Two Novel Carbazole Dyes for Dye-Sensitized Solar Cells with Open-Circuit Voltages up to 1 v Based on Br-/Br3- Electrolytes

Article abstract of DOI:10.1021/ol9022936

"Chemical Equation Presented" Dye-sensitized solar cells (DSCs) based on two novel carbazole dyes (TC301 and TC306) and a Br^-/Br ₃^- redox mediator In dried CH₃CN solutions as electrolytes yielded a V_oc

Full text of DOI:10.1021/ol9022936

ORGANIC
LETTERS
2009
Vol. 11, No. 23
5542-5545
Two Novel Carbazole Dyes for
Dye-Sensitized Solar Cells with
Open-Circuit Voltages up to 1 V Based
on Br-/Br3 Electrolytes
-
Chao Teng,† Xichuan Yang,*,† Chunze Yuan,† Chaoyan Li,† Ruikui Chen,†
Haining Tian,† Shifeng Li,† Anders Hagfeldt,†,‡ and Licheng Sun*,†,§
State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on
Molecular DeVices, Dalian UniVersity of Technology (DUT), 158 Zhongshan Road, 116012
Dalian, China, School of Chemical Science and Engineering, Center of Molecular DeVices,
Physical Chemistry, Royal Institute of Technology (KTH), Teknikringen 30, 10044 Stockholm,
Sweden, and School of Chemical Science and Engineering, Department of Chemistry, Organic
Chemistry, Royal Institute of Technology (KTH), Teknikringen 30, 10044 Stockholm, Sweden
lichengs@kth.se; yangxc@dlut.edu.cn; hagfeldt@kth.se
Received October 15, 2009
ABSTRACT
-
Dye-sensitized solar cells (DSCs) based on two novel carbazole dyes (TC301 and TC306) and a Br-/Br3 redox mediator in dried CH3CN
solutions as electrolytes yielded a Voc of 1.156 V and a η value of 3.68% and a Voc of 0.939 V and a η value of 5.22% under simulated AM 1.5,
respectively. The dyes TC301 and TC306 have more positive HOMO levels (1.59 and 1.38 V vs NHE) than the redox potential of Br-/Br3--based
electrolytes, which have sufficient driving force to regenerate dyes. Under similar conditions with an I-/I3- instead of a Br-/Br3- redox mediator,
DSCs sensitized by the dyes TC301 and TC306 produced a Voc of 0.696 V and a η value of 2.36% and a Voc of 0.621 V and a η value of 4.10%,
respectively.
Dye-sensitized solar cells (DSCs) have attracted much
attention during the past years due to their low production
cost and relatively high solar energy conversion efficiency.1,2
DSCs based on Ru-complex photosensitizers such as N719
and redox mediator I-/I3 couple can provide record high
solar energy-to-electricity conversion efficiencies (η) of up
to 12% at simulated AM 1.5 irradiation.3 Despite its good
performance, one main drawback of the I-/I3- system is the
-
mismatch between its redox potential E(I-/I3 ) (0.5 V vs
-
† Dalian University of Technology.
‡ Department of Chemistry, Organic Chemistry, Royal Institute of
Technology.
(1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737.
(2) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269.
(3) Gra¨tzel, M. J. Photochem. Photobiol. A 2004, 164, 3.
§ Department of Chemistry, Organic Chemistry, Royal Institute of
Technology.
10.1021/ol9022936 CCC: $xxxx
Published on Web 11/09/2009
 2009 American Chemical Society

