High-performance organic materials for dye-sensitized solar cells: Triarylene-linked dyads with a 4-tert-butylphenylamine donor

Article abstract of DOI:10.1002/asia.201100777

A series of organic dyes were prepared that displayed remarkable solar-to-energy conversion efficiencies in dye-sensitized solar cells (DSSCs). These dyes are composed of a 4-tert-butylphenylamine donor group (D), a cyanoacrylic-acid acceptor group (A), a

Full text of DOI:10.1002/asia.201100777

FULL PAPERS
DOI: 10.1002/asia.201100777
High-Performance Organic Materials for Dye-Sensitized Solar Cells:
Triarylene-Linked Dyads with a 4-tert-Butylphenylamine Donor
Yuan Jay Chang,[a] Po-Ting Chou,[a] Szu-Yu Lin,[b] Motonori Watanabe,[b] Zhi-Qian Liu,[c]
Ju-Ling Lin,[b] Kew-Yu Chen,[c] Shih-Sheng Sun,[b] Ching-Yang Liu,[d] and
Tahsin J. Chow*[b]
Abstract: A series of organic dyes were
prepared that displayed remarkable
solar-to-energy conversion efficiencies
in dye-sensitized solar cells (DSSCs).
These dyes are composed of a 4-tert-
butylphenylamine donor group (D), a
cyanoacrylic-acid acceptor group (A),
and a phenylene-thiophene-phenylene
(PSP) spacer group, forming a D-p-A
system. A dye containing a bulky tert-
butylphenylene-substituted carbazole
(CB) donor group showed the highest
performance, with an overall conver-
sion efficiency of 6.70%. The perfor-
mance of the device was correlated to
the structural features of the donor
groups; that is, the presence of a tert-
butyl group can not only enhance the
electron-donating ability of the donor,
but can also suppress intermolecular
aggregation. A typical device made
with the CB-PSP dye afforded a maxi-
mum photon-to-current conversion ef-
ficiency (IPCE) of 80% in the region
400–480 nm, a short-circuit photocur-
rent density Jsc =14.63 mAcmÀ2, an
open-circuit photovoltage Voc =0.685 V,
and a fill factor FF=0.67. When che-
nodeoxycholic acid (CDCA) was used
as a co-absorbent, the open-circuit volt-
age of CB-PSP was elevated signifi-
cantly, yet the overall performance de-
creased by 16–18%. This result indicat-
ed that the presence of 4-tert-butyl-
phenyl substituents can effectively in-
hibit self-aggregation, even without
CDCA.
Keywords: carbazoles
·
dyads
·
organic materials · solar cells · triar-
ylenes
Introduction
tion owing to their improved efficiency and lower cost. Ever
since the report by OꢀRegan and Grꢁtzel in 1991 on high-
performance dye-sensitized solar cells made with ruthenium
complexes, research in this area has boomed.[1] To date,
three landmark polypyridyl ruthenium(II) complexes have
achieved maximum power-conversion efficiencies of over
11%: N3, N719, and the black dye.[2] Compared to metallic
materials, organic dyes have the advantages of environmen-
tal friendliness, higher structural flexibility, lower cost, and
easier preparation. A wide variety of organic dyes have
been reported in the literature, including derivatives of cou-
marin,[3] indoline,[4] cyanine,[5] merocyanine,[6] hemicyanine,[7]
perylene,[8] dithiensilole,[9] spirobifluorene,[10] and porphy-
rin,[11] whilst most of them exhibited energy-to-electricity
conversion efficiencies in the range 5–9%. In our previous
studies, we investigated a series of organic dyads with a
donor-p-acceptor (D-p-A) structure, in which an amine
donor group (D) was functionalized with a cyanoacrylic-acid
acceptor group (A) through a triaryl linkage (p).[12] The
DSSCs made with these D-p-A dyads exhibited remarkable
quantum efficiency. Among others, the structure of the elec-
tron-donating group is one of the crucial factors that influ-
ence the conversion efficiency of solar energy into electricity
in DSSCs. Herein, we report our efforts on the design of dif-
ferent electron donors, in particular those with bulky 4-tert-
butylphenyl substituents. The fabrication process was also
modified in order to obtain the highest efficiency with these
new dyes.
Under the global strain of over-consumed fossil fuels and
the need to reduce the greenhouse effect, there is an urgent
demand for finding clean and renewable energy sources.
Solar energy is widely recognized as a potential way to help
solve this problem. Over the past two decades, dye-sensi-
tized solar cells (DSSCs) have received considerable atten-
[a] Dr. Y. J. Chang, P.-T. Chou
Department of Chemistry
National Taiwan University
Taipei 106 (Taiwan)
[b] S.-Y. Lin, Dr. M. Watanabe, J.-L. Lin, Prof. Dr. S.-S. Sun,
Prof. Dr. T. J. Chow
Institute of Chemistry
Academia Sinica
Taipei 115 (Taiwan)
Fax : (+886)2-27884179
E-mail: tjchow@chem.sinica.edu.tw
[c] Z.-Q. Liu, Prof. Dr. K.-Y. Chen
Department of Chemical Engineering
Feng Chia University
Taichung 40724 (Taiwan)
[d] Prof. Dr. C.-Y. Liu
Department of Applied Chemistry
Chinese Culture University
Taipei 111 (Taiwan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100777.
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Chem. Asian J. 2012, 7, 572 – 581

Results and Discussion
quently reduced by sodium borohydride to give 4-(thiophen-
2-yl)benzenamine. The amine group was then modified in
the next step by the simultaneous addition of two 4-tert-
phenyl substituents using a Buchwald–Hartwig coupling re-
action to produce compound 3-DTB in 50% yield.[13] The
chain-length was further elongated by inserting another
phenylene unit through a Stille coupling reaction with 4-bro-
mobenzaldehyde to yield compound 4-DTB. The formation
of DTB-PSP was completed by a Knoevenagel condensation
with a cyanoacetic acid to build up the cyanoacrylic acid ac-
ceptor unit.[14]
Synthesis and Chemical Characterization
The structures of the dyes are shown in Scheme 1. A 4-tert-
butylphenyl substituent is placed on the amine moiety of all
of the compounds. Their synthetic sequences are outline in
Five compounds containing unsymmetrical amine groups
were prepared according to a slightly modified procedure.
All of these syntheses were started from the corresponding
secondary amines, onto which a 4-tert-butylphenyl group
was added, either through an Ullmann coupling reaction
(for compound 1-CB) or through a Buchwald–Hartwig cou-
pling reaction (for the other 1-X compounds, i.e. X=TB,
5TB, 6TB, and PT).[15] The phenyl group of compound 1-X
was brominated using N-bromosuccinimide to form com-
pound 2-X,[16] which was then transformed into compounds
3-X and 4-X through similar procedures to those used in the
formation of compound DTB-PSP in yields of 63–90%. In
the final step, the cyanoacrylic acid moiety was generated
via Knoevenagel condensation to form the five X-PSP
dyes.[14] All of the final products crystallized as deep-colored
solids, and their structures were confirmed by spectroscopic
analysis.
Scheme 1. Structure of the dyes with a PSP triaryl linkage.
