Benzotriazole-containing D-π-A conjugated organic dyes for dye-sensitized solar cells

Article abstract of DOI:10.1002/asia.201201173

A series of metal-free benzotriazole-based dipolar dyes have been developed as sensitizers for dye-sensitized solar cells (DSSCs). Different heteroaromatic rings such as furan, thiophene, and selenophene, were used in combination with benzotriazole as the conjugated spacer group. Light harvesting, charge recombination, and electron injection of the cells fabricated are affected by the heteroaromatic ring used in the spacer. The DSSC with the thiophene-containing dye has the highest conversion efficiency of 6.20 %, which reaches 85 % of the standard cell based on N719. Benzotriazole-based dyes with different heteroaromatic rings in the conjugated spacer have been synthesized and used to fabricate dye-sensitized solar cells. Heteroaromatic rings with heavier heteroatoms led to greater dark current (Se>S>O). The best conversion efficiency reaches 85 % of the N719-based standard cell under standard global AM 1.5 G illumination. Copyright

Full text of DOI:10.1002/asia.201201173

FULL PAPER
DOI: 10.1002/asia.201201173
Benzotriazole-Containing D–p–A Conjugated Organic Dyes for Dye-
Sensitized Solar Cells
Yung-Sheng Yen,[a, b] Chang-Tze Lee,[c] Chih-Yu Hsu,[a] Hsien-Hsin Chou,[a]
Yung-Chung Chen,[a] and Jiann T. Lin*[a, c]
Abstract: A series of metal-free benzo-
triazole-based dipolar dyes have been
developed as sensitizers for dye-sensi-
tized solar cells (DSSCs). Different
heteroaromatic rings such as furan,
thiophene, and selenophene, were used
in combination with benzotriazole as
the conjugated spacer group. Light har-
vesting, charge recombination, and
electron injection of the cells fabricated
are affected by the heteroaromatic ring
used in the spacer. The DSSC with the
thiophene-containing dye has the high-
est conversion efficiency of 6.20%,
which reaches 85% of the standard cell
based on N719.
Keywords: benzotriazole
· charge
recombination · heterocycles · solar
cells · sensitizers
Introduction
dyes is advantageous in solid-state DSSCs because a thinner
TiO2 film can be used to facilitate electron transport. This
characteristic also allows metal-free dyes to complement the
deficit in the absorption of ruthenium dyes for using in co-
sensitized DSSCs with improved performance.[6]
Organic sensitizers normally have a D–p–A dipolar archi-
tecture, which has a p-electron-rich moiety as the donor
(D), a p-electron-deficient moiety as the acceptor (A), and
a p-conjugated spacer as the bridge between the donor and
the acceptor. Previously, we developed metal-free sensitizers
with an electron-deficient benzothiadiazole (BT) unit in the
conjugated spacer,[7] and found that the BT unit effectively
shifted the electronic absorption band to a longer wave-
length. The best photon-to-electricity conversion efficiency
of the DSSCs only reached approximately 70% of the stan-
Dye-sensitized solar cells (DSSCs) have attracted considera-
ble attention owing to their high incident solar-light-to-elec-
tricity conversion efficiency and low cost of production since
the innovative report by Grꢀtzel and O’Reagen in 1991.[1]
Currently, DSSCs sensitized by polypyridyl RuII complexes
have achieved remarkably high conversion efficiencies that
exceed 11% under air mass (AM 1.5G) illumination.[2]
More recently, porphyrin-based DSSCs have made a break-
through and achieved an efficiency of 12.3%, which even
surpasses that of ruthenium-based dyes for DSSCs.[3] Al-
though the ruthenium complexes showed high efficiency and
long-term stability, they are quite expensive and hard to
purify. Besides, the metal-to-ligand charge-transfer (MLCT)
band in ruthenium dyes seldom exceeds 25000mꢀ1 cmꢀ1. In
comparison, porphyrin compounds normally have only weak
or negligible absorption in the wavelength region between
the Soret and Q bands. Considerable research efforts have
also been focused on the development of new metal-free or-
ganic dye sensitizers[4] and a record high efficiency of 10.3%
has been achieved.[5] The intense absorption of metal-free
dard cell based on cis-[RuACHTUNRTGENNUG(NCS)2ACHTUNTGNER(NUNG dcbpy)2] (dcbpy=2,2’-bi-
pyridyl-4,4’-dicarboxylate) owing to partial charge trapping
of the BT unit. Although we[8a] and other research
groups[8b–f] found that significantly improved efficiencies
could be achieved with BT-based sensitizers after appropri-
ate modification of the conjugated spacer, partial charge
trapping still remained.
