Incorporating a new 2 H-[1,2,3]Triazolo[4,5-c ]pyridine moiety to construct D-A-π-A organic sensitizers for high performance solar cells

Article abstract of DOI:10.1021/ol501163b

Two new organic dyes (PTN1 and NPT1) of the configuration D-A-π-A, based on 2H-[1,2,3]triazolo[4,5-c]pyridine (PT) as a central linker, have been synthesized and used as the sensitizers for dye-sensitized solar cells. Compared with pyridal[2,1,3]thiadiazole-containing congeners, the new dyes have conversion efficiencies nearly 1 order higher due to alleviation of charge trapping. The best conversion efficiency of the cell reaches 6.05% (under AM 1.5G irradiation). Upon addition of the coadsorbent CDCA, the efficiency is boosted to 6.76%, which reaches ～90% of the standard based on N719.

Full text of DOI:10.1021/ol501163b

Letter
pubs.acs.org/OrgLett
Incorporating a New 2H‑[1,2,3]Triazolo[4,5‑c]pyridine Moiety To
Construct D−A−π−A Organic Sensitizers for High Performance Solar
Cells
Sumit Chaurasia, Wei-I Hung, Hsien-Hsin Chou, and Jiann T. Lin*
Institute of Chemistry, Academia Sinica, 115 Nankang, Taipei, Taiwan
S
* Supporting Information
ABSTRACT: Two new organic dyes (PTN1 and NPT1) of the
configuration D−A−π−A, based on 2H-[1,2,3]triazolo[4,5-c]-
pyridine (PT) as a central linker, have been synthesized and
used as the sensitizers for dye-sensitized solar cells. Compared with
pyridal[2,1,3]thiadiazole-containing congeners, the new dyes have
conversion efficiencies nearly 1 order higher due to alleviation of
charge trapping. The best conversion efficiency of the cell reaches
6.05% (under AM 1.5G irradiation). Upon addition of the
coadsorbent CDCA, the efficiency is boosted to 6.76%, which
reaches ∼90% of the standard based on N719.
rganic dyes as light harvesters for dye-sensitized solar
cells (DSSCs) have attracted considerable attention for
triazole ring,9 leading to the more electron-rich character of the
BTz. Though the electron-withdrawing −NN− and −C
N− groups10 render the BTz to be electron-accepting11 and
capable of electron transporting,12 charge trapping is greatly
alleviated in BTz-based dipolar sensitizers13 compared to the
BT-based congeners.
In our previous report, we successfully synthesized a series of
new dyes using an electron-deficient pyridal[2,1,3]thiadiazole14
(PyT) entity as an auxiliary acceptor and obtained the
maximum efficiency of 4.24%. Though PyT dyes had an even
longer wavelength absorption compared with their BTz
congeners, they exhibited significant charge trapping and
deteriorated the cell performance. Therefore, we decided to
replace PyT with a 2H-[1,2,3]triazolo[4,5-c]pyridine (PT)
entity so as to reach a compromise between light harvesting
and charge trapping. To the best of our knowledge, there is no
report on PT-based sensitizers for DSSCs. Herein, we report
the synthesis and DSSC performance of two new PT-based
organic dyes PTN1 and NPT1 for the first time. The power
conversion efficiency up to 6.76% along with a high VOC (0.70
V) was achieved with NPT1-based DSSC.
Our synthetic approach to regioregular PT-containing dyes is
centered on the chemistry of 2H-[1,2,3]triazolo[4,5-c]pyridine
(PT). The key precursor, 4,7-dibromo-2-hexyl-2H-[1,2,3]-
triazolo[4,5-c]pyridine (PTBr2, 1), was synthesized in two
steps by following a modified reported method.15 The alkyl
chain present at the nitrogen atom is beneficial to the solubility
in common organic solvents and may possibly suppress the
close contact of the electrolytes toward the TiO2 surface (vide
infra).16 The first Stille coupling of the asymmetric PTBr2
O
the past two decades after the seminal report of Gratzel and co-
̈
workers on ruthenium complexes.1 The record high conversion
efficiency (η) of 13.0% has been achieved with a Zn-porphyrin
complex-based DSSC recently.2 In addition to ruthenium
complexes and porphyrin dyes, metal-free sensitizers have also
been intensively studied because of their molecular design
flexibility and low cost.3 Very recently, an impressively high cell
efficiency of 11.5% has also been achieved with a metal-free
dye.3d
A conventional metal-free dye has a common structural motif
D−π−A,4 where D is the electron donor, A is the electron
acceptor, and π is the conjugated spacer between the two.
