Highly efficient and thermally stable organic sensitizers for solvent-free dye-sensitized solar cells

Article abstract of DOI:10.1002/anie.200703852

(Chemical Equation Presented) A sense for sensitizers: Devices based on JK-46 (see structure) and a volatile electrolyte yielded an extremely high overall conversion efficiency of 8.6% under AM 1.5 sunlight. Solar cells fabricated employing the JK-46 sens

Full text of DOI:10.1002/anie.200703852

Angewandte
Chemie
DOI: 10.1002/anie.200703852
Solar Cells
Highly Efficient and Thermally Stable Organic Sensitizers for Solvent-
Free Dye-Sensitized Solar Cells**
Hyunbong Choi, Chul Baik, Sang Ook Kang, Jaejung Ko,* Moon-Sung Kang,
Md. K. Nazeeruddin,* and Michael Grätzel
Increasing energy demands and concerns about global
warming have led to a greater focus on renewable energy
sources during the last years. Dye-sensitized solar cells
(DSSCs) have a significant potential to be used as low-cost
devices for conventional p–n junction solar cells.[1] Several
RuII polypyridyl complexes have achieved power conversion
efficiencies above 11% in standard global air mass 1.5,
thereby showing a good stability.[2] Some organic dyes with
interesting photophysical and electrochemical properties can
be used as promising sensitizers. Recently, an impressive
photovoltaic performance was obtained using organic dyes
that showed efficiencies between 6 and 9%.[3] However,
organic dyes are known to be less stable than ruthenium
complexes, which is probably a result of the formation of
excited triplet states and unstable radicals under light
irradiation. Another disadvantage of organic dyes in DSSCs
is the formation of aggregates on the semiconductor surface,
which leads to self-quenching and reduces electron injection
into TiO2.[4] Therefore, engineering of organic sensitizers with
an enhanced stability and a reduced tendency toward
aggregation is paramount. Recently, a successful approach
was introduced by incorporating a nonplanar bis-dimethyl-
fluorenylamino moiety[5] and a terthiophene unit[6] into the
organic framework, which not only suppresses aggregate
formation but also increases the molar extinction coefficient
of the organic sensitizer. Herein, we report novel organic
sensitizers, coded as JK-45 and JK-46, which consist of: 1) a
tailored dimethylfluorenylamino moiety that ensures greater
resistance to degradation when exposed to light and/or high
temperatures, because it possesses a bipolar character that
allows the formation of both stable cation and anion
radicals;[7] 2) conducting thiophene units with aliphatic
chains that enhance the interface and the tolerance towards
water in the electrolytes; 3) a cyanoacrylic acid moiety, which
acts as acceptor and anchoring group.
The sensitizers JK-45 and JK-46 are readily synthesized in
three steps, which are illustrated in Scheme 1. The Suzuki
coupling of 1[8] and 2[9] with 1.2 equivalents of 2-(3’,4,3’’-tri-
n-hexyl-2,2’:5’,2’’-terthio-phen-5-yl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane[10] yields 3 and 4. The terthiophene derivatives
were converted into their corresponding carbaldehydes 5 and
6 by means of a Vilsmeier–Haack reaction.[11] The aldehydes 5
and 6, upon reaction with cyanoacetic acid in the presence of
piperidine in acetonitrile, produced the JK-45 and JK-46 dyes.
The visible absorption spectrum of JK-45 exhibits two
maxima at 430 nm (e = 34800mꢀ1 cmꢀ1) and 369 nm (e =
60200mꢀ1 cmꢀ1), which are attributed to the p–p* transitions
of the conjugated molecule. Under similar conditions, the JK-
46 sensitizer exhibits absorption bands at 430 nm (e =
29200mꢀ1 cmꢀ1) and 372 nm (e = 35000mꢀ1 cmꢀ1; Figure 1).
