Thieno[3,4-b]thiophene-based organic dyes for dye-sensitized solar cells

Article abstract of DOI:10.1002/chem.201200012

New dipolar sensitizers containing an ethyl thieno[3,4-b]thiophene-2- carboxylate (ETTC) entity in the conjugated spacer have been synthesized in two isomeric forms. These compounds were used as the sensitizers of n-type dye-sensitized solar cells (DSSCs). The best conversion efficiency (5.31 %) reaches approximately 70 % of the N719-based (7.41 %) DSSC fabricated and measured under similar conditions. The ETTC-containing compounds exhibit a bathochromic shift of the absorption compared to their thiophene congeners due to the quinoid effect, however, charge-trapping at the ester group of ETTC was found to jeopardize the electron injection and lower the cell efficiency. Charge trapping is alleviated as the ester group of ETTC is replaced with a hydrogen atom, as evidenced from the theoretical computation.

Full text of DOI:10.1002/chem.201200012

DOI: 10.1002/chem.201200012
ThienoACHTUNGTRENNUNG[3,4-b]thiophene-Based Organic Dyes for Dye-Sensitized Solar Cells
Yung-Chung Chen,[a] Hsien-Hsin Chou,[a] Ming Chih Tsai,[a, b] Sheng-Yu Chen,[a]
Jiann T. Lin,*[a] Ching-Fa Yao,*[b] and Kellen Chen[c]
Abstract: New dipolar sensitizers con-
taining an ethyl thieno[3,4-b]thio-
fabricated and measured under similar
conditions. The ETTC-containing com-
pounds exhibit a bathochromic shift of
the absorption compared to their thio-
phene congeners due to the quinoid
effect, however, charge-trapping at the
ester group of ETTC was found to
jeopardize the electron injection and
lower the cell efficiency. Charge trap-
ping is alleviated as the ester group of
ETTC is replaced with a hydrogen
atom, as evidenced from the theoreti-
cal computation.
AHCTUNGTRENNUNG
phene-2-carboxylate (ETTC) entity in
the conjugated spacer have been syn-
thesized in two isomeric forms. These
compounds were used as the sensitizers
of n-type dye-sensitized solar cells
(DSSCs). The best conversion efficien-
cy (5.31%) reaches approximately
70% of the N719-based (7.41%) DSSC
Keywords: dye-sensitized
solar
cells · electron transfer · metal-free
sensitizers · quantum chemistry ·
quinoids
Introduction
these two classes of dyes, metal-free dyes normally absorb
at shorter wavelength, which results in a lower cell efficien-
cy. Nonetheless, a promising cell efficiency of>10% was re-
ported recently by Wang and co-workers.[6]
To keep pace with ever increasing demand of energy supply
accompanying global economyꢀs growth, search for alterna-
tive energy sources has become one of the most important
issues for governments and scientists. Though nuclear power
plant plays an important role in mankind civilization, the
recent disaster in Japan arouses some doubts on its long-
term reliability. Among different choices of new energy
sources, solar energy is considered to be very attractive be-
cause of its sustainability and abundant supply. In spite of
good solar-to-electricity conversion efficiency, the high price
prohibits large scale application of silicon-based solar cells.
Therefore, more and more researchers turn their interests to
organic photovoltaic cells (OPVs)[1] and dye-sensitized solar
cells (DSSCs).[2] Considerable progress has been made on
dye-sensitized solar cells for the past two decades since the
seminal report by OꢀReagen and Grꢁtzel in 1991.[3] Efficien-
cies surpassing 11% have been achieved by ruthenium dyes-
Electron-transfer dynamics strongly affects the perfor-
mance of DSSCs,[7] and this dynamics certainly correlates
with the molecular structure of the sensitizer.[8] Therefore,
even though light harvesting of sensitizers is of paramount
importance, exploration of sensitizers for better correlation
of structure with cell performance should not be over-
looked. With this in mind, we set out to search for possible
candidates which have similar structures for fair comparison.
