5-formylfuran-2-ylboronic acid
as the reagent. The product was purified by column chromatography on
silica gel (CH2Cl2/petroleum ether, 3:1 v/v) to give a red solid (80 mg).
Yield: 34.6%; 1H NMR (400 MHz, CDCl3): d=9.63 (s, 1H), 7.89 (m,
4H), 7.83 (d, J=8.32 Hz, 2H), 7.65 (d, J=8.37 Hz, 2H), 7.37 (s, 1H),
7.31 (s, 1H), 7.29 (d, J=3.70 Hz, 2H), 7.13 (m, 5H), 6.89 (m, 2H), 3.82
(t, J=9.00 Hz, 9.00 Hz, 1H), 3.75 (m, 4H), 2.28 (s, 3H), 1.98 (m, 2H),
1.86 (m, 2H), 1.73 (m, 2H), 1.57 (m, 4H), 1.24 (m, 4H), 0.82 ppm (m,
6H); HRMS (ESI): m/z calcd for C49H46N3O4: 740.3488 [MÀH]À; found:
740.3489.
the NSFC/China (21190033), and by the National Basic Research 973
Program (2011CB8068400).
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Compound 6: The synthesis resembled that of compound 2, but with
compound 4 as the starting material and 4-formylphenyl boronic acid as
the reagent. The product was purified by column chromatography on
silica gel (CH2Cl2/petroleum ether, 3:1 v/v) to give a red solid (80 mg).
Yield: 34.6%; 1H NMR (400 MHz, CDCl3): d=10.07 (s, 1H), 7.97 (m,
4H), 7.90 (d, J=8.40 Hz, 2H), 7.80 (t, J=6.30 Hz, 8.07 Hz, 4H), 7.72 (d,
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6.95 (d, J=8.32 Hz, 1H), 3.83 (m, 4H), 3.70 (m, 2H), 2.35 (s, 3H), 2.07
(m, 2H), 1.93 (m, 2H), 1.85 (m, 4H), 1.65 (m, 6H), 1.32 ppm (m, 6H);
HRMS (ESI): m/z calcd for C51H48N3O3: 750.3696 [MÀH]À; found:
750.3698.
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ID01: The synthesis resembles that of compound TD02, but with com-
pound 5 as the starting material. The product was purified by column
chromatography on silica gel (CH2Cl2/EtOH, 10:1 v/v) to give a dark-red
1
solid (42 mg). Yield: 51.8%; H NMR (500 MHz, CDCl3): d=8.01 (d, J=
8.45 Hz, 2H), 7.89 (t, J=6.71 Hz, 6.71 Hz, 4H), 7.84 (s, 1H), 7.74 (d, J=
8.40 Hz, 2H), 7.45 (s, 1H), 7.38 (d, J=9.54 Hz, 1H), 7.28 (s, 1H), 7.20 (q,
J=8.52 Hz, 8.52 Hz, 8.40 Hz, 4H), 7.05 (d, J=3.80 Hz, 1H), 6.96 (d, J=
8.16 Hz, 1H), 3.82 (m, 5H), 2.02 (m, 1H), 1.93 (s, 3H), 1.82 (m, 2H),
1.62 (m, 8H), 1.31 (m, 4H), 0.87 ppm (m, 6H); 13C NMR (125 MHz,
CDCl3): d=216.0, 211.7, 201.2, 176.7, 175.2, 171.7, 168.4, 165.6, 162.7,
157.5, 156.6, 148.5, 147.3, 140.1, 136.4, 129.3, 128.0, 126.8, 126.3, 125.8,
120.3, 115.7, 115.6, 112.9, 110.7, 97.5, 95.7, 89.3, 85.1, 82.6, 81.7, 72.7, 66.8,
65.0, 62.9, 59.1, 57.5, 49.6, 45.5, 42.1, 31.6, 29.7, 29.2, 26.4, 24.5, 22.8, 20.7,
20.1, 13.4 ppm; HRMS (ESI): m/z calcd for C52H47N4O5: 807.3546
[MÀH]À; found: 807.3538.
ID02: The synthesis resembled that of compound TD02, but with com-
pound 6 as the starting material. The product was purified by column
chromatography on silica gel (CH2Cl2/EtOH, 10:1 v/v) to give a red solid
(30 mg). Yield: 52.6%; 1H NMR (500 MHz, CDCl3): d=7.97 (m, 4H),
7.90 (d, J=8.40 Hz, 2H), 7.80 (t, J=6.30 Hz, 8.07 Hz, 4H), 7.72 (d, J=
8.30 Hz, 2H), 7.52 (s, 1H), 7.43 (s, 1H), 7.37 (d, J=8.36 Hz, 1H), 7.20
(m, 4H), 6.95 (d, J=8.32 Hz, 1H), 4.85 (m, 1H), 3.87 (m, 1H), 3.83 (m,
4H), 2.35 (s, 3H), 2.07 (m, 1H), 1.93 (m, 2H), 1.85 (m, 1H), 1.60–1.70
(m, 5H), 1.25–1.35 (m, 5H), 0.85–0.90 ppm (m, 6H); 13C NMR
(125 MHz, CDCl3): d=215.6, 213.7, 204.1, 197.6, 193.3, 166.1, 161.6,
152.4, 149.4, 131.1, 129.9, 129.3, 125.6, 123.4, 120.3, 120.2, 117.9, 113.7,
110.3, 109.0, 69.5, 67.8, 66.3, 58.6, 57.3, 47.2, 45.4, 36.7, 35.2, 33.9, 31.6,
29.6, 27.7, 24.6, 19.8, 13.7, 11.8 ppm; HRMS (ESI): m/z calcd for
C54H49N4O4: 817.3754 [MÀH]À; found: 817.3759.
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Diketopyrrolopyrrole Complexes for Panchromatic DSCs
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Received: July 13, 2012
Revised: August 22, 2012
Published online: && &&, 0000
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FULL PAPER
Biosensors
Panchromatic’s labyrinth: A series of
metal-free dyes that were bridged by
a diketopyrrolopyrrole moiety were
synthesized. DSCs based on ID01
showed a broader incident-photon-to-
current-conversion efficiency (IPCE)
spectrum over the whole visible range
close to 800 nm. Intensity-modulated
photovoltage spectroscopy (IMVS)
and impedance spectroscopy (IS) of
the relationship between benzene and
furan moieties as spacers were also
performed.
Sanyin Qu, Chuanjiang Qin,
Ashraful Islam, Jianli Hua, Han Chen,
He Tian,* Liyuan Han*
&&&& — &&&&
Tuning the Electrical and Optical
Properties of Diketopyrrolopyrrole
Complexes for Panchromatic Dye-
Sensitized Solar Cells
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