normal hydrogen electrode, NHE)4 and that of the dye (ca.
1.0 V vs NHE), resulting in an excessive driving force of
0.5 eV and a energy loss incurred during dye regeneration
which is one of the main factors limiting the performance
of current DSCs. Thus, the search for alternative redox
couples with a more positive redox potential than I-/I3- is a
current research topic of high priority. Because the potential
5
of Br-/Br3- redox couple E(Br-/Br3 ) (about 1.1 V vs NHE)
-
is more positive than that of I-/I3- redox couple, a large Voc
enhancement is expected if the Br-/Br3 couple is used in
-
place of the I-/I3 couple for DSCs containing dyes which
-
have also more positive HOMO levels than E(Br-/Br3 ).
-
Therefore, it is crucial to design dyes whose HOMO and
LUMO levels match the redox potential of Br-/Br3- and the
conduction band of TiO2, respectively. Here we report two
novel metal-free organic carbazole dyes TC301 and TC306
(Figure 1), which have more positive oxidation potentials
than E(Br-/Br3 ). By using the novel dyes and Br-/Br3
-
-
redox couple, a Voc of 1.15 V and a η of 3.68% for DSC
sensitized by the dye TC301 and a Voc of 0.939 V and a η
of 5.22% for DSC sensitized by the other dye TC306 were
achieved, respectively. The value of 1.15 V is the highest
Voc ever reported for a single DSC so far.
Figure 2. (a) Absorption and emission spectra of the dyes TC301
and TC306 in dried CH3CN solutions (2 × 10-5 M). (b) Absorption
spectra of the dyes TC301 and TC306 on TiO2 films immerged in
dried CH3CN solutions with 0.8 M LiBr and without electrolyte.
(Ecb, -0.5 V vs NHE) (Table 1). The relatively large energy
gap between the LUMO and the Ecb provides the possibility
for the addition of 4-tert-butylpyridine (TBP) to the elec-
trolyte, which can shift the Ecb more negatively and,
consequently, improve the voltage and the total efficiency.7
The first oxidation potentials (Eox) corresponding to the
HOMO levels of the dyes were measured by cyclic volta-
mmetry (CV) in dried DMF solutions. The Eox were
determined to be 1.59 V (vs NHE) for the dye TC301 and
1.38 V (vs NHE) for the dye TC306, which were more
Figure 1. Molecular structures of two novel metal-free organic
carbazole dyes TC301 and TC306.
The spectra of the dyes TC301 and TC306 in dried
CH3CN solutions showed absorption bands with ηmax at 374
and 429 nm, respectively (Figure 2a). The molar extinction
coefficients, ε(ηmax), were determined to be 12300 and 20900
M-1 cm-1 at the absorption maxima. The absorption spectra
of the dyes attached on TiO2 films (Figure 2b) with ηmax
appearing at 384 and 411 nm. In the presence of 0.8 M LiBr,
the ηmax attached on TiO2 films were red-shifted 10 nm for
the dye TC301 and 14 nm for the dye TC306, respectively,
and also the absorption intensities have increased.
positive than E(Br-/Br3 ), indicating that the oxidized dyes
-
formed after electrons injection into the conduction band of
TiO2 could accept electrons from Br-/Br3- redox mediators
thermodynamically.
Dye-sensitized solar cells based on a Br-/Br3 redox
-
mediator (devices A) containing 0.9 M 1,2-dimethyl-3-
butylimidazolium bromide (DMBIBr), 0.08 M Br2, and 0.5
M TBP electrolyte in dried CH3CN solutions yielded a Voc
of 1.156 V and a η value of 3.68% for the dye TC301 and
a Voc of 0.939 V and a η value of 5.22% for the dye TC306
(Figure 3a,c and Table 2). DSCs containing 0.9 M DMBIBr,
0.8 M LiBr, 0.08 M Br2, and 0.5 M TBP electrolyte in dried
It is well-known that adsorption/intercalation of potential-
determining cations such as Li+ into the porous TiO2
electrode can broaden the absorption of dye on TiO2.6 Red-
shift of absorptions of the dyes on TiO2 films lead to E0-0
changes; thus, LUMO levels of the dyes changed from -1.23
to -1.09 V for the dye TC301 and from -1.06 to -0.96 V
for the dye TC306, which were sufficiently more negative
than the potential of conduction band edge of TiO2 electrode
CH3CN solutions also based on Br-/Br3 redox mediator
-
(devices B) yielded a Voc of 1.041 V and a η value of 3.66%
for the dye TC301 and a Voc of 0.915 V and a η value of
5.07% for the dye TC306. Under the same conditions, DSCs
based on I-/I3 redox mediators (devices C) containing 0.6
-
(4) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49.
(5) Wang, Z.; Sayama, K.; Sugihara, H. J. Phys. Chem. B 2005, 109
(47), 22449.
(7) Boschloo, G.; Ha¨ggmanand, L.; Hagfeldt, A. J. Phys. Chem. B 2006,
110, 13144.
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Tachiya, M. J. Phys. Chem. B 2005, 109, 16406.
(8) Hagberg, D.; Edvinsson, T.; Marinado, T.; Boschloo, G.; Hagfeldt,
A.; Sun, L. Chem. Commun. 2006, 2245.
Org. Lett., Vol. 11, No. 23, 2009
5543