Their molar extinction coefficients are higher than that of the
standard N719 dye (14000mꢀ1 cmꢀ1).[13] When JK-45 and JK-
46 are excited within their p!p* bands, they exhibit a strong
luminescence maximum at around 650 nm, with E0ꢀ0 tran-
sition energies of 2.50 and 2.35 eV, respectively. The excited-
*
state oxidation potentials (Eox) of the dyes (JK-45: ꢀ1.48 V
[*] H. Choi, C. Baik, Prof. Dr. S. O. Kang, Prof. Dr. J. Ko
Department of New Material Chemistry
Korea University, Jochiwon, Chungnam 339-700 (Korea)
Fax: (+82)41-867-5396
and JK-46: ꢀ1.36 V, versus NHE, normal hydrogen elec-
trode) are much more negative than the conduction-band
edge of TiO2, which is located at ꢀ0.5 V (vs. NHE), thus
providing a thermodynamic driving force for efficient elec-
tron injection.
E-mail: jko@korea.ac.kr
Homepage: http://dssc.korea.ac.kr
Dr. Md. K. Nazeeruddin, Prof. Dr. M. Grätzel
LPI, Institut des Sciences et IngØnierie Chimiques, FacultØ des
Sciences de Base
École Polytechnique FØdØrale de Lausanne
1015 Lausanne (Switzerland)
Molecular-orbital calculations illustrate that the HOMO
of JK-46 is localized over the fluorenylamino unit through
benzo [b]thiophene and the LUMO is localized over the
cyanoacrylic unit through thiophene (Figure 2). Examination
of the HOMO and LUMO of both sensitizers indicates that
the HOMO–LUMO excitation moves the electron distribu-
tion from the bis(9.9-dimethylfluoren-2-yl)amino unit to the
cyanoacrylic acid moiety, thus allowing an efficient photo-
induced electron transfer from the dye to the TiO2 electrode.
The photocurrent action spectra of the devices composed
of JK-45 and JK-46 were obtained using an electrolyte
comprising 0.6m 1,2-dimethyl-3-n-propylimidazolium iodide
(DMPImI), 0.1m LiI, 0.05m I2, and 0.5m 4-tert-butylpyridine
in acetonitrile (Figure 3). The incident-photon-to-current
conversion efficiency (IPCE) of JK-46 exceeds 70% in the
Fax: (+41)21-693-4111
E-mail: mdkhaja.nazeeruddin@epfl.ch
Dr. M.-S. Kang
Energy & Environment Lab.
Samsung Advanced Institute of Technology (SAIT)
Yongin, 446-712 (Korea)
[**] This work was supported by the Korea Science and Engineering
Foundation (KOSEF) through the National Research Lab. Program
funded by the Ministry of Science and Technology (No.
M10500000034-06J0000-03410) and BK-21 (2006).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2008, 47, 327 –330
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
efficiency (h) were recorded over a period of
1000 h. After 1000 h of light soaking, the Voc of
JK-46 decreased by 77 mV, but the loss was
compensated by an increase in the short-circuit
current density from 11.71 to 13.44 mAcmꢀ2,
while the fill factor was very stable. The long-
term stability of the device is remarkable,
because the initial efficiency of 6.82% slightly
increased to 7.03% during the 1000 h light
soaking test. Only three ruthenium polypyridyl
sensitizers have resisted light-soaking stress for
1000 h while retaining an efficiency over 7%.[15]
This is the first time that organic-sensitized
DSSCs with an efficiency above 7% have
passed the light-soaking test. The enhanced
stability of JK-45 and JK-46 can be attributed
to the bis-dimethylfluorenyl-amino moiety cou-
pled to the substituted hexyl chains, which
prevents water-induced dye desorption from
the TiO2 surface. The JK-2 sensitizer reported
in reference [7] does not contain hexyl chains
and is hence less stable than JK-45 and JK-46.