Benzotriazole (BTA), in which the sulfur atom of the ben-
zothiadiazole is replaced by NR (R=alkyl group), is consid-
ered as a moderate electron-deficient heterocycle[9] and has
been widely used as the acceptor of semiconducting poly-
mers for bulk heterojunction solar cells.[10] However, unlike
BT, the BTA unit has a higher lowest unoccupied molecular
orbital (LUMO) energy level than the benzothiadiazole unit
because of the higher basicity of the nitrogen atom than the
sulfur atom.[10b] With our continuing interest in metal-free
sensitizers for DSSC applications, we therefore decided to
incorporate the BTA moiety in the conjugated spacer of the
dye because the charge trapping effect found in the BT con-
gener might be alleviated. Though a slight blue shift in the
absorption spectra is expected for the BTA compound com-
[a] Dr. Y.-S. Yen, Dr. C.-Y. Hsu, Dr. H.-H. Chou, Dr. Y.-C. Chen,
Prof. Dr. J. T. Lin
Institute of Chemistry
Academia Sinica
Taipei, 115 (Taiwan)
E-mail: jtlin@gate.sinica.edu.tw
[b] Dr. Y.-S. Yen
Assistant Research Scholar of the National
Science Council of ROC
[c] C.-T. Lee, 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/asia.201201173.
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Jiann T. Lin et al.
Scheme 1. Synthesis of dyes 1–5. DMF=N,N-dimethylformamide.
pared to the BT congener, incorporation of an alkyl chain at
the 2-position of the BTA unit might not only improve the
solubility of the dye, but also help to suppress the dark cur-
rent by blocking the electrolytes from close contact with the
TiO2 surface. Herein, we report a new series of dyes with
a benzotriazole entity in the conjugated spacer. DSSCs
using these dyes as the sensitizers will be also discussed.
Two papers were published while this research was still on-
going.[11]
Results and Discussion
Synthesis of the Materials
Figure 1. The absorption spectra of the dyes in THF.
The benzotriazole-based organic dyes (1–5) were synthe-
sized as illustrated in Scheme 1. A Stille coupling reaction[12]
between 4,7-dibromo-2-hexylbenzotriazole and appropriate
stannyl compounds provided substrates 2a–e, which under-
went subsequent formylation by Vilsmeier reaction to afford
3a–e. Intermediates 5a–e were obtained from a Stille cou-
pling reaction of 3a–e with tributylstannyl derivatives. Final-
ly, a classical Knoevenagel reaction between 2-cyanoacetic
acid and 5a–e in acetic acid led to the desired products.
These new dyes were fully char-
a very intense absorption band around 454–478 nm (molar
extinction coefficient (e)=4.10–4.88ꢂ104 mꢀ1 cmꢀ1) attributa-
ble to the p–p* transition with charge-transfer character
(from the arylamine donor to the 2-cyanoacrylic acid accept-
or). The maximum absorption wavelength of 1–3 (1:
460 nm; 2: 462 nm; 3, 478 nm) and the molar extinction co-
efficient (1: 4.10ꢂ104 mꢀ1 cmꢀ1
; ; 3:
2: 4.64ꢂ104 mꢀ1 cmꢀ1
acterized
with
1H NMR,
Table 1. Electrooptical parameters of the dyes.
Dye labs (eꢂ10ꢀ4 [mꢀ1 cmꢀ1])[a] [nm] lem[a] [nm] E1/2 (ox)[b] [mV] Eox[c] [V] E0ꢀ0[c] [eV] E0ꢀ0
13C NMR spectroscopy, and
HRMS.
*
[d] [V]
1
2
3
4
5
460 (4.10), 362 (2.83), 306 (2.01) 565
390 (101)
459 (107)
423 (165)
513 (112)
461 (109)
1.09
1.16
1.12
1.21
1.16
2.42
2.39
2.32
2.43
2.36
ꢀ1.23
ꢀ1.13
ꢀ1.10
ꢀ1.12
ꢀ1.10
462 (4.64), 304 (1.97)
478 (4.85), 306 (2.12)
454 (4.88), 306 (2.62)
462 (4.35), 302 (1.87)
583
593
580
584
Optical Properties
The absorption spectra of the
dyes in a solution of THF are
displayed in Figure 1, and the
corresponding data are present-
[a] The absorption spectra were recorded in THF. [b] The cyclic voltammetric studies were conducted in
CH2Cl2. Scan rate: 100 mVsecꢀ1, Electrolyte: (n-C4H9)4NPF6; DEp is the separation between the anodic and
cathodic peaks in units of mV. Potentials are quoted with reference to the internal ferrocene standard (E1/2
+265 mV vs Ag/AgNO3). [c] E0ꢀ0 was determined from intersection of absorption and emission spectra in
=
ed in Table 1. All the dyes have THF. [d] E0ꢀ0*: The excited state oxidation potential vs NHE.
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4.85ꢂ104 mꢀ1 cmꢀ1) increased in the order of 1<2<3. These
observations are consistent with the electronegative effect
reported by Wang et al. on metal-free dyes and Ru com-
plexes that contain similar five-membered heteroaromatic
units,[13] that is, more electronegative atoms (O>S>Se) in
the five-membered heteroaromatic units will result in a blue
shift of the absorption band and a decreased molar extinc-
tion coefficient. The band at approximately 300 nm is attrib-
uted to the localized p–p* transitions. An extra band at
362 nm for 1 could also be due to localized p–p* transitions.