Charge transfer from the donor to the acceptor is beneficial to
red shifting the absorption spectra and facilitating electron
injection from the photoexcited dye molecule to the TiO2
photoanode. In order to extend the absorption to a longer
wavelength region for better light harvesting, the D−π−A
design can be modified with insertion of an auxiliary acceptor,
described as the D−A−π−A approach.5 High cell efficiencies
up to 9% have been proven possible with D−A−π−A type
sensitizers.6 In our earlier reports, we found that incorporation
of electron deficient entities such as benzo[2,1,3]thiadiazole
(BT) and the cyanovinyl entity in the conjugated spacer
significantly red shift the absorption. However, we also found
that charge trapping at the electron deficient unit of the spacer
could jeopardize charge separation and result in inferior cell
performance.7 Compared with the BT entity, dyes or polymers
based on a benzotriazole (BTz) entity have a higher LUMO
energy level and wider HOMO/LUMO gap,8 because the lone
pair on the nitrogen atom of BTz is more basic than that on the
sulfur atom of BT and hence is more easily donated into the
Received: April 22, 2014
Published: May 22, 2014
© 2014 American Chemical Society
3052
dx.doi.org/10.1021/ol501163b | Org. Lett. 2014, 16, 3052−3055

Organic Letters
Letter
selectively occurred at the position adjacent to the nitrogen.17
Consequently, two isomers (Figure 1), PTN1 (N atom facing
Table 1. Electro-optical Parameters of the Dyes
λabs (ε × 104
HOMO/
d
a
b
c
e
f
M−1 cm−1
[nm]
)
λmax
E1/2 (ox)
[mV]
LUMO
E0−0
E0−0*
[V]
dyes
[nm]
[eV]
[eV]
PTN1 464 (3.99),
482
548 (90) 5.75/−
2.38
−1.11
390 (2.14)
3.37
NPT1 476 (4.47),
476
553 (83) 5.75/−
2.37
−1.06
386 (1.95)
3.38
a
b
Measured in THF (solution). Absorption maxima of film (adsorbed
c
on TiO2). Measured in CH2Cl2. scan rate = 100 mV s−1; electrolyte =
[(n-C4H9)4][NPF6]; Eox = (Epa + Epc)/2, ΔEp = (Epa − Epc) where Epa
and Epc are peak anodic and cathodic potentials, respectively.
Oxidation potential reported is adjusted to the potential of ferrocene
d
which was used as an internal reference. HOMO and LUMO
energies are calculated using formula HOMO = 5.2 + (E1/2 − EFc),
e
LUMO = E0−0 − HOMO. Optical band gap energy gap, E0−0, was
derived from the intersection of absorption and emission spectra.
E0−0*: the excited state oxidation potential vs NHE.
f
Figure 1. Structures of the dyes.
toward the acceptor) and NPT1 (N atom facing away from the
acceptor), were successfully synthesized by following the
pathways described in Scheme 1. The new dyes were
characterized through 1H, 13C NMR, and HRMS spectroscopy.
nm. The former can be attributed to the aromatic π−π*
transition of triphenylamine, and the later broad band can be
attributed to the intramolecular charge transfer (ICT) from the
triphenylamine donor to the cyanoacetic acid acceptor with
some delocalized π−π* transition character.18 The λmax value of
the ICT band is 464 and 476 nm for PTN1 and NPT1,
respectively. As expected, the absorption maxima of PT-based
dyes are significantly blue-shifted (50−60 nm) compared with
the PyT-based dyes,14 which can be attributed to the higher
basicity of the nitrogen atom of the PT unit. This observation is
in conformity with the trend found in benzotriazole- and
benzothiadiazole-based dyes and is also supported by
theoretical computations (vide infra).13a It is noteworthy that
the ICT band of PT-based dyes have much higher molar
extinction coefficient (PTN1: 39900, NPT1: 44700) than PyT
based dyes (SC-PyTN1: 19300, SC-NPyT1: 35100), implying
a stronger π−π* transition character in the former and can be
attributed to the better planarity of the conjugated spacer in the
former (vide infra).