Photophysical and Electrochemical Properties
Scheme 2. Compound DTB-PSP is the only dye containing
two 4-tert-butylphenyl substituents and its structure possess-
es a two-fold symmetry. The synthesis of DTB-PSP started
from para-bromonitrobenzene, onto which a thiophene unit
was added using the Stille coupling reaction. The nitro
group of the product 2-(4-nitrophenyl)thiophene was subse-
The absorption spectra of the dyes in dilute tetrahydrofuran
(3ꢃ10À5 m) are shown in Figure 1. Each of these compounds
exhibit two or three major absorption bands in the ranges
302–307 nm, 341–356 nm, and 428–460 nm, respectively. The
À
first two bands were assigned to localized aromatic p p*
Scheme 2. Synthesis. 1) [PdCl2ACTHUNGTREN(UNG PPh3)2], 2-(tributylstannyl)thiophene, DMF, 908C; 2) SnCl2·2H2O, NaBH4, EtOH; 3) [Pd2ACTHUNGTRE(UNNNG dba)3], dppf, 4-bromo-tert-butyl-
benzene, toluene, 908C; 4) NBS, DMF, RT; 5) (a) nBuLi, tributyltin chloride, THF, À788C; followed by aq. HCl, (b) [PdCl
CHTUNGTRENNUNG
hyde, DMF, 908C; 6) cyanoacetic acid, NH4OAc, AcOH, ca. 90–1008C. DMF=N,N-dimethylformamide, dba=dibenzylideneacetone, dppf=1,1’-bis(di-
phenylphosphino)ferrocene, NBS=N-bromosuccinimide, THF=tetrahydrofuran.
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T. J. Chow et al.
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Figure 1. Absorption spectra in THF.
Figure 2. Adsorption spectra of the dyes adsorbed on the surface of TiO2.
transitions, and the last one to a charge-transfer (CT) transi-
tion. The major absorption band of DTB-PSP is slightly
red-shifted to that of TB-PSP, owing to the presence of the
extra tert-butyl substituent. The influence of ring-size was in-
vestigated by comparing compounds 5TB-PSP and 6TB-
PSP, which have extinction coefficients of e=2.89ꢃ
104 mÀ1 cmÀ1 and 2.64ꢃ104 mÀ1 cmÀ1, respectively (Table 1).
These values are consistent with their oscillator strengths as
determined by TDDFT (0.82 and 0.79, respectively). In the
series of CB-PSP, TB-PSP, and PT-PSP, the lmax value of
the major absorption band appeared at 428, 433, and
446 nm, respectively. The longest-wavelength absorption of
PT-PSP was ascribed to the additional electron-donating
effect of the sulfur atom. The dihedral angles between the
aminophenyl and thiophenyl rings were calculated to be
À26.448, À22.938, and À22.898 for compounds CB-PSP, TB-
PSP, and PT-PSP, respectively, and this indicated that a
good amount of planarity had persisted. Of all the com-
pounds tested, CB-PSP showed the highest molar-extinction
coefficient (4.00ꢃ104 mÀ1 cmÀ1), which correlated well with
its highest oscillator strength (0.92, calculated by TDDFT).
The absorption spectra of the dyes, after they were chemi-
sorbed onto the surface of TiO2, displayed a mild blue shift
with respect to those in solution (Figure 2). Such a phenom-
enon has been observed previously, and was attributed to
the decrease in electron-accepting ability of the cyanoacry-
late group comparing to a free carboxylic acid.[17] The ab-
sorption band of CB-PSP on TiO2 possesses the longest
wavelength (lmax =418 nm) and displayed the smallest blue
shift with respect to its absorption in solution. However, in
the excited state, their conformation may be readjusted to a
different geometry from those in the ground state. The pres-
ence of a conformational twist in the excited state may ben-
efit the quantum efficiency of the DSSC, as a non-planar ge-
ometry could retard the rate of charge-recombination and
therefore increase the photocurrent.
The first oxidation potentials (Eox), corresponding to the
HOMO level of dyes, were measured by cyclic voltammetry
(CV) in tetrahydrofuran, and the results are shown in
Table 1. The relative potentials decrease in the order CB-
PSP (0.91)>TB-PSP (0.73)>DTB-PSP (0.69)>PT-PSP
(0.56)>6TB-PPS (0.54)>5TB-PSP (0.51). The LUMO
levels of the sensitizers were estimated by the values of Eox
and the 0–0 band gaps, whilst the latter values were ob-
tained at the intersection of the absorption and emission
spectra. The band-gap energies decrease along with the elec-
tron-donating power of the substituents in the trend: TB-
PSP (2.49)>DTB-PSP (2.46), CB-PSP (2.46)>5TB-PSP
(2.28), and PT-PSP (2.39)>6TB-PSP (2.26). The estimated
LUMO levels of all of dyes tested are sufficiently higher
than the conductive band level of TiO2 (ca. À0.5 V vs.
Table 1. Calculated (TDDFT/B3LYP) and experimental parameters of all of the dyes.[a]
[f]
[h]
Dye
HOMO/LUMO[b] [eV] Band gap[b]
f[b]
labs[c]nm
labs (film) [nm] HOMO[d]/LUMO[e] [eV] Eox [V] E0À0[g] [V] Ered [V]
[e/MÀ1 cmÀ1
]
DTB-PSP
TB-PSP
5TB-PSP
CB-PSP
6TB-PSP
PT-PSP
À4.98/À2.58
À5.03/À2.61
À4.96/À2.52
À5.29/À2.58
À4.95/À2.53
À5.04/À2.61
2.40
2.42
2.44
2.71
2.42
2.43
0.71 439 (34000)
0.70 433 (32600)
0.82 460 (28900)
0.92 428 (40000)
0.79 454 (26400)
0.49 446 (33900)
405
401
415
418
406
408
À5.19/À2.73
À5.23/À2.74
À5.01/À2.73
À5.41/À2.95
À5.04/À2.78
À5.06/À2.67
0.69
0.73
0.51
0.91
0.54
0.56
2.46
2.49
2.28
2.46
2.26
2.39
À1.77
À1.76
À1.77
À1.55
À1.72
À1.83
[a] f=Oscillator strength for the lowest-energy transition; e=absorption coefficient; Eox =oxidation potential; E0À0 =0–0 transition energy measured at
the intersection of absorption and emission spectra. [b] TDDFT/B3LYP calculated values. [c] Absorptions measured in THF. [d] Oxidation potentials of
dyes (10À3 m) in THF, 0.1m (n-C4H9)4NPF6, scan rate 50 mVsÀ1 (vs. Fc+/Fc). [e] LUMO calculated by HOMO+E0À0. [f] Eox calculated by HOMO+4.5
(eV) (vs. NHE). [g] E0À0 determined from the intersection of absorption and emission in THF. [h] Ered calculated by EoxÀE0À0
.
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Chem. Asian J. 2012, 7, 572 – 581

High-Performance Organic Materials for Dye-Sensitized Solar Cells
NHE; NHE=normal hydrogen electrode, whilst their
effect, but also a stronger dipole moment and a higher ab-
sorptivity. The electron-density distributions in the HOMO
and LUMO are illustrated in the Supporting Information,
Figure S34. The electron density of the HOMO is mainly lo-
calized on the amine moiety, whilst that of the LUMO is
mainly localized on the cyanoacrylic acid group. Upon pho-
toexcitation, an electron migrates from the donor (D) to the
acceptor (A), thereby forming a charge-shifted dipolar state.
In both of the ground and excited states, the electron densi-
ties of D and A are heavily coupled with the orbitals in the
central bridge linkage (B). The difference in Mꢄlliken
charge shift surrounding the D, B, and A segments, before
(S0 state) and after (S1 state) the photoexcitation, can be
clearly depicted in a bar chart (see the Supporting Informa-
tion, S33).
HOMO levels are sufficiently lower than that of the electro-
À
lyte ion pair IÀ/I3 (ca. 0.4 V vs. NHE).[18] The electronic
structures thus ensure a favorable exothermic flow of charge
throughout the photoelectronic conversion (Figure 3). In
Photovoltaic Performance
The performances of the devices under solar conditions
(AM 1.5) are listed in Table 2. The J–V curves for each of
the individual dyes are shown in Figure 4. A comparison be-
Table 2. Photovoltaic parameters of devices for different donor of PSP
series under AM 1.5 conditions.[a]
Figure 3. Potential energy levels of the dyes.
