New donor-π-acceptor type triazatruxene derivatives for highly efficient dye-sensitized solar cells

Article abstract of DOI:10.1021/ol402931u

A new class of organic dyes based on triazatruxene have been designed and synthesized for dye-sensitized solar cells. The photoelectronic properties of these donor-π-acceptor dyes can be tuned by changing π-conjugated linkers. The best performance was found for triazatruxene dye TD1, wherein, with thiophene as the conjugated linker and cyanoacrylic acid as the acceptor, a power conversion efficiency up to 6.10% was achieved.

Full text of DOI:10.1021/ol402931u

ORGANIC
LETTERS
2013
Vol. 15, No. 23
6034–6037
New Donor-π-Acceptor Type
Triazatruxene Derivatives for Highly
Efficient Dye-Sensitized Solar Cells
Xing Qian,† Yi-Zhou Zhu,*,† Jian Song,‡ Xue-Ping Gao,‡ and Jian-Yu Zheng*,†
State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University,
Tianjin, 300071, China, and Institute of New Energy Material Chemistry, Nankai
University, Tianjin, 300071, China
zhuyizhou@nankai.edu.cn; jyzheng@nankai.edu.cn
Received October 10, 2013
ABSTRACT
A new class of organic dyes based on triazatruxene have been designed and synthesized for dye-sensitized solar cells. The photoelectronic propertiesof
these donor-π-acceptor dyes can be tuned by changing π-conjugated linkers. The best performance was found for triazatruxene dye TD1, wherein, with
thiophene as the conjugated linker and cyanoacrylic acid as the acceptor, a power conversion efficiency up to 6.10% was achieved.
Dye-sensitized solar cells (DSSCs) have been consider-
ably developed due to their significant potential to be used as
low-cost devices for solar energy-to-electricity conversion.1
Great efforts in many aspects covering semiconductors,
counter electrodes, electrolytes, and sensitizers have been
devoted to improving the power conversion efficiency.2 In
these cells, the sensitizers play a key role in their performance
and remain one of the most studied areas.3 Many new
sensitizers have been reported, and currently there are two
major categories of sensitizers: metalÀorganic complexes
and metal-free organic dyes.4 The metalÀorganic
complexes mainly include polypyridyl Ru(II) complexes,
porphyrins and phthalocyanines, which have shown pro-
mising performance.5 However, some problems such as
toxicity, limited resource, and high cost inevitably coexist.
On the other hand, metal-free organic sensitizers have
attracted considerable attention for their structural diver-
sity, high molar extinction coefficient, ease of structural
tuning, and low cost.6
Triazatruxene (TAT), in which three indole units are
combined by one benzene ring, can be considered as
an extended delocalized π-system. The replacement of
5,10,15-C atoms of truxene by three N atoms endows the
resultant triazatruxene with excellent electron-donating
† State Key Laboratory and Institute of Elemento-Organic Chemistry.
‡ Institute of New Energy Material Chemistry.
€
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r
10.1021/ol402931u
Published on Web 11/13/2013
2013 American Chemical Society

derivatives have been used to improve the power conver-
sion efficiency of DSSCs and significant improvements
have been achieved.13cÀe Triazatruxene in which three
carbazole units share one benzene ring, compared to a
single carbazole system, has an enlarged π-system and may
therefore increase the electron donating ability of the D-π-
A type dye.12 So we present the D-π-A type dyes consisting
of a triazatruxene moiety acting as the electron donor and
2-cyanoacylic acid acting as the electron acceptor/anchoring
group. Aromatic rings such as thiophene, furan, and benzene
have been selected to act as the π-conjugated linkers and
resulted in sensitzers TD1, TD2, and TD3, respectively. The
molecular structures of these dyes are shown in Figure 1. The
syntheses of these dyes involved two major steps: (1) Suzuki
cross-coupling of brominated triazatruxene and substituted
aromatic aldehydes produced the π-extended triazatruxene
bearing aldehydes; (2) Knoevenagel reaction of the resulting
aldehydes and cyanoacetic acid afforded the target sensitizers
TD1À3. Details of the synthesis are shown in Scheme 1.
Figure 1. Structures of the triazatruxene-based dyes TD1À3.
potential for intramolecular charge transfer.7,8 Owing to
its unique discotic π-extended and electron-rich aromatic
structure, this C3 symmetric fused indole trimer has been
widely used to produce electroactive discotic liquid-
crystalline materials,9 hole transporting materials with high
hole mobility,10 organic-light-emitting diodes (OLEDs),11
and two-photon absorption (TPA) materials.8,12 However,
to the best of our knowledge, there are no reports about the
application in dye-sensitized solar cells. Inspired by its
proven strong intramolecular charge transfer characteristic,
synthetic flexibility, and high stability, we report herein the
design, synthesis, and characterization of a series of new
organic sensitizers based on triazatruxene.
Scheme 1. Synthetic Procedures of TD1À3
Figure 2. UVÀvis absorption spectra of TD1À3 in THF and on
TiO2 films (inset).
The UVÀvis absorption spectra of TD1À3 are depicted
in Figure 2. The data for the absorption, electrochemical
properties, and frontier orbital energy levels are summar-
ized in Table 1. Each of these organic dyes exhibits two
major absorption bands at 280À350 and 400À550 nm in
THF solution. The short wavelength bands can be as-
signed to the localized aromatic πÀπ* transitions, while
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ꢀ
´
ꢀ
ꢀ
ꢀ
´
rez, R.; Gomez-Lor, B.
´
The donor-π-acceptor (D-π-A) structure has been com-
monly involved in most organic dyes owing to its efficient
intramolecular charge transfer (ICT) characteristic, which
is important for light harvesting.13 The excited state gen-
erated by an ICT transition from donor to acceptor is
responsible for the injection into the conduction band of
the semiconductors.14 Recently, the D-π-A type carbazole
ꢀ
(9) (a) Gomez-Lor, B.; Alonso, B.; Omenat, A.; Serrano, J. L. Chem.
Commun. 2006, 5012. (b) Luo, J.; Zhao, B.; Shao, J.; Lim, K. A.; Chan,
H. S. O.; Chi, C. J. Mater. Chem. 2009, 19, 8327. (c) Zhao, B.; Liu, B.;
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Org. Lett., Vol. 15, No. 23, 2013
6035