The aggregation of the dye can be ruled out because the ab-
sorption profile is unaffected at lower dye concentrations.
The absorption band of 4 exhibited a hypsochromic shift
compared with that of 2, most likely the hexyl group teth-
ered at the thiophene ring of 4 rendered the conjugated
spacer less planar (see below) and deteriorated the electron-
ic communication between the donor and the acceptor. The
absorption wavelength of compound 2 is shorter (ca. 71 nm)
than its congener[8d] in which the benzotriazole entity is re-
placed with a benzothiadiazole entity, thereby indicating
that the more electron-deficient benzothiadiazole unit can
more effectively red shift the absorption. However, a higher
tendency of charge trapping and dye aggregation in benzo-
thiadiazole dyes could jeopardize the charge transfer and
consequently electron injection from the excited dye mole-
cule into the TiO2 conduction band. Considering that incor-
poration of two electron-withdrawing fluorine groups at the
5- and 6-positions of the 2-(2-butyloctyl)-2H-benzo[d]-
pounds 1 and 4 have less blue shifts in the absorption, this
might be because there is higher degree of aggregation upon
dye adsorption. A somewhat higher degree of J-aggregation
compared with other benzotriazole dyes[11] might stem from
the more planar structure of the dyes in this study. The ag-
gregation was alleviated after addition of chenodeoxycholic
acids (CDCA) as the co-adsorbent (Figure S1, see the Sup-
porting Information). All the compounds are weakly emis-
sive (Figure S2, see the Supporting Information), and the
large Stokes shift (>4000 cmꢀ1) is consistent with the
charge-transfer nature of the emission (see below).
Electrochemical Properties
The highest occupied molecule orbital (HOMO) and
LUMO of the dyes should match the potential of the con-
duction band edge of TiO2 and the electrolyte redox poten-
tial, respectively, for facile electron injection and dye regen-
eration. Therefore, the electrochemical properties of the
dyes were investigated. The electrochemical data of the
compounds are listed in Table 1, and the representative
cyclic voltammograms are shown in Figure 3. The first
ACHTUNGTRENNUNG[1,2,3]triazole unit in a conjugated polymer results in a pro-
found effect in the hole mobility of the polymer and thus
the photovoltaic performance,[10b] we also synthesized 5 for
comparison. The absorption spectra of 2 and 5 are nearly su-
perimposable, thus indicating that fluorine atoms have a neg-
ligible electronic effect on the 2-(2-butyloctyl)-2H-benzo[d]-
ACHTUNGTRENNUNG
TiO2 film are shown in Figure 2. A blue shift in the absorp-
tion spectra relative to that in the solution might be caused
by deprotonation of the carboxylic acid group.[14] The spec-
tra are also broadened at the long-wavelength side; this in-
dicates possible existence of J-aggregation of the dyes. Com-
Figure 3. Cyclic voltammograms of 1–5 in deoxygenated CH2Cl2 contain-
ing 0.1m TBAPF6 at 258C. Ferrocene (Fc) was added as an internal stan-
dard. All potentials are in volts vs Ag/AgNO3 (0.01m in MeCN; the scan
rate is 100 mVsꢀ1).
quasi-reversible one-electron redox wave can be attributed
to the oxidation of the arylamine. Another quasi-reversible
redox wave at a higher potential can be ascribed to the oxi-
dation of the conjugated spacer. The excited state potential
(E0ꢀ0*) of the sensitizer was estimated from the first oxida-
tion potential (Eox) at the ground state and the zero–zero
excitation energy (E0ꢀ0) estimated from the intersection of
the absorption and the emission spectra. The E0ꢀ0* values
(ꢀ1.10 to ꢀ1.23 V vs normal hydrogen electrode (NHE),
see Table 1) deduced are more negative than the conduc-
tion-band-edge energy level of the TiO2 electrode (ꢀ0.5 V
vs NHE)[15] and the first oxidation potentials of the dyes
ꢀ
Figure 2. The absoprtion spectra of the dyes on TiO2.
(1.09 to 1.21 V vs NHE) are more positive than the Iꢀ/I3
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redox couple (ca. 0.4 V vs NHE).[16] These results assure fa-
vorable electron injection and regeneration of the dye after
electron injection.
broadened IPCE spectra compared to the absorption spec-
tra of the dyes in the solution (Figure 1). Depending on the
dyes, J-aggregation may improve the cell performance due
to better light harvesting,[17] or jeopardize the cell perfor-
mance due to quenching of the excited dye molecules.[18] In
our case, the IPCE spectra have significant contribution
from absorption at labs >550 nm. The DSSC based on 3 has
IPCE spectra covering the same wavelength range as that of
N719, which is consistent with the longest absorption wave-
length of 3. Because of the intense absorption of the dyes,
the DSSCs of 2 and 3 have IPCE values surpassing those of
N719 at wavelengths below 460 nm, and the DSSC of 4 has
IPCE values comparable with those of N719 at wavelengths
below 450 nm. However, the IPCE values of the DSSCs are
significantly lower than those of N719 at longer wavelength.