Dye
Jsc [mAcmÀ2
]
Voc [V]
FF
h
[b] [%]
t[c] [ms]
particular, the lowest HOMO potential-energy level was
found in CB-PSP, which implies that the device made with
it may show the most efficient charge regeneration.
DTB-PSP
TB-PSP
5TB-PSP
CB-PSP
6TB-PSP
PT-PSP
N719
13.96
13.32
12.60
14.63
8.48
0.67
0.66
0.63
0.685
0.57
0.68
0.73
0.66
0.61
0.61
0.67
0.65
0.64
0.62
6.14
5.43
4.87
6.70
3.14
6.32
7.24
17.76 (4.97)
17.17 (4.68)
16.66 (4.30)
19.44 (7.23)
14.85 (3.98)
18.56 (5.68)
22.11 (19.9)
14.12
15.95
Theoretical Calculations
To gain a better understanding of the electronic configura-
tions, the compounds were examined by theoretical models
using a Gaussian 03 program.[19] The molecular geometries
were optimized by a B3LYP hybrid functional with a 6-
31G* basis set, and then the electronic configurations of
both the ground and excited states were computed by time-
dependent density functional theory (TDDFT) with the
B3LYP functional. The HOMO/LUMO energy levels and
the corresponding band-gaps are listed in Table 1 (see the
Supporting Information, Tables S1–S3 and Figures S33–S35)
and they are remarkably consistent with the experimental
values. According to the optimized molecular geometry, the
bridge conformations of 5TB-PSP and 6TB-PSP are more
coplanar than CB-PSP and PT-PSP (S35). The moieties
with strongly electron-donating substituents displayed larger
oscillator strengths as well as lower oxidation potentials.
The molecular dipole moment of compound 5TB-PSP
(11.00 debye) was about equal to that of compound 6TB-
PSP (10.95 debye) in the ground state. The dipole moment
of compound TB-PSP (9.58 debye) was somewhat smaller
than those of compounds CB-PSP (10.99 debye) and PT-
PSP (11.05 debye). The latter two also exhibited higher
molar extinction coefficients for more-efficient light-harvest-
ing. Compared to compound TB-PSP, the presence of two
tert-butyl substituents in dye DTB-PSP exhibits not only a
smaller band-gap owing to a stronger electron-donating
[a] Jsc =short-circuit photocurrent density; Voc =open-circuit photovolt-
age; FF=fill factor; h=total-power-conversion efficiency. [a] Perfor-
mance of DSSC measured in a 0.25 cm2 working area on an FTO (8 W/
square) substrate. [b] Lifetime of injected electrons measured by transi-
ent photovoltage at open circuit in the presence of LiI electrolyte (0.5m)
in MeCN. Values in parantheses were obtained by fitting the middle-fre-
quency in the Bode phase plots through the expression t=1/
f is the frequency.
ACHTUNGTRENUN(NG 2pf), where
tween the performances of DTB-PSP and TB-PPS is worth
mentioning. The former exhibited a higher short-circuit cur-
rent, a higher open-circuit voltage, and a higher field factor
(Jsc =13.96mAcmÀ2, Voc =0.68 V, FF=0.66) than the latter
(Jsc =13.32mAcmÀ2, Voc =0.66 V, FF=0.61). As a result, the
overall conversion efficiency of the former is better than the
latter (6.26% vs. 5.43%). In the IPCE plot of DTB-PSP, the
spectrum covers a broader wavelength range than TB-PSP,
and also shows a higher conversion efficiency (Figure 5). A
comparison between 5TB-PSP and 6TB-PSP indicated that
a five-membered ring is preferable owing to a more-planar
geometry. The current density of 5TB-PSP (12.60mAcmÀ2)
is significantly higher than that of 6TB-PSP (8.48mAcmÀ2),
as is the conversion efficiency 4.87% vs. 3.14%. For com-
pounds CB-PSP and PT-PSP, their short-circuit currents
maintain
a constant value in the range of 14.12–
14.63mAcmÀ2, which was slightly better than that of TB-
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T. J. Chow et al.
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long decay lifetime (18.56ms).[20] It is noteworthy that the
fill factors of dyes CB-PSP (0.67) and PT-PSP (0.64) were
again higher than that of compound TB-PSP (0.61). Among
all of the dyes tested, the best performance was found with
CB-PSP, which showed a maximal IPCE value of 80% and
an overall conversion efficiency of 6.70%, which is quite
close to that of the well-known ruthenium complex N-719.
The incident photon to current conversion efficiency (IPCE)
was higher than 70% in the 390–540 nm region (Figure 5).
Electrochemical Impedance Spectra and the Influence of
CDCA
Electrochemical impedance spectroscopy (EIS) was per-
formed to further elucidate the photovoltaic properties of
these dyes.[21] EIS analysis of the DSSCs made with these
sensitizers were taken under a forward bias of À0.73 V in
the dark. In the Nyquist plot (Figure 6) a major semicircle
Figure 4. Comparison of the J–V curves of DSSCs made from the dyes
with N719.
Figure 5. Comparison of the IPCE plots of DSSCs made from the dyes
with N719.
Figure 6. J--V curves of DSSC made from CB-PSP with and without
CDCA.
PSP (13.32mAcmÀ2). This result complies well with their
higher absorptivity in the charge-transfer transitions
(Table 1, also see the Supporting Information, Figure S2).
The open-circuit voltages of CB-PSP and PT-PSP (0.685 V
and 0.68 V, respectively) were also higher than that of TB-
PSP (0.66 V). These values benefited from the reduction in
the rate of charge recombination at the nanocrystalline/dye/
redox-electrolyte interface, and was demonstrated by the
trend in dark currents, which appeared in the order CB-
PSP<PT-PSP<TB-PSP (Figure 4). Further support was
found by examining the decay in the transient photovoltages
of the assembled devices. The measurements were done on
an open circuit in the presence of LiI electrolyte in acetoni-
trile using a pulsed Nd:YAG laser (see the Supporting Infor-
mation, Figure S42). The decay lifetimes of the dyes are
listed in Table 2, and compound CB-PSP indeed showed the
longest lifetime (19.44ms). A longer electron lifetime indi-
cates a slower charge recombination, and therefore leads to
a larger Voc value. This rationale is also true for compound
PT-PSP, whose high value of Voc is also supported by its
was observed in the frequency range 20–100 Hz, which is re-
lated to the transport process of injected electrons at the in-
terfaces between TiO2 and the electrolyte/dye.[22] The
charge-recombination resistance (Rrec) can be deduced by
fitting the curves using Z-view software.[23] The resistance
value is related to the charge recombination rate at the TiO2
surface of the DSSC; for example, a smaller Rrec value indi-
cates a faster charge recombination and therefore a larger
dark current (Figure 5). The radius of the semicircle increas-
es in the order 6TB-PSP (23.9 ohm)<5TB-PSP
(30.5 ohm)<TB-PSP (33.1 ohm)<DTB-PSP (40.4 ohm)<
PT-PSP (46.3 ohm)<CB-PSP (47.1 ohm) in the dark. This
trend appears to be consistent with the Voc values of 6TB-
PSP (0.57 V)<5TB-PSP (0.63 V)<TB-PSP (0.66 V)ꢀ
DTB-PSP (0.67 V)<PT-PSP (0.68 V)ꢀCB-PSP (0.685 V).
In Bode phase plots, the peak position of the middle fre-
quency is related to the electron lifetime, for example, a
shift to low frequency corresponds to a longer electron life-
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Chem. Asian J. 2012, 7, 572 – 581

High-Performance Organic Materials for Dye-Sensitized Solar Cells
time. The injected electron lifetime (t) can be estimated di-
rectly by fitting the plots into the equation t=1/ACTHNUGTRNEUNG(2pf),
where f is the frequency. The lifetimes estimated by EIS
were compared with those obtained by the transient photo-
voltages. The trends in both series are consistent with one
another, as shown in Table 2.
pressing dark current was clear, and consequently the Voc
value was increased. However, the Jsc values decreased sub-
stantially in the presence of CDCA, presumably caused by a
reduction in the loading amount of the dye on the TiO2 sur-
face. This result indicated that self-aggregation of CB-PSP
did not happen to a significant degree, even without CDCA.