To understand the electron-injection prop-
Scheme 1. Synthesis of JK-45 and JK-46.
erty and the change in Voc of JK-45 and JK-46 more
clearly, we measured the electron-diffusion coefficients
and lifetimes of the photoelectrode. Figure 5 shows the
electron-diffusion coefficients (De) and the lifetimes (te)
of DSSCs prepared using different dyes (namely, N719,
JK-45, and JK-46) as a function of Jsc. The De values were
obtained using a time constant (tc), which was deter-
mined by fitting the decay of the photocurrent transient
to exp(ꢀt/tc) and the TiO2 film thickness (w) using the
equation: De = w2/(2.77tc). The t value was also deter-
mined by fitting the decay of the photovoltage transient
to exp(ꢀt/t). The Jsc values in the x axis increased with an
increase in the initial laser intensity controlled by ND
filters with different optical densities. The De values of
the photoanodes adsorbing the JK-45 and JK-46 dyes are
very similar to those of N719 under identical short-
circuit-current conditions, which demonstrates that the
Figure 1. Absorption and emission spectra of JK-45 (black solid line) and JK-
46 (gray solid line) in THF and absorption spectra of JK-45 (black dashed
line) and JK-46 (gray dashed line) adsorbed on a TiO2 film. The emission
spectra were obtained using the same solution by exciting at 430 nm at
298 K.
spectral range: 400–610 nm, thereby reaching its maximum
value (namely, 82%) at 482 nm. Under solar-simulated light
irradiation (that is, 100 mWcmꢀ2; 1.5 AM Global), the JK-46-
based DSSC exhibited
a short-circuit current (Jsc) of
17.45 mAcmꢀ2, an open-circuit voltage (Voc) of 0.664 V, and
a fill factor (FF) of 0.742, which corresponds to an overall
efficiency of 8.60%. Using solvent-free ionic-liquid electro-
lytes composed of 0.2m I2, 0.5m NMBI, and 0.1m GuNCS in
PMII/EMINCS (13:7), the JK-46 sensitizer yielded a strik-
ingly high conversion efficiency of 7%. To the best of our
knowledge, this is the highest efficiency ever reported for
DSSCs based on organic sensitizers.[14] Figure 4 shows the
photovoltaic performance during long-term accelerated aging
of JK-45 and JK-46 sensitized solar cells using an ionic liquid
at AM 1.5 and 608C. The values of Jsc, Voc, FF, and the overall
Figure 2. Isodensity surface plots of the HOMO and LUMO of a) JK-45
and b) JK-46.
328
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Angewandte
Chemie
Figure 5. Electron-diffusion coefficients De (a) and lifetimes te (b) in
&
*
*
the photoelectrodes adsorbing different dyes. : N719, : JK-45, : JK -
46.
Figure 3. a) J–V curve and b) IPCE spectra of N719 (1), JK-45 (2), and
JK-46 (3).
were observed between the cells that used different dyes. The
electron-recombination rates seem to be determined by the
molecular structures of the dyes as a result of their different
coverage on the TiO2 surface. The electron lifetimes are
consistent with the Voc values shown in Table 1. The presence
of hexyl groups in the JK-45 and JK-46 dyes increases the
electron lifetime (t) as a result of the steric hindrance, which
leads to an increase in Voc and the solar-to-electric conversion
efficiency.[12]
In conclusion, we designed and synthesized two novel
organic dyes (namely, JK-45 and JK-46). A solar-cell device
based on the sensitizer JK-46 and a volatile electrolyte
yielded an overall conversion efficiency of 8.60%, whereas
the conversion efficiency of a device based on the same
sensitizer and a solvent-free ionic-liquid electrolyte was 7%
(both devices were tested under AM 1.5 sunlight). The JK-46-
based solar cells fabricated using a solvent-free ionic-liquid
electrolyte exhibited an excellent stability under light soaking
at 608C for 1000 h.
~
Figure 4. Evolution of the solar-cell parameters with JK-45 ( ) and JK-
46 