Consequently, the best conversion efficiency of the DSSCs
in this study only reaches approximately 85% of the N719-
based standard cell. The device efficiencies are in the order
of 2>5>4>3>1. The efficiency of the device based on 2
(6.20%) is significantly higher than that based on the benzo-
thiadiazole congener (h=3.81%), which is only 42% of the
N719-based standard cell (h=8.97%).[8d]
Photovoltaic Devices
DSSCs were fabricated using these dyes as the sensitizers,
with an effective area of 0.25 cm2, nanocrystalline anatase
TiO2 particles, and the electrolyte composed of 0.05m I2/
0.5m LiI/0.5m tert-butylpyridine in an acetonitrile solution.
The performance parameters of the DSSCs fabricated using
these dyes as the sensitizers under AM 1.5G illumination
are listed in Table 2. The photocurrent–voltage (J–V) curves
Table 2. Performance parameters of DSSCs constructed using the dyes.
Dye
JSC [mAcmꢀ2
]
VOC [V]
FF
h [%]
1
2
3
4
10.30
14.60
14.10
13.10
13.20
15.10
0.68
0.65
0.60
0.69
0.68
0.74
0.62
0.66
0.67
0.65
0.67
0.66
4.37
6.20
5.66
5.89
6.01
7.33
5
N719
Light absorption efficiency obviously plays an important
role in cell performance. The DSSC based on 1 exhibited
the lowest short-circuit current (JSC), which is in conformity
with its weaker charge-transfer absorption compared with
the other dyes. Compared to the DSSC that is based on 2,
the significantly lower JSC value of the DSSC based on 4
might be attributed to the shorter absorption and the lower
adsorbed dye density of 4 than that of 2. The adsorbed dye
densities of the sensitizers on the TiO2 surface were 3.69ꢂ
10ꢀ7, 3.29ꢂ10ꢀ7, 4.19ꢂ10ꢀ7, 1.83ꢂ10ꢀ7, and 3.72ꢂ10ꢀ7
mol cmꢀ2 for 1, 2, 3, 4, and 5, respectively. The lower dye
density value of 4 than 2 might be due to the increased
steric congestion imparted by 4 than 2 owing to the presence
of two extra functional groups and less planar structure of 4.
DSSCs of 2 and 3 have larger JSC values than the others;
this could be ascribed to their intense charge-transfer bands.
Compared with 2, 5 has a lower JSC value; this is likely
owing to less-efficient light harvesting. In contrast, the
open-circuit voltage (VOC) value is higher in the cell of 5
owing to more effective suppression of the dark current (see
below). The precise role of the fluorine substituents needs
further investigation. The DSSC of 2 has a higher VOC than
that of its congener (0.54 V)[8d] in which the benzotriazole
entity is replaced with a benzothiadiazole entity. Similar to
the observation of Wang et al,[19c] such a change could be
relevant to the greater affinity of the more polarizable ben-
and the incident photon-to-current conversion efficiencies
(IPCEs) of the cells are shown in Figure 4 and 5, respective-
ly. J-aggregation of the dyes (see above) is evident from the
Figure 4. J–V curves and dark currents of DSSCs based on the dyes.
ꢀ
zothiadiazole entity towards I2 (or I3 ). The lower VOC
values for the DSSCs based on 2 and 3 could also be as-
cribed to the higher affinity of the sulfur and selenium
ꢀ
atoms towards I2 (or I3 ) in the two compounds, which in-
creased the local concentration of iodine in the vicinity of
the TiO2 surface. A similar trend was found in furan- and
thiophene-containing BTA dyes.[12b] There have been many
examples illustrating that dye molecules with polarizable
sulfur-containing thiophene moieties have a stronger inter-
Figure 5. IPCE plots of the DSSCs using the dyes and N719.
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ꢀ
action with polarizable I3 and/or I2, thus leading to more
[19]
facile charge recombination (dark current) and lower VOC
.
A weak interaction between selenophene and I2 may be an
indication that tetrahydroselenophene is capable of forming
a complex with molecular iodine.[20] OꢃRegan pointed out
that the I2 binding with dyes results in facile charge recom-
[21]
bination and lower VOC
.