Scheme 1. Synthetic Scheme for PTN1 and NPT1 Dyes
Figure S5 depicts the absorption spectra of the organic dyes
adsorbed on a porous TiO2 nanoparticle film (4 μm in
thickness). Upon adsorption on the TiO2, the dyes PTN1 and
NPT1 exhibited a red shift of 18 nm and ∼0 nm, respectively,
indicating that J-aggregation of the dye molecules is less serious
for the NPT1 dye and is evidenced from its higher cell
efficiency (vide infra).
UV−vis absorption spectra of both dyes in THF are
presented in Figure 2, and their corresponding data are
summarized in Table 1. The absorption spectra of both show
two prominent bands in the regions 350−400 nm and 400−550
Electrochemical properties of PTN1 and NPT1 were studied
by cyclic voltammetry (CV) in CH2Cl2 solutions. The cyclic
voltammograms are shown in Figure S1, and the relevant data
are compiled in Table 1. The dyes exhibit a reversible one-
electron redox couple attributed to the oxidation of the
arylamine. The NPT1 is oxidized at a higher potential than
PTN1 due to the shorter distance between the electronegative
nitrogen atom of PT and the arylamine in the former, similar to
the trend observed in the PyT-based dyes.14 The HOMO
energy level deduced from the redox potentials for PTN1 and
NPT1 were found to be the same (5.75 eV), similar to the
trend found in the PyT dyes.14 The excited state potential (E0−0
*), estimated from the first oxidation potential at the ground
state and E0−0, are more negative (Table 1) than the
conduction band edge of TiO2 (−0.5 V vs NHE), ensuring
an efficient electron injection process from the excited state of
the dyes into the TiO2 electrode. In contrast, the more positive
oxidation potential of the dyes compared with that of the I−/I3−
Figure 2. UV−vis absorption spectra of dyes in THF (10−5 M).
3053
dx.doi.org/10.1021/ol501163b | Org. Lett. 2014, 16, 3052−3055

Organic Letters
Letter
redox couples (∼0.4 V vs NHE) indicates that regeneration of
the dyes is thermodynamically feasible.
The charge excitation behavior for the dyes is further
examined via density functional calculations. Selected frontier
orbitals of the dyes are shown in Figure 3. The results from the
Figure 3. Frontier orbitals of the dyes (HOMO and LUMO).
time independent approach are compiled in Table S1. The
HOMO is mainly distributed from the arylamine extending to
the conjugated bridge. In comparison, the LUMO is mainly
distributed from the 2-cyanoacrylic acid extending to the
bridge. Therefore, charge transfer from the arylamine donor to
the 2-cyanoacrylic acid acceptor is evident. Figure S4 displays
the changes in the Mulliken charge for the dyes upon vertical
excitation, as calculated via TDDFT (data Table S1, Supporting
Information). Compared with the PyT unit in PyTN1 and
NPyT1 dyes14 as well as BT unit in the S1 dye,7a the Mulliken
charge change (Δq) and the product of the oscillator strength
and the Mulliken charge change (f × Δq) of the PT unit in
both PTN1 and NPT1 dyes for S0 → S1 transition are less
negative, indicating the charge trapping effect of the PT unit in
both PTN1 and NPT1 dyes is alleviated significantly. In our
previous report,19 we found that the more negative f × Δq value
of the acceptor (anchor) could be used as an approximation for
the electronic coupling strength from the dye molecule to the
attached TiO2 nanoparticle, and the more negative f × Δq value
was corresponding with the larger short-circuit current of the
cell. In this study, we also found that the acceptor of the new
dyes had a more negative f × Δq value in the S0 → S1 transition
(PTN1: −0.20; NPT1: −0.23) compared with PyTN1 (−0.08)
and NPyT1 (−0.02) dyes, and thus larger JSC values (PTN1:
9.32; NPyT1: 13.37; PyTN1: 0.72; NPT1: 2.41 mA cm−2).
