Engineering diketopyrrolopyrrole sensitizers for highly efficient dye-sensitized solar cells: Enhanced light harvesting and intramolecular charge transfer

Article abstract of DOI:10.1039/c4ra02009j

Two asymmetric DPP dyes with a D-π-A structure are reported, where DPP is used as a bridge to connect the triphenylamine donor and cyanoacetic acid acceptor. The compact dyes exhibit high light absorption ability covering the whole visible spectral region. The most efficient cell exhibited a short-circuit current density of 17.72 mA cm^-2, an open-circuit voltage of 725 mV, and a fill factor of 74%, yielding a power conversion efficiency of 9.51% under standard test conditions (irradiation of 1000 W m^-2, air mass = 1.5 G), which is the highest value reported for DPP-based DSCs employing an I ^-/I₃^- redox couple. Furthermore, the DPP dyes show remarkable stability under long-term irradiation over 1000 h. Considering the facile synthesis and excellent stability, the DPP sensitizer would be a promising option for highly efficient DSCs. The results show that judicious molecular engineering is crucial for constructing highly efficient charge transfer sensitizers in DSCs.

Full text of DOI:10.1039/c4ra02009j

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PAPER
Engineering diketopyrrolopyrrole sensitizers for
highly efficient dye-sensitized solar cells: enhanced
light harvesting and intramolecular charge
transfer†
Cite this: RSC Adv., 2014, 4, 16906
ab
Jin-Hua Huang,ab Ke-Jian Jiang,
Fang Zhang,ab Wei Wu,ab Shao-Gang Li,ab
*
*
Lian-Ming Yangab and Yan-Lin Song
ab
Two asymmetric DPP dyes with a D–p–A structure are reported, where DPP is used as a bridge to connect the
triphenylamine donor and cyanoacetic acid acceptor. The compact dyes exhibit high light absorption ability
covering the whole visible spectral region. The most efficient cell exhibited a short-circuit current density
of 17.72 mA cmꢀ2, an open-circuit voltage of 725 mV, and a fill factor of 74%, yielding a power conversion
efficiency of 9.51% under standard test conditions (irradiation of 1000 W mꢀ2, air mass ¼ 1.5 G), which is
the highest value reported for DPP-based DSCs employing an Iꢀ/I3 redox couple. Furthermore, the DPP
ꢀ
Received 8th March 2014
Accepted 26th March 2014
dyes show remarkable stability under long-term irradiation over 1000 h. Considering the facile synthesis
and excellent stability, the DPP sensitizer would be a promising option for highly efficient DSCs. The results
show that judicious molecular engineering is crucial for constructing highly efficient charge transfer
sensitizers in DSCs.
DOI: 10.1039/c4ra02009j
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performance pigments due to its exceptional photochemical,
mechanical and thermal stability.5 Following its successful
investigations in a series of optical electronic devices,6 it was
Introduction
As one of the key components in dye-sensitized solar cells
(DSCs), the sensitizer exerts a signicant inuence on the device
power conversion and stability.1 With ruthenium complexes2a–d
and zinc porphyrin dyes,2e–g high power conversion efficiencies
of up to 8–12% have been reported. On the other hand, metal-
free organic dyes, usually with an electron donor–p bridge–
electron acceptor (D–p–A) conguration, have been actively
pursued due to their molecular tailoring exibility and raw
material abundance, and comparable efficiencies have been
achieved.3 In the D–p–A organic dyes, triarylamine and cya-
noacrylic acid were widely used as donor and acceptor, respec-
tively, and various p-conjugated linkers were employed to
bridge the donor and acceptor units to create a large number of
D–p–A dyes for DSCs. It is well known that a linker is of para-
mount importance in tuning the molecular energy gap, and the
electronic and steric structures, strongly affecting device
performances.4
also employed as a linker in development of sensitizers for DSCs
in recent years.7 Following the rst report in 2010,7a Han's group
introduced a strong electron donor indoline in a DPP dye, giving
efficiency of 7.4%.7d Recently, asymmetric DPP sensitizer DPP07
with intense absorption in the red/near-IꢀR region were prepared
with efficiencies of ꢁ8.6% using a Iꢀ/I3 based electrolyte.8 In
our previous report, a compact DPP dye ICD-1, was designed,
where diphenylamine unit was directly connected to phenyl–
DPP unit with D–p–A structure, giving a power conversion
efficiency of 8.61%. The result demonstrated that the electron
communication in ICD-1 is more favourable as compare with
that in the similar structured sensitizer ICD-2 with a triphe-
nylamine donor.9 Encouraged by the result, we focused on the
further structural optimization of DPP-based dyes for
improving light harvesting ability. Here, two compact asym-
metric DPP dyes (ICD-8 and ICD-9), were designed, and an
ꢀ
efficiency up to 9.51% was achieved in a DSC with an Iꢀ/I3
Diketopyrrolopyrrole (DPP) chromophore has a uniquely
planar conjugated bicyclic structure with electron-withdrawing
property, and its derivatives are extensively used as high-
based electrolyte under standard AM 1.5 conditions. The
chemical structures of the dyes are shown in Fig. 1.
aBeijing National Laboratory for Molecular Sciences (BNLMS), P. R. China
bKey Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, P. R. China
Results and discussion
The synthetic route of ICD-8 and ICD-9 is described in Fig. 2.
Their UV-Vis absorption spectra examined in dichloromethane
solutions were displayed in Fig. 3, and the relevant parameters
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra02009j
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Fig. 1 Molecular structures of DPP07, ICD-8 and ICD-9.
Fig.
3
Absorption spectra of ICD-8 and ICD-9 measured in
are collected in Table 1. Both the dyes show broader absorption
spectra with two absorption bands covering a wide range in the
visible region. The maximum absorption peak of ICD-9 is at
581 nm, which is 21 nm red shi compared to that (560 nm) of
the similar dye DPP07 where an additional phenyl unit is
included.8a More importantly, the absorption spectrum of ICD-9
covers the whole visible range, while the latter lacks absorption
between 360–450 nm.8a The result indicates that ICD-9 dye is
compact in structure, and possesses highly potential light har-
vesting ability as sensitizer in DSCs. In the case of ICD-8, the
maximum absorption peak (lmax, 620 nm) was signicantly red
shied by 39 nm as compared with that in ICD-9. The large
difference can be explained by the increased dihedral twist at
the Ph–DPP–Th bonds in the presence of two large ethylhexyl
units in ICD-9. The bulky units can attenuate the tendency of
dye aggregation on TiO2 surface, and improve the device
performance.
dichloromethane solution.
delocalized over the whole triphenylamine and DPP units, while
their LUMOs are on the cyanoacrylic acid group and part of the
DPP macrocycle. Thus electron communication may be
favourable between the donor and the acceptor in both the dyes,
allowing an efficient electron transfer from dye to TiO2 electrode
under light irradiation. The larger twist angels are expected to
localize the triphenylamine radical cation away from the
acceptor, and might be benecial for retarding the back elec-
tron transfer from TiO2 to the oxidized dye and improving the
device open-circuit voltage.8a
To further investigate the molecular energy levels, cyclic
voltammetry measurements were performed in a 0.1 M
dichloromethane solution of tetrabutylammoniumhexa-
uorophosphate with ferrocene as internal standard at 0.63 V
vs. NHE. The rst oxidation potentials (Es+/0) of ICD-8 and ICD-9
were observed to be 0.92 and 0.93 V vs. NHE, respectively, which
are assigned to the oxidation of triphenylamine units. Both the
potential values are substantially more positive than the iodide/
tri-iodide couple redox (0.4 V vs. NHE), indicating that the
In order to gain insight into the geometrical conguration
and electron distribution of the frontier orbitals of the two dyes,
density functional theory (DFT) calculations were made on a
B3LYP/6-31G level as shown in Fig. 4. As shown in Fig. 4, the
frontier HOMOs of both the dyes are homogeneously
Fig. 2 Synthetic routes of dye ICD-8 and ICD-9. (i) KOBut, NMP; (ii) Pd(OAc)2, P(But)3, t-BuONa, PhMe, Ar, 90 ꢂC; (iii) n-BuLi, (n-Bu)3SnCl, ꢀ78 ꢂC;
PdCl2(PPh3)2, Ar, 65 ꢂC; (iv) cyanoacetic acid, piperidine, THF, Ar, reflux.
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ground-state sensitizer regeneration is energetically favorable
in DSC.10
The optical transition energies (E0–0) were 1.81 eV for ICD-8
and 1.89 eV for ICD-9, estimated from the intersection of their
absorption and emission spectra. excited-state redox potential,
Es+/*, determined by subtracting E0–0 from Es+/0, were ꢀ0.89 V
for ICD-8 and ꢀ0.96 eV for ICD-9. Both the values are negative
enough to allow their excited state electron transfer into the
TiO2 conduction band (ꢀ0.5 V vs. NHE).11
For preparation of the DSCs, a double TiO2 electrode con-
taining an 11 mm transparent layer and a 6 mm scattering layer
was employed. The electrodes were prepared by screen printing
method, and sintered at 500 ꢂC in air for 30 min, followed with
treatment by 0.05 M titanium tetrachloride solution and
subsequent heat treatment at 500 ꢂC for 30 min.12 The prepared
TiO2 electrodes were stained in the dye solution of 0.2 mM ICD-
8 or ICD-9 in a mixture of acetonitrile, tert-butylalcohol and
tetrahydrofuran (1 : 1 : 0.2, v/v/v). In the solution, 20 mM
3a,7a-dihydroxy-5b-cholic acid (chenodeoxycholic acid) was
added as co-adsorbent to prevent dye aggregation on TiO2
surface. The nal device was prepared in a sandwich-type,
where the dyed TiO2 electrode and a Pt-counter electrode was
sealed with a hot-melt lm, and lled with an Iꢀ/I3ꢀ electrolyte
between them. The electrolyte contains 0.68 M dimethyl
imidiazolium iodide, 0.05 M iodine, 0.10 M LiI, 0.05 M guani-
dinium thiocyanate, and 0.40 M tert-butylpyridine in the
mixture of acetonitrile and valeronitrile (85 : 15, v/v).
Fig. 5a shows action spectra in the form of monochromatic
incident photon-to-current conversion efficiencies (IPCEs).
Both the dye-based DSCs exhibited a strong and broad response
in the entire visible spectral region with the highest value of
85% and 83% at 593 nm for ICD-9 and ICD-8, respectively. In
the spectra, ICD-8 showed slightly higher response in the long
wavelengths 660–800 nm but much lower IPCE values at the
other spectral regions, compared to ICD-9, which is in good
agreement with absorption spectra. Both the dyes are rare
examples having panchromatic spectral response, because most
organic dyes show narrow absorption bands, could not absorb
efficiently over the entire visible in DSCs with a single sensi-
tizer.13 The compact ICD-9 exhibits a broader spectral response,
Fig. 4 Frontier molecular orbitals of the HOMO and LUMO for ICD-8
and ICD-9, calculated with DFT on a B3LYP/6-31+G(d) level (iso-
density ¼ 0.03 au).
as compared to the dye DPP07 mentioned above, where the
device lacks efficient response between 400 and 500 nm.8a
Fig. 5b shows the photocurrent density–voltage characteris-
tics for DSCs with ICD-8 and ICD-9 measured under AM 1.5 G
simulated solar light at a light intensity of 100 mW cmꢀ2. ICD-9
based-device gave a short circuit photocurrent density (Jsc) of
17.72 ꢃ 0.20 mA cmꢀ2, an open circuit voltage (Voc) of 725 ꢃ 10
mV, and a ll factor (FF) of 0.74 ꢃ 0.02, corresponding to an
overall conversion efficiency (h), derived from the equation
h ¼ Jsc ꢄ Voc ꢄ FF/light intensity, of 9.51%, while ICD-8 based
device gave Jsc of 16.76 ꢃ 0.20 mA cmꢀ2, Voc of 683 ꢃ 10 mV, FF
of 0.66 ꢃ 0.02, and h of 7.56% under the same conditions. The
large differences in the efficiencies come mainly from the
differences in the Voc and FF between them. In the given
systems with different dyes, generally, the Voc is strongly
affected by the charge recombination at the dyed TiO2 interface
due to the dye aggregation. Considering their structural differ-
ence, the higher Voc can be assigned to the effective aggregation
suppression from branched 2-ethyl-hexyl chains in ICD-9.14
For further elucidating the large difference in Vocs for ICD-8
and ICD-9, electrochemical impedance spectroscopy (EIS) was
performed to investigate the FTO/TiO2/dye interfaces. The
Table 1 Photoelectrochemical properties of ICD-8 and ICD-9, and their solar cell performance
3 [L molꢀ1 cmꢀ1
]
E0–0 [eV]
1.81
Es+/0c [V]
0.92
Es+/*d [V]
ꢀ0.89
Jsc [mA cmꢀ2
16.76 ꢃ 0.20
17.72 ꢃ 0.20
]
Voc [mV]
683 ꢃ 10
725 ꢃ 10
FF
he [%]
7.56
b
Dye
lmaxa [nm]
ICD-8
ICD-9
620
339
581
393
53 000
39 800
57 800
35 400
0.66 ꢃ 0.02
0.74 ꢃ 0.02
1.89
0.93
ꢀ0.96
9.51
a
Absorption in CH2Cl2 solutions (1 ꢄ 10ꢀ5 M) at rt. b E0–0 values were estimated from the intersection of the absorption and emission spectra. c The
oxidation potentials of the dyes were measured in CH2Cl2 solutions with tetrabutylammoniumhexauorophosphate (TBAPF6, 0.1 M) as electrolyte,
Pt wires as working and counter electrode, Ag/Ag+ as reference electrode; calibrated with ferrocene/ferrocenium (Fc/Fc+) as an internal reference and
converted to NHE by addition of 630 mV. d The estimation was determined by subtracting E0–0 from Es+/0
.
The data were recorded under AM 1.5 G
e
simulated solar light at a light intensity of 100 mW cmꢀ2, and represents the average of three devices, where TiO2 lms with 11 mm thick
nanocrystalline layer and 6 mm thick scattering layer were used with an electrolyte containing 0.68 M dimethyl imidiazolium iodide, 0.05 M
iodine, 0.10 M LiI, 0.05 M guanidinium thiocyanate, and 0.40 M tert-butylpyridine in a mixture of acetonitrile and valeronitrile (85/15, v/v), and
each data was averaged by three parallel samples.
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Fig. 6 Variations of the photovoltaic parameters (Jsc, Voc, FF, and h)
with aging time for the DSSC device based on RD-II under one-sun
light soaking.
initial value. The high stability may come from strong adsorp-
tion ability of the compact dye on TiO2 surface, and will be
favourable for the practical application.
Conclusions
In conclusion, two new asymmetric DPP dyes with a D–p–A
structure were designed with high light absorption ability
covering the whole visible spectral region. The panchromatic
dye ICD-9 exhibits high power conversion efficiency of 9.51%
under standard AM 1.5 conditions, which is the highesꢀt values
reported for DPP dye-based DSCs employing a Iꢀ/I3 redox
couple. Furthermore, the DPP dyes show remarkable stability
under long-term irradiation over 1000 h. Considering the facile
synthesis and excellent stability, the DPP sensitizer would be a
promising option for high efficient DSCs. Further improvement
of the performance could be expected for DPP sensitizers
through further extending spectral response for high short
circuit photocurrent, and employing Co2+/Co3+ redox couple for
high open circuit voltage.16 More importantly, judicious
molecular engineering is crucial for constructing highly effi-
cient charge transfer sensitizers in DSCs. The results would
provide valuable, basic guidelines for rational designs of D–p–A
molecules for high-performance DSCs and other optoelectronic
devices.
Fig. 5 a) Incident photon-to-electron conversion efficiency (IPCE)
action; (b) I–V characteristics measured under AM 1.5 conditions; (c)
electrochemical impedance spectra measured in the dark at a forward
bias of 0.6 V for the DSCs employing ICD-8 and ICD-9.
Nyquist plots of DSCs were recorded in the dark with a forward
bias of 0.6 V, as shown in Fig. 5c. Two semicircles from le to
right in the Nyquist plot were assigned to the resistances of
charge transfer (RPt) on the Pt counter electrode and charge
recombination (Rr) at the interface of the TiO2/electrolyte.15
Clearly, the semicircle for Rr is larger in ICD-9 than that in ICD-
Experimental
General information
8, indicating the effective suppression of charge recombination Unless otherwise noted, all solvents were treated by standard
at the interface of the TiO2/electrolyte in ICD-9 based device, methods before use. All reagents were commercially available
which is consistent with the observed high Voc value.
and used without further purication unless indicated other-
For testing long-term stability, photovoltaic performance of wise. Thin lay all chemicals were analytical grade and used as
ICD-9 based devices was recorded discontinuously over the received. NMR spectra were recorded on a BRUKER AVANCE
entire 1000 h testing period at room temperature under one-sun 400 MHz instruments. The residual solvent protons (1H) were
light soaking. As shown in Fig. 6, the photovoltaic parameters used as internal standards. 1H-NMR data are presented as
Jsc, Voc and FF of the ICD-9 based cell changed slightly with the follows: chemical shi in ppm (d) downeld from tetrame-
time, and the nal conversion efficiency retained 92% of its thylsilane (multiplicity, coupling constant (Hz), integration).
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The following abbreviations are used in reporting NMR data: s, chromatography to afford an dark solid (812 mg, 90%). 1H-NMR
singlet; br.s, broad singlet; d, doublet; t, triplet; q, quartet; dd, (CDCl3, 400 MHz) d (ppm): 0.86–0.90 (m, 6H), 1.27–1.31 (m,
doublet of doublets; m, multiplet. Mass spectra were taken on a 30H), 1.75 (4H), 3.86 (d, J ¼ 7.6 Hz, 2H), 4.04 (t, J ¼ 7.6 Hz, 2H),
Bruker Daltonics Inc. APEXII FT-ICR spectrometer. The material 7.03 (d, J ¼ 8.8 Hz, 2H), 7.13 (d, J ¼ 6.8 Hz, 4H), 7.34 (d, J ¼ 8.8
3-(4-bromophenyl)-6-(thiophen-2-yl)-2,5-dihydro-pyrrolo[3,4-c]- Hz, 4H), 7.59 (d, J ¼ 4.0 Hz, 1H), 7.77 (d, J ¼ 8.8 Hz, 2H), 7.83 (d,
pyrrole-1,4-dione (DPP) were prepared according to the litera- J ¼ 8.4 Hz, 2H), 7.93 (d, J ¼ 8.4 Hz, 2H), 8.86 (d, J ¼ 4.4 Hz, 1H),
ture procedures.17
10.02 (s, 1H).
Synthesis of 2a
Synthesis of ICD-8
In an oven-dried 250 mL three-necked ask, 8 g (18 mmol) DPP An oven-dried 100 mL one-necked ask was charged with
(1) and 4.03 g (36 mmol) potassium tert-butoxide were dissolved compound 4 (4.2 ꢄ 10ꢀ4 mol), cyanoacetic acid (8 ꢄ 10ꢀ3 mol),
in 150 mL of dry NMP, the solution was vigorously stirred at 10 mL of dry THF and 0.5 mL piperidine. The solution was
60 ꢂC for 30 min, aer which time 20.8 g (108 mmol) of heated to reux for 15 h and the colour turned to deep red. Aer
1-bromohexane was dropwised over a period of 90 min. The cooling to room temperature, dichloromethane and a diluted
reaction was maintained for another 20 hours. Aer cooling to solution of hydrochloric acid were added and the organic phase
room temperature, toluene was added and the organic phase was washed with water (3 ꢄ 20 mL) and brine (3 ꢄ 20 mL). The
was ltered through a silica-gel pad. The ltrate was washed combined organic phase was dried on MgSO4, ltered and
with water (3 ꢄ 50 mL) and brine (3 ꢄ 50 mL). The combined evaporated under reduced pressure and the residue puried by
organic phases were evaporated under reduced pressure and silica-gel column chromatography to give a dark-purple solid
1
the residue puried by silica-gel column chromatography to (78%). H-NMR (DMSO, 400 MHz) d (ppm): 0.79–0.85 (m, 6H),
give the desired product. 2.5 g (22%) of bright orange needles 1.19–1.34 (m, 30H), 1.54–1.60 (m, 4H), 3.78 (2H), 3.93 (2H), 6.83
were obtained.
(d, J ¼ 8.0 Hz, 4H), 7.10 (d, J ¼ 8.0 Hz, 4H), 7.42 (d, J ¼ 8.0 Hz,
4H), 7.76 (d, J ¼ 4.4 Hz, 2H), 7.76–7.98 (m, 8H), 8.83 (d, J ¼ 4 Hz,
1H); MS (MALDI-TOF): m/z found: 912.7 (M+).
Synthesis of 3a
An oven-dried 100 mL three-necked ask was charged with
compound 2a (0.6 g, 1.0 mmol), Pd(OAc)2 (22 mg, 0.1 mmol),
Synthesis of 2b
PtBu3 (20 mg, 0.1 mmol), bis(tert-butylphenyl)amine (0.48 g, In an oven-dried 250 mL three-necked ask, 8 g (18 mmol) DPP
1.30 mmol), tBuONa (0.48 g, 5.0 mmol). The ask was evacuated (1) and 4.03 g (36 mmol) potassium tert-butoxide were dissolved
and backlled with nitrogen, with the operation being repeated in 150 mL of dry NMP, the solution was vigorously stirred at
twice. Then 20 mL toluene was added and the mixture was 60 ꢂC for 30 min, aer which time 20.8 g (108 mmol) of
heated at 90 ꢂC for 16 h. Aer cooling to room temperature, 1-bromo-2-ethylhexane was dropwised over a period of 90 min.
dichloromethane were added and the organic phase was ltered The reaction was maintained for another 20 hours. Aer cooling
through a silica-gel pad. The ltrate was with water (3 ꢄ 20 mL) to room temperature, toluene was added and the organic phase
and brine (3 ꢄ 20 mL). The combined organic phases was dried was ltered through a silica-gel pad. The ltrate was washed
on MgSO4, ltered and evaporated under reduced pressure and with water (3 ꢄ 50 mL) and brine (3 ꢄ 50 mL). The combined
the residue puried by silica-gel column chromatography to organic phases were evaporated under reduced pressure and
1
give a purple solid (0.61 g, 62%). H-NMR (CDCl3, 400 MHz) d the residue puried by silica-gel column chromatography to
(ppm): 0.80–0.86 (m, 6H), 1.26–1.31 (m, 25H), 1.54 (s, 5H), 1.71 give the desired product. 2.5 g (22%) of bright orange needles
(s, 4H), 3.83 (t, J ¼ 8.0 Hz, 2H), 4.00 (t, J ¼ 8.0 Hz, 2H), 7.03 (d, were obtained.
J ¼ 4.8 Hz, 3H), 7.12 (d, J ¼ 8.8 Hz, 4H), 7.33 (d, J ¼ 8.4 Hz, 4H),
7.60 (d, J ¼ 5.2 Hz, 1H), 7.75 (d, J ¼ 8.8 Hz, 2H), 8.84 (d, J ¼ 3.2 Synthesis of 3b
Hz, 1H).
An oven-dried 100 mL three-necked ask was charged with
compound 2b (0.6 g, 1.0 mmol), Pd(OAc)2 (22 mg, 0.1 mmol),
Synthesis of 4a
PtBu3 (20 mg, 0.1 mmol), bis(tert-butyl phenyl)amine (0.48 g,
In an oven-dried 250 mL three-necked ask, compound (3a) 1.30 mmol), tBuONa (0.48 g, 5.0 mmol). The ask was evacuated
(0.8 g, 1 mmol) and dried THF (50 mL) were added under N2 and backlled with nitrogen, with the operation being repeated
atmosphere. n-Butyl lithium (0.42 mL, 2.4 M in hexane, 1 twice. Then 20 mL toluene was added and the mixture was
mmol), was added dropwise at ꢀ78 C stirred for 1 h with the heated at 90 ꢂC for 16 h. Aer cooling to room temperature,
ꢂ
temperature maintained ꢀ78 ꢂC. then the mixture was warmed dichloromethane were added and the organic phase was ltered
up to ꢀ40 ꢂC for 3 h and cooled to ꢀ78 ꢂC again, tributyltin through a silica-gel pad. The ltrate was with water (3 ꢄ 20 mL)
chloride (0.27 mL, 1 mmol) was added one portion. The mixture and brine (3 ꢄ 20 mL). The combined organic phases was dried
was warmed to room temperature slowly and stirred for another on MgSO4, ltered and evaporated under reduced pressure and
24 h. Then 4-bromobenzaldehyde (0.28 g, 1.5 mmol) and the residue puried by silica-gel column chromatography to
1
PdCl2(PPh3)2 (120 mg, 0.171 mmol) were added, the solution give a purple solid (0.61 g, 62%). H-NMR (CDCl3, 400 MHz) d
ꢂ
was stirred at 65 C for another 15 h, cooled and concentrated (ppm): 0.80–0.94 (m, 12H), 1.28–1.38 (m, 28H), 1.61–1.68 (m,
on the rotary evaporator. The residue was subjected to column 6H), 1.68 (s, 2H), 3.83 (t, J ¼ 7.6 Hz, 2H), 3.99 (t, J ¼ 7.6 Hz, 2H),
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7.03 (d, J ¼ 4.8 Hz, 3H), 7.12 (d, J ¼ 8.8 Hz, 4H), 7.33 (d, J ¼ 8.4 30 min, followed by treating with a 40 mM aqueous solution of
Hz, 4H), 7.60 (d, J ¼ 5.2 Hz, 1H), 7.75 (d, J ¼ 8.8 Hz, 2H), 8.84 (d, TiCl4 for 30 min at 75 ꢂC and subsequent sintering at 500 ꢂC for
J ¼ 3.2 Hz, 1H).
30 min. The thickness of TiO2 lms was measured by a proler,
Sloan, Dektak3. The electrodes were immersed in a dye bath
containing 0.2 mM ICD-8 or ICD-9 and 20 mM 3a,7a-dihydroxy-
5b-cholic acid (chenodeoxycholic acid) in 4-tert-butanol–aceto-
nitrile mixture–tetrahydrofuran (1 : 1 : 0.2, v/v) and kept for 24 h
at room temperature. The dyed electrodes were then rinsed with
the mixed solvent to remove excess dye. A platinum-coated
counter electrode was prepared according to the reporter,19 and
two holes were drilled on its opposite sides. The two electrodes
were sealed together with a 25 mm thick thermoplastic Surlyn
frame. An electrolyte solution was then introduced through one
of the two holes in the counter electrode, and the holes were
sealed with the thermoplastic Surlyn. The electrolyte contains
0.68 M dimethyl imidiazolium iodide, 0.05 M iodine, 0.10 M LiI,
0.05 M guanidinium thiocyanate, and 0.40 M tert-butylpyridine
in the mixture of acetonitrile and valeronitrile (85 : 15, v/v). All
the devices were prepared with a photoactive area of about
0.3 cm2, and a metal mask of 0.165 cm2 was covered on the
device for photovoltaic property measurements.
Synthesis of 4b
In an oven-dried 250 mL three-necked ask, compound (3b)
(0.8 g, 1 mmol) and dried THF (50 mL) were added under N2
atmosphere. n-Butyl lithium (0.42 mL, 2.4 M in hexane, 1
ꢂ
mmol), was added dropwise at ꢀ78 C stirred for 1 h with the
temperature maintained ꢀ78 ꢂC. then the mixture was warmed
up to ꢀ40 ꢂC for 3 h and cooled to ꢀ78 ꢂC again, tributyltin
chloride (0.27 mL, 1 mmol) was added one portion. The mixture
was warmed to room temperature slowly and stirred for another
24 h. then 4-bromobenzaldehyde (0.28 g, 1.5 mmol) and
PdCl2(PPh3)2 (120 mg, 0.171 mmol) were added, the solution
ꢂ
was stirred at 65 C for another 15 h, cooled and concentrated
on the rotary evaporator. The residue was subjected to column
chromatography to afford an dark solid (812 mg, 90%). 1H-NMR
(CDCl3, 400 MHz) d (ppm): 0.76–0.94 (m, 12H), 1.27–1.38
(m, 29H), 1.60–1.66 (m, 6H), 1.68 (s, 1H), 3.86 (d, J ¼ 7.6 Hz, 2H),
4.0 (d, J ¼ 7.6 Hz, 2H), 7.03 (d, J ¼ 8.8 Hz, 2H), 7.11 (d, J ¼ 8.8 Hz,
4H), 7.32 (d, J ¼ 8.4 Hz, 4H), 7.58 (d, J ¼ 4 Hz, 1H), 7.72 (d, J ¼
8.8 Hz, 2H), 7.82 (d, J ¼ 8.4 Hz, 2H), 7.93 (d, J ¼ 8 Hz, 2H), 8.85
(d, J ¼ 4 Hz, 1H), 10.03 (s, 1H).
Characterization
The photocurrent–voltage (I–V) characteristics were recorded at
room temperature using a computer-controlled Keithley 2400
source meter under air mass (AM) 1.5 simulated illumination
(100 mW cmꢀ2, Oriel, 67005), and each data was averaged by
three parallel samples. The action spectra of monochromatic
incident photo-to-current conversion efficiency (IPCE) for solar
cells were performed using a commercial setup (PV-25 DYE,
JASCO). A 300 W xenon lamp was employed as light source for
generation of a monochromatic beam. Calibrations were per-
formed with a standard silicon photodiode. IPCE is dened by
IPCE(l) ¼ hcJsc/e4l, where h is Planck's constant, c is the speed
of light in a vacuum, e is the electronic charge, l is the wave-
length in meters (m), Jsc is the short-circuit photocurrent
density (A mꢀ2), and 4 is the incident radiation ux (W mꢀ2).
Synthesis of ICD-9
An oven-dried 100 mL one-necked ask was charged with
compound 4b (4.2 ꢄ 10ꢀ4 mol), cyanoacetic acid (8 ꢄ 10ꢀ3
mol), 10 mL of dry THF and 0.5 mL piperidine. The solution was
heated to reux for 15 h and the colour turned to deep red. Aer
cooling to room temperature, dichloromethane and a diluted
solution of hydrochloric acid were added and the organic phase
was washed with water (3 ꢄ 20 mL) and brine (3 ꢄ 20 mL). The
combined organic phase was dried on MgSO4, ltered and
evaporated under reduced pressure and the residue puried by
silica-gel column chromatography to give a dark-purple solid
(78%). 1H-NMR (DMSO, 400 MHz) d (ppm): 0.78–0.88 (m, 12H),
1.16–1.38 (m, 29H), 1.55–1.62 (m, 6H), 1.74 (s, 1H), 3.80 (s, 2H),
3.87 (d, J ¼ 6.8 Hz, 2H), 6.84 (d, J ¼ 8.4 Hz, 4H), 7.10 (d, J ¼ 8.4
Hz, 4H), 7.41 (d, J ¼ 8.4 Hz, 4H), 7.76 (d, J ¼ 4.4 Hz, 2H), 7.86–
7.99 (m, 6H), 8.80 (d, J ¼ 4 Hz, 1H); MS (MALDI-TOF): m/z found:
968.5 (M+).
Acknowledgements
This work is supported by the National Nature Science Foun-
dation (Grant no. 21174149, 51173190, 21073203, and
21121001), the National 863 Program (no. 2011AA050521), and
the 973 Program (2009CB930404, 2011CB932303, and
2011CB808400).
DSC fabrication
The nanocrystalline TiO2 pastes (particle size, 20 nm) were
prepared using a previously reported procedure.18 Fluorine
doped thin oxide (FTO, 4 mm thickness, 10 ohms per sq, Nip-
pon Sheet Glass, Japan) conducting electrodes were washed
with soap and water, followed by sonication for 10 min in
acetone and isopropanol, respectively. Following a drying
period, the electrodes were then submerged in a 40 mM
aqueous solution of TiCl4 for 30 min at 75 ꢂC, and then washed
by water and ethanol. On the electrodes, an 11 mm thick
nanocrystalline TiO2 layer and 6 mm thick TiO2 light scattering
layer (particle size, 400 nm, PST-400C) were prepared by screen-
printing method. The TiO2 electrodes were heated at 500 ꢂC for
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