The binding coefficients for
iodine and ruthenium dyes were measured by I2 titration
curves of dye-sensitized TiO2 film. We also examined I2 ti-
tration on dye-loaded TiO2 films for probing the interaction
between iodine and dyes 1 (furan), 2 (thiophene) and 3 (se-
lenophene). The changes in the absorption spectra of the
dyed films during I2 titration are shown in Figure S3 (see the
Supporting Information). Possibly due to weak interaction
of the dyes with iodine, no obvious new peaks and isosbestic
points were found in the titration spectra, thus preventing us
from obtaining binding coefficients. Nonetheless, the de-
creased intensity of the absorbance spectra of the dye with
a higher I2 concentration in the order of decreasing rate (3>
2>1) is consistent with the difference in VOC. The similar
trend in the VOC found in furan and thiophene congeners re-
ported by Hua et al.[11b] could be due to the same reason.
As aforementioned, the extra functional groups in com-
pound 4 affect the planarity of the conjugated spacer; this
results in less dye adsorption as well as a blue shift in the
the absorption compared to 2. It is worthy of note that the
DSSC based on 4 has a higher VOC and a smaller dark cur-
rent (see below) than that of 2. It is believed that extra func-
tional groups help to decrease the void space among the dye
molecules and block the close contact of the iodine mole-
cules with the dyes. This also points out that other factors
should not be overlooked, however, the interaction between
iodine and heteroaromatic rings may play an important role.
Faster recombination of the electrons in the TiO2 film
with the oxidized electrolyte will lead to a larger dark cur-
rent. The J–V curves of the DSSCs under dark conditions
are shown in Figure 4. The dark current value decreases in
the order of 3>2>5>4>1. The trend roughly conforms to
that obtained from the electrochemical impedance studies.
Electrochemical impedance spectroscopy (EIS) can provide
important information on the interfacial charge recombina-
tion and redox reaction process in DSSCs. Figure 6 shows
the Nyquist plots under a forward bias of ꢀ0.55 V in the
dark. The middle-frequency regime is mainly related to the
electron transport from the conduction band of TiO2 to I2
Figure 6. Nyquist plots of DSSCs in darkness.
and dark currents for DSSCs of 4 and 5 might be due to
a different recombination rate of the electron with the oxi-
dized dye, which is reflected in the EIS studies. The electron
transport resistance (Rtr) of DSSCs can be deduced from the
radius of the intermediate frequency semicircle in the Ny-
quist plots (Figure S4, see the Supporting Information) ob-
tained from the electrochemical impedance measurement
under illumination (100 mWcmꢀ2, open-circuit voltage con-
ditions). The Rtr value decreases in the order of
1
(31.18 W)>4 (17.31 W)>3 (15.91 W)>5 (15.33 W)>2
(14.24 W). The lower electron transport resistance will assist
electron collection and improve the cell efficiency. The
much higher Rtr value for the DSSC based on 1 is in con-
formity with its lowest JSC value. The difference in the Rtr
value is small in DSSCs of other dyes.
Theoretical Approach
To correlate the molecular structures of the dyes with the
performance of DSSCs, we carried out density functional
theory (DFT) as well as time-dependent DFT (TDDFT) cal-
culations at the B3LYP/6-31G* level of theory using a suite
of Q-Chem 4.0 software. The results for the theoretical ap-
proach are summarized in Table S1 (see the Supporting In-
formation). Compound 1 has a nearly planar structure in the
optimized structure owing to the small amount of steric bulk
imparted by the furan entity. In comparison, the dihedral
angle between the phenyl ring of the triphenylamine and
the larger thiophene or selenophene entity is larger than 208
(Figure 7), and the largest twist angle is found in 4; this is
due to the presence of the hexyl substituent in the thio-
phene ring. Selected frontier orbitals of the dyes are shown
in Figure S5 (see the Supporting Information). The HOMO
in these compounds is mainly distributed from the arylamine
to the conjugated spacer, while the LUMO is largely distrib-
uted from 2-cyanoacrylic acid to the spacer. Figure 8 shows
the detachment–attachment plots of dyes 1, 2, and 3 upon
the S0!S1 transition, which features the points exactly
where the hole densities (detachment) and the electron den-
sities (attachment) reside. For all the compounds, the lowest
energy transition (S0!S1) is nearly 100% of the HOMO!
ꢀ
(or I3 ) in the electrolyte. The charge recombination resist-
ance (Rrec) at the TiO2 surface can be deduced from the
radius of the intermediate frequency by fitting curves using
Z-view software.
The Rrec value increases in the order of 3 (85.42 W)<2
(244.87 W)<4 (385.80 W)<5 (417.55 W)<1 (545.43 W). As
discussed earlier, the low affinity of the oxygen atom (in
ꢀ
furan) towards I2 (or I3 ) can therefore more effectively sup-
press the charge recombination in the DSSC of 1. In con-
trast, the high affinity of the selenium atom (in seleno-
ꢀ
phene) towards I2 (or I3 ) results in a more facile charge re-
combination. The slight discrepancy between the EIS data
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(Table S1, see the Supporting Information). Figure 9 dis-
plays the changes in Mulliken charges of the dyes for the
S0!S1 and S0!S2 transitions. Although there is negative
charge at BTA, the Ac entity has an even higher negative
charge. This is very different to those of BT dyes, in which
the BT entity has a more negative Mulliken charge than the
acceptor.[7,8a] Clearly, a charge trapping effect is found in the
BT dyes,[7,8a] which is detrimental to electron injection is
greatly alleviated in the BTA dyes.