In recent years, we have developed some series of amine-
based metal-free dipolar sensitizers for n-type DSSCs in
which the sensitizers were adsorbed on the nanocrystalline
TiO2, a n-type semiconductor.[9] In one case where thiazole
entity was used in the conjugated spacer, we found that the
sensitizer with heteroatoms on the thiazole ring away from
the acceptor exhibited much better cell performance than its
isomer having heteroatoms closer to the acceptor.[9i] There-
and Zn-porphyrin dye-based[5] DSSCs. Compared with
[4]
fore,
the
ethyl
thienoACTHNGUTERNNU[G 3,4-b]thiophene-2-carboxylate
(ETTC) entity was chosen for our purpose based on the fol-
lowing reasons: 1) ETTC and congeners have been widely
used for construction of low-band gap polymers for OPV ap-
plications because of their favorable quinoid structure;[1c,10]
this characteristic may help to redshift the absorption spec-
tra. 2) ETTC can be inserted in the conjugated spacer where
the sulfur atom of the thiophene-2-carboxylate is either
closer to or away from the acceptor. Therefore, the two pos-
sible isomeric structures allow systematic studies for com-
parison. In this paper we will report the syntheses of two
series of isomeric sensitizers based on ETTC and their ap-
plications in n-type DSSCs. In addition, quantum chemistry
computation was carried out to elucidate the different be-
haviors of the two isomers.
[a] Y.-C. Chen, H.-H. Chou, M. C. Tsai, S.-Y. Chen, J. T. Lin
Institute of Chemistry, Academia Sinica
11529, Nankang, Taipei (Taiwan)
Fax : (+886)783-1237
E-mail: jtlin@chem.sinica.edu.tw
[b] M. C. Tsai, C.-F. Yao
Department of Chemistry
National Taiwan Normal University
117, Taipei (Taiwan)
E-mail: cheyaocfh@ntnu.edu.tw
[c] K. Chen
AU Optronics Corporation
30078, Hsinchu (Taiwan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200012.
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FULL PAPER
Results and Discussion
Knoevenagel condensation of MCT-na (or FTT-na) with cy-
anoacetic acid afforded the desired products.
Synthesis of the materials: The structures of new dyes are il-
lustrated in Figure 1. The compounds are classified into two
types of isomers, MCT-n and FTT-n (n=1–5). The syntheti-
Optical properties: The optical absorption spectra of the
dyes in THF are displayed in Figure 2 and the data are col-
lected in Table 1. The long
wavelength
absorption
at
>470 nm is from more delocal-
ized p–p* transition with
charge-transfer character, and
the bands at <400 nm are due
to localized p–p* transition.
The lmax (or E0ꢀ0 from the in-
tersection of the absorption and
emission spectra) value of the
charge transfer band for the
two series of compounds de-
creases in the order of MCT-
5> MCT-2> MCT-4> MCT-
1> MCT-3 and FTT-5> FTT-
2> FTT-4> FTT-1> FTT-3, re-
spectively. Replacement of
a phenyl unit by a thienyl unit
(5 vs. 1) results in considerable
red shift of the lmax value,
which is consistent with the
trend normally observed in
Figure 1. Structures of MCT, FTT dyes and referenced dyes (1P-PSS and L1).
nonlinear
optical
chromo-
cal protocol of the compounds is illustrated in Scheme 1.
ThienoACHTUNGTRENNUNG[3,4-b]thiophene-2-carboxylic acid ethyl ester (1) was
phores[13] and is attributed to a lower delocalization energy
of thiophene than benzene.[14] Similarly, 2 and 4 have
a larger lmax value than 3 despite of planarity of the fluorene
entity. Though furan has smaller resonance energy com-
pared to thiophene, compounds with a thiophene entity in
the spacer have a shorter lmax value than their furan conge-
ners, that is, 2 versus 4. This phenomenon is in conformity
with the trend observed in nonlinear optical chromophor-
es.[13b,d] Dye 3 absorbs at shorter wavelength than 1, indicat-
prepared according to the published procedures.[11] Formyla-
tion of 1 via Vilsmeyer–Haack reaction led to two isomers,
2a (35%) and 2b (18%). Compound 3a (or 3b) was ob-
tained from 2a (or 2b) via bromination with NBS. The two
compounds then underwent palladium catalyzed Stille cou-
pling[12] reaction with appropriate stannyl reagents to pro-
vide formyl compounds MCT-na (or FTT-na). Finally, the
Scheme 1. Synthesis of the dyes.