Dye-sensitized solar cells with an effective area of 0.16 cm2
and the electrolyte composed of 0.05 M I2/0.5 M LiI/0.5 M
tert-butylpyridine (TBP) in acetonitrile solution were fabricated
to explore the potentials of new dyes as the sensitizers. The
incident photo-to-current conversion efficiency (IPCE) plots of
the cells and the typical photocurrent−voltage J−V curves of
the devices under the illumination of AM 1.5G (100 mW cm−2)
as well as the dark current are shown in Figure 4. The
performance parameters for the cells are summarized in Table
2. The highest IPCE performance was observed for the dye
NPT1 in the range 350 to 650 nm, with a maximum value of
∼75% at 520 nm (∼85% for NPT1 + 30 mM CDCA; see
Figures 4b and S9). The power conversion efficiency of the cell
is 3.69% and 6.05% for PTN1 and NPT1, respectively. In
contrast, PyTN1 (0.19%) and NPyT1 (0.82%) have much
Figure 4. Current density−voltage/dark-current curves (a) and IPCE
plots (b) of DSSCs based on PTN1 and NPT1 dyes.
Table 2. Device Performance Parameters of the Dyes
dyes
VOC/V
0.57
JSC/mA/cm2
FF
η %
PTN1
PTN1
NPT1
NPT1
N719
9.32
0.70
0.73
3.69
4.85
a
b
a
0.61
10.83
0.66 0.00
0.70
13.37 0.31
13.41
0.69 0.01
0.72
6.05 0.15
6.76
0.76
13.93
0.66
7.53
a
b
Dye with addition of 30 mM CDCA. Taken from three
measurements.
lower cell efficiencies.14 The short-circuit current (JSC) value of
the PT cells (PTN1: 9.32; NPT1: 13.37 mA cm−2) is also
much higher than that of the PyT cells (PyTN1: 0.72; NPyT1:
2.41 mA cm−2). The nearly 1 order improvement of the
efficiency and significantly higher JSC value from PyT dyes to
PT dyes can be attributed to serious charge trapping in the
former (vide infra, see computation). It is interesting to note
that the cell performance of NPT1 is much better than that of
PTN1. This outcome could be ascribed to several reasons: (1)
NPT1 has more intense ICT absorption; (2) NPT1 has a
higher dye loading by ∼18% (PTN1: 3.12 × 10−7 mol/cm2 and
NPT1: 3.68 × 10−7 mol/cm2); (3) NPT1 can suppress the
dark current more effectively (vide infra, see electrochemical
impedance spectra (EIS)); (4) there is better electron transport
in the cell of NPT1 (vide infra, see EIS).
Although the long alkyl chain at the PT entity may help to
ease dye aggregation,16 some degree of dye aggregation still
occurs (Figure S5). As dye aggregation tends to quench the
excited dye molecule and jeopardizes the electron injection
efficiency,20 we also fabricated DSSCs from the dyes containing
30 mM of chenodeoxycholic acid (CDCA) as the coadsorbent.
The cell performance was improved significantly for both dyes:
NPT1, 0.72 V, 10.63 mA cm−2, 6.76%; PTN1, 0.61 V, 10.83
mA cm−2, 4.85%. It is believed that both dye aggregation and
charge recombination are eased. Alleviation of dye aggregation
3054
dx.doi.org/10.1021/ol501163b | Org. Lett. 2014, 16, 3052−3055

Organic Letters
Letter
(2) Mathew, S.; Yella, A.; Gao, P.; Humphrey-Baker, R.; Curchod, B.
F. E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin,
can be supported by the blue shift of the absorption spectra of
PTN1 and NPT1 when CDCA was added (Figure S5). Dark
current suppression, on the other hand, is supported by
comparison of EIS with and without addition of CDCA (vide
infra, EIS).
M. K.; Gratzel, M. Nat. Chem. 2014, 6, 242−247.
̈
(3) (a) Gratzel, M. Acc. Chem. Res. 2009, 42, 1788−1798.