The addition of bulky tert-butyl substituents did have a pro-
found effect in preventing aggregation.
In a previous report, it has been shown that co-depositing
chenodeoxycholic acid (CDCA) with organic dyes on TiO2
can effectively retard the charge recombination, and conse-
quently increase the electron lifetime. CDCA also an effect
of preventing intermolecular aggregation of the dyes, there-
by enhancing the short-circuit current.[22] This method was
applied to our compound CB-PSP. With the addition of
CDCA, we observed an increase of the open circuit, yet sur-
prisingly we found a decrease in the short-circuit current
(Table 3). With 1 mm and 10 mm co-absorptions of CDCA,
the Jsc values decreased from 14.63 mAcmÀ2 to 12.52 and
11.40 mAcmÀ2, respectively, as did the FF values (from 0.67
to 0.63 and 0.66, respectively). The overall quantum efficien-
cy dropped to 5.47–5.61%. The addition of CDCA to CB-
PSP did not have a favorable influence on device perfor-
mance (Figure 6 and Figure 7). The reason for this phenom-
enon can be explored more clearly by examining the EIS
spectra. When the dye was co-absorbed with CDCA, the di-
ameter of the major semicircles in the Nyquist plot was en-
larged along with the increase in CDCA concentration
(Figure 7). The low-frequency peak in the Bode plot was
also shifted to lower frequency. Therefore, the effect of de-
Conclusions
The organic dyes reported here have several advantages:
1) easy handling and preparation from low-cost starting ma-
terials, 2) intense absorption in the visible-light region owing
to efficient intramolecular-charge-transfer transitions,
3) stable charge-separated states with long lifetimes, and
4) depressed dark-current because of the presence of bulky
4-tert-butylphenyl substituents.
Among the series of dyes tested, some structural factors
can be correlated with device performance. The dyes may
be divided into three groups based on their structural fea-
tures: 1) those with two aryl substituents on the nitrogen
donor atom (5TB-PSP and 6TB-PSP) 2) those with three
aryl substituents on the nitrogen donor atom (TB-PSP and
DTB-PSP); and 3) those with a fused ring between the aryl
groups (CB-PSP and PT-PSP). The DSSC performance was
found to be in the order of group 3>group 2>group 1,
whilst the highest performance was observed for compound
CB-PSP.
Table 3. Photovoltaic parameters of devices made with CB-PSP with and
without CDCA.[a]
In the group 1 dyes, the current density of compound
5TB-PSP (12.60 mAcmÀ2) is significantly higher than that
of 6TB-PSP (8.48 mAcmÀ2), as a result of higher ring pla-
narity. The conversion efficiencies of the two compounds
are 4.87% and 3.14%, respectively. For the dyes in groups 2
and 3, the addition of an additional aryl substituent on the
nitrogen atom lowered the HOMO levels substantially, thus
promoting the injection efficiency of electrons from the
electrolyte to the oxidized dyes. The dyes in groups 2 and 3
also exhibited higher absorptivity, which led to better IPCE
values than those in group 1.
Dye[b]
CDCA [mm]
Jsc [mAcmÀ2
]
Voc [V]
FF
h[c] [%]
CB-PSP
0
1
10
14.63
12.52
11.40
0.685
0.71
0.72
0.67
0.63
0.66
6.70
5.61
5.47
[a] Jsc =short-circuit photocurrent density; Voc =open-circuit photovolt-
age; FF=fill factor; h=total-power-conversion efficiency. [b] Concentra-
tion of dye is 3ꢃ10À4 m in THF. [c] Performance of DSSC measured in a
0.25 cm2 working area on an FTO (8 W/square) substrate under AM 1.5
conditions.
In the group 2 dyes, a comparison between compounds
DTB-PSP and TB-PPS indicated that the former exhibited
higher values in all three parameters, that is, short-circuit
current, open-circuit voltage, and field factor. The presence
of two bulky 4-tert-butyl substituents in compound DTB-
PSP provided better shielding between the electrolyte and
the TiO2 surface, so that the dark current was reduced and
the Voc value increased. The electron-donating nature of the
4-tert-butyl group also elevated the HOMO level, narrowed
down the band gap, and widened the absorption spectrum of
the dye. The IPCE plot of DTB-PSP covered a broader
wavelength range than that of TB-PSP. As a result, the over-
all conversion efficiency of the former is better than the
latter (6.26% vs. 5.43%).
Among all of the dyes tested, CB-PSP (group 3) showed
Figure 7. EIS Nyquist plots of CB-PSP with and without CDCA at
À0.73 V bias in the dark.
the highest molar extinction coefficient (4.00ꢃ104 mÀ1 cmÀ1),
Chem. Asian J. 2012, 7, 572 – 581
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577

T. J. Chow et al.
FULL PAPERS
on a Jasco-550 spectrophotometer. Emission spectra were obtained on a
Hitachi F-4500 spectrofluorimeter. The emission spectra in solution were
measured in spectroscopy grade solvent at a detection angle of 908. The
redox potentials were measured using cyclic voltammetry on a CHI 620
analyzer. The data were collected and analyzed using electrochemical
analysis software. All measurements were carried out in THF containing
0.1m tetrabutylammonium hexaflourophosphate (TBAPF6) as the sup-
porting electrolyte under ambient conditions after purging for 10 min
with N2. The conventional three-electrode configuration was employed,
which consisted of a glassy-carbon working electrode, a platinum counter
electrode, and a Ag/AgNO3 (0.1m) reference electrode calibrated with
ferrocene/ferrocenium (Fc/Fc+) as an internal reference with a scan rate
of 50 VsÀ1. Mass spectra were recorded on a VG70–250S mass spectrom-
eter. The cell containing the solution of the sample (1 mm) and the sup-
porting electrolyte were thoroughly purged with nitrogen gas before
measurements were taken. The oxidation potentials of organic dyes were
estimated by averaging the anode and cathode peak potentials. The
HOMO and LUMO values were calculated with reference to the ferro-
cene oxidation potential by using the following equations: HOMO=
which is consistent with the high oscillator strength (0.92)
calculated by TDDFT. The two phenyl moieties in the car-
bazole moiety maintained a better planar geometry than the
isolated phenyl groups in the triarylamine group. CB-PSP
possesses the lowest oxidation potential of all of the dyes,
which thus affords an increased efficiency for charge regen-
eration in the device. The Voc values of CB-PSP and PT-
PSP were among the highest observed (0.685 V and 0.68 V,
respectively). These values benefited by reducing the
charge-recombination rate in the nanocrystalline/dye/redox-
electrolyte interface. This phenomenon can be rationalized
by the long electron lifetimes measured by both the decay
of transient photovoltage (Table 2; also see the Supporting
Information, Figure S42) and the large Rrec values obtained
by EIS analyses (Figure 8). The device made with CB-PSP
E
ox+4.8 eV; LUMO=HOMOÀE0À0, where the HOMO of ferrocene was
set at 4.8 eV.