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5431

J. T. Lin, C.-F. Yao et al.
important roles in this behavior (see below). Another inter-
esting feature is that the charge transfer band of MCT com-
pounds has longer lmax and smaller molar extinction coeffi-
cient than that of FTT congeners. This may be attributed to
the better charge trapping effect of the ester moiety in MCT
compounds (see below), which significantly lowers the
LUMO energy level and reduce the transition probability
from the ground state to the first excited state. Emission
spectra of the compounds follow the same trend as the ab-
sorption spectra except for the order of 1 and 4. The longer
emission wavelength of 1 than 4 suggests that there is more
geometrical relaxation for the lowest emissive excited state
of the former.
Electrochemical properties: The electrochemical properties
of the dyes were investigated by using cyclic voltammetry
(CV) method, and relevant data are presented in Table 1.
Selected cyclic voltammograms are shown in Figure 3. A
Figure 2. Absorption spectra of a) MCT and b) FTT dyes in THF solu-
tions.
ing that an extra phenylene in 3 jeopardizes the electronic
communication between the donor and the acceptor.
Figure 3. Cyclic voltammograms of the dyes recorded in CH2Cl2 (scan
rate 100 mVsꢀ1).
It is very interesting that the ETTC entity is better than
the thiophene entity in redshifting the absorption band. For
example, the lmax values of MCT-1 (or FTT-1) and MCT-2
(or FTT-2) are larger than those of 1P-PSS[15] (lmax
=
quasi-reversible one-electron oxidation wave detected at
about 380–613 mV more positive than ferrocene/ferroceni-
um is attributed to the removal of electron from the aryl-
461 nm, e=27100mꢀ1 cmꢀ1) and L1[16] (lmax =444 nm, e=
27490mꢀ1 cmꢀ1), respectively. Likely, both quinoid structure
of ETTC and the electron-withdrawing ester moiety play
AHCTUNGTREGaNNUN mine. Similar to our previous observations, direct attach-
ment of electron excessive thio-
phene at the nitrogen atom of
arylamine significantly lowers
the oxidation potential of the
arylamine.[17] As expected, com-
pounds MCT-5 and FTT-5 have
the lowest oxidation potentials
among all. Compounds without
electron excessive thiophene or
furan ring in the spacer, MCT-
1, MCT-3, FTT-1, and FTT-3,
are oxidized at higher poten-
tials. A second quasi-reversible
one-electron oxidation wave at
a higher potential was discerni-
ble for MCT-2, MCT-4, FTT-2,
and FTT-4, indicating the pres-
ence of thiophene or furan
Table 1. Electrooptical parameters of the dyes.