̈
(b) Hagfeldt, A.; Boschloo, G.; Sun, L. C.; Kloo, L.; Pettersson, H.
Chem. Rev. 2010, 110, 6595−6663. (c) Ning, Z.; Fu, Y.; Tian, H.
Energy Environ. Sci. 2010, 3, 1170−1181. (d) Zhang, M.; Wang, Y.; Xu,
M.; Ma, W.; Li, R.; Wang, P. Energy Environ. Sci. 2013, 6, 2944−2949.
(4) (a) Mishra, A.; Fischer, M. K. R.; Bauerle, P. Angew. Chem., Int.
Ed. 2009, 48, 2474−2499. (b) Hagfeldt, A.; Boschloo, G.; Sun, L.;
Kloo, L.; Petterssen, H. Chem. Rev. 2010, 110, 6595−6663. (c) Yen,
Y.-S.; Chou, H.-H.; Chen, Y.-C.; Hsu, C.-Y.; Lin, J. T. J. Mater. Chem.
2012, 22, 8374−8747. (d) Zhou, H.; Yang, L.; Stuart, A. C.; Price, S.
C.; Liu, S.; You, W. Angew. Chem., Int. Ed. 2011, 50, 2995−2998.
(e) Zhang, S.; Yang, X.; Numata, Y.; Han, L. Energy Environ. Sci. 2013,
6, 1443−1464.
Figure S6 shows the EIS of DSSCs measured in the dark.
The intermediate frequency semicircle in the Nyquist plot
represents the charge transfer phenomena between the TiO2
surface and the electrolyte, i.e., the dark current. The resistance
toward the dark current is larger for the cell of NPT1,
indicating its more effective suppression of dark current. This is
consistent with the smaller dark current (Figure 4a) and the
larger VOC (Table 2) measured. More compact packing of the
NPT1 dye likely more efficiently blocks the electrolytes from
approaching the TiO2 surface. EIS was also measured for
DSSCs of NPT1 with different concentrations of CDCA (0, 10,
and 30 mM). The resistance toward the dark current increases
as the concentration of CDCA increases (Figure S7).
Figure S8 shows the Nyquist plots upon illumination of 100
mW cm−2 under open-circuit conditions. The radius of the
intermediate frequency semicircle in the Nyquist plot will be
used to deduce the electron-transport resistance, and the
smaller radius means lower electron transport resistance. There
is no significant difference found in the electron transport
resistance between PTN1 (12.85 Ohm) and NPT1 (12.27
Ohm).
(5) Wu, Y.; Zhu, W. Chem. Soc. Rev. 2013, 42, 2039−2058.
(6) (a) Qu, S.; Qin, C.; Islam, A.; Wu, Y.; Zhu, W.; Hua, J.; Tian, H.;
Han, L. Chem. Commun. 2012, 48, 6972−6974. (b) Pei, K.; Wu, Y.;
Islam, A.; Zhang, Q.; Han, L.; Tian, H.; Zhu, W. ACS Appl. Mater.
Interfaces 2013, 5, 4986−4995.
(7) (a) Velusamy, M.; Thomas, K. R. J.; Lin, J. T.; Hsu, Y.-C.; Ho, K.-
C. Org. Lett. 2005, 7, 1899−1902. (b) Chou, H.-H.; Chen, Y.-C.;
Huang, H.-J.; Lee, T.-H.; Lin, J. T.; Tsai, C.; Chen, K. J. Mater. Chem.
2012, 22, 10929−10938. (c) Lin, R. Y.-Y.; Lee, C.-P.; Chen, Y.-C.;
Peng, J.-D.; Chu, T.-C.; Chou, H.-H.; Yang, H.-M.; Lin, J. T.; Ho, K.-
C. Chem. Commun. 2012, 48, 12071−12073.
(8) Zhang, Z. H.; Peng, B.; Liu, B.; Pan, C. Y.; Li, Y. F.; He, Y. H.;
Zhou, K. H.; Zou, Y. P. Polym. Chem. 2010, 1, 1441−1447.
(9) Price, S. C.; Stuart, A. C.; Yang, L. Q.; Zhou, H. X.; You, W. J.