Table 1. Absorption, Emission, and Electrochemical Properties
of the Dyes TC301 and TC306
c
λmax
(nm) in
emissionb Eox
λmax
(nm)
λmax
Eox - E0-0
(V vs
NHE)
f
(V vs (nm) on E0-0
d
dyes CH3CNa
NHE) TiO2
384d
(V)
2.82
2.68
2.44
2.34
-1.23
-1.09
-1.06
-0.96
TC301
TC306
374
429
527
553
1.59
392e
411d
425e
1.38
a Absorptions in dried CH3CN solutions (2 × 10-5 M) at rt. b Emission
spectra in dried CH3CN solutions (2 × 10-5 M) at rt. c The oxidation
potentials of the dyes were measured in dried DMF solutions with 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte (working
electrode: glassy carbon; reference electrode: Ag/Ag+; calibrated with
ferrocene/ferrocenium (Fc/Fc+) as an internal reference and converted to
NHE by addition of 630 mV.8 Counter electrode: Pt). d Absorptions of the
dyes loaded on TiO2 films immerged in dried CH3CN solutions. e Absorptions
of the dyes loaded on TiO2 films immerged in 0.8 M LiBr in dried CH3CN
solutions. f E0-0 were determined by onset wavelengths of the coresponding
absorption spetra on TiO2 films.
Figure 3. Photocurrent density vs voltage curves and IPCE action
spectra for dye-sensitized solar cells sensitized by the dyes TC301
(a, b) and TC306 (c, d) with different electrolytes: device A based
on 0.9 M DMBIBr, 0.08 M Br2, and 0.5 M TBP electrolyte in dried
CH3CN sloutions; device B based on 0.9 M DMBIBr, 0.8 M LiBr,
0.08 M Br2, and 0.5 M TBP electrolyte in dried CH3CN solutions;
device C based on 0.6 M DMPII, 0.06 M LiI, 0.04 M I2, and 0.4
M TBP electrolyte in dried CH3CN solutions.
M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.06
M LiI, 0.04 M I2, and 0.4 M TBP in dried CH3CN solutions
produced a Voc of 0.696 V and a η value of 2.36% for the
dye TC301 and a Voc of 0.621 V and a η value of 4.10% for
the dye TC306. After replacement of I-/I3 with Br-/Br3
-
-
in TC301 dye-sensitized solar cells, device A yielded a
significant increasement in Voc (460 mV) but a decrease in
Jsc (0.42 mA cm-2). Device B yielded a significant increase-
ment in both Voc (345 mV) and Jsc (0.5 mA cm-2). Significant
A. For DSCs sensitized by the dye TC306, the IPCE value
for device C is broader than that for devices A and B. These
results are in agreement with absorption spectra of the dyes
attached to TiO2 with 0.8 M LiBr in CH3CN solutions and
photocurrent generation of the devices.
increases in Voc by using Br-/Br3 -based electrolytes instead
-
of I-/I3- were attributed to the enlarged energy level between
the redox potential of the electrolyte and the Fermi level of
TiO2. The Voc of device B is lower than that of device A;
this is probably due to the positive shift of the conduct band
edge of TiO2 in the presence of Li+ ions.9,10 The higher Jsc
in device B was attributed to the broad absorption of TC301
Table 2. Photovoltaic Performance of DSCs Based on the Dyes
TC301 and TC306 in Different Electrolytes As Shown in
Figure 3
on TiO2 in the presence of Li+ ions. Replacement of I-/I3
-
dye
Jsc (mA cm-2
)
Voc (V)
ff
η (%)
with Br-/Br3- in TC306 dye-sensitized solar cells, compared
to device C, yielded a large increase in Voc (about 300 mV)
for both device A and device B also due to the enlarged
energy level between the redox potential of the electrolyte
and the Fermi level of TiO2.
For DSCs sensitized by the dye TC301, the incident
photon-to-current conversion efficiencies (IPCEs) were be-
tween 75% and 80% around 400 nm and IPCE plateaus were
between 360 and 460 nm (Figure 3b). For DSCs sensitized
by the dye TC306, IPCEs were between 71% and 80%
around 450 nm, and IPCE plateaus were between 350 and
500 nm (Figure 3d). One can see from parts b and d of Figure
3 that devices A and B produced larger IPCE values below
400 nm compared to device C. This phenomenon is attribu-
tive to the lower absorption of bromine than that of iodine
in this spectral region. For DSCs sensitized by the dye
TC301, we found the IPCE value for device C is slightly
lower than that for device B and higher than that for device
device A
device B
device C
device A
device B
device C
4.00
4.93
4.42
7.12
7.36
8.76
1.156
1.041
0.696
0.939
0.915
0.621
0.796
0.713
0.767
0.781
0.752
0.754
3.68
3.66
2.36
5.22
5.07
4.10
TC301
TC306
Figure 4 shows the electrochemical impedance spec-
tra11-14 for devices A-C under forward bias (-0.7 V) in
the dark. The larger semicircles in the Nyquist plots are
attributed to the charge-transfer processes occurring at TiO2/
dye/electrolyte interface. Evidently, the charge-transfer re-
sistance at the interface was increased remarkably from
(11) Adachi, M.; Sakamoto, M.; Jiu, J.; Ogata, Y.; Isoda, S. J. Phys.
Chem. B 2006, 110 (28), 13872
(12) Wang, Q.; Moser, J.; Gra¨tzel, M. J. Phys. Chem. B 2005, 109 (31),
.
14945
.
(13) Wang, Z.; Koumura, N.; Cui, Y.; Takahashi, M.; Sekiguchi, H.;
Mori, A.; Kubo, T.; Furube, A.; Hara, K. Chem. Mater. 2008, 20 (12),
3993
(14) Kuang, D.; Uchida, S.; Humphry-Baker, R.; Zakeeruddin, S.;
Gra¨tzel, M. Angew. Chem., Int. Ed. 2008, 47 (10), 1923
(9) Redmond, G.; Fitzmaurice, D. J. Phys. Chem. B 1993, 97, 1426
.
.
(10) Lindstrom, H.; Sodergren, S.; Solbrand, A.; Rensmo, H.; Hjelm,
J.; Hagfeldt, A.; Lindquist, S.-E. J. Phys. Chem. B 1997, 101, 7717
.
.
5544
Org. Lett., Vol. 11, No. 23, 2009

Although very high open-circuit voltage with a good
energy conversion efficiency was achieved, we found that
stability of DSCs based on Br-/Br3 electrolytes and the
-
novel dyes TC301 and TC306 was not very good because
of bromination reaction of bromine with unsaturated bonds
of the dyes especially in the presence of light irradiation.
To achieve long-term stable DSCs based on Br-/Br3
-
electrolytes, we are synthesizing new dyes without reactive
position which could react with bromine.
In summary, we have demonstrated that a DSC consisting
of Br-/Br3 redox mediator and the organic dye TC301
-
accomplished a very high open-circuit voltage (up to 1.15
V) with a good energy conversion efficiency of 3.68%. And
for the dye TC306-sensitized solar cell, a high open-circuit
voltage (0.939 V) with a considerably good energy conver-
sion efficiency of 5.22% was achieved with a Br-/Br3 -
-
containing electrolyte. These results suggest that the Br-/
Figure 4.
Electrochemical impedance spectra, scanned from 10-1
Br3 couple could be a promising alternative to the I-/I3
-
-
to 105 Hz at room temperature, for the dye TC301 and devices
A-C, respectively: (a) Nyquist plots; (b) Bode phase plots. The
cells were measured at -0.7 V in the dark. The alternate current
(AC) amplitude was set at 10 mV.
couple for those dyes with more positive HOMO levels in
DSCs. Further efforts on finding new dye with broader
absorption to get even better DSC performance are in
progress.
Acknowledgment. We gratefully acknowledge the finan-
cial support of this work from the following sources: the
National Natural Science Foundation of China (Grant No.
20633020), the Ministry of Science and Technology (MOST),
the Swedish Energy Agency, the K&A Wallenberg Founda-
tion, and the Swedish Research Council.
device C to devices A and B. The electron lifetime (τ) in
TiO2 film for DSCs sensitized by the dye TC301 are 98 and
93 ms for device A and B, which are much larger than that
for device C (38 ms). Both results suggest that the charge
recombination was inhibited upon replacing I-/I3- with Br-/
-
Br3 . We can conclude that significant increasements in Voc
by using Br-/Br3 -based electrolytes instead of I-/I3 were
attributed to not only the enlarged energy level between
the redox potential of the electrolyte and the Fermi level of
the conduction band of TiO2 but also the suppressed charge
recombination.
-
-
Supporting Information Available: Fabrication of the
nanocrystalline TiO2 solar cells, synthesis and spectra data
of the dyes TC301 and TC306. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL9022936
Org. Lett., Vol. 11, No. 23, 2009
5545