Dye
labs (eꢃ10ꢀ4 mꢀ1 cmꢀ1
)
lem
Eox
HOMO/LUMO E0ꢀ0
[eV]
E0ꢀ0
*
[nm][a]
[nm][a] [mV][b]
[eV][c] [V][d]
MCT-1 328 (1.92), 524 (2.29)
MCT-2 302 (2.76), 546 (3.42)
MCT-3 313 (2.62), 496 (2.81)
MCT-4 302 (2.78), 553 (3.62)
644
661
612
624
545 (119)
444 (124), 681 (153) 5.24/3.31
549 (196)
446 (100), 680 (na)
434 (113)
613 (121)
448 (110), 684 (113) 5.25/3.29
523 (86)
452 (90), 704 (na)
380 (109)
5.35/3.30
2.05
1.93
2.09
1.96
1.82
2.12
1.96
2.18
2.00
1.88
ꢀ0.70
ꢀ0.69
ꢀ0.74
ꢀ0.71
ꢀ0.59
ꢀ0.71
ꢀ0.71
ꢀ0.86
ꢀ0.75
ꢀ0.70
5.35/3.26
5.25/3.29
5.23/3.41
5.41/3.29
MCT-5 329 (2.39), 382 (1.37), 570 (2.92) 694
FTT-1
FTT-2
FTT-3
FTT-4
FTT-5
302 (3.13), 509 (3.47)
312 (3.10), 536 (3.49)
310 (4.40), 477 (3.58)
310 (3.11), 523 (3.05)
398 (1.12), 548 (2.80)
629
623
593
618
664
5.32/3.14
5.25/3.25
5.18/3.30
[a] Recorded in THF solutions at 298 K. [b] Recorded in THF solutions. Eox =1/2
A
where Epa and Epc are peak anodic and cathodic potentials, respectively. Oxidation potential reported is adjust-
ed to the potential of ferrocene which was used as an internal reference. The values in parentheses are the
peak separation of cathodic and anodic waves. Scan rate: 100 mVsꢀ1. [c] The bandgap, E0ꢀ0, was derived from
the intersection of the absorption and emission spectra. [d] E0ꢀ0*: The excited state oxidation potential versus
NHE.
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Organic Dyes for Dye-Sensitized Solar Cells
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helped to stabilize the dications formed. There is no obvious
trend in the first oxidation potential between the isomeric
pair of MCT and FTT.
Photocurrent–voltage characteristics: The dye-sensitized
solar cells were fabricated by using these dyes as the sensi-
tizers for nanocrystalline anatase TiO2. Typical solar cells,
with an effective area of 0.25 cm2, were fabricated with an
electrolyte composed of 0.05m I2/0.5m LiI/0.5m 4-tert-butyl-
pyridine in acetonitrile solution. The device performance
statistics under AM 1.5 illumination are collected in Table 2.
Figure 4 and Figure 5 show the photocurrent–voltage (J–V)
curves and the incident photon-to-current conversion effi-
Table 2. DSSCs performance parameters of the dyes.
Sample
VOC
[V]
JSC
[mAcmꢀ2
FF
h
Dye loading
G
]
[%]
[10ꢀ7 molcmꢀ2
]
MCT-1
MCT-2
MCT-3
MCT-4
MCT-5
FTT-1
FTT-2
FTT-3
FTT-4
FTT-5
N719
0.56
0.52
0.58
0.50
0.44
0.61
0.54
0.64
0.53
0.51
0.72
8.80
4.86
8.14
3.64
2.92
12.6
9.21
12.2
7.52
6.64
16.1
0.69
0.69
0.70
0.68
0.52
0.66
0.66
0.68
0.66
0.65
0.64
3.40
1.72
3.30
1.23
0.66
5.03
3.26
5.31
2.65
2.17
7.41
2.73
3.68
5.29
4.63
3.09
3.72
4.14
1.78
3.12
2.57
1.46
Figure 5. IPCE plots for the DSSCs using a) MCT and b) FTT dyes.
ciencies (IPCE) of the cells. The conversion efficiencies
appear to be lower than that of N719-based cell (N719=
bis(tetrabutylammonium)-cis-di(thiocyanato)-N,N’-bis(4-
carbox-ylato-4’-carboxylic
acid-2,2’-bipyridine)ruthenium)
fabricated and measured under similar conditions. FTT com-
pounds exhibited better cell efficiencies than MCT conge-
ners. DSSCs based on FTT-1 and FTT-3 have the best per-
formance among all, with efficiencies reaching about 70%
of the standard cell.