Carbazole, para-bromonitrobenzene, 1,2,3,4-tetrahydroquinoline, 2-meth-
ylindoline, 4-bromo-tert-butylbenzene, palladium(II) acetate (PdACHTUNGTRENNUNG(OAc)2),
1,1’- bis(diphenylphosphanyl) ferrocene (dppf), 1,4-dibromobenzene,
10H-phenothiazine, n-butyllithium (1.6m in n-hexane), trans-dichlorobis(-
triphenylphosphine) palladium(II) ([PdCl2ACTHNUTRGEN(UNG PPh3)2]), N,N-dimethylforma-
mide, 2-(tributylstannyl)thiophene, 4-bromobenzaldehyde, N-bromosucci-
nimide (NBS), diphenylamine, tin(II) chloride dihydrate (SnCl2·2H2O),
sodium borohydride, tri-n-butyltin chloride, cyanoacetic acid, ammonium
acetate, and glacial acetic acid, were purchased from ACROS, Alfa,
Merck, Lancaster, TCI, Sigma–Aldrich, and Showa, and purified where
necessary. Chromatographic separations were carried out by using silica
gel from Merck, Kieselgel si 60 (40–63 mm). The synthesis of compounds
1-X, 2-X, and 3-X, along with their spectra are provided in the Support-
ing Information.
Synthesis2-(4-Nitrophenyl)thiophene
To a three-necked flask containing a mixture of para-bromonitrobenzene
Figure 8. EIS Nyquist plots of the dyes at À0.73 V bias in the dark (i.e.,
minus the imaginary part of the impedance -Z“ vs. the real part of the
impedance Zꢀ when sweeping the frequency).
(6.22 g, 30.95 mmol), [PdCl2ACHTNUTRGNEG(UN PPh3)2] (0.65 g, 0.9 mmol), and 2-tributyl-
stannylthiophene (26.5 mL, 71.0 mmol) was added DMF (50 mL). The re-
action mixture was stirred at 908C for 24 h. After cooling, the reaction
was quenched by adding MeOH and saturated KF (aq., 45 mL). The mix-
ture was extracted with CH2Cl2 and the organic layer dried over anhy-
drous MgSO4. Evaporation of the solvent gave a crude product that was
purified by column chromatography on silica gel (CH2Cl2/n-hexane, 1:3).
The yellow solid was isolated in 80% yield (5.07 g, 24.76 mmol), m.p.:
138–1408C. 1H NMR (500 MHz, CDCl3): d=8.21 (dt, 2H, J=9.0,
2.5 Hz), 7.71 (dt, 2H, J=9.0, 2.5 Hz), 7.45 (dd, 1H, J=4.0, 1.0 Hz), 7.42
(dd, 1H, J=4.0, 1.0 Hz), 7.13 ppm (dd, 1H, J=5.0, 3.5 Hz). 13C NMR
(125 MHz, CDCl3): d=146.5, 141.5, 140.5, 128.6, 127.6, 125.9, 125.6,
124.3 ppm. MS (EI, 70 eV): m/z (%): 205 [M+] (100); HRMS calcd for
C10H7O2NS: 205.0197; found: 205.0196.
showed the lowest dark current (Figure 4). Therefore, the
CB-PSP dye exhibited the highest Jsc value of
14.63 mAcmÀ2, a Voc value of 0.685 V, and a FF value of
0.67, corresponding to a conversion efficiency (h) value of
6.7%.
In particular, the prevention of aggregation by the bulky
4-tert-butyl substituents was clearly demonstrated by the co-
absorption experiment with CDCA. The open-circuit volt-
age increased upon the addition of CDCA, whilst the short-
circuit current decreased. This decrease in the short circuit
current was caused by a decrease in loading amount. We
also found that self-aggregation on the surface of TiO2 did
not occur to a significant degree on these dyes.
4-(Thiophen-2-yl)aniline
A mixture of SnCl2·2H2O (19.26 g, 85.35 mmol) and 2-(4-nitrophenyl)-
AHCTUNGERTGtNNUN hiophene (3.5 g, 17.07 mmol) in degassed absolute EtOH was placed in
a three-necked flask under a nitrogen atmosphere, and heated to reflux.
Following this, a suspension of NaBH4 (323 mg, 8.54 mmol) in EtOH was
slowly added dropwise to this reaction and the mixture was heated to
reflux with stirring. After cooling, the reaction was quenched by adding
deionized water in an ice bath. To the resulting solution was added
NaOH (1 m) to render it strongly basic, and the mixture extracted with
EtOAc. The organic layer was dried over anhydrous MgSO4 and evapo-
rated under vacuum. The product was purified by column chromatogra-
phy on silica gel (CH2Cl2/n-hexane, 1:1). Yellow-green solid was obtained
in 90% yield (2.69 g, 15.36 mmol), m.p. 72–748C. 1H NMR (400 MHz,
CDCl3): d=7.44 (d, 2H, J=8.4 Hz), 7.19 (d, 1H, J=4.8 Hz), 7.17 (d, 1H,
J=3.2 Hz), 7.06 (t, 1H, J=4.0 Hz), 6.69 ppm (d, 2H, J=8.4 Hz).
Experimental Section
General Procedures
All reactions and manipulations were carried out under a nitrogen at-
mosphere. Solvents were distilled freshly according to standard proce-
1
dures. H and 13C NMR spectra were recorded on a Brucker (AV 400/AV
500 MHz) spectrometer in CDCl3 or [D6]DMSO. Chemical shifts are re-
ported downfield of tetramethylsilane. Absorption spectra were recorded
578
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Chem. Asian J. 2012, 7, 572 – 581

High-Performance Organic Materials for Dye-Sensitized Solar Cells
13C NMR (100 MHz, CDCl3): d=146.0, 145.0, 127.8, 127.1, 125.1, 123.1,
121.3, 115.3 ppm. MS (EI, 70 eV): m/z (%): 175 [M+] (100); HRMS calcd
for C10H9NS: 175.0456; found: 175.0456.
1H), 7.86 (d, 2H, J=8.5 Hz), 7.74 (d, 2H, J=8.0 Hz), 7.47 (d, 2H, J=
8.5 Hz), 7.41 (d, 1H, J=4.0 Hz), 7.27 (d, 2H, J=8.5 Hz), 7.25 (d, 2H, J=
8.0 Hz), 7.21 (d, 1H, J=4.0 Hz), 7.11 (d, 2H, J=8.5 Hz), 7.06–7.04 (m,
5H), 1.31 ppm (s, 9H). 13C NMR (125 MHz, CDCl3): d=191.2, 147.9,
147.3, 146.5, 146.1, 144.4, 140.4, 140.1, 134.8, 130.4, 129.2, 127.1, 126.4,
126.1, 126.0, 125.4, 124.5, 124.4, 123.2, 123.0, 122.8, 34.3, 31.3 ppm. MS
(FAB, 70 eV): m/z (%) 488 [M+H+] (100); HRMS calcd for C33H30NOS:
488.2048; found: 488.2053.
(N,N-Di-tert-butylphenyl)-4-(thiophen-2-yl)aniline (3-DTB)
A
mixture of 4-(thiophen-2-yl)aniline (1.5 g, 8.56 mmol), [Pd2ACTHNGUTERNN(UG dba)3]
(296 mg, 0.32 mmol), dppf (284 mg, 0.51 mmol), 1-tert-butyl-4-bromoben-
zene (4.54 mL, 21.4 mmol), and sodium tert-butoxide (4.11 g, 42.8 mmol)
in dry toluene was placed in a three-necked flask under a nitrogen atmos-
phere and was stirred at 908C for 15 h. After cooling, the reaction was
quenched by adding H2O, and then was extracted with EtOAc. The or-
ganic layer was dried over anhydrous MgSO4 and evaporated under
vacuum. The product was purified by column chromatography on silica
gel (n-hexane). Compound 3-DTB was obtained in 50% yield as a
yellow solid, m.p. 208–2108C. 1H NMR (400 MHz, CDCl3): d=7.47 (d,
2H, J=8.4 Hz), 7.29 (d, 4H, J=8.0 Hz), 7.23 (d, 2H, J=4.0 Hz), 7.08–
7.05 (m, 7H), 1.34 ppm (s, 18H). 13C NMR (100 MHz, CDCl3): d=147.6,
145.8, 144.8, 144.5, 127.9, 127.7, 126.6, 126.0, 124.0, 123.7, 122.9,
121.9 ppm. MS (EI, 70 eV): m/z (%): 439 [M+] (100); HRMS calcd for
C30H33NS: 439.2334; found: 439.2331.
