Novel triphenylamine donors with carbazole moieties for organic sensitizers toward cobalt(II/III) redox mediators

Article abstract of DOI:10.1021/ol5018036

A series of D-π-A organic dyes, X72-75, containing novel triarylamine electron donors have been synthesized for dye-sensitized solar cells (DSCs). The superiority of the asymmetric design of the triphenylamine electron donor over the symmetrical triphenylamine when applied in organic dyes for cobalt cells has been observed. Using X72 with the cobalt(II/III) redox shuttle resulted in an overall power conversion efficiency (PCE) of 9.18%, outperforming the state-of-the-art dye C218 under the same conditions.

Full text of DOI:10.1021/ol5018036

Letter
pubs.acs.org/OrgLett
Novel Triphenylamine Donors with Carbazole Moieties for Organic
Sensitizers toward Cobalt(II/III) Redox Mediators
Yulin Tan, Mao Liang,* Ziyi Lu, Yequan Zheng, Xinli Tong, Zhe Sun, and Song Xue*
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, Department of Applied Chemistry, Tianjin University
of Technology, Tianjin 300384, P.R. China
S
* Supporting Information
ABSTRACT: A series of D−π−A organic dyes, X72−75,
containing novel triarylamine electron donors have been
synthesized for dye-sensitized solar cells (DSCs). The
superiority of the asymmetric design of the triphenylamine
electron donor over the symmetrical triphenylamine when
applied in organic dyes for cobalt cells has been observed.
Using X72 with the cobalt(II/III) redox shuttle resulted in an
overall power conversion efficiency (PCE) of 9.18%, out-
performing the state-of-the-art dye C218 under the same
conditions.
ye-sensitized solar cells (DSCs) have attracted consid-
interesting properties would be produced as a result of
judiciously varying the terminal donors.
D
erable attention since the initial work from Gratzel and
̈
co-workers.1 Compared to inorganic silicon-based solar cells,
low costs, and good efficiency are prominent features of
DSCs.1b In the past two decades, tremendous research efforts
have been devoted to designing and synthesizing donor−π−
acceptor (D−π−A) organic sensitizers,2−12 which currently
reach more than 10% power-conversion efficiency (PCE).13
Recently, the combination of the cobalt(II/III) redox shuttles
with high-absorption coefficient organic D−π−A dyes has
brought forth a new opportunity to make a further break-
through in efficiency. Fabrication of DSCs utilizing the
[Co(bpy)3]2+/3+ redox couple and SM315 exhibited a high
photovoltage of 0.91 V and demonstrated a record 13% PCE at
full sun illumination.14 Note that significant control of charge
recombination and fine-tuning of the molecular energy level are
still big challenges for the cobalt cells. To meet the
requirements for effective light harvesting and control of the
charge recombination to the cobalt(III) complexes, molecular
engineering of the electron donor is a feasible strategy in
developing organic sensitizers toward cobalt(II/III) redox
mediators.
In this study, three carbazole moieties (9-hexyl-3-(2-
(hexyloxy)phenyl)-9H-carbazole, 2-HPC; 9-hexyl-3-(4-
(hexyloxy)phenyl)-9H-carbazole, 4-HPC; 3,9-bis(4-(hexyloxy)-
phenyl)-9H-carbazole, BHPC) were employed to construct
four novel triarylamine electron donors, enabling modulation of
the steric properties and the molecular energy level of organic
dyes. The synthesized organic dyes (X72, X73, X74, and X75)
are shown in Figure 1, with the synthetic methodology of the
X72−74 being highlighted in Scheme 1.
Compounds 5a−c were obtained by the Suzuki coupling
reaction of 1a,b with 2, followed by reduction of the nitro
group and subsequent reaction with 1-bromo-4-(hexyloxy)-
benzene. These compounds were allowed to react with an aryl
bromide 6 to produce 7a−c. Aldehydes 8a−c were synthesized
by a Vilsmeier−Haack reaction of 7a−c with POCl3 and DMF.
Triarylamines are well-known electron donors that are widely
used in organic sensitizers, owing to their strong electron-
donating ability and excellent hole-transporting ability.3 It is
valuable to note that most triarylamines are symmetrical,
namely, when two identical donors are substituted on an aniline
(e.g., dihexyloxy-substituted triphenylamine or a bis-dimethyl-
fluorenylamino moiety).3 Our strategy for obtaining highly
efficient organic dyes toward the cobalt(II/III) redox shuttles is
focused on asymmetric triphenylamine donors. This delicate
architecture provides an opportunity to fine-tune the steric
properties and the molecular energy level of organic dyes
through the introduction of two donor moieties with different
functions. By this approach, new families of sensitizers with
Figure 1. Structure of the synthesized dyes and C218.