To elucidate the factors affecting the performance of the
cells, the adsorbed dye densities of the sensitizers on TiO2
surface were measured (Table 2). Obviously dye density
alone is not the decisive factor for the cell performance. For
example, the dye density of MCT-3 is nearly three times
compared with FTT-3. However, the conversion efficiency
of DSSC based on MCT-3 is only 62% of the cell based on
FTT-3. Both open-circuit voltage (VOC) and short-circuit cur-
rent (JSC) of FTT compounds are higher than those of MCT
series. The higher JSC values of the FTT-based DSSCs are
most likely stemmed from more efficient electron injection
from the dye to the TiO2 surface (see below), besides partial
contribution from larger molar extinction coefficients of
FTT than MCT. Effective suppression of dark current nor-
mally leads to a higher VOC. The dark currents of DSSCs
fabricated were also checked (Figure 6). The cells of 1 and 3
in both series have lower dark currents and therefore higher
VOC values. In comparison, cells of 5 have the highest dark
Figure 4. Current density/voltage curves of DSSCs based on the a) MCT
and b) FTT dyes under AM 1.5 solar simulator of 100 mWcmꢀ2. Cell
area=0.25 cm2.
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J. T. Lin, C.-F. Yao et al.
ed. The electron transport resistance of 5, 4, and 2 in both
series of compounds is significantly larger than that of 3 and
1, and decreases in the order of MCT-5@ MCT-4> MCT-2
and FTT-5@ FTT-4> FTT-2, respectively. This is consistent
with the better cell performance of 1- and 3-based DSSCs.
Moreover, MCT-based cells have larger resistance than
FTT-based cells, which is in conformity with their lower effi-
ciencies.
Theoretical approach: Quantum chemistry computation was
conducted for the compounds in order to gain further in-
sight for the correlation between the structure and the phys-
ical property as well as the cell performance, and the results
for the theoretical approach are included in Table S1 in the
Supporting Information. Computations on compounds
MCT-6 (or FTT-6), and MCT-7 (or FTT-7), in which the
ester group in MCT-1 (or FTT-1) and MCT-2 (or FTT-2)
was replaced by a hydrogen atom (Figure 1), were also car-
ried out for comparison. The computed frontier orbitals of
the compounds and their corresponding energy states are in-
cluded in Figure S3 and Figure S4 in the Supporting Infor-
mation, respectively. In the optimized structure, no signifi-
cant deviation from planarity was found in the spacer be-
tween the ETTC and 2-cyanoacrylic acid or heteroaromatic
ring (Figure 7). The dihedral angles between ETTC and
phenyl or fluorenyl ring (ca. 31 to 358) are larger than those
between ETTC and thienyl or furyl ring (ca. 10 to 208). The
larger dihedral angle will jeopardize the charge transfer and
result in blue shift of the absorption band: MCT-1, MCT-3,
FTT-1 and FTT-3 have smaller lmax values among the series
of compounds (see above). The HOMO orbitals in these
compounds are mainly composed of the diarylamine with
contributions from the neighboring spacer extending to
ETTC entity and the 2-cyanoacrylic acid. In comparison, the
LUMO orbitals are mainly composed of 2-cyanoacrylic acid
moiety and ETTC extending to the spacer. The charge
transfer character of the S0!S1 transition is therefore evi-
dent. Figure 8 shows HOMO and LUMO orbitals of four
representative compounds, MCT-2, FTT-2, MCT-6 and FTT-
6 (see below).
The Mulliken charges shifts for the S1 and S2 states were
also calculated from the TDDFT results. Differences in the
Mulliken charges in the excited and ground states were cal-
culated and grouped into several segments (Figure S5 in the
Supporting Information), diphenylamino (Am), aromatic
ring (Ph, Flu6, T, or F), ETTC (Ttc2 or Tt’c2) and 2-cyanoa-
crylic acid (Ac), to estimate the extent of charge separation
upon excitation (Table S1 in the Supporting Information).
Figure S5 displays the changes in Mulliken charges of all
dyes for the S0!S1 and S0!S2 transitions. There is signifi-
cant charge transfer from the arylamine donor to the 2-cya-
noacrylic acid acceptor for S0!S1 transition, and the nega-
tive charge accumulated at 2-cyanoacrylic acid (Ac) ranges
from ꢀ0.19 to ꢀ0.31. There is even higher negative charge
accumulated at the ETTC entity (ꢀ0.34 to ꢀ0.53). Com-
pared to FTT compounds, MCT compounds were found to
have more negative charge accumulated at the ETTC unit.