4-(5-(1-(4-tert-Butylphenyl)-2-methylindolin-5-yl)thiophen-2-
yl)benzaldehyde (4–5TB)
Compound 4–5TB was synthesized according to the same procedure as
compound 4-DTB. Compound 4–5TB was obtained as an orange solid in
70% yield, m.p. 159–1618C. 1H NMR (400 MHz, CDCl3): d=10.00 (s,
1H), 7.89 (d, 2H, J=8.0 Hz), 7.77 (d, 2H, J=8.0 Hz), 7.44–7.41 (m, 4H),
7.34 (d, 1H, J=8.4 Hz), 7.21 (d, 2H, J=8.8 Hz), 7.19 (d, 1H, J=4.0 Hz),
6.80 (d, 1H, J=8.4 Hz), 4.46–4.41 (m, 1H), 3.39 (dd, 1H, J=15.2,
8.8 Hz), 2.82 (dd, 1H, J=15.6, 8.8 Hz), 1.37 (s, 9H), 1.35 ppm (s, 3H).
13C NMR (100 MHz, CDCl3): d=191.4, 149.2, 147.4, 146.3, 140.4, 140.0,
139.2, 134.5, 130.5, 130.2, 126.2, 126.1, 125.2, 124.2, 122.4, 122.1, 121.6,
108.0, 60.2, 36.8, 34.3, 31.4, 20.1 ppm. MS (FAB, 70 eV): m/z (%): 451
[M+] (100); HRMS calcd for C30H29NOS: 451.1970; found: 451.1967.
4-(5-(4-(Bis(4-tert-butylphenyl)amino)phenyl)thiophen-2-yl)benzaldehyde
(4-DTB)
4-(5-(10-(4-tert-Butylphenyl)-10H-phenothiazin-3-yl)thiophen-2-
yl)benzaldehyde (4-PT)
BuLi (5.4 mL, 8.54 mmol, 1.6m in n-hexane) was added dropwise to a so-
lution of 3-DTB (2.5 g, 5.69 mmol) in dry THF at À788C in a three-
necked flask under a nitrogen atmosphere. The solution was cooled to
À408C and stirred with a magnetic bar for 30 min. The solution was
cooled again to À788C and to it was added tri-n-butyltin chloride
(2.8 mL, 16.1 mmol). The reaction was warmed to room temperature and
stirred overnight. The reaction mixture was quenched by the addition of
H2O, and was extracted with CH2Cl2. The combined organic solution was
dried over anhydrous MgSO4, and dried under vacuum. The crude prod-
uct was dissolved in dry DMF, then to it was added 4-bromobenzalde-
Compound 4-PT was synthesized according to the same procedure as
compound 4-DTB. Compound 4-PT was obtained as an orange solid in
81% yield, m.p. 199–2018C. 1H NMR (500 MHz, CDCl3): d=7.85 (dd,
2H, J=8.4, 1.6 Hz), 7.71 (d, 2H, J=8.4 Hz), 7.60 (dd, 2H, J=8.4,
2.0 Hz), 7.37 (d, 1H, J=3.9 Hz), 7.29 (dd, 2H, J=8.4, 2.0 Hz), 7.24 (s,
1H), 7.14 (d, 1H, J=3.9 Hz), 7.07 (dd, 1H, J=8.6, 2.1 Hz), 6.99 (dd, 1H,
J=7.4, 1.8 Hz), 6.78–6.85 (m, 2H), 6.17 (d, 1H, J=7.8 Hz), 6.15 (d, 1H,
J=8.5 Hz), 1.40 ppm (s, 9H). 13C NMR (125 MHz, CDCl3): d=191.3,
151.7, 145.0, 144.2, 143.8, 140.6, 140.0, 137.7, 134.9, 130.4, 130.2, 128.1,
127.7, 127.0, 126.6, 126.0, 125.5, 124.3, 123.6, 123.2, 122.5, 120.4, 119.0,
115.9, 34.8, 31.4 ppm. MS (FAB, 70 eV): m/z (%) 517 [M+] (100); HRMS
calcd for C33H27NOS2: 517.1534; found: 517.1529.
hyde (1.0 g, 5.41 mmol) and [PdCl2ACHTNUGTRNEUNG(PPh3)2] (119 mg, 0.17 mmol). The so-
lution was heated to 908C for 24 h and then cooled. The reaction was
quenched by the addition of MeOH and saturated KF (aq., 15 mL). The
mixture was extracted with CH2Cl2, and the organic layer dried over an-
hydrous MgSO4. Evaporation of the solvent gave a crude product, which
was purified by column chromatography on silica gel (CH2Cl2/n-hexane,
4-(5-(1-(4-tert-Butylphenyl)-1,2,3,4-tetrahydroquinolin-6-yl)thiophen-2-
yl)benzaldehyde (4–6TB)
1:1). 4-DTB was obtained as
a yellow solid in 90% yield (2.78 g,
5.12 mmol), m.p. 235–2378C. 1H NMR (400 MHz, CDCl3): d=10.01 (s,
1H), 7.90 (d, 2H, J=8.0 Hz), 7.77 (d, 2H, J=8.4 Hz), 7.49 (d, 2H, J=
8.4 Hz), 7.44 (d, 1H, J=4.0 Hz), 7.31 (d, 4H, J=8.8 Hz), 7.24 (d, 1H, J=
4.0 Hz), 7.09 (d, 4H, J=8.8 Hz), 7.07 (d, 2H, J=8.4 Hz), 1.35 ppm (s,
18H). 13C NMR (100 MHz, CDCl3): d=191.4, 148.2, 146.2, 144.5, 140.3,
140.2, 134.7, 130.5, 126.7, 126.4, 126.1, 125.4, 124.4, 123.1, 122.3, 34.3,
31.4 ppm. MS (FAB, 70 eV): m/z (%): 544 [M+H+ (100); HRMS calcd
for C37H38NOS: 544.2674; found: 544.2682.
Compound 4–6TB was synthesized according to the same procedure as
compound 4-DTB. Compound 4–6TB was obtained as a yellow solid in
64% yield, m.p. 177–1798C. 1H NMR (500 MHz, CDCl3): d=9.96 (s,
1H), 7.85 (d, 2H, J=8.3 Hz), 7.72 (d, 2H, J=8.3 Hz), 7.37–7.39 (m, 3H),
7.30 (s, 1H), 7.16–7.19 (m, 3H), 7.13 (d, 1H, J=3.8 Hz), 6.67 (d, 1H, J=
8.5 Hz), 3.64 (t, 2H, J=5.6 Hz), 2.89 (t, 2H, J=6.3 Hz), 2.04–2.09 (m,
2H), 1.33 ppm (s, 9H). 13C NMR (125 MHz, CDCl3): d=191.3, 147.5,
147.2, 145.0, 144.8, 140.4, 139.3, 134.5, 130.4, 126.8, 126.4, 126.0, 125.2,
124.9, 124.1, 123.8, 123.2, 122.1, 115.1, 51.1, 34.4, 31.4, 27.8, 22.5 ppm. MS
(FAB, 70 eV): m/z (%): 451 [M+] (100); HRMS calcd for C30H29NOS:
451.1970; found: 451.1979.