the long wavelength ones correspond to intramolecular charge
transfer (ICT) transitions.15 The intensity of πÀπ* transitions
is higher than that of the ICT transitions. The absorption
maxima (λmax) ofTD1 (476nm) andTD2 (442 nm) at the long
wavelength region are more red-shifted in comparison to
that of TD3 (422 nm). That can be ascribed to the better
electron delocalization over the whole molecule with
electron-rich thiophene or furan as the conjugated linker.
As shown in Figure 2, when anchoring on mesoporous
TiO2 films, these dyes all show significantly broadened and
red-shifted absorption spectra compared with that mea-
sured in THF solutions.
level. Theelectron distributionsof the HOMO and LUMO
of these dyes are shown in Figure 3. For the HOMO, the
electron density is uniformly distributed along the triaza-
truxene unit and the π-spacer bridging unit, whereas the
LUMO shows localized electron distributions though the
cyanoacrylic acid and its adjacent π-spacer due to the
intramolecular charge transfer along the π-conjugated
skeleton. These electron distributions will therefore ensure
efficient electron injection from the dye to the conduction
band of TiO2.
Table 1. Photophysical, Electrochemical Data for TD1À3a
λmax
nm
/
ε/
Eox
/
E0À0
V
/
Ered
/
HOMO/
dye
M
À1 cmÀ1
V
V
LUMO/eV
TD1
318
476
316
442
317
422
75 336
39 319
0.91
0.90
0.93
2.03
2.07
2.13
À1.12 À5.41/À3.38
À1.17 À5.40/À3.33
À1.20 À5.43/À3.30
TD2
TD3
100 937
40 459
112 046
38 102
a E0À0 values (zerothÀzeroth transition energies) were estimated
NHE vs the vacuum level was set to 4.5 V.16
Figure 3. Electron distributions of the HOMO and LUMO of
the dyes TD1, TD2, and TD3.
from the onset absorption wavelength in THF. Ered = Eox À E0À0
.
Cyclic voltammetry (CV) measurement was used to
study the redoxbehavior ofTD1, TD2, and TD3 inCH2Cl2
with a 100 mV sÀ1 scan rate and calibrated against ferro-
cene (0.63 V vs. NHE),17 using 0.1 M tetrabutylammo-
nium hexafluorophosphate as the supporting electrolyte.
The oxidation potential of the organic dyes was deter-
mined from the peak potentials by CV, and the oxidation
potential (Eox) corresponded to the highest occupied mo-
lecular orbital (HOMO), while the lowest unoccupied
molecular orbital (LUMO) could be calculated from
Eox À E0À0. As shown in Table 1, these triazatruxene-based
sensitizers show a first reversible oxidation wave at 0.91
(TD1), 0.90 (TD2), and 0.93 V (TD3), respectively. As a
result, the ground-state oxidation potentials of the three dyes
are higher than the redox potential of the iodide/triiodide
redox couple (0.4 V vs. NHE) in all cases. On the other hand,
the LUMO of these dyes (À1.12, À1.17, and À1.20 V vs.
NHE, respectively) are also more negative than the conduc-
tion band of TiO2 (À0.5 V vs. NHE). These values allow an
effective electron injection into the conduction band (CB) of
TiO2 and ensure the regeneration of the oxidized form of the
dyes in a DSSC.
Table 2. Photovoltaic Performance of TD1À3 and Reference
N719a
dye
Voc/mV
Jsc/mA cmÀ2
ff
PCE %
TD1
TD2
TD3
N719
670
654
686
734
14.7
13.6
11.6
16.9
0.62
0.62
0.64
0.63
6.10
5.50
5.11
7.82
a Active area is 0.16 cm2. Electrolyte is composed of 0.3 M 1,2-
dimethyl-3-propylimidazolium iodide (DMPII), 0.1 M LiI, 0.05 M I2,
and 0.5 M 4-tert-butyl pyridine in acetonitrile.
Liquid DSSCs were fabricated to test the photovoltaic
performance of TD1, TD2, and TD3. (The method for the
cell fabrication and photovoltaic characterization are de-
scribed in the Supporting Information (SI).) The DSSC
performance parameters of TD1À3 and the standard N719
dye are displayed in Table 2. The photocurrent densityÀ
voltage (JÀV) curves of these dyes are plotted in Figure 4.
TheDSSC performancesof all the dyesare evaluatedunder
AM 1.5 G irradiation at 100 mW cmÀ2 with a 0.16 cm2 active
surface area. The solar cell based on TD1 showed a power
conversion efficiency (PCE) of 6.10%, with a short-circuit
photocurrent density (Jsc) of 14.7 mA cmÀ2, an open-circuit
photovoltage (Voc) of 670 mV, and a fill factor (ff) of 0.62
under standard conditions. When the conjugating π-spacer
thiophene was replaced by furan, the Voc and Jsc of the
resulting TD2 (Voc: 654 mV; Jsc: 13.6 mA cmÀ2) were both
decreased. The solar cell based on TD2 finally showed an
efficiency of 5.50%, with a fill factor of 0.62. At the same
To obtain deep insight into the molecular structure and
frontier molecular orbitals of TD1, TD2, and TD3, the
geometries of these dyes were optimized by density func-
tional theory (DFT) calculations at the B3LYP/6-31G(d)
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6036
Org. Lett., Vol. 15, No. 23, 2013