Am. Chem. Soc. 2011, 133, 4625−4631.
(10) Newkome, G. R.; Paudler, W. W. Contemporary Heterocyclic
Chemistry; John Wiley: New York, 1982.
In summary, we have successfully designed and synthesized
two new organic sensitizers (PTN1 and NPT1) containing 2H-
[1,2,3]triazolo[4,5-c]pyridine as the central linkage for high-
performance dye-sensitized solar cells. Compared to pyridal-
[2,1,3]thiadiazole-based dyes, the conversion efficiency of the
DSSCs is nearly 1 order higher. The best cell performance has a
(11) Palmer, M. H.; Simpson, I.; Wheeler, J. R. Z. Naturforsch. 1981,
36a, 1246−1252.
V
OC of 0.66 V, JSC of 13.37 mA/cm2, and conversion efficiency
(12) Ahn, S.-H.; Czae, M.-Z.; Kim, E.-R.; Lee, H.; Han, S.-H.; Noh,
J.; Hara, M. Macromolecules 2001, 34, 2522−2527.
of 6.05%. Upon CDCA addition, the efficiency was further
boosted to 6.76% reaching ∼90% of the standard cell based on
N719. Significant improvement of the cell performance of the
new dyes compared to their pyridal[2,1,3]thiadiazole congeners
can be attributed to the alleviation of charge trapping of the
[1,2,3]triazolo[4,5-c]pyridine moiety compared with the
pyridal[2,1,3]thiadiazole moiety.
(13) (a) Yen, Y.-S.; Lee, C.-T.; Hsu, C.-Y.; Chou, H.-H.; Chen, Y.-C.;
Lin, J. T. Chem.Asian J. 2013, 8, 809−816. (b) Mao, J.; Guo, F.;
Ying, W.; Wu, W.; Li, J.; Hua, J. Chem.Asian J. 2012, 7, 982−991.
(14) Chaurasia, S.; Hsu, C.-Y.; Chou, H.-H.; Lin, J. T. Org. Electron.
2014, 15, 378−390.
(15) (a) Tanimoto, A.; Yamamoto, T. Adv. Synth. Catal. 2004, 346,
1818−1823. (b) Patel, D. G. (Dan); Feng, F.; Ohnishi, Y.-y.; Abboud,
K. A.; Hirata, S.; Schanze, K. S.; Reynolds, J. R. J. Am. Chem. Soc. 2012,
134, 2599−2612.
ASSOCIATED CONTENT
■
(16) Cui, Y.; Wu, Y.; Lu, X.; Zhang, X.; Zhou, G.; Miapeh, F. B.; Zhu,
W.; Wang, Z.-S. Chem. Mater. 2011, 23, 4394−4401.
S
* Supporting Information
(17) Handy, S. T.; Wilson, T.; Muth, A. J. Org. Chem. 2007, 72,
8496−8500.
Experimental procedures and full spectroscopic data for all new
dyes. This material is available free of charge via the Internet at
http://pubs.acs.org.
(18) Yang, H. Y.; Yen, Y. S.; Hsu, Y. C.; Chou, H.-H.; Lin, J. T. Org.
Lett. 2010, 12, 16−19.
(19) Huang, S.-T.; Hsu, Y.-C.; Yen, Y.-S.; Chou, H.-H.; Lin, J. T.;
Chang, C.-W.; Hsu, C.-P.; Tsai, C.; Yin, D.-J. J. Phys. Chem. C 2008,
112, 19739−19747.
(20) (a) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo,
A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. J. Phys. Chem. B
2003, 107, 597−606. (b) Hara, K.; Tchibana, Y.; Ohga, Y.; Shinpo, A.;
Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. Sol. Energy Mater. Sol.
Cells 2003, 77, 89−103.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jtlin@gate.sinica.edu.tw.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Academia Sinica and Ministry of Science and
Technology, Taiwan, for financial support, and the Instrumen-
tal Center of Institute of Chemistry (AS).
REFERENCES
(1) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737−740.
■
̈
3055
dx.doi.org/10.1021/ol501163b | Org. Lett. 2014, 16, 3052−3055