4-(5-(9-(4-tert-Butylphenyl)-9H-carbazol-3-yl)thiophen-2-yl)benzaldehyde
(4-CB)
Compound 4-CB was synthesized according to the same procedure as
compound 4-DTB. A yellow solid of 4-CB was obtained in 75% yield,
m.p. 251–2538C. 1H NMR (400 MHz, CDCl3): d=10.02 (s, 1H), 8.42 (s,
1H), 8.21 (d, 1H, J=7.6 Hz), 7.92 (d, 2H, J=8.4 Hz), 7.83 (d, 2H, J=
8.4 Hz), 7.71 (d, 1H, J=8.4 Hz), 7.65 (d, 2H, J=8.4 Hz), 7.52–7.44 (m,
6H), 7.41 (d, 1H, J=3.6 Hz), 7.36–7.32 (m, 1H), 1.46 ppm (s, 9H).
13C NMR (100 MHz, CDCl3): d=191.4, 150.7, 147.4, 141.6, 140.8, 140.5,
140.3, 134.8, 134.5, 130.5, 126.8, 126.5, 126.3, 126.1, 125.9, 125.4, 124.1,
123.8, 123.4, 123.0, 120.4, 120.1, 117.6, 110.4, 110.2, 34.8, 31.4 ppm. MS
(FAB, 70 eV): m/z (%) 485 [M+] (100); HRMS calcd for C33H27NOS:
485.1813; found: 485.1814.
(E)-3-(4-(5-(4-(Bis(4-tert-butylphenyl)amino)phenyl)thiophen-2-
yl)phenyl)-2-cyanoacrylic acid (DTB-PSP)
A mixture of compound 6-DTB (1.05 g, 1.93 mmol), cyanoacetic acid
(165 mg, 1.93 mmol), and ammonium acetate (38 mg, 0.48 mmol) in
acetic acid was placed in a three-necked flask under a nitrogen atmos-
phere and was stirred at 90–1008C for 12 h. After cooling, the reaction
was quenched by adding distilled water and extracted with CH2Cl2. The
organic layer was dried over anhydrous MgSO4 and evaporated under
vacuum. The product was purified by column chromatography on silica
gel (CH2Cl2/acetic acid, 19:1). The black solid was isolated in 61% yield
(718 mg, 1.17 mmol), m.p. 278–2808C. 1H NMR (400 MHz, [D6]DMSO):
d=8.26 (s, 1H), 8.07 (d, 2H, J=8.4 Hz), 7.87 (d, 2H, J=8.4 Hz), 7.74 (d,
1H, J=4.0 Hz), 7.59 (d, 2H, J=8.8 Hz), 7.46 (d, 1H, J=3.6 Hz), 7.36 (d,
4H, J=8.8 Hz), 7.00 (d, 4H, J=8.8 Hz), 6.93 (d, 2H, J=8.4 Hz),
1.28 ppm (s, 18H).13C NMR (100 MHz, [D6]DMSO): d=163.7, 148.0,
146.4, 145.3, 144.5, 140.3, 137.9, 131.8, 131.0, 127.6, 126.9, 126.8, 126.6,
4-(5-(4-(4-tert-Butylphenyl)phenyl)amino)phenyl)thiophen-2-
yl)benzaldehyde (4-TB)
Compound 4-TB was synthesized according to the same procedure as
compound 4-DTB. Compound 4-TB was obtained as a yellow solid in
71% yield, m.p. 157–1598C. 1H NMR (500 MHz, CDCl3): d=9.97 (s,
Chem. Asian J. 2012, 7, 572 – 581
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T. J. Chow et al.
FULL PAPERS
125.7, 124.7, 124.5, 122.1, 34.5, 31.6 ppm. MS (FAB, 70 eV): m/z (%) 611
[M+H+] (100); HRMS calcd for C40H39N2O2S: 611.2732; found: 611.2741.
1H, J=3.8 Hz), 7.39 (d, 2H, J=8.5 Hz), 7.31–7.34 (m, 2H), 7.21 (dd, 1H,
J=8.5, 2.0 Hz), 7.16 (d, 2H, J=8.5 Hz), 6.51 (d, 1H, J=8.5 Hz), 3.56 (t,
2H, J=5.5 Hz), 2.82 (t, 2H, J=5.5 Hz), 1.95 (m, 2H), 1.27 ppm (s, 9H).
13C NMR (125 MHz, [D6]DMSO): d=163.8, 153.2, 147.3, 146.5, 145.1,
145.0, 139.0, 138.3, 132.0, 130.5, 127.7, 126.8, 125.5, 125.1, 124.4, 124.3,
123.2, 123.0, 117.2, 115.0, 51.1, 34.6, 31.6, 27.6, 22.4 ppm. MS (FAB,
70 eV): m/z (%) 518 [M+] (100); HRMS calcd for C33H30N2O2S:
518.2028; found: 518.2024.
(E)-3-(4-(5-(9-(4-tert-Butylphenyl)-9H-carbazol-3-yl)thiophen-2-
yl)phenyl)-2-cyanoacrylic acid (CB-PSP)
Compound CB-PSP was synthesized according to the same procedure as
compound DTB-PSP. Compound CB-PSP was obtained as a dark-red
solid in 70% yield, m.p. 298–3008C. 1H NMR (400 MHz, CDCl3): d=
8.66 (s, 1H), 8.36 (d, 1H, J=7.6 Hz), 8.32 (s, 1H), 8.12 (d, 2H, J=
8.0 Hz), 7.93 (d, 2H, J=8.0 Hz), 7.83 (d, 1H, J=4.0 Hz), 7.80 (d, 1H, J=
8.4 Hz), 7.72 (d, 2H, J=8.0 Hz), 7.67 (d, 1H, J=4.0 Hz), 7.59 (d, 2H, J=
8.0 Hz), 7.49–7.39 (m, 3H), 7.34 (t, 1H, J=7.2 Hz), 1.41 ppm (s, 9H).
13C NMR (100 MHz, CDCl3): d=163.8, 153.6, 150.7, 146.6, 141.2, 140.4,
140.2, 138.3, 134.4, 132.1, 130.6, 127.9, 127.4, ,127.2, 126.6, 126.0, 125.7,
124.7, 124.6, 123.7, 123.0, 121.4, 120.8, 117.9, 116.9, 110.8, 110.4, 35.0,
31.6 ppm. MS (FAB, 70 eV): m/z (%): 552 [M+] (100); HRMS calcd for
C36H28N2O2S: 552.1871; found: 552.1869.
Fabrication and Characterization of DSSCs
The FTO-conducting glass (FTO glass, fluorine-doped tin-oxide over-
layer, transmission>90% in the visible region, sheet resistance 8 W
squareÀ1), titanium dioxide pastes of titanium-nanoxide T/SP and titani-
um-nanoxide R/SP were purchased from Solaronix. A thin film of TiO2
(ca. 16–18 mm thick) was coated onto a 0.25 cm2 FTO glass substrate, and
the thickness was measured by Veeco Dektak 150. The substrate was im-
mersed in a THF solution containing 3ꢃ10À4 m dye sensitizers for at least
12 h, then rinsed with anhydrous acetonitrile and dried. Another piece of
FTO that was covered with sputtered platinum (100 nm thick) was used
as a counter electrode. The active area was controlled at a dimension of
0.25 cm2 by adhering polyester tape (60 mm thick) onto the platinum elec-
trode. The photocathode was placed on top of the counter electrode and
was tightly clipped together to form a cell. Electrolyte was then injected
into the seam between two electrodes. An acetonitrile solution containing
LiI (0.5m), I2 (0.05m), and 4-tert-butylpyridine (0.5m) was used as the
electrolyte. A device made from a commercial dye (N719) under the
same conditions (3ꢃ10À4 m, Solaronix S.A., Switzerland) was used as a
reference. The cell parameters were obtained under an incident beam of
light (intensity 100 mWcmÀ2, measured by a thermopile probe; Oriel
71964) that was generated by a 300 W Xe lamp (Oriel 6258) passing
through an AM 1.5 filter (Oriel 81088). The current–voltage parameters
of DSSCs were recorded by a potentiostat/galvanostat model CHI650B
(CH Instruments, USA). The light intensity was further calibrated by an
Oriel reference solar cell (Oriel 91150) and adjusted to be 1.0 sun. The
monochromatic quantum efficiency was recorded through a monochro-
mator (Oriel 74100) at short-circuit condition. Electrochemical impe-
dance spectra of DSSCs were recorded by an Impedance/Gain-Phase an-
alyzer (SI 1260, Solartron). The frequencies explored were in the range
of 10 mHz to 65 kHz. The alternating current (ac) amplitude was set at
20 mV. The photovoltage transients of assembled devices were recorded
using a digital oscilloscope (Tektronix, TDS 3012b). Pulsed-laser excita-
tion was applied by a Q-Switched Nd:YAG laser (Quanrel, brilliant B)
with a 10 Hz repetition rate at 532 nm and a 7 ns pulse width at half-
height. The laser energy of 3 mJcmÀ2 to cover the area of the device size
was slightly larger than 0.25 cm2. The recombination lifetime of injected
electrons with oxidized dyes was measured by transient photovoltages on
an open circuit in the presence of LiI electrolyte (0.5m) in acetonitrile.
The average electron lifetime can be approximately estimated by fitting a
(E)-3-(4-(5-(4-(N-(4-tert-butylphenyl)-N-phenyl)amino)phenyl)thiophen-
2-yl)phenyl)-2-cyanoacrylic acid (TB-PSP)
Compound TB-PSP was synthesized according to the same procedure as
compound DTB-PSP. Compound TB-PSP was obtained as a dark-red
solid in 64% yield, m.p. 274–2768C. 1H NMR (400 MHz, [D6]DMSO):
d=8.30 (s, 1H), 8.09 (d, 2H, J=8.0 Hz), 7.88 (d, 2H, J=8.0 Hz), 7.76 (d,
1H, J=3.6 Hz), 7.61 (d, 2H, J=8.4 Hz), 7.49 (d, 1H, J=3.6 Hz), 7.38–
7.31 (m, 4H), 7.09–7.05 (m, 3H), 7.02 (d, 2H, J=8.4 Hz), 6.97 (d, 2H,
J=8.4 Hz), 1.29 ppm (s, 9H). 13C NMR (100 MHz, [D6]DMSO): d=
163.7, 147.8, 147.2, 146.6, 145.2, 144.4, 140.3, 138.1, 131.9, 130.8, 130.0,
127.8, 127.0, 126.9, 125.7, 124.9, 124.7, 124.6, 123.8, 122.7, 34.5, 31.6 ppm.
MS (FAB, 70 eV): m/z (%) 555 [M+H+] (100); HRMS calcd for
C36H31N2O2S: 555.2106; found: 555.2113.
(E)-3-(4-(5-(1-(4-tert-Butylphenyl)-2-methylindolin-5-yl)thiophen-2-
yl)phenyl)-2-cyanoacrylic acid (5TB-PSP)
Compound 5TB-PSP was synthesized according to the same procedure
as compound DTB-PSP. Compound 5TB-PSP was obtained as a black
solid in 65% yield, m.p. 267–2698C. 1H NMR (400 MHz, [D6]DMSO):
d=8.29 (s, 1H), 8.08 (d, 2H, J=8.4 Hz), 7.85 (d, 2H, J=8.4 Hz), 7.72 (d,
1H, J=3.6 Hz), 7.50 (s, 1H), 7.44–7.35 (m, 4H), 7.22 (d, 2H, J=8.4 Hz),
6.74 (d, 1H, J=8.4 Hz), 4.53–4.44 (m, 1H), 3.40–3.36 (m, 1H), 2.76 (dd,
1H, J=16.2, 7.6 Hz), 1.30 (s, 9H), 1.25 ppm (s, 3H).13C NMR (100 MHz,
[D6]DMSO): d=163.8, 148.9, 146.7, 145.8, 139.9, 138.8, 132.0, 130.8,
127.7, 127.4, 126.5, 125.5, 125.4, 124.0, 123.2, 122.5, 121.5, 107.8, 59.6,
36.4, 34.5, 31.6, 20.2 ppm. MS (FAB, 70 eV): m/z (%) 518 [M+] (100);
HRMS calcd for C33H30N2O3S: 518.2028; found: 518.2026.
(E)-3-(4-(5-(10-(4-tert-Butylphenyl)-10H-phenothiazin-3-yl)thiophen-2-
yl)phenyl)-2-cyanoacrylic acid (PT-PSP)
decay of the open-circuit voltage transient to exp
and t is an average time constant before recombination.
G
Compound PT-PSP was synthesized according to the same procedure as
compound DTB-PSP. Compound PT-PSP was obtained as a dark-red
solid in 60% yield, m.p. 302–3048C. 1H NMR (500 MHz, [D6]DMSO):
d=7.85 (s, 1H), 7.80 (d, 2H, J=8.5 Hz), 7.65–7.68 (m, 5H), 7.43 (d, 1H,
J=3.9 Hz), 7.37 (d, 1H, J=2.2 Hz), 7.33 (d, 2H, J=8.5 Hz), 7.22 (dd,
2H, J=8.5, 2.1 Hz), 7.05 (dd, 1H, J=7.6, 1.5 Hz), 6.91 (td, 1H, J=7.6,
1.5 Hz), 6.83 (td, 1H, J=7.3, 1.0 Hz), 6.08 (d, 1H, J=8.5 Hz), 6.07 (d,
1H, J=5.8 Hz), 4.70 (s, 2H), 1.34 ppm (s, 9H). 13C NMR (125 MHz,
[D6]DMSO): d=193.4, 167.7, 166.8, 151.7, 143.8, 143.7, 143.6, 140.7,
137.7, 136.3, 133.6, 132.2, 132.0, 130.4, 128.3, 128.2, 127.9, 127.3, 127.1,
126.1, 125.0, 124.9, 123.5, 123.3, 121.6, 120.2, 118.7, 116.4, 116.2, 45.6,
35.0, 31.6 ppm. MS (FAB, 70 eV): m/z (%) 584 [M+] (100); HRMS calcd
for C36H28N2O2S2: 584.1592; found: 584.1594.
Quantum Chemistry Computation
All organic dyes were optimized by using B3LYP/6–31G* hybrid func-
tional. Geometry optimizations were performed to locate the minima on
the potential energy surface, in order to predict the equilibrium structure
of a given molecule. For the excited states, a time-dependent density-
functional theory (TDDFT) with the B3LYP functional was employed.
All analyses were performed under Q-Chem 3.0 software. The frontier-
orbital plots of HOMO and LUMO were drawn by using Gaussian 03.
All calculations were performed using the Gaussian 03 suite of programs.
(E)-3-(4-(5-(1-(4-tert-Butylphenyl)-1,2,3,4-tetrahydroquinolin-6-
yl)thiophen-2-yl)phenyl)-2-cyanoacrylic acid (6TB-PSP)
Acknowledgements
Compound 6TB-PSP was synthesized according to the same procedure
as compound DTB-PSP. Compound 6TB-PPS was obtained as a black
solid in 52% yield, m.p. 234–2368C. 1H NMR (500 MHz, [D6]DMSO):
d=8.24 (s, 1H), 8.03 (d, 2H, J=8.5 Hz), 7.81 (d, 2H, J=8.5 Hz), 7.67 (d,
Financial support from the National Science Council and Academia
Sinica are gratefully acknowledged. Special thanks to Professor C.-P. Hsu
(Institute of Chemistry, Academia Sinica) for his assistance with Quan-
tum chemistry computations.
580
www.chemasianj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 572 – 581

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Received: September 14, 2011
Published online: December 23, 2011
Chem. Asian J. 2012, 7, 572 – 581
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
581