Conclusions
New metal-free dyes comprising heteroaromatic ring, benzo-
triazole, and a heteroaromatic ring (furan, thiophene, or se-
lenophene) as the conjugated spacer between the arylamine
donor and 2-cyanoacrylic acceptor have been synthesized
and used as the sensitizers for DSSCs. Compared to benzo-
thiazole-containing congeners, there is less charge trapping
in the benzotriazole unit, which is beneficial to electron in-
jection. The heteroaromatic ring has great influence on the
cell performance. The light-harvesting efficiency of the dyes
decreases in the order of selenophene>thiophene>furan,
and the efficiency of dark current suppression decreases in
the order of furan>thiophene>selenophene. The thio-
phene-containing dye has the best cell performance, and the
conversion efficiency reaches 6.20%, which is approximately
85% of the standard cell based on N719.
Figure 7. Schematic division and dihedral angles of molecules.
Experimental Section
General Information
Unless otherwise specified, all the reactions were performed under an at-
mosphere of nitrogen using standard Schlenk techniques. All solvents
used were purified by standard procedures, or purged with nitrogen
before use. 1H NMR spectra were recorded on a Bruker 300 MHz or
400 MHz spectrometer. Absorption spectra were recorded on a Cary 50
probe UV/Vis spectrophotometer. All chromatographic separations were
carried out on silica gel (60m, 230–400 mesh). Fast atom bombardment
(FAB) mass spectra were recorded on a VG70-250S mass spectrometer.
Figure 8. Calculated detachment–attachment plot of dyes 1, 2, and 3.
LUMO transition, thus confirming its charge-transfer char-
acter (arylamine to 2-cyanoacrylic acid). The high absorp-
tion intensity (calc. oscillator strength=0.69–1.08) of the
lowest energy band is attributed
to the prominent contribution
of the spacer to both the
HOMO and LUMO. The Mul-
liken charges for the S1 and S2
states were calculated from the
TDDFT results. Differences in
the Mulliken charges in the ex-
cited and the ground states
were calculated and grouped
into several segments, triphe-
nylamine (Am), heteroaromatic
ring (F, T, Se, or T6), benzotria-
zole (Bta6, Bta6F), heteroaro-
matic ring (F’, T’, Se’, or T6’),
and 2-cyanoacrylic acid (Ac), to
estimate the extent of charge
separation
upon
excitation Figure 9. Plot of the difference in the Mulliken charges between the ground state and the excited state.
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Elementary analyses were performed on a Perkin–Elmer 2400 CHN ana-
lyzer. The starting material 4,7-dibromo-2-hexyl-2H-benzotriazole was
synthesized according to the literature method.[22]
sis calcd (%) for C42H35N5O4: C 74.87, H 5.24, N 10.39; found: C 74.64, H
5.19, N, 10.44.
(Z)-2-Cyano-3-(5-(7-(5-(4-(diphenylamino)phenyl)thiophen-2-yl)-2-hexyl-
2H-benzo[d]ACTHNUTRGNEUG[N 1,2,3]triazol-4-yl)thiophen-2-yl)acrylic acid (2)
Assembly and characterization of DSSCs
1H NMR ([D8]THF, 400 MHz): d=8.40 (s, 1H), 8.30 (d, J=4.4 Hz, 1H),
8.10 (d, J=4.0 Hz, 1H), 7.95 (d, J=4.0 Hz, 1H), 7.91 (d, J=7.6 Hz, 1H),
7.75 (d, J=7.6 Hz, 1H), 7.62 (d, J=8.4 Hz, 2H), 7.41 (d, J=3.6 Hz, 1H),
7.27 (t, J=8.8 Hz, 4H), 7.10 (t, J=7.6 Hz, 4H), 7.05 (d, J=6.0 Hz, 2H),
7.03 (d, J=7.2 Hz, 2H), 4.90 (t, J=7.2 Hz, 2H), 2.22 (m, 2H), 1.40 (m,
6H), 0.90 ppm (t, J=7.2 Hz, 3H); 13C NMR ([D8]THF, 100 MHz): d=
163.1, 148.8, 147.7, 147.5, 145.4, 145.3, 141.9, 141.7, 138.4, 137.7, 135.7,
129.3, 129.2, 128.1, 127.6, 126.4, 125.7, 124.5, 124.3, 123.4, 123.1, 121.7,
121.4, 115.6, 99.1, 56.7, 31.2, 29.8, 26.2, 22.4, 13.3 ppm; MS (FAB): m/z:
706.23 [M+]; elemental analysis calcd (%) for C54H59N5O2S2: C 74.19, H
6.80, N 8.01; found: C 73.96, H 6.75, N, 8.11.