Received: June 23, 2014
Published: July 24, 2014
© 2014 American Chemical Society
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Letter
Scheme 1. Schematic Diagram for the Synthesis of X72−74
Table 1. Optical and Electrochemical Properties of the Dyes
λ
E0−0
b
ED/D+
c
ED*/D+
d
maxa
a
dye
(nm)
ε (M−1 cm−1
)
(V)
(V)
(V)
X72
X73
X74
X75
C218
556
550
539
553
546
60 590
61 850
53 760
54 170
59 000
2.01
2.03
2.04
2.04
2.06
0.86
0.88
0.85
0.80
0.90
−1.16
−1.15
−1.19
−1.24
−1.16
b
a
The absorption spectra were measured in CH2Cl2 solutions. E0−0
values were estimated from the intersections of normalized absorption
c
and emission spectra (λint, E0−0 = 1240/λint); see Figure S1. ED/D+ was
recorded by cyclic voltammograms of the dye-loaded TiO2 film.
d
ED*/D+ was calculated from ED/D+ − E0−0
.
maximum absorption wavelength (λmax = 539 nm) and an
attenuated maximum molar absorption coefficient (ε = 53 760
M−1 cm−1), indicating that the substituent alteration from 2-
HPC to 4-HPC brings forth a weakened intramolecular charge
transfer interactions. A 4-(hexyloxy)phenyl unit was introduced
on the carbazole nitrogen atom for X73 leading to a
bathochromic shift of 11 nm, as well as increasing the
absorption intensity when compared to the X74, owing to
the extended π-conjugation. On the other hand, upon
incorporating two carbazole moieties, i.e. 2-HPC and 4-HPC,
in triarylamine, X75 shows a distinct advantage over other dyes
in the range of the absorption band. Note that all carbazole
dyes exhibit a broader absorption compared with the UV−vis
data recorded from the C218, which is beneficial to light-
harvesting and thus short-circuit current enhancement.
We further carried out cyclic voltammogram measurements
(Figure S2) to determine the accurate ED/D+ values of these dye
sensitized films (Table 1), by averaging the anodic and cathodic
peak potentials. Generally, the carbazole based dyes have a
lower first oxidation potential than that of the C218 dye,
suggesting the feature of being electron-richer for 2-HPC, 4-
HPC, and BHPC moieties. Note that the ED/D+ of X75 (0.80
V) is much lower than those of X72−74 (0.85−0.88 V) as
result of two carbazole donors being introduced, resulting in an
attenuated driving force for the regeneration of the oxidized
dye. It is obvious that the asymmetric design of triphenylamine
donor can be used to fine-tune the organic dye HOMO level so
as to achieve efficient dye regeneration and at the same time
maximize the open circuit potential. The calculated excited-
state potentials of these dyes (−1.15 to −1.24 V vs NHE) are
placed sufficiently above the TiO2 conduction band edge to
ensure no energetic barriers for the electron injection.
Subsequently, the target dyes were obtained via Knoevenagel
condensation reaction of the aldehydes 8a−c with cyanoacetic
acid in the presence of a catalytic amount of piperidine. Note
that the yields of 8a−c (red route) through the Suzuki reaction
between a borate complex and 6-bromo-4,4-dihexyl-4H-cyclo-
penta[2,1-b:3,4-b′]dithiophene-2-carbaldehyde are <30%. Fur-
thermore, the products are very difficult to purify by column
chromatography. In this work, the synthetic strategy incorpo-
rating a Buchwald−Hartwig reaction as the key step has been
established for facile synthesis of 8a−c (blue route).
Compounds 5a−c underwent a Buchwald−Hartwig amination,
giving the desired 7a−c with a 50−70% yield. A route that is
different from the conventional way of triarylamine dyes
synthesis. The synthetic route to X75 is shown in Scheme S1 in
the Supporting Information (SI). The detail characterization
data of the dyes are also provided in the SI.
UV−vis absorption spectra of dyes in dichloromethane are
presented in Figure 2, and their corresponding data are
Further insight into the molecular structure and frontier
molecular orbitals of these dyes was performed via density
functional calculations. An optimized geometrical configuration
of the dyes can be found in Figure S3, and selected frontier
orbitals of the dyes are shown in Figure S4. Examination of the
HOMO and LUMO of X72−75 indicates that HOMO−
LUMO excitation moves the electron distribution from the
triarylamine donor to the cyanoacrylic acid moiety via a π-space
group, allowing efficient charge separation and electron
injection.
Figure 2. Absorption spectra of dyes in DCM.
summarized in Table 1. All dyes exhibit a single prominent
band with the absorption maximum around ca. 540−560 nm.
Among these sensitizers, the X72 dye with 2-HPC based
triarylamine electron donor presents the longest maximum
absorption wavelength of 556 nm, with a maximum molar
absorption coefficient of 60 590 M−1 cm−1. By comparison,
constructing the X74 dye with 4-HPC leads to a blue-shifted
Owing to the poor solubility of X72−75 in the routine DSC
dye bath solvents (e.g., t-BuOH and acetonitrile (MeCN)), a
binary solvent of DCM and t-BuOH was our preliminary choice
for the adsorption of dyes on a mesoporous titania film.
However, this mixed solvent does not lead to satisfied
photovoltaic performance (Table S1), owing to a low grafting
density of dyes on titania (Figure S5). By contrast, use of a
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Letter
ternary dye bath solvent, including MeCN with DCM and t-
BuOH dramatically improved DSC device performance. Based
on a mixed solvent of t-BuOH−MeCN−DCM (1.5:1.5:2
volume ratio), DSC devices were fabricated using the carbazole
dyes to compare the influence of the electron donor variations
on the photovoltaic parameters. The photocurrent density
voltage curves (J−V) of the devices employing the Co-bpy
electrolyte under AM 1.5 irradiation (100 mW cm−2) were
recorded, and the detailed photovoltaic parameters were
compiled in Table 2. As Figure 3a presents, the short-circuit
dyes closely follow the absorption spectra of the dye-grafted
translucent titania films in contact with the Co-bpy electrolyte
for cell fabrication (Figure S6). As shown in Figure 3b, the X72
cell exhibits a broad plateau with a higher IPCE summit
compared to the X73 and X74 counterparts, which contributes
to the higher JSC of X72. It is valuable to note that the X75
confers a lower IPCE from 350 to 600 nm as compared to
those obtained with X72−74, a result that can mainly be
attributed to the small driving force for regeneration (reasons
have been provided in pp S67−68 in SI). Furthermore, for Co-
bpy cells, the driving force for regeneration of X75 is only 0.24
eV, much lower than those of X72−74. Assuming that an
energy gap of 0.4 eV is necessary for efficient regeneration,15
this may induce inferior regeneration efficiency. Nevertheless,
this loss in IPCE is compensated by a gain in longer
wavelength, resulting in a JSC value as high as that for X73.
By contrast, the IPCE response of C218 is relatively narrower
than other dyes, despite its high response in this spectral region.
To examine the energetic and dynamic origins of the
observed VOC difference, we performed charge-extraction and
transient photovoltage decay measurements. As presented in
Figure 4a, cells made with X72 and C218 display a similar
Table 2. DSCs Performace Parameters of the Investigated
Dyes
dye
JSC (mA cm−2
)
VOC (mV)
FF
PCE/%
X72
X73
X74
X75
C218
14.6
13.5
14.0
13.8
13.8
925
937
913
901
865
0.68
0.67
0.68
0.67
0.69
9.18
8.47
8.69
8.33
8.35
Figure 3. (a) J−V curves for DSCs based on the dyes under
illumination of AM 1.5 G simulated sunlight (100 mW cm−2). (b)
IPCE spectra of the same DSCs.
Figure 4. (a) Charge density at open circuit and (b) electron lifetime
as a function of charge density for DSCs.
photocurrent density (JSC), open-circuit photovoltage (VOC),
and fill factor (FF) of the X74 cell are 14.0 mA cm−2, 913 mV,
and 0.68, respectively, affording a power conversion efficiency
(PCE) of 8.69%. Furthermore, an improved JSC of 14.6 mA
cm−2 concomitant with an increased VOC of 925 mV was
achieved with the X72 dye, contributing to a higher PCE of
9.18%, indicative of the superiority of 2-HPC over 4-HPC
when applied in triarylamine organic dyes. Despite the highest
VOC, the X73 cell exhibits a much lower PCE compared to the
X72, primarily owing to the significantly attenuated photo-
currents. It is noteworthy that, in comparison to the case of the
C218 dye, the employment of the X72−74 brings forth
evidently enhanced PCE, demonstrating the superiority of the
asymmetric design of the triphenylamine electron donor over
the symmetrical triphenylamine when applied in organic dyes
for cobalt cells. One may argue that this is not true because X75
shows a comparable PCE of C218. Worthy of note, the HOMO
level of X75 is significantly influenced by the two carbazole
units rather than the 2-(hexyloxy)phenyl or 4-(hexyloxy)phenyl
group; at the same time, the regeneration and recombination
kinetics in cobalt cells are closely related to the HOMO level of
the dyes. From this point of view, the triarylamine in X75 can
be thought to be a symmetrical structure.
extracted electron density (dn) at the same potential bias,
indicative of a fixed conduction-band (CB) edge of titania with
respect to the electrolyte Fermi level. The dye alteration from
X72 to X74 caused the TiO2 bands to be shifted upward by
about 30 mV. Possible causes are thought to be correlated with
the dye coverage and the interaction between TiO2 and TBP.16
However, the relatively higher conduction band edge of the
X74 cell does not lead to an improved VOC, due to the
attenuated electron lifetime. As the photocurrent decay
measurements indicate (Figure 4b), for these asymmetric
triphenylamine dyes, the 2-HPC donor leads to the longest
lifetimes, whereas the 4-HPC and BHPC donors shorten the
lifetimes. Furthermore, X75 featuring a bulky triphenylamine
donor exhibits a shortened lifetime, a result that is thought to
be correlated with the small driving force for the regeneration
of the oxidized dye, because an inefficient regeneration of dye
cations also promotes the interfacial recombination of titania
electrons with electron-accepting species in an electrolyte and/
or dye cations. For C218, a shorter electron lifetime has been
measured compared to devices incorporating X72−75, giving a
clear clue regarding its smaller photovoltage.
In summary, we have synthesized four D−π−A organic
sensitizers with novel triarylamine donors for DSC applications
through a facile method where the π-conjugated bridge was
introduced into dyes via a Buchwald−Hartwig reaction. Not
We further carried out the incident photon-to-collected
electron conversion efficiency (IPCE) measurements of the
cobalt cells. The IPCE spectra of the devices made with these
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Organic Letters
Letter
(11) Murakami, T. N.; Koumura, N.; Uchiyama, T.; Uemura, Y.;
Obuchi, K.; Masaki, N.; Kimura, M.; Mori, S. J. Mater. Chem. A 2013,
1, 792−798.
only was the electron-donating ability stronger in the absence
of carbazole moieties but also the unfavorable recombination
between the electrolyte and the TiO2 surface was reduced as
well. The device based on the sensitizer X72 gave an overall
conversion of 9.18%, which is higher than that of the state-of-
the-art dye C218 under the same conditions. This work
provides a new paradigm for developing new D−π−A organic
sensitizers for DSCs employing the cobalt redox couples.
(12) Liu, B.; Wang, B.; Wang, R.; Gao, L.; Huo, S.; Liu, Q.; Li, X.;
Zhu, W. J. Mater. Chem. A 2014, 2, 804−812.
(13) (a) Cao, Y.; Cai, N.; Wang, Y.; Li, R.; Yuan, Y.; Wang, P. Phys.
Chem. Chem. Phys. 2012, 14, 8282−8286. (b) Tsao, H. N.; Burschka,
J.; Yi, C.; Kessler, F.; Nazeeruddin, M. K.; Gratze, M. Energy Environ.
̈
Sci. 2011, 4, 4921−4924. (c) Yella, A.; Humphry-Baker, R.; Curchod,
B. F. E.; Astani, N. A.; Teuscher, J.; Polander, L. E.; Mathew, S.;
Moser, J.-E.; Tavernelli, I.; Rothlisberger, U.; Gratzel, M.;
̈
ASSOCIATED CONTENT
■
Nazeeruddin, M. K.; Frey, J. Chem. Mater. 2013, 25, 2733−2739.
(d) Yang, J.; Ganesan, P.; Teuscher, J.; Moehl, T.; Kim, Y. J.; Yi, C.;
Comte, P.; Pei, K.; Holcombe, T. W.; Nazeeruddin, M. K.; Hua, J.;
S
* Supporting Information
Experimental details, characterization data for all new
compounds, details of the DSC fabrication, cyclic voltammo-
gram, emission spectra, molecular orbitals, and cell character-
izations. This material is available free of charge via the Internet
at http://pubs.acs.org.
Zakeeruddin, S. M.; Tian, H.; Gratzel, M. J. Am. Chem. Soc. 2014, 136,
̈
5722−5730.
(14) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B.
F. E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin,
M. K.; Gratzel, M. Nat. Chem. 2014, 6, 242−247.
̈
(15) Feldt, S. M.; Lohse, P. W.; Kessler, F.; Nazeeruddin, M. K.;
Gratzel, M.; Boschloo, G.; Hagfeldt, A. Phys. Chem. Chem. Phys. 2013,
̈
AUTHOR INFORMATION
■
15, 7087−7097.
Corresponding Authors
(16) Zhang, J.; Yao, Z.; Cai, Y.; Yang, L.; Xu, M.; Li, R.; Zhang, M.;
Dong, X.; Wang, P. Energy Environ. Sci. 2013, 6, 1604−1614.
*E-mail: liangmao717@126.com.
*E-mail: xuesong@ustc.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support from the
National 973 Program (No. 2011CBA00702), the National
Science Foundation of China (No. 21373007, 21376179), and
the Tianjin Natural Science Foundation (13JCZDJC32400,
14JCYBJC21400).
REFERENCES
■
(1) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737−740.
̈
(b) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H.
Chem. Rev. 2010, 110, 6595−6663.
(2) Yen, Y.-S.; Chou, H.-H.; Chen, Y.-C.; Hsu, C.-Y.; Lin, J. T. J.
Mater. Chem. 2012, 22, 8734−8747.
(3) Liang, M.; Chen, J. Chem. Soc. Rev. 2013, 42, 3453−3488.
(4) Cai, L.; Moehl, T.; Moon, S.-J.; Decoppet, J.-D.; Humphry-Baker,
R.; Xue, Z.; Liu, B.; Zakeeruddin, S. M.; Grazel, M. Org. Lett. 2014, 16,
̈
106−109.
(5) Lim, B.; Margulis, G. Y.; Yum, J.-H.; Unger, E. L.; Hardin, B. E.;
Gratzel, M.; McGehee, M. D.; Sellinger, A. Org. Lett. 2013, 15, 784−
̈
787.
(6) (a) Gao, P.; Tsao, H. N.; Gratzel, M.; Nazeeruddin, M. K. Org.
̈
Lett. 2012, 14, 4330−4333. (b) Gao, P.; Kim, Y. J.; Yum, J.-H.;
Holcombe, T. W.; Nazeeruddin, M. K.; Gratzel, M. J. Mater. Chem. A
̈
2013, 1, 5535−5544.
(7) (a) Velusamy, M.; Thomas, K. R. J.; Lin, J. T.; Hsu, Y.-C.; Ho, K.-
C. Org. Lett. 2005, 7, 1899−1902. (b) Chaurasia, S.; Hung, W.-I;
Chou, H.-H.; Lin, J. T. Org. Lett. 2014, 16, 3052−3055.
(8) (a) Wang, Z.; Liang, M.; Wang, H.; Wang, P.; Cheng, F.; Sun, Z.;
Xue, S. ChemSusChem 2014, 7, 795−803. (b) Li, G.; Liang, M.; Wang,
H.; Sun, Z.; Wang, L.; Wang, Z.; Xue, S. Chem. Mater. 2013, 25,
1713−1722. (c) Sun, Z.; Zhang, R.-K.; Xie, H.-H.; Wang, H.; Liang,
M.; Xue, S. J. Phys. Chem. C 2013, 117, 4364−4373. (d) Wang, Z.;
Wang, H.; Liang, M.; Tan, Y.; Cheng, F.; Sun, Z.; Xue, S. ACS Appl.
Mater. Interfaces 2014, 6, 5768−5778.
(9) Zhu, W.; Wu, Y.; Wang, S.; Li, W.; Li, X.; Chen, J.; Wang, Z.-S.;
Tian, H. Adv. Funct. Mater. 2011, 21, 756−763.
(10) Koumura, N.; Wang, Z.-S.; Mori, S.; Miyashita, M.; Suzuki, E.;
Hara, K. J. Am. Chem. Soc. 2006, 128, 14256−14257.
3981
dx.doi.org/10.1021/ol5018036 | Org. Lett. 2014, 16, 3978−3981