higher efficiency observed for TD1 is mainly due to the
relatively larger Jsc.
Electrochemical impedance spectroscopy (EIS) is a power-
ful technique of characterizing the important interfacial
charge transfer and carrier transportation process in DSSCs.18
EIS Nyquist plots and Bode phase plots measured in the dark
under forward bias (À0.65 V) are shown in Figure S1 (SI).
The larger semicircle in the lower frequency range represents
the interfacial charge transfer resistances (Rct) at the TiO2/
dye/electrolyte interface.19 The fitted Rct increases in the
order TD2 (55 Ω) < TD1 (110 Ω) < TD3 (249 Ω). This
trend appears to be consistent with the values of open circuit
voltage. A larger Rct commonly indicates that the electron
recombination in the devices is strongly reduced, thus result-
ing in a smaller dark current and a larger value of open-circuit
photovoltage.20 The electron lifetime (τ), another important
parameter for DSSCs, could be extracted from the peak
frequency (f) at a lower frequency region in EIS Bode plots
using τ = 1/(2πf).21 In general, a longer electron lifetime
corresponds to a larger value of Voc. The electron lifetime
(τ) was estimated to follow the trend TD2 (16 ms) < TD1
(32 ms) < TD3 (47 ms), which is in good agreement with the
above-mentioned result.
In conclusion, three new D-π-A type organic sensitizers
TD1, TD2, and TD3 composed of a triazatruxene donor, a
cyanoacrylic acid acceptor, and an aromatic π-bridge have
been successfully designed and synthesized. All sensitizers
have shown significant performance in dye-sensitized solar
cells (over 5% power conversion efficiency), and the high-
est PCE value is up to 6.1% with a thiophene as the linker.
The results demonstrate that the triazatruxene-based sen-
sitizer can produce high photocurrent density and is a
promising candidate in DSSCs. Further studies for triaza-
truxene-based sensitizers with higher efficiency through
molecular modifications are ongoing in our laboratory.
Figure 4. Photocurrent densityÀvoltage (JÀV) curves and the
IPCE spectra (inset) of TD1À3 and N719.
time, the solar cell based on TD3 (with a benzene ring as the
conjugating π-spacer; Voc: 686 mV; Jsc: 11.6 mA cmÀ2
)
showed an efficiency of 5.11%, with a fill factor of 0.64. The
best efficiency of TD1 reached about 78% of the ruthenium
dye N719-based standard cell fabricated and measured
under the same conditions. To analyze the difference be-
tween the Jsc values, the incident photon-to-current conver-
sion efficiencies (IPCEs) as a function of incident wave-
length for DSSCs based on these dyes are plotted as the inset
in Figure 4. As shown in IPCE spectra, the dyes TD1À3 can
all efficiently convert the light to photocurrents in the region
from 400 to 700 nm. However, the onsets of the IPCE
spectra for TD1À3 are at 750, 730, and 690 nm, respectively,
which are significantly broadened compared with those of
their UVÀvis absorption spectra in THF. The IPCE value
changing tendency of these dyes is in accordance with their
UVÀvis absorption spectra in THF solutions. Moreover,
the TD1 based cell gave over 60% IPCE values from 410 to
610 nm with a maximum IPCE value of 78% at 480 nm, and
its photocurrent signal is up to about 750 nm. The high
IPCE value and broader absorption of TD1 explained the
high Jsc value from the JÀV measurement. The somewhat
Acknowledgment. The authors wish to thank the 973
Program (2011CB932502) and NSFC (Nos. 21172126 and
21272123) for their generous financial support.
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Interfaces 2012, 4, 1822.
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Supporting Information Available. Detailed experimen-
tal procedures and characteristic data are provided. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 23, 2013
6037