Figure 6. Dark current of DSSCs based on a) MCT and b) FTT dyes.
currents and the lowest VOC among all. Somewhat higher
VOC found in the FTT-based DSSCs cannot be rationalized
by the dark current. We speculate the FTT dyes adsorbed
on TiO2 nanoparticles raised the Fermi level compared the
MCT dyes. The aforementioned arguments on dark current
were supported by the electrochemical impedance spectro-
scopic (EIS) studies carried out in the dark. The intermedi-
ate frequency semicircle in the electrochemical impedance
spectra (Figure S1 in the Supporting Information) represents
the charge transfer phenomena between the TiO2 surface
and the electrolyte, that is, dark current. Efficient suppres-
sion of the dark current leads to larger semicircle. The re-
sistance towards the dark current (the size of semicircle) de-
creases in the order of MCT-3> MCT-1> MCT-2> MCT-
4> MCT-5 and FTT-3> FTT-1> FTT-2> FTT-4> FTT-5,
respectively.
Electrochemical impedance upon illumination of
100 mWcmꢀ2 was also measured under open-circuit condi-
tion, and the Nyquist plots of DSSCs with different dyes are
shown in Figure S2 in the Supporting Information. The
radius of the intermediate frequency semicircle (10ꢀ2–
105 Hz) in the Nyquist plot will reflect electron-transport re-
sistance, and the smaller radius means the lower electron
transport resistance. Though overlapping of the first and the
second semicircles prevented us from unambiguous calcula-
tion of the electron transport resistance in most cases, the
order of the electron transport resistance could be estimat-
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Organic Dyes for Dye-Sensitized Solar Cells
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Moreover, the ester of ETTC in MCT has higher negative
charge than that in FTT. Though the extra thiophene ring of
ETTC will lead to better stabilization during formation of
the quinoid structure, the charge trapping effect of the ester
moiety may lower the cell efficiencies of MCT-based DSSCs
due to less efficient electron injection. Similar behavior was
found in sensitizers incorporating a cyanovinyl entity in the
spacer.[9f] Computations on compounds MCT-6 (or FTT-6),
and MCT-7 (or FTT-7) offer us an opportunity for closer in-
spection of the role of the ester group in these compounds.
Note that Ttc2 or Tt’c2 used for FTT and MCT compounds
have now been changed to Tt and Tt’, respectively. The fꢃ
Dq value (the product of the oscillator strength and the
Mulliken charge changes in the terminal Ac group) of the
Ac entity for the S0!S1 transition, an approximate measure
for the electronic coupling strength from the dye molecule
to the attached TiO2 nanoparticle, increases in both FTT
and MCT dyes: FTT-1: ꢀ0.18, FTT-6: ꢀ0.21; MCT-1: ꢀ0.13,
MCT-6: ꢀ0.21; FTT-2: ꢀ0.18, FTT-7: ꢀ0.25; MCT-2: ꢀ0.18,
MCT-7: ꢀ0.24. Therefore, we anticipate that there is more
facile electron injection and the efficiencies of DSSCs can
be further improved as the ester group of ETTC is replaced
by a hydrogen atom or an alkyl group. It is also interesting
to note that MCT and FTT compounds have comparable ab-
sorption wavelength and the oscillator strength as the ester
group of ETTC is replaced by a hydrogen atom, indicating
the impact of the ester group on the electronic properties of
the molecules.