The photoanode used was a 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 a fluorine-doped tin oxide (FTO) glass substrate[23] with a di-
mension of 0.5ꢂ0.5 cm2, and the film thickness was measured by a profil-
ometer (Dektak3, Veeco/Sloan Instruments Inc., USA). A platinized
FTO produced by thermopyrolysis of H2PtCl6 was used as a counter elec-
trode. The TiO2 thin film was dipped into the THF solution containing
3ꢂ10ꢀ4 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 they were tightly clipped together to form a cell. The electrolyte was
then injected into the space and the cell was sealed with the Torr Seal
cement (Varian, MA, USA). The electrolyte was composed of 0.5m lithi-
um iodide (LiI), 0.05m iodine (I2), and 0.5m 4-tert-butylpyridine that was
dissolved in acetonitrile.
2-Cyano-3-(5-{7-[5-(4-diphenylamino-phenyl)-selenophen-2-yl]-2-hexyl-
2H-benzotriazol-4-yl}-selenophen-2-yl)-acrylic acid (3)
1H NMR ([D8]THF, 400 MHz): d=8.43 (s, 1H), 8.32 (d, J=4.0 Hz, 1H),
8.29 (d, J=4.0 Hz, 1H), 8.06 (d, J=4.4 Hz, 1H), 7.92 (d, J=7.6 Hz, 1H),
7.72 (d, J=8.0 Hz, 1H), 7.56–7.54 (m, 2H), 7.26 (d, J=8.0 Hz, 4H), 7.11
(d, J=8.0 Hz, 4H), 7.05–7.01 (m, 4H), 4.90 (t, J=7.2 Hz, 2H), 2.22 (m,
2H), 1.41 (m, 6H), 0.90 ppm (t, J=7.2 Hz, 3H); 13C NMR ([D8]THF,
100 MHz): d=164.3, 156.2, 153.3, 149.9, 148.9, 148.6, 143.7, 143.5, 143.0,
142.7, 142.4, 131.6, 131.5, 130.3, 129.0, 128.7, 127.8, 126.3, 125.6, 125.3,
124.7, 124.5, 124.2, 123.0, 117.4, 100.0, 57.8, 32.3, 30.9, 27.3, 23.5,
14.5 ppm; MS (FAB): m/z: 802.12 [M+]; elemental analysis calcd (%) for
C42H35N5O2Se2: C 63.08, H 4.41, N 8.76; found: C 63.22, H 4.61, N 8.55.
Iodine titration of dye-loaded TiO2 films
The dye-loaded TiO2 films on 1 mm glass substrates were placed in
a one-sided 10 mm cuvette containing acetonitrile (2 mL). Upon the ad-
dition of a 0.01m I2 solution into the cuvette for the desired concentra-
tion, the measurement of the spectrum of the film and solution can be
made, and subsequent rotation of the cuvette by 90 degrees allows mea-
surement of the spectrum of the solution alone. The absorbance of the
dye-loaded TiO2 film alone was obtained by:
Afilm =Afilm+solutio-
nꢀ0.9Asolution, where the factor 0.9 is due to the fraction of the thickness
of the substrate (1 mm) based on the beam pathway (10 mm). For a more
detailed method please refer to Ref. [21].
2-Cyano-3-(5-{7-[5-(4-diphenylamino-phenyl)-4-hexyl-thiophen-2-yl]-2-
hexyl-2H-benzotriazol-4-yl}-3-hexyl-thiophen-2-yl)-acrylic acid (4)
1H NMR ([D8]THF, 400 MHz): d=8.52 (s, 1H), 8.15 (s, 1H), 8.02 (s,
1H), 7.80 (d, J=7.6 Hz, 1H), 7.63 (d, J=7.6 Hz, 1H), 7.39 (d, J=8.4 Hz,
2H), 7.31 (t, J=8.0 Hz, 4H), 7.18 (d, J=7.6 Hz, 4H), 7.13 (d, J=8.4 Hz,
2H), 7.08 (t, J=7.6 Hz, 2H), 4.87 (t, J=7.2 Hz, 2H), 2.93 (t, J=7.6 Hz,
2H), 2.77 (t, J=7.6 Hz, 2H), 2.25 (m, 2H), 1.75 (m, 4H), 1.52–1.36 (m,
18H), 1.33 ppm (t, J=7.2, 9H); 13C NMR ([D8]THF, 100 MHz): d=
165.0, 156.3, 149.1, 149.0, 148.9, 144.3, 143.5, 143.2, 141.0, 140.7, 138.1,
132.0, 131.6, 131.1, 130.6, 129.7, 127.3, 126.1, 125.9, 124.6, 124.3, 123.0,
122.8, 117.3, 99.0, 58.0, 33.1, 33.0, 32.7, 32.6, 32.3, 31.2, 30.6, 30.4, 30.2,
30.1, 27.6, 24.0, 23.9, 23.8, 14.9, 14.89, 14.86 ppm; MS (FAB): m/z: 874.42
[M+]; elemental analysis calcd (%) for C54H59N5O2S2: C 74.19, H 6.80, N
8.01; found: C 73.96, H 6.75, N, 8.11.
