Dipolar compounds containing fluorene and a heteroaromatic ring as the conjugating bridge for high-performance dye-sensitized solar cells

Article abstract of DOI:10.1002/chem.200903151

A novel series of dipolar or-ganic dyes containing diarylamine as the electron donor, 2-cyanoacrylic acid as the electron acceptor, and fluorene and a heteroaromatic ring as the conju-gating bridge have been developed and characterized. These metal-free dyes exhibited very high molar extinction coefficients in the electronic absorption spectra and have been successfully fab-ricated as efficient nanocrystalline TiO₂ dye-sensitized solar cells (DSSCs). The solar-energy-to-electricity conversion efficiencies of DSSCs ranged from 4.92 to 6.88%, which reached 68-96% of a standard device of N719 fabricated and measured under the same conditions. With a TiO₂ film thickness of 6 μm, DSSCs based on these dyes had photo-currents surpassing that of the N719-based device. DFT computation results on these dyes also provide detailed structural information in connection with their high cell performance.

Full text of DOI:10.1002/chem.200903151

DOI: 10.1002/chem.200903151
Dipolar Compounds Containing Fluorene and a Heteroaromatic Ring as the
Conjugating Bridge for High-Performance Dye-Sensitized Solar Cells
Chih-Hsin Chen,[a] Ying-Chan Hsu,[a] Hsien-Hsin Chou,[a] K. R. Justin Thomas,[a]
Jiann T. Lin,*[a, b] and Chao-Ping Hsu[a]
Abstract: A novel series of dipolar or-
ganic dyes containing diarylamine as
the electron donor, 2-cyanoacrylic acid
as the electron acceptor, and fluorene
and a heteroaromatic ring as the conju-
gating bridge have been developed and
characterized. These metal-free dyes
exhibited very high molar extinction
coefficients in the electronic absorption
spectra and have been successfully fab-
ricated as efficient nanocrystalline TiO2
dye-sensitized solar cells (DSSCs). The
solar-energy-to-electricity conversion
efficiencies of DSSCs ranged from 4.92
to 6.88%, which reached 68–96% of a
standard device of N719 fabricated and
measured under the same conditions.
With a TiO2 film thickness of 6 mm,
DSSCs based on these dyes had photo-
currents surpassing that of the N719-
based device. DFT computation results
on these dyes also provide detailed
structural information in connection
with their high cell performance.
Keywords: donor–acceptor
sys-
tems · energy conversion · fluorenes ·
solar cells · sulfur heterocycles
Introduction
conversion efficiency (h) of up to ~5%.[3] In contrast, ruthe-
nium–polypyridyl complex-based DSSCs were reported to
have an even higher h value of ~11%.[4]
In recent years, the search for renewable energy has re-
ceived worldwide attention because of the fast depletion of
fossil fuel reserves and global warming as well as environ-
mental pollution problems accompanying fossil fuel con-
sumption. Solar-energy conversion devices that convert
photon energy into electricity are very attractive due to the
unlimited solar-energy supply. Inorganic semiconductor
solar cells, such as silicon solar cells, have been developed
over several decades and have important applications, such
as satellites and solar houses. However, silicon materials are
very expensive and low-cost alternatives, such as organic
semiconductors[1] and organic dye-sensitized solar cells
(DSSCs),[2] have attracted considerable interest from both
academic and industrial communities. To date, polymer-
based solar cells can produce a solar-energy-to-electricity
After Grꢀtzelꢁs seminal reports on a DSSC that utilized a
RuII-based N3 dye,[5] cis-[Ru
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
4,4’,4’’-tricarboxy-2,2’:6’,2’’-terpyridine), a few DSSCs with
similar RuII–bipyridine or terpyridine dyes[7] have been re-
ported with efficiencies higher than 10%, a value close to
an amorphous silicon-based photovoltaic cell. There are two
important factors contributing to the high performance of
the devices: 1) the presence of strong metal-to-ligand
charge-transfer (MLCT) absorption extending to >700 nm
allows efficient light-harvesting in the red to near IR region
and 2) efficient electron transfer from the commonly used
electrolyte, Iꢀ, to the ruthenium dye cation.[8] In recent
years, metal-free dipolar organic sensitizers have also been
actively investigated. Relative to ruthenium dyes, organic
compounds have several advantages: 1) they have larger
molar extinction coefficients, which is beneficial to light-har-
vesting, 2) their structural modification is more flexible, and
3) they are of lower cost. Considerable progress has been
made on metal-free dipolar organic sensitizers,[9] and recent-
ly Wang and co-workers reported an impressive high conver-
sion efficiency of 9.8% with excellent stability for a DSSC
based on a metal-free sensitizer.[10] DSSCs that utilize metal-
free organic sensitizers, such as coumarin-,[11] indoline-,[12]
cyanine-,[13] hemicyanine-,[14] merocyanine-,[15] perylene-,[16]
[a] Dr. C.-H. Chen, Dr. Y.-C. Hsu, Dr. H.-H. Chou, Dr. K. R. J. Thomas,
Prof. Dr. J. T. Lin, Prof. Dr. C.-P. Hsu
Institute of Chemistry, Academia Sinica
Taipei, 115 (Taiwan)
Fax : (+886)2-27831237
E-mail: jtlin@chem.sinica.edu.tw
[b] Prof. Dr. J. T. Lin
Department of Chemistry, National Central University
Chungli, 320 (Taiwan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903151.
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FULL PAPER
thiophene-,[11b,17] and triarylamine-based[18] compounds were
reported to have good conversion efficiencies.
In a previous study, we found that DSSCs based on (E)-2-
um hydrobromide to form the corresponding phosphonium
salt, 3) Wittig reaction of the phosphonium salt with thio-
phene-2-carbaldehyde or 2-formylthiazole to afford the in-
termediate with an internal vinyl unit as a mixture of E and
Z isomers, 4) pure E isomer was obtained by adding a cata-
lytic amount of iodine into the CH2Cl2 solution of the E/Z
mixture in the dark, and 5) palladium-catalyzed Buchwald–
cyano-3-(5-{9,9-diethyl-7-[naphthalen-1-ylACTHUNRTGNEUNG(phenyl)amino]-
9H-fluoren-2-yl}thiophen-2-yl)acrylic acid[17b] exhibited an
impressive conversion efficiency (>90% of the standard cell
from N3). Possibly the high efficiency of the DSSC stems
from the high molar extinction coefficient of the electronic
absorption band even though the absorption maximum ap-
pears at a much shorter wavelength than that of N3. It is
our speculation that the diarylamine donor with a fluorenyl
conjugation in the sensitizer plays an important role for the
high performance of DSSCs. In this paper, we report a
series of metal-free dipolar compounds containing fluorene
and heteroaromatic rings as the conjugating bridge. The ef-
fects of the donor and the conjugating bridge on the photo-
physical properties of the dyes and on the performance of
DSSCs that use these dyes as the sensitizers are discussed.
ꢀ
Hartwig C N bond-formation reactions between arylbro-
mides and diarylamines. All the intermediates were further
formylated then reacted with cyanoacetic acid in glacial
acetic acid by a Knoevenagel reaction to form the desired
products. The coupling constant (J ~15.7 Hz) between the
1
two vicinal vinyl protons in the H NMR spectra unambigu-
ously identifies the E configuration of the dyes. The dyes
are dark-brown and -purple powders, respectively, and are
readily soluble in common organic solvents, such as THF,
CH2Cl2, and CHCl3.
Results and Discussion
Synthesis of materials: Com-
pounds (1–8) synthesized in this
study are composed of a diaryl-
amine moiety as the electron
donor,
a conjugating bridge
consisting of a fluorenyl unit
and a heteroaromatic ring, and
2-cyano-acrylic acid as the elec-
tron acceptor and anchoring
group to the TiO2. The synthet-
ic protocols for 2–8 are illustrat-
ed in Schemes 1 and 2. In
Scheme 1, two key steps were
adopted to obtain the inter-
mediates that had no internal
vinyl unit between the fluorenyl
unit and the heteroaromatic
ring: 1) selective conversion of
one of the aromatic bromides
to an arylamine through
a
ꢀ
Buchwald–Hartwig C N bond-
formation reaction and 2) a
Suzuki coupling reaction of the
corresponding boron reagent
with aryl bromide. Whereas in
Scheme 2, the key steps to
obtain the intermediates with
an internal vinyl unit between
the fluorenyl unit and the heter-
oaromatic ring are: 1) reduction
of 7-bromo-9,9-dihexyl-9H-fluo-
rene-2-carbaldehyde to the cor-
responding alcohol by sodium
borohydride, 2) treating the al-
cohol with triphenylphosphoni-
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3185

J. T. Lin et al.
due to the charge-transfer tran-
sition mixed with more delocal-
ized p–p* transitions. The
charge-transfer character of the
longer wavelength absorption is
supported by the following ob-
servations: replacement of the
2-cyanoacrylic acid moiety in 7
by a formyl group and a hydro-
gen atom, respectively, leads to
a decreasing of lmax by 40 nm
(467 vs. 427 nm in THF) and
69 nm (467 vs. 398 nm in THF).
The energy of the band II is de-
pendent on the spacer used. For
example, the charge-transfer
and/or the p–p* transition de-
crease in energy when an ethyl-
ene unit is inserted between the
fluorene moiety and the hetero-
aromatic ring, that is, lmax(2)<
lmax(4) and lmax(6)<lmax(7).
Consistent with literature re-
ports,[19] the thiazole moiety
was found to be beneficial in
lowering the energy of the
charge-transfer transition, that
is, lmax(8)ꢁlmax(6)>lmax(2). It
is interesting to note that the
molar extinction coefficient of
the thiazole derivative is lower
than that of the thiophene con-
gener if the heteroatoms are
closer to the acceptor (2-cya-
noacrylic acid), that is, e(8)<
e(2). On the contrary, the thia-
zole compound has a higher
molar extinction coefficient if
the heteroatoms are away from
the acceptor, that is, e(2)<e(6).
Similar to some metal-free di-
polar sensitizers,[17c,g] a blueshift
of the charge-transfer band in a
more polar solvent, that is, neg-
ative solvatochromism, is de-
tected in these compounds. For
example, the lmax value decreas-
es by 20 nm on going from THF
to CH3CN. This may be attrib-
uted to the strong interaction of
polar solvent molecules with
the sensitizer, which weakens
Scheme 1. Synthetic route for 2, 3, 6, and 8: i) diphenylamine, [Pd
1,1’-bis(diphenylphosphino)ferrocene (dppf), tBuONa, toluene; ii) nBuLi, B
bromo-2,3-dihydrothieno[3,4-b-1,4]dioxine or 2-bromothiazole, [Pd(PPh3)4], Na2CO3 (2m, aq), toluene; iv) n-
BuLi, DMF or n-formylmorpholine; v) 2-(1,3-dioxolan-2-yl)-5-iodothiazole, [PdACTHNUTRGNE(UNG PPh3)4], Na2CO3 (2m, aq), tol-
uene, then acetic acid; vi) 2-cyanoacetic acid/NH4OAc/acetic acid.
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Scheme 2. Synthetic route for 4, 5, and 7: i) NaBH4, THF; ii) HPPh3Br, CHCl3; iii) a) thiophene-2-carboxalde-
hyde or 2-formylthiazole, tBuOK, THF; b) I2, CH2Cl2; iv) diphenylamine or N,N-bis(9,9-diethyl-9H-fluoren-2-
yl)amine, PdACHTUNGTRENNUNG(OAc)2, PACHTUNGTRENNUNG(tBu)3, tBuONa, toluene; v) nBuLi, DMF or n-formylmorpholine, THF; vi) 2-cyanoace-
tic acid, NH4OAc, acetic acid.
ꢀ
Photophysical properties: The absorption spectra of the
dyes in THF are shown in Figure 1 and the photophysical
data are collected in Table 1. The band ranging from 300–
400 nm (band I) can be attributed to the localized p–p*
transition, whereas the band at >400 nm (band II) is likely
the O H bond of the carboxylic acid and consequently de-
creases the electron-withdrawing nature of the COOH
group.[17g,18d] Similar behavior was reported in the rutheni-
um-containing black dye.[6]
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High-Performance Dye-Sensitized Solar Cells
FULL PAPER
(0.94 to 1.16 V vs NHE) are more positive than that of the
Iꢀ/I3ꢀ redox couple (0.4 V vs NHE).[17g,22]
Photovoltaic devices: Dye-sensitized solar cells with an ef-
fective area of 0.25 cm2 were fabricated by using the com-
pounds adsorbed on nanocrystalline anatase TiO2 as the sen-
sitizers and an acetonitrile solution containing I2 (0.05m)/LiI
(0.5m)/tert-butyl pyridine (0.5m) as the electrolyte. The
device performance statistics under AM 1.5 illumination are
collected in Table 2. The photocurrent-voltage (J-V) curves
and the incident photon-to-current conversion efficiencies
(IPCE) of the cells are plotted in Figures 3 and 4, respec-
tively. All the devices showed moderate to good power con-
version efficiencies ranging from 4.92 to 6.88%. These
values correspond to 68–96%
Figure 1. Absorption spectra of dyes recorded in THF.
of the standard cell of rutheni-
um dye N719 fabricated and
Table 1. Electrooptical parameters of the dyes.
[a]
[c]
[d]
Dye labs (eꢃ10ꢀ4 mꢀ1 cmꢀ1
)
[nm]
E1/2 (DEp)[b] [mV]
HOMO/LUMO [eV] E0ꢀ0 [eV] E0ꢀ0
*
[V]
measured under similar condi-
tions. The impressively high
conversion efficiencies of sever-
al devices can be attributed to
the very high molar extinction
coefficients of the dye (e>
35000mꢀ1 cmꢀ1 except 7 and 8),
1
2
3
4
5
6
7
8
421 (5.29), 333 (1.39)
427 (4.26), 348 (2.04), 308 (2.52) 419 (94)
460 (5.51), 343 (2.03), 309 (2.94) 390 (69)
462 (3.60), 358 (1.51), 302 (1.84) 375 (76), 888 (106) 5.18/2.93
461 (3.94), 374 (5.00) 242 (78), 841 (120) 5.04/2.85
448 (5.13), 348 (3.08), 310 (3.10) 456 (92)
467 (2.74), 355 (1.86), 312 (2.07) 407 (84)
452 (3.39), 352 (3.07), 313 (2.82) 435 (100)
435 (65)
5.24/2.66
5.22/2.84
5.19/2.87
2.58
2.38
2.32
2.25
2.19
2.33
2.21
2.32
ꢀ1.44
ꢀ1.26
ꢀ1.23
ꢀ1.17
ꢀ1.25
ꢀ1.17
ꢀ1.10
ꢀ1.18
5.26/2.93
5.21/3.00
5.24/2.92
which
well
exceed
20000mꢀ1 cmꢀ1, an uppermost
[a] Recorded in THF. [b] Recorded in CH2Cl2. scan rate=100 mVsꢀ1; electrolyte=(n-C4H9)4NPF6; DEp is the
separation between the anodic and cathodic peaks. Potentials are quoted with reference to the internal ferro-
cene standard (E1/2 =+212 mV vs Ag/AgNO3). [c] The bandgap, E0ꢀ0, was derived from the observed optical
edge. [d] E0ꢀ0*: the excited-state oxidation potential vs. NHE.
value observed for most ruthe-
Electrochemical properties: The electrochemical properties
of the dyes were studied by cyclic voltammetry (CV) and
differential pulse voltammetry (DPV). Representative cyclic
voltammograms are shown in Figure 2 and the relevant CV
data are collected in Table 1. All dyes exhibit a quasi-rever-
sible one-electron redox wave (242–456 mV vs Fc/Fc+) at-
tributed to the oxidation of the arylamine. The oxidation po-
tentials (Eox) of the compounds decrease in the order of 6>
8ꢁ1>2>7>3>4>5. The very low oxidation potential in 5
is consistent with our previous observation that the elec-
tron-excessive fluoren-2-yl moiety can significantly increase
the electron density at the attached nitrogen atom.[20] The
heteroaromatic ring in the spacer also affects the oxidation
potential of the arylamine, and the electron-deficient thia-
zole moiety results in a higher Eox value relative to the elec-
tron-excessive thiophene moiety, that is, Eox(6)>Eox(2)>
Eox(3). Moreover, the shorter distance between the amine
and the thiazole leads to a higher Eox value: 6>7. It is con-
ceivable that electron injection from the excited dye to TiO2
is energetically favored since the excited-state potential
(E0ꢀ0*) of the sensitizer, estimated from the difference be-
tween the first oxidation potential at the ground state and
the zero–zero excitation energy E0ꢀ0, is more negative
(ꢀ1.10 to ꢀ1.44 V vs NHE, see Table 1) than the conduc-
tion-band-edge energy level of the TiO2 electrode, ꢀ0.5 V vs
Figure 2. Cyclic voltammograms of 4 and 5 recorded in CH2Cl2 (scan
rate: 100 mVsꢀ1 (top). Differential pulse voltammograms in the oxidation
region (bottom).
NHE.[21] Dye regeneration should also be energetically fa-
vored in view that the first oxidation potentials of the dyes
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J. T. Lin et al.
Table 2. Performance parameters of DSSCs based on dyes 1–8 and
circuit currents (JSC) in 8-based DSSCs can be attributed to
the less efficient light harvesting of the sensitizers after
being adsorbed on the TiO2 surface. Though high-perfor-
mance DSSCs were reported for sensitizers containing the
bis(fluoren-2-yl)amino donor,[17f,18b,c] sensitizer 5 with such a
moiety was found to have a device efficiency inferior to 2,
which contains a diphenylamino donor. Relative to 2, both
JSC and VOC (open-circuit voltage) dropped in the 5-based
cell. The shorter recombination lifetime of the photoinjected
electrons with the oxidized dye (vide infra) and the smaller
adsorption density of 5 may be the main cause of the lower
photocurrent of the device. The higher-lying HOMO of 5
may also decrease the driving force of dye regeneration by
N719.[a]
Dye
Jsc [mAcmꢀ2
]
Voc [V]
FF
h [%]
1
2
3
4
5
6
7
8
16.06
15.82
14.32
15.67
13.65
14.55
13.66
10.40
16.08
0.67
0.68
0.68
0.65
0.64
0.71
0.67
0.66
0.72
0.64
0.63
0.64
0.64
0.67
0.67
0.64
0.72
0.63
6.76
6.78
6.23
6.46
5.78
6.88
5.83
4.92
7.19
N719
[a] Experiments were conducted by using TiO2 photoelectrodes with ap-
proximately 16 mm thickness and a 0.25 cm2 working area on the FTO
(15 W/sq.) substrates.
ꢀ
the electrolyte, Iꢀ/I3 . Accordingly, more competitive recom-
bination (vide infra) of the photoinjected electrons with oxi-
dized dye and oxidized mediator will lead to decreased VOC
and JSC. The lower VOC value of the 5-based DSSC is sup-
ported by its higher dark current (see Figure S1 in the Sup-
porting Information). DSSCs of 4 with a similarly high dark
current also had a relatively low VOC. DSSCs based on 6
have the highest conversion efficiency (6.88%) of all. This
outcome is attributed to four factors: 1) compound 6 has a
high molar extinction coefficient and long absorption wave-
length, 2) the cell has an impressively high VOC value, 3) the
cell has the longest recombination lifetime of the photoin-
jected electrons with the oxidized dye (vide infra), and
4) the cell has the lowest dark current. It is important to
note that the heteroaromatic ring in the conjugating bridge
is very critical for the high performance of DSSCs in this
study. For example, compound 1’ formed by stripping off the
thiophene ring from 1 has a device efficiency at least 10%
lower than that of 1.[23] A significantly larger molar extinc-
tion coefficient of 1 than 1’ (52900 vs. ~26000mꢀ1 cmꢀ1)
clearly results in better light-harvesting of the former even
though the two dyes have comparable absorption wave-
lengths. The importance of the fluorenyl unit for the high
performance of the cells is also realized from the following
observations: 1) 1 has a larger molar extinction coefficient
than its congener in which the fluorenyl entity is replaced
by a biphenyl entity, (E)-2-cyano-3-{5’-[4’’-(naphthylphenyla-
mino)biphenylene]thiophen-2’-yl}acrylic acid,[24] and the
former has device efficiency 20% higher than that of the
latter, 2) when the fluorenyl entity of 2 is replaced with a
phenyl entity, the resulting compound, 5-[4-(diphenylamino-
phenyl)]thiophene-2-cyanoacrylic acid, has a device efficien-
cy ~27% lower than that of 2.[25] A significantly larger
molar extinction coefficient was also noticed in the com-
pound with the fluorenyl entity, 1. Insertion of a vinyl entity
between the fluorene unit and the heteroaromatic ring leads
to a redshift of the absorption band at the expense of cell ef-
ficiency, that is, (4)<(2) and (7)<(6). The much shorter re-
combination lifetime of the photoinjected electrons with the
oxidized dye is likely an important factor leading to lower
efficiencies of DSSCs based on sensitizers with a vinyl
entity, 4, 5, and 7. However, more studies are needed to elu-
cidate the role of the vinyl entity. Increasing the number of
the thienylfluorene segments also results in the inferior effi-
Figure 3. J-V curves of DSSCs based on the dyes 1–8.
Figure 4. IPCE plots for the DSSCs by using dyes 1–8.
nium-based sensitizers.[5–7] Consequently, strong absorption
appears to compensate well for the much shorter absorption
wavelength of the metal-free dyes relative to the ruthenium-
based sensitizers.
The adsorbed densities of the dyes on the TiO2 surface
were checked (1: 2.7ꢃ10ꢀ7, 2: 2.5ꢃ10ꢀ7, 3: 2.8ꢃ10ꢀ7, 4: 3.0ꢃ
10ꢀ7, 5: 7.9ꢃ10ꢀ8, 6: 2.9ꢃ10ꢀ7, 7: 3.3ꢃ10ꢀ7, 8: 2.7ꢃ
10ꢀ7 molcmꢀ2) and found to differ within 25% except for 5.
The smaller density of 5 is likely due to the greater steric
congestion of the two fluorenyl moieties at the donor. Com-
paratively lower conversion efficiencies and smaller short-
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High-Performance Dye-Sensitized Solar Cells
FULL PAPER
ciency of the DSSC because of the deviation of the two suc-
cessive aromatic segments from coplanarity for the dyes
with a longer spacer.[17b]
The recombination lifetime of the photoinjected electron
(tR) with the oxidized dye was measured by a transient pho-
tovoltage at open circuit. The electrolyte used was 0.5m of
LiClO4 or LiI in CH3CN. I2 was omitted to avoid the possi-
bility of a dark current. The resulting tR values are listed in
Table 3. The tR values are comparable and range from 0.16
Table 3. Recombination lifetime of the photoinjected electrons.
Dye
1
2
3
4
5
6
7
8
N719
tR [ms] 9.43 6.37 4.63 3.85 1.49 10.30 4.86 6.07 8.83
Figure 5. IPCE of the DSSCs fabricated with 6 mm TiO2 films by using
dyes 1, 6, and N719.
to 0.20 ms in the presence of LiClO4. However, they are
slightly smaller than that of the N719-based device. In the
presence of LiI, regeneration of the sensitizer by the electro-
lyte retards the recombination and significantly increases tR
values, ranging from 1.49 to 10.30 ms relative to 8.83 ms for
the N719-based device. The longest electron lifetime found
in 6 is consistent with the highest VOC value for the 6-based
cell. The shortest lifetime in 5 is attributed to the slower dye
regeneration because of the higher HOMO level of the dye
(vide supra). In addition, when the vinyl group is inserted
into the spacer, both tR and VOC values decrease, that is,
tR(2)>tR(4) and VOC(2)>VOC(4). The same trend was also
found for compounds 6 and 7.
The use of thinner films of TiO2 is demanding for DSSCs
employing solvent-free ionic liquid electrolytes or solid-state
organic hole-transporting materials because of the limited
electron diffusion length.[26] However, optical absorption
will be sacrificed with the use of the thinner TiO2 film. It is
possible to settle this dilemma if the sensitizers possess suffi-
ciently high molar extinction coefficients. Sensitizers 1 and 6
possessing high molar extinction coefficients were, therefore,
tested for DSSCs with thinner sensitized-TiO2 films. The
performance parameters of the DSSCs with TiO2 films of
different thickness are listed in Table 4. With the thicknesses
of the TiO2 films at 6 mm, both the photocurrents and the
power conversion efficiencies of devices that utilize 1 and 6
were larger than that of the N719-based device. In the
region between 500 and 350 nm the IPCEs of 1 and 6 clearly
exceeded that of N719 (Figure 5). There should be the op-
portunity of panchromatic co-sensitization by using 1 or 6
with a sensitizer absorbing in the near-IR region.[27]
Theoretical approach: To gain insight into the relationship
between the structure and photophysical properties of the
dyes, some density functional calculations at the B3LYP/6-
31G* level were conducted. The computed frontier orbitals
of the compounds and their corresponding energy states are
included in Figure S2 (see the Supporting Information) and
Figure 6, respectively. In the optimized structure, no signifi-
cant deviation from planarity was found in the spacer be-
tween the arylamine and 2-cyanoacrylic acid (see Figure S3
in the Supporting Information). The dihedral angles be-
tween any two neighboring segments are ꢂ3.08 for com-
pounds 1, 2, 4, 5, and 7. In comparison, compounds with a
thiazolyl (abbreviated as Tha) or substituted thiophene (ab-
breviated as edoT) group have a larger dihedral angle be-
tween the fluorenyl segment (abbreviated as Flu) and the
heteroaromatic ring, ranging from 16.1 to 21.38 (compounds
3, 6, and 8). Further TD-DFT calculations of the dyes at the
same level were also performed (see the Experimental Sec-
tion), and the results are listed in Table S1 (see the Support-
ing Information). The HOMO and HOMOꢀ1 orbitals in
Table 4. Performance parameters of the DSSCs fabricated with different
TiO2 films.
Thickness
Dye
VOC [V]
JSC [mAcmꢀ2
]
h [%]
FF
1
6
0.69
0.72
0.76
13.53
11.85
11.48
6.15
5.89
5.79
0.66
0.70
0.66
6 mm[a]
N719
1
6
0.68
0.71
0.73
14.47
13.02
13.62
6.38
6.38
6.63
0.65
0.69
0.67
10 mm[a]
N719
[a] Without a 300 nm-sized light-scattering layer.
Figure 6. Computed energy states of the compounds.
Chem. Eur. J. 2010, 16, 3184 – 3193
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3189

J. T. Lin et al.
these compounds are mainly composed of the diarylamine
and fluorenyl fragments, with some contribution from the
vinyl entity and the heteroaromatic ring. In comparison, the
LUMO and LUMO+1 orbitals are mainly composed of the
heteroaromatic ring and a 2-cyanoacrylic acid moiety, with
some contribution from one of the phenyl rings in the fluo-
renyl fragment and the vinyl entity. Consistent with the elec-
trochemical data, compound 5 containing a more electron-
releasing bis(trifluorenyl)amino group was calculated to
have the highest HOMO energy level, ꢀ4.77 eV. For all
molecules, the S1 state consists of a HOMO!LUMO transi-
tion exclusively, whereas the S2 state consists mainly of a
HOMOꢀ1!LUMO transition (76–95%). Both transitions
are typical p–p* transitions with charge-transfer character.
Based on the calculated oscillator strength, it is likely that
the observed band with the lowest energy is a combination
of S0!S1 and S0!S2 transitions. The calculated excitation
energies of the vinyl-free dyes are in the order of 1-
leads to a different surface dipole of TiO2 and affects the
tendency of electron recombination.
Conclusion
In summary, we have synthesized a class of dipolar com-
pounds containing a fluorenyl segment and a heteroaromatic
ring as the conjugating bridge. These metal-free sensitizers
could be efficiently adsorbed on nanocrystalline anatase
TiO2 particles for the dye-sensitized solar cells application.
The very strong absorption of the dyes results in favorable
light harvesting and a high performance of the DSSCs has
been achieved. The maximum solar-energy-to-electricity
conversion efficiency under AM 1.5 irradiation is 6.88%,
reaching 96% of the standard cell of ruthenium dye N719
fabricated and measured under similar conditions. Density
functional studies provided further support on the high cell
performance of these dyes. DSSCs with a thinner film of
TiO2 had photocurrents surpassing that of the N719-based
device, which will pave the way for a future panchromatic
approach using co-sensitizers or an application in solid-state
DSSCs.
ACHTUNGTRENNUNG(2.24 eV)>2ACHTUNGTRENNUNG(2.16 eV)>6AHCTUNGERTG(NNUN 2.08 eV)>8ACTHNUGTREN(NUNG 1.98 eV), and the
trend is in parallel with the spectroscopic data. For the dyes
containing an internal vinyl entity, that is, 4, 5, and 7, their
corresponding energy gaps are smaller than those of the
vinyl-free analogues due to the elongated p-conjugation.
The Mulliken charges for the S1, S2, and S3 states were
also calculated from the TD-DFT results. Differences in the
Mulliken charges in the excited and ground states were cal-
culated and grouped into several segments, diarylamine
(Am, Am’, or Am’’), fluorene (Flu or Flu’), vinyl (v), hetero-
aromatic ring (T, edoT, Tha, or Tha’), and 2-cyanoacrylic
acid (Ac), to estimate the extent of charge separation upon
excitation (see Table S1 in the Supporting Information). Fig-
ure S4 (see the Supporting Information) displays the
changes in Mulliken charges of the dyes for the S0!S1 and
S0!S2 transitions. In general, there is significant charge
transfer from the arylamine donor to the 2-cyanoacrylic acid
acceptor for all S0!Sn (n=1–3) transitions, that is, signifi-
cant positive and negative charges are located at the donor
and acceptor, respectively. The negative charge accumulated
at 2-cyanoacrylic acid (Ac) was found to be larger in the
vinyl-free dyes 1, 2, 3, 6, and 8 (ꢀ0.37–0.44) than that in the
others (ꢀ0.32–0.34 for 4, 5, and 7). For the S0!S1 and S0!
S2 transitions of dyes 6, 7, and 8, the electron-withdrawing
nature of thiazolyl group (Tha) is evident from their greater
accumulation of Mulliken charges (ꢀ0.28–0.33 for S0!S1;
ꢀ0.13–0.15 for S0!S2) than the corresponding thienyl ana-
logues.
It is interesting to note that sensitizer 8, with heteroatoms
on the thiazole ring closer to the acceptor, has significantly
lower device efficiency than its isomer 6, containing heteroa-
toms away from the acceptor, after the dye concentration on
TiO2 is corrected. This outcome can be rationalized by two
factors: 1) there is better light-harvesting for 6 and 2) com-
pound 6 has a longer electron recombination lifetime. It is
known that the work function of the semiconductor can be
tuned by the interface dipole created by the organic mole-
cules anchored at the surface of the semiconductor.[28] We
speculate that the different orientation of the thiazolyl ring
Experimental Section
General information: Unless otherwise specified, all the reactions were
performed under a nitrogen atmosphere by using standard Schlenk tech-
niques. THF was distilled from sodium and benzophenone under a nitro-
gen atmosphere. DMF and CH2Cl2 were distilled from CaH2 under a ni-
trogen atmosphere. The synthesis of starting materials and intermediates
are described in the Supporting Information. The synthesis of 1 was de-
scribed in our previous study.[17b] All chromatographic separations were
carried out on silica gel (60M, 230–400 mesh). 1H and 13C NMR spectra
were recorded on a Bruker AMX400 or AV500 spectrometer. Mass spec-
tra (FAB) were recorded on a VG70–250S mass spectrometer. Elementa-
ry analyses were performed on a Perkin–Elmer 2400 CHN analyzer.
Cyclic voltammetry experiments were performed with a CH Instruments
electrochemical analyzer. All measurements were carried out at room
temperature with a conventional three electrode configuration consisting
of a platinum working electrode, a platinum wire auxiliary electrode, and
a nonaqueous Ag/AgNO3 reference electrode. The E1/2 values were deter-
mined as (Epa +Epc)/2, in which Epa and Epc are the anodic and cathodic
peak potentials, respectively. The potentials are quoted against the ferro-
cene internal standard. The solvent used in all experiments was THF, and
the supporting electrolyte was 0.1m tetrabutylammonium hexafluoro-
phosphate. Electronic absorption spectra were obtained on a Cary 50
Probe UV/Vis spectrophotometer.
Synthesis of 2a–7a: The syntheses of compounds 2a, 3a, and 6a and
compounds 4a, 5a, and 7a follow similar procedures. Therefore, only the
synthesis of 2a and 4a will be described in detail. The characterization
details for other compounds are provided in the Supporting Information.
9,9-Dihexyl-N,N-diphenyl-7-(thiophen-2-yl)-9H-fluoren-2-amine
Dry toluene (40 mL) was added to a flask containing a mixture of 7-(di-
phenylamino)-9,9-dihexyl-9H-fluoren-2-yl-boronic acid (2.40 g,
4.4 mmol), 2-bromothiophene (0.65 g, 4.0 mmol), Na2CO3 (2m in H2O,
8.0 mL, 16.0 mmol), and [Pd(PPh3)4] (0.19 g, 0.16 mmol). After the reac-
(2a):
AHCTUNGTRENNUNG
tion mixture had been refluxed for 24 h, the solvent was removed and
the residue was extracted with CH2Cl2/brine. The organic layer was dried
over anhydrous MgSO4, then filtered and dried. The crude product was
further purified by column chromatography by using CH2Cl2/hexanes
(1:2) as the eluent to give 2a as a pale-yellow solid (2.1 g, 68%).
3190
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ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 3184 – 3193

High-Performance Dye-Sensitized Solar Cells
FULL PAPER
1H NMR (CDCl3): d=7.61–7.58 (m, 2H), 7.52–7.44 (m, 3H), 7.25–7.21
(m, 5H), 7.10–7.06 (m, 5H), 7.00–6.93 (m, 4H), 1.88–1.83 (m, 4H), 1.24–
1.09 (m, 12H), 0.75 (t, J=7.1 Hz, 6H), 0.70–0.60 ppm (m, 4H); FAB-MS:
m/z: 583.4 [M+].
by using CH2Cl2/hexanes (1:1) as the eluent to give 8b as a pale-yellow
solid (1.35 g, 55%). H NMR (CDCl3): d=9.95 (s, 1H), 8.29 (s, 1H), 7.63
(d, J=8.0 Hz, 1H), 7.59 (dd, J=8.0, 1.6 Hz, 1H), 7.55 (d, J=8.0 Hz, 1H),
7.50 (d, J=1.2 Hz, 1H), 7.26–7.22 (m, 4H), 7.12–7.08 (m, 5H), 7.03–6.99
(m, 3H), 1.90–1.83 (m, 4H), 1.12–1.04 (m, 12H), 0.77 (t, J=7.2 Hz, 6H),
0.70–0.63 ppm (m, 4H); FAB-MS: m/z: 612.2 [M+].
1
(E)-9,9-Dihexyl-N,N-diphenyl-7-[2-(thiophen-2-yl)vinyl]-9H-fluoren-2-
amine (4a): A mixture of diphenylamine (0.19 g, 1.1 mmol), 2-[2-(7-
bromo-9,9-dihexyl-9H-fluoren-2-yl)vinyl]thiophene (0.52 g, 1.0 mmol),
Synthesis of compounds 2–8: Compounds 2–8 were synthesized by similar
procedures. Only the synthesis of 2 will be described in detail. The char-
acterization details for other compounds are provided in the Supporting
Information.
sodium tert-butoxide (0.14 g, 1.5 mmol), PdACTHNUTRGENUG(N OAc)2 (4.5 mg, 0.02 mmol),
tri(tert-butyl)phosphine (5.5 mg, 0.02 mmol), and dry toluene (20 mL)
was refluxed under nitrogen for 16 h. After cooling, the reaction mixture
was quenched with water and the mixture was extracted with CH2Cl2.
The combined organic layer was washed with brine and dried over anhy-
drous MgSO4. After filtration and removal of the solvent, the crude prod-
uct was further purified by column chromatography by using CH2Cl2/
hexane (1:1) as the eluent to yield 4a as a pale-yellow oil (0.43 g, 70%).
1H NMR (CDCl3): d=7.70 (d, J=8.4 Hz, 2H), 7.66 (s, 1H), 7.53 (dd, J=
8.0, 1.6 Hz, 1H), 7.48 (d, J=16.0 Hz, 1H), 7.35 (d, J=5.2 Hz, 1H), 7.31–
7.26 (m, 4H), 7.18–7.16 (m, 2H), 7.08–7.01 (m, 9H), 1.90–1.80 (m, 4H),
1.12–1.02 (m, 12H), 0.78 (t, J=7.2 Hz, 6H), 0.70–0.64 ppm (m, 4H);
FAB-MS: m/z: 609.9 [M+].
(E)-2-Cyano-3-{5-[7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl]thio-
phen-2-yl} acrylic acid (2): Acetic acid (10 mL) was added to a mixture
of 5-[7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl]thiophene-2-carbal-
dehyde (0.61 g, 1.0 mmol), cyanoacetic acid (0.085 g, 1.0 mmol), and am-
monium acetate (0.019 g, 0.25 mmol). The mixture was heated at 1208C
for 16 h and then allowed to cool to room temperature. Water was added
and the resulting solid was filtered and washed with water to afford the
crude product. It was further purified by column chromatography on
silica gel by using CH2Cl2 then CH2Cl2/acetic acid (5:1) as the eluents to
yield 2 as a dark-red solid (0.45 g, 66%). 1H NMR (CDCl3): d=8.34 (s,
1H), 7.89 (d, J=4.0 Hz, 1H), 7.67 (d, J=8.0 Hz, 1H), 7.61 (d, J=8.0 Hz,
1H), 7.54–7.57 (m, 2H), 7.48 (d, J=4.0 Hz, 1H), 7.25 (dd, J=8.0, 8.0 Hz,
4H), 7.08–7.12 (m, 5H), 7.00–7.04 (m, 3H), 1.84–1.95 (m, 4H), 1.03–1.14
(m, 12H), 0.77 (t, J=7.1 Hz, 6H), 0.60–0.68 ppm (m, 4H); 13C NMR
(CDCl3): d=167.2, 157.3, 152.7, 151.8, 148.0, 147.8, 143.2, 140.4, 134.1,
130.4, 129.2, 125.8, 124.2, 123.2, 122.9, 121.8, 120.9, 120.5, 119.7, 118.7,
115.9, 95.8, 55.3, 40.1, 31.5, 29.5, 23.8, 22.5, 14.0 ppm; FAB-MS m/z: 678.3
[M+]; HRMS: m/z: calcd for C45H46N2O2S: 678.3280; found: 678.3283
[M+]; elemental analysis calcd (%) for C45H46N2O2S: C 79.61, H 6.83, N
4.13; found: C 79.75, H 7.15, N 4.05.
Synthesis of 2b–8b: The syntheses of compounds 2b–5b and compounds
6b and 7b, follow similar procedures. Therefore, only the synthesis of 2b,
6b, and 8b will be described in detail. The characterization details for
the other compounds are provided in the Supporting Information.
5-[7-(Diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl]thiophene-2-carbalde-
hyde (2b): nBuLi (1.6m in hexane, 1.38 mL, 2.2 mmol) was added drop-
wise at ꢀ788C to a solution of 2a (1.17 g, 2.0 mmol) in anhydrous THF
(10 mL). After the solution had been stirred at ꢀ788C for 1 h, dry DMF
(1 mL) was added slowly. After the addition, the solution was stirred at
ꢀ788C for 30 min, then warmed to room temperature and stirred over-
night. The reaction was quenched by the addition of dilute HCl aqueous
solution and the mixture was extracted with CH2Cl2. The combined or-
ganic layer was washed with brine and dried over anhydrous MgSO4.
After filtration and removal of the solvent, the crude product was further
purified by column chromatography by using CH2Cl2/hexanes (1:1) as the
Assembly and characterization of DSSCs: The TiO2 electrode with a
0.25 cm2 geometric area was immersed in an acetonitrile/tert-butanol mix-
ture (1:1) containing bis(tetrabutyl-ammonium)-cis-di(thiocyanato)-N,N’-
bis(4-carboxylato-4’-carboxylic acid-2,2’-bipyridine)ruthenium(II) (3ꢃ
10ꢀ4 m, N719, Solaronix S.A., Switzerland) or in a THF solution contain-
ing organic sensitizers (3ꢃ10ꢀ4 m) for at least 12 h. A platinized FTO was
used as a counter electrode and was controlled to give an active area of
0.36 cm2 by adhered polyester tape with a thickness of 60 mm. After rins-
ing with CH3CN or THF, the photoanode was placed on top of the coun-
ter electrode and they were tightly clipped together to form a cell. Elec-
trolyte was then injected into the space and the cell was sealed with Torr
Seal cement (Varian, MA, USA). The electrolyte was composed of lithi-
um iodide (LiI, 0.5m), iodine (I2, 0.05m), and 4-tert-butylpyridine (TBP,
0.5m) dissolved in acetonitrile. A 0.6ꢃ0.6 cm2 cardboard mask was clip-
ped onto the device to constrain the illumination area. The photoelectro-
chemical characterizations on the solar cells were carried out by using a
modified light source, a 300 W Xe lamp (Oriel 6258) equipped with a
water-based IR filter, and AM 1.5 filter (Oriel 81088). Photocurrent-volt-
age characteristics of the DSSCs were recorded with a potentiostat/galva-
1
eluent to yield 2b as a yellow solid (0.89 g, 73%). H NMR (CDCl3): d=
9.87 (s, 1H), 7.73 (d, J=4.0 Hz, 1H), 7.64–7.61 (m, 2H), 7.55–7.53 (m,
2H), 7.42 (d, J=4.0 Hz, 1H), 7.25–7.21 (m, 4H), 7.11–7.08 (m, 5H),
7.02–6.98 (m, 3H), 1.88–1.84 (m, 4H), 1.23–1.09 (m, 12H), 0.75 (t, J=
7.1 Hz, 6H), 0.70–0.60 ppm (m, 4H); FAB-MS: m/z: 611.3 [M+].
2-[7-(Diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl]thiazole-5-carbalde-
hyde (6b): nBuLi (1.6m in hexane, 1.38 mL, 2.2 mmol) was added drop-
wise at ꢀ788C to a solution of 6a (1.17 g, 2.0 mmol) in anhydrous THF
(10 mL). The solution was stirred at ꢀ788C for 1 h, then a solution of N-
formylmorpholine (0.30 mL, 3.0 mmol) in anhydrous THF (10 mL) was
added slowly. After the addition, the solution was stirred at ꢀ788C for
30 min, then warmed to room temperature and stirred overnight. Saturat-
ed NH4Cl solution was added and the solution was stirred at room tem-
perature for 15 min. The organic phase was separated and the aqueous
layer was extracted with CH2Cl2. The combined organic layer was
washed with brine and dried over anhydrous MgSO4. After filtration and
removal of the solvent, the crude product was further purified by column
chromatography by using CH2Cl2/hexanes (3:2) as the eluent to yield 6b
as a yellow solid (0.86 g, 70%).
nostat (CHI650B, CH Instruments, USA) at
a light intensity of
100 mWcmꢀ2 measured by a thermopile probe (Oriel 71964). The light
intensity was further calibrated by an Oriel reference solar cell (Oriel
91150) and adjusted to be 1.0 sun. The monochromatic quantum efficien-
cy was recorded through a monochromator (Oriel 74100) at short circuit
condition. The intensity of each wavelength was in the range of 1 to
5-[7-(Diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl]thiazole-2-carbalde-
hyde (8b): Dry toluene (40 mL) was added to a flask containing a mix-
ture of 7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl boronic acid
(2.40 g, 4.4 mmol), 2-(1,3-dioxolan-2-yl)-5-iodothiazole (1.13 g, 4.0 mmol),
3 mWcmꢀ2
.
The photovoltage transients of assembled devices were recorded with a
digital oscilloscope (LeCroy, WaveSurfer 24Xs). Pulsed laser excitation
was applied by a Q-switched Nd:YAG laser (Continuum, model Minilite
II) with a 1 Hz repetition rate at 532 nm and a 5 ns pulse width at half
height. The beam size was slightly larger than 0.5ꢃ0.5 cm2 to cover the
area of the device with an incident energy of 1 mJcmꢀ2. The recombina-
tion lifetime of photoinjected electrons with oxidized dyes was measured
by transient photovoltages at open circuit in the presence of LiClO4 or
LiI electrolyte (0.5m). The average electron lifetime can be estimated ap-
proximately by fitting a decay of the open circuit voltage transient with
Na2CO3 (2m in H2O, 8.0 mL, 16.0 mmol), and [PdACHTNUGTRNEGNU(PPh3)4] (0.19 g,
0.16 mmol). After the reaction mixture had been refluxed for 24 h, the
solvent was removed and the residue was extracted with CH2Cl2/brine.
The organic layer was dried over anhydrous MgSO4, then filtered and
pumped dry. Glacial acetic acid (25 mL) was added to the crude product
and the mixture was heated to 508C. After the solution turned clear,
water (4 mL) was added and the solution was stirred at 508C for another
3 h. The solution was cooled and iced water (50 mL) was added. The pre-
cipitate formed was filtered, washed with water and methanol, and then
dried. The crude product was further purified by column chromatography
exp
G
recombination.
Chem. Eur. J. 2010, 16, 3184 – 3193
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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J. T. Lin et al.
Computational methodology: Geometrical optimization of the molecules
at the density functional level was carried out with Q-Chem[30] software
by using the B3LYP exchange-correlation functional and the 6-31G*
basis set. The gas-phase time-dependent DFT (TD-DFT) calculations at
the same level were performed on the basis of the geometry in the
ground state. TD-DFT calculations were previously employed to charac-
terize excited states with charge-transfer character.[31] In some cases, un-
derestimation of the excitation energies was observed.[31,32] Therefore, in
the present work, we characterize the electronic configuration and avoid
drawing conclusions from the excitation energy.
[9] A. Mishra, M. K. R. Fischer, P. Bꢀuerle, Angew. Chem. 2009, 121,
2510–2536; Angew. Chem. Int. Ed. 2009, 48, 2474–2499.
[10] G. Zhang, H. Bala, Y. Cheng, D. Shi, X. Lv, Q. Yu, P. Wang, Chem.
Commun. 2009, 2198–2200.
[11] a) K. Hara, K. Sayama, Y. Ohga, A. Shinpo, S. Suga, H. Arakawa,
Chem. Commun. 2001, 569–570; b) K. Hara, M. Kurashige, Y. Dan-
oh, C. Kasada, A. Shinpo, S. Suga, K. Sayama, H. Arakawa, New J.
Chem. 2003, 27, 783–785.
[12] a) T. Horiuchi, H. Miura, S. Uchida, Chem. Commun. 2003, 3036–
3037; b) T. Horiuchi, H. Miura, S. Uchida, J. Photochem. Photobiol.
A 2004, 164, 29–32; c) T. Horiuchi, H. Miura, K. Sumioka, S.
Uchida, J. Am. Chem. Soc. 2004, 126, 12218–12219; d) S. Ito, S. M.
Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet, P. Comte,
M. K. Nazeeruddin, P. Pꢄchy, M. Takata, H. Miura, S. Uchida, M.
Grꢀtzel, Adv. Mater. 2006, 18, 1202–1205.
[13] A. Ehret, L. Stuhl, M. T. Spitler, J. Phys. Chem. B 2001, 105, 9960–
9965.
Acknowledgements
[14] a) Z.-S. Wang, F.-Y. Li, C.-H. Huang, Chem. Commun. 2000, 2063–
2064; b) Q.-H. Yao, F.-S. Meng, F.-Y. Li, H. Tian, C.-H. Huang, J.
Mater. Chem. 2003, 13, 1048–1053.
We thank the National Science Council and Academia Sinica for finan-
cial support.
[15] a) A. C. Khazraji, S. Hotchandani, S. Das, P. V. Kamat, J. Phys.
Chem. B 1997, 101, 4693–4700; b) K. Sayama, K. Hara, N. Mori, M.
Satsuki, S. Suga, S. Tsukagoshi, Y. Abe, H. Sughara, H. Arakawa,
Chem. Commun. 2000, 1173–1174; c) K. Sayama, S. Tsukagoshi, T.
Mori, K. Hara, Y. Ohga, A. Shinpou, Y. Abe, S. Suga, H. Arakawa,
Sol. Energy Mater. Sol. Cells 2003, 80, 47–71.
[16] a) S. Ferrere, A. Zaban, B. A. Gregg, J. Phys. Chem. B 1997, 101,
4490–4493; b) S. Ferrere, B. A. Gregg, New J. Chem. 2002, 26,
1155–1160.
[17] a) M. Velusamy, K. R. J. Thomas, J. T. Lin, Y.-C. Hsu, K.-C. Ho,
Org. Lett. 2005, 7, 1899–1902; b) K. R. J. Thomas, J. T. Lin, Y.-C.
Hsu, K.-C. Ho, Chem. Commun. 2005, 4098–4100; c) D. P. Hagberg,
T. Edvinsson, T. Marinado, G. Boschloo, A. Hagfeldt, L. Sun, Chem.
Commun. 2006, 2245–2247; d) S.-L. Li, K.-J. Jiang, K.-F. Shao, L.-
M. Yang, Chem. Commun. 2006, 2792–2794; e) N. Koumura, Z.-S.
Wang, S. Mori, M. Miyashita, E. Suzuki, K. Hara, J. Am. Chem. Soc.
2006, 128, 14256–14257; f) S. Kim, J. K. Lee, S. O. Kang, J. Ko, J.-H.
Yum, S. Fantacci, F. De Angelis, D. Di Censo, M. K. Nazeeruddin,
M. Grꢀtzel, J. Am. Chem. Soc. 2006, 128, 16701–16707; g) K. R. J.
Thomas, Y.-C. Hsu, J. T. Lin, K.-M. Lee, K.-C. Ho, C.-H. Lai, Y.-M.
Cheng, P.-T. Chou, Chem. Mater. 2008, 20, 1830–1840.
[18] a) M. Liang, W. Wu, F. Cai, P. Chen, B. Peng, J. Chen, Z. Li, J. Phys.
Chem. C 2007, 111, 4465–4472; b) H. Choi, J. K. Lee, K. Song, S. O.
Kang, J. Ko, Tetrahedron 2007, 63, 3115–3121; c) I. Jung, J. K. Lee,
K. H. Song, K. Song, S. O. Kang, J. Ko, J. Org. Chem. 2007, 72,
3652–3658; d) S.-T. Huang, Y.-C. Hsu, Y.-S. Yen, H.-H. Chou, J. T.
Lin, C.-W. Chang, C.-P. Hsu, C. Tsai, D.-J. Yin, J. Phys. Chem. C
2008, 112, 19739–19747; e) Z. Ning, H. Tian, Chem. Commun. 2009,
5483–5495; f) Z. Ning, Q. Zhang, H. Pei, J. Luan, C. Lu, Y. Cui, H.
Tian, J. Phys. Chem. C 2009, 113, 10307–10313.
[19] a) Y.-K. Wang, C.-F. Shu, E. M. Breitung, R. J. McMahon, J. Mater.
Chem. 1999, 9, 1449–1452; b) P. R. Varanasi, A. K.-Y. Jen, J. Chan-
drasekhar, I. N. N. Namboothiri, A. Rathna, J. Am. Chem. Soc.
1996, 118, 12443–12448.
[20] K. R. J. Thomas, M. Velusamy, J. T. Lin, C.-H. Chuen, Y.-T. Tao,
Chem. Mater. 2005, 17, 1860–1866.
[21] K. Hara, T. Sato, R. Katoh, A. Furube, Y. Ohga, A. Shinpo, S. Suga,
K. Sayama, H. Sugihara, H. Arakawa, J. Phys. Chem. B 2003, 107,
597–606.
[22] A. Hagfeldt, M. Grꢀtzel, Chem. Rev. 1995, 95, 49–68.
[23] A. Baheti, P. Tyagi, K. R. J. Thomas, Y.-C. Hsu, J. T. Lin, J. Phys.
Chem. C 2009, 113, 8541–8547.
[24] Y. J. Chang, T. J. Chow, Tetrahedron 2009, 65, 4726–4734.
[25] a) D. P. Hagberg, T. Marinado, K. M. Karlsson, K. Nonomura, P.
Qin, G. Boschloo, T. Brinck, A. Hagfeldt, L. Sun, J. Org. Chem.
2007, 72, 9550–9556; b) W.-H. Liu, I.-C. Wu, C.-H. Lai, C.-H. Lai,
P.-T. Chou. Y.-T. Li, C.-L. Chen, Y.-Y. Hsu, Y. Chi, Chem. Commun.
2009, 5152–5154.
[1] a) C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, Adv. Funct. Mater.
2001, 11, 15–26; b) B. Maennig, J. Drechsel, D. Gebeyehu, P. Simon,
F. Kozlowski, A. Werner, F. Li, S. Grundmann, S. Sonntag, M. Koch,
K. Leo, M. Pfeiffer, H. Hoppe, D. Meissner, N. S. Sariciftci, I.
Riedel, V. Dyakonov, J. Parisi, Appl. Phys. A 2004, 79, 1–14; c) C. J.
Brabec, Sol. Energy Mater. Sol. Cells 2004, 83, 273–292; d) H.
Hoppe, N. S. Sariciftci, J. Mater. Res. 2004, 19, 1924–1945.
[2] a) A. Hagfeldt, M. Grꢀtzel, Acc. Chem. Res. 2000, 33, 269–277;
b) M. Grꢀtzel, J. Photochem. Photobiol. C 2003, 4, 145–153; c) M.
Grꢀtzel, Inorg. Chem. 2005, 44, 6841–6851; d) N. Robertson,
Angew. Chem. 2006, 118, 2398–2405; Angew. Chem. Int. Ed. 2006,
45, 2338–2345.
[3] a) W. Ma, C. Yang, X. Gong, K. Lee, A. J. Heeger, Adv. Funct.
Mater. 2005, 15, 1617–1622; b) J. Hou, H.-Y. Chen, S. Zhang, G. Li,
Y. Yang, J. Am. Chem. Soc. 2008, 130, 16144–16145.
[4] a) M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E.
Mueller, P. Liska, N. Vlachopoulos, M. Grꢀtzel, J. Am. Chem. Soc.
1993, 115, 6382–6390; b) M. Grꢀtzel, J. Photochem. Photobiol. A
2004, 164, 3–14; c) M. K. Nazeeruddin, F. DeAngelis, S. Fantacci, A.
Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru, M. Grꢀtzel, J. Am.
Chem. Soc. 2005, 127, 16835–16847; d) Y. Chiba, A. Islam, Y. Wata-
nabe, R. Komiya, N. Koide, L. Han, Jpn. J. Appl. Phys. Part 2 2006,
45, L638–L640; e) F. Gao, Y. Wang, J. Zhang, D. Shi, M. Wang, R.
Humphrey-Baker, P. Wang, S. M. Zakeeruddin, M. Grꢀtzel, Chem.
Commun. 2008, 2635–2637.
[5] B. OꢁReagen, M. Grꢀtzel, Nature 1991, 353, 737–740.
[6] M. K. Nazeeruddin, P. Pꢄchy, T. Renouard, S. M. Zakeeruddin, R.
Humphrey-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklov-
er, L. Spiccia, G. B. Deacon, C. A. Bignozzi, M. Grꢀtzel, J. Am.
Chem. Soc. 2001, 123, 1613–1624.
[7] a) P. Wang, S. M. Zakeeruddin, J. E. Moser, R. Humphry-Baker, P.
Comte, V. Aranyos, A. Hagfeldt, M. K. Nazeeruddin, M. Grꢀtzel,
Adv. Mater. 2004, 16, 1806–1811; b) M. K. Nazeeruddin, T. Bessho,
Le Cevey, S. Ito, C. Klein, F. De Angelis, S. Fantacci, P. Comte, P.
Liska, H. Imai, M. Grꢀtzel, J. Photochem. Photobiol. A 2007, 185,
331–337; c) F. Gao, Y. Wang, J. Zhang, D. Shi, M. Wang, R. Hum-
phrey-Baker, P. Wang, S. M. Zakeeruddin, M. Grꢀtzel, Chem.
Commun. 2008, 2635–2637; d) T. Bessho, E. Yoneda, J.-H. Yum, M.
Guglielmi, I. Tavernmelli, H. Imai, U. Rothlisberger, M. K. Nazeer-
uddin, M. Grꢀtzel, J. Am. Chem. Soc. 2009, 131, 5930–5934; e) J.-H.
Yum, I. Jung, C. Baik, J. Ko, M. K. Nazeeruddin, M. Grꢀtzel,
Energy Environ. Sci. 2009, 2, 100–102; f) Y. Cao, Y. Bai, Q. Yu, Y.
Cheng, S. Liu, D. Shi, F. Gao, P. Wang, J. Phys. Chem. C 2009, 113,
6290–6297; g) B.-S. Chen, K. Chen, Y.-H. Hong, W.-H. Liu, T.-H.
Li, C.-H. Lai, P.-T. Chou, Y. Chi, G.-H. Lee, Chem. Commun. 2009,
5844–5846.
[8] Y. Tachibana, S. A. Haque, I. P. Mercer, J. R. Durant, D. R. Klug, J.
Phys. Chem. B 2000, 104, 1198–1205.
3192
www.chemeurj.org
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 3184 – 3193

High-Performance Dye-Sensitized Solar Cells
FULL PAPER
[26] a) L. Schmidt-Mende, M. Grꢀtzel, Thin Solid Films 2006, 500, 296–
301; b) H. J. Snaith, A. J. Moule, C. Klein, K. Meerholz, R. H.
Friend, M. Grꢀtzel, Nano Lett. 2007, 7, 3372–3376; c) H. J. Snaith,
C. S. Karthikeyan, A. Petrozza, J. Teuscher, J. E. Moser, Md. K. Na-
zeeruddin, M. Thelakkat, M. Grꢀtzel, J. Phys. Chem. C 2008, 112,
7562–7566.
[27] a) J.-J. Cid, J.-H. Yum, S.-R. Jang, Md. K. Nazeeruddin, E. Martꢅnez-
Ferrero, E. Palomares, J. Ko, M. Grꢀtzel, T. Torres, Angew. Chem.
2007, 119, 8510–8514; Angew. Chem. Int. Ed. 2007, 46, 8358–8362;
b) H. Choi, S. Kim, S. O. Kang, J. Ko, M.-S. Kang, J. N. Clifford, A.
Forneli, E. Palomares, Md. K. Nazeeruddin, M. Grꢀtzel, Angew.
Chem. 2008, 120, 8383–8387; Angew. Chem. Int. Ed. 2008, 47, 8259–
8263.
[28] a) H. Ishii, K. Sugiyama, E. Ito, K. Seki, Adv. Mater. 1999, 11, 605–
625; b) S. Braun, W. R. Salaneck, M. Fahlman, Adv. Mater. 2009, 21,
1450–1472.
[29] S. Rꢆhle, M. Greenshtein, S.-G. Chen, A. Merson, H. Pizem, C. S.
Sukenik, D. Cahen, A. Zaban, J. Phys. Chem. B 2005, 109, 18907–
18913.
2006, 3279–3289; c) Y. Shao, L. F. Molnar, Y. Jung, J. Kussmann, C.
Ochsenfeld, S. T. Brown, A. T. B. Gilbert, L. V. Slipchenko, S. V.
Levchenko, D. P. OꢁNeill, R. A. Jr DiStasio, R. C. Lochan, T. Wang,
G. J. O. Beran, N. A. Besley, J. M. Herbert, C. Y. Lin, T. V. Voorhis,
S. H. Chien, A. Sodt, R. P. Steele, V. A. Rassolov, P. E. Maslen, P. P.
Korambath, R. D. Adamson, B. Austin, J. Baker, E. F. C. Byrd, H.
Dachsel, R. J. Doerksen, A. Dreuw, B. D. Dunietz, A. D. Dutoi,
T. R. Furlani, S. R. Gwaltney, A. Heyden, S. Hirata, C.-P. Hsu, G.
Kedziora, R. Z. Khalliulin, P. Klunzinger, A. M. Lee, M. S. Lee,
W. Z. Liang, I. Lotan, N. Nair, B. Peters, E. I. Proynov, P. A. Pienia-
zek, Y. M. Rhee, J. Ritchie, E. Rosta, C. D. Sherrill, A. C. Simmo-
nett, J. E. Subotnik, H. L. III Woodcock, W. Zhang, A. T. Bell, A. K.
Chakraborty, Phys. Chem. Chem. Phys. 2006, 8, 3172–3191.
[31] a) H. M. Vaswani, C.-P. Hsu, M. Head-Gordon, G. R. Fleming, J.
Phys. Chem. B 2003, 107, 7940–7946; b) Y. Kurashige, T. Nakajima,
S. Kurashige, K. Hirao, Y. Nishikitani, J. Phys. Chem. A 2007, 111,
5544–5548.
[32] A. Dreuw, M. Head-Gordon, J. Am. Chem. Soc. 2004, 126, 4007–
4016.
[30] a) N. Hirata, J.-J. Lagref, E. J. Palomares, J. R. Durrant, M. K. Na-
zeeruddin, M. Grꢀtzel, D. Di Censo, Chem. Eur. J. 2004, 10, 595–
602; b) J. R. Durrant, S. A. Haque, E. Palomares, Chem. Commun.
Received: November 17, 2009
Published online: January 29, 2010
Chem. Eur. J. 2010, 16, 3184 – 3193
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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