Conclusion
In summary, we have synthesized two series of isomeric sen-
sitizers containing a ethyl thienoACTHNUGRTNEUNG[3,4-b]thiophene-2-carboxyl-
ate (ETTC) entity in the conjugated spacer, MCT (with the
sulfur atom of the thiophene-2-carboxylate in ETTC lying
away from the acceptor) and FTT (with the sulfur atom of
the thiophene-2-carboxylate in ETTC lying closer to the ac-
ceptor). We also explored their applications in n-type
DSSCs. The best conversion efficiency reaches 5.31%,
which is 71% of the N719-based DSSC fabricated and mea-
sured under similar conditions. The MCT isomer has longer
wavelength absorption with weaker absorption intensity
than the FTT isomer. Both MCT and FTT compounds ex-
hibit bathochromic shift of the charge transfer absorption
compared to their thiophene congeners due to energetically
favorable quinoid structure of ETTC than thiophene. How-
ever, charge-trapping of the ester moiety at the ETTC
entity hampers efficient electron injection, and there is
more serious charge trapping in MCT dyes. As the ester
group of ETTC is replaced with a hydrogen atom, the
charge trapping can be alleviated and better charge injection
is expected.
Figure 7. Schematic division and dihedral angles of molecules.
Figure 8. HOMO and LUMO orbitals of dyes MCT-2, FTT-2, MCT-6,
and FTT-6.
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J. T. Lin, C.-F. Yao et al.
A platinized FTO produced by thermopyrolysis of H2PtCl6 was used as
a counter electrode. The TiO2 thin film was dipped into the THF solution
containing 3ꢃ10ꢀ4 m dye sensitizers for at least 12 h. After rinsing with
THF, the photoanode adhered with a polyester tape of 30 mm in thickness
and with a square aperture of 0.36 cm2 was placed on top of the counter
electrode and tightly clipping them together to form a cell. Electrolyte
was then injected into the space and then sealing the cell with the Torr
Sealꢄ cement (Varian, MA, USA). The electrolyte was composed of 0.5m
lithium iodide (LiI), 0.05m iodine (I2), and 0.5m 4-tert-butylpyridine that
was dissolved in acetonitrile.
Experimental Section
General information: Unless otherwise specified, all the reactions were
performed under nitrogen atmosphere using standard Schlenk tech-
niques. All solvents used were purified by standard procedures, or
purged with nitrogen before use. All chromatographic separations were
carried out on silica gel (60m, 230–400 mesh). 1H NMR and 13C NMR
spectra were recorded on a Bruker 400 MHz spectrometer. Mass spectra
(FAB) were recorded on a VG70–250S mass spectrometer. Elementary
analyses were performed on a Perkin–Elmer 2400 CHN analyzer. Ab-
sorption spectra were recorded on a Cary 50 probe UV/Vis spectropho-
tometer. Fluorescence spectra were recorded on a Hitachi F-4500 Spec-
trophotometer. Cyclic voltammetry experiments were performed with
a CHI-621B electrochemical analyzer. All measurements were carried
out at room temperature with a conventional three electrode configura-
tion consisting of a platinum working electrode, an auxiliary electrodes
and a non-aqueous Ag/AgNO3 reference electrode. The photoelectro-
chemical characterizations on the solar cells were carried out using an
Oriel Class A solar simulator (Oriel 91195 A, Newport Corp.). Photocur-
rent–voltage characteristics of the DSSCs were recorded with a potentio-
stat/galvanostat (CHI650B, CH Instruments, Inc.) at a light intensity of
100 mWcmꢀ2 calibrated by an Oriel reference solar cell (Oriel 91150,
Newport Corp.). The monochromatic quantum efficiency was recorded
through a monochromator (Oriel 74100, Newport Corp.) at short circuit
condition. The intensity of each wavelength was in the range of 1 to
3 mWcmꢀ2. Electrochemical impedance spectra (EIS) were recorded for
Quantum chemistry calculations: The computation were performed with
Q-Chem 4.0 software.[19] Geometry optimization of the molecules were
performed using hybrid B3LYP functional and 6–31G* basis set. For
each molecule, a number of possible conformations were examined and
the one with the lowest energy was used. The same functional was also
applied for the calculation of excited states using time-dependent density
functional theory (TD-DFT). There exist a number of previous works
that employed TD-DFT to characterize excited states with charge-trans-
fer character.[20] In some cases underestimation of the excitation energies
was seen.[21] Therefore, in the present work, we use TD-DFT to visualize
the extent of transition moments as well as their charge-transfer charac-
ters, and avoid drawing conclusions from the excitation energy.
Acknowledgements
DSSC under illumination at open-circuit voltage (VOC
) or dark at
ꢀ0.42 V potential at room temperature. The frequencies explored ranged
from 10 mHz to 100 kHz. The TiO2 nanoparticles and N719 were pur-
chased from Solaronix S. A., Switzerland.
We acknowledge the support of the Academia Sinica (AC) and NSC
(Taiwan), AUO Optronics Corp. and the Instrumental Center of Institute
of Chemistry (AC).
Synthesis of ethyl 4-(4-(diphenylamino)phenyl)-6-formylthieno
b]thiophene-2-carboxylate (FTT-1a): In a two-necked flask was loaded
a mixture of ethyl-4-bromo-6-formylthieno[3,4-b]thiophene-2-carboxylate
(3b, 0.14 g, 0.44 mmol), N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)aniline (0.20 g, 0.53 mmol), Pd(PPh3)4 (30 mg), 2m
ACHTUNGTNER[NUNG 3,4-
AHCTUNGTRENNUNG
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AHCTUNGTRENNUNG
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FTT-1a in 89% yield. H NMR (400 MHz, CDCl3): d=9.85 (s, 1H), 7.99
(s, 1H), 7.58 (d, J=8.8 Hz, 2H), 7.32 (t, J=7.6 Hz, 4H), 7.17–7.09 (m,
8H), 4.38 (q, J=7.6 Hz, 2H), 1.39 ppm (t, J=7.2 Hz, 3H).
Synthesis (E)-2-cyano-3-(4-(4-(diphenylamino)phenyl)-2-
of
(ethoxycarbonyl)thienoACHTUNGTRENNUNG[3,4-b]thiophen-6-yl)acrylic acid (FTT-1): A mix-
ture of FTT-1a (0.19 g, 0.39 mmol), cyanoacetic acid (65 mg, 0.78 mmol),
acetonitrile (20 mL), and piperidine (5 drops) was placed into a 300 mL
flask and refluxed for overnight. After cooling, the organic precipitate
was collected by filtration, and washed with ether and water. The crude
product was purified by column chromatography using EtOH/CH2Cl2
(1:9 by vol) as the eluent. The product was further recrystallized with
ether/hexane to provide FTT-1 in 60% yield. 1H NMR (400 MHz,
[D8]THF): d=8.48 (s, 1H), 8.20 (s, 1H), 7.90 (d, J=8.4 Hz, 2H), 7.50 (t,
J=7.6 Hz, 4H), 7.35 (d, J=7.6 Hz, 4H), 7.13 (t, J=7.6 Hz, 2H), 7.07(d,
J=8.4 Hz, 2H), 4.40 (q, J=7.2 Hz, 2H), 1.39 ppm (t, J=7.2 Hz, 3H);
13C NMR (125 MHz, [D8]THF): d=164.2, 162.5, 152.8, 151.0, 147.8,
147.4, 142.6, 141.1, 140.5, 130.6, 129.9, 126.7, 126.0, 125.4, 124.6, 122.7,
120.7, 117.0, 97.8, 62.7, 14.7 ppm; MS (FAB): m/z 551.1 (M+); elemental
analysis calcd (%) for C31H22N2O4S2: C 67.62, H 4.03, N 5.09; found: C
67.78, H 4.01, N 4.95.
Other compounds are described in detail in the Supporting Information.
Assembly and characterization of DSSCs: The photoanode used was the
TiO2 thin film (12 mm of 20 nm particles as the absorbing layer and 6 mm
of 400 nm particles as the scattering layer) coated on FTO glass sub-
strate[18] with a dimension of 0.5ꢃ0.5 cm2, and the film thickness mea-
sured by a profilometer (Dektak3, Veeco/Sloan Instruments Inc., USA).
5436
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Chem. Eur. J. 2012, 18, 5430 – 5437

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Published online: March 15, 2012
Chem. Eur. J. 2012, 18, 5430 – 5437
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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