Quantum chemistry computation experiments
The computations were performed with Q-Chem 4.0 software.[24] Geome-
try 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 TDDFT. A number of previous studies exist that have em-
ployed TDDFT to characterize excited states with charge-transfer char-
acter.[25] In some cases, an underestimation of the excitation energies was
observed.[25,26] Therefore, in the present study, we use TDDFT to visual-
ize the extent of transition moments as well as their charge-transfer char-
acters, and avoid drawing conclusions from the excitation energy.
2-Cyano-3-(5-{7-[5-(4-diphenylamino-phenyl)-thiophen-2-yl]-5,6-difluoro-
2H-benzotriazol-4-yl}-thiophen-2-yl)-acrylic acid (5)
The synthetic details for the intermediates of the desired products (1–5)
are described in the Supporting Information.
1H NMR ([D8]THF, 400 MHz): d=8.45 (d, J=4.4 Hz, 1H), 8.43 (s, 1H),
8.41 (d, J=4.0 Hz, 1H), 8.04 (d, J=3.6 Hz, 1H), 7.63 (d, J=8.4 Hz, 2H),
7.45 (d, J=2.4 Hz, 1H), 7.27 (t, J=7.8 Hz, 4H), 7.13–7.02 (m, 8H), 4.89
(t, J=7.4 Hz, 2H), 2.20 (m, 2H), 1.46–1.32 (m, 6H), 0.90 ppm (t, J=
7.2 Hz, 3H); 13C NMR ([D8]THF, 100 MHz): d=164.1, 150.6, 149.3,
148.9, 148.8, 148.6, 146.4, 146.2, 141.7, 138.8, 138.6, 137.7, 133.6, 133.5,
131.8, 131.7, 131.2, 130.3, 128.8, 127.7, 125.7, 124.4, 123.8, 117.0,
102.0,58.1, 32.3, 30.8, 27.3, 23.5, 14.5 ppm; MS (FAB): m/z: 741.20 [M+];
elemental analysis calcd (%) for C42H33F2N5O2S2: C 68.00, H 4.48, N
9.44; found: C 67.99, H 4,50, N, 9.52.
General synthetic procedures for 1–5
Acetic acid (10 mL) was added to a flask containing a mixture of carbal-
dehyde precursor (1.0 mmol), cyanoacetic acid (1.3 mmol), and ammoni-
um acetate (0.03 mmol). The mixture was heated at 1008C for 8 h, and al-
lowed to cool to room temperature. The resulting solid was filtered and
washed with distilled water, diethyl ether, and methanol to give a dark
red solid. Further purification by silica-gel chromatography using di-
chloromethane/acetic acid (50:1) as the eluent afforded the pure product.
2-Cyano-3-{5-[2-hexyl-7-(5-{4-[phenyl-(1-vinyl-propenyl)-amino]-phenyl}-
furan-2-yl)-2H-benzotriazol-4-yl]-furan-2-yl}-acrylic acid (1)
1H NMR ([D8]THF, 400 MHz): d=8.16 (d, J=7.6 Hz, 1H), 8.02 (s, 1H),
8.10 (d, J=3.6 Hz, 1H), 7.76 (d, J=8.8 Hz, 2H), 7.69 (d, J=3.6 Hz, 1H),
7.62 (d, J=3.2 Hz, 1H), 7.46 (d, J=3.2 Hz, 1H), 7.27 (t, J=8.0 Hz, 4H),
1H), 7.09–7.12 (m, 6H), 7.02 (t, J=7.2 Hz, 2H), 6.89 (d, J=3.2 Hz, 1H),
4.87 (t, J=7.2 Hz, 2H), 2.20 (m, 2H), 1.39 (m, 6H), 0.90 ppm (t, J=
7.2 Hz, 3H); 13C NMR ([D8]THF, 100 MHz): d=164.3, 156.9, 155.9,
150.3, 149.5, 148.8, 148.7, 142.2, 141.5, 138.2, 130.3, 126.1, 125.6, 124.7,
124.3, 124.2, 122.7, 120.7, 117.8, 116.7, 115.9, 114.9, 107.9, 99.5, 57.7, 32.3,
31.0, 27.3, 23.5, 14.4 ppm; MS (FAB): m/z: 674.27 [M+]; elemental analy-
Acknowledgements
We acknowledge the support of the Academia Sinica (AC) and NSC
(Taiwan) and the Instrumental Center of Institute of Chemistry (AC).
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Received: December 11, 2012
Published online: February 7, 2013
Chem. Asian J. 2013, 8, 809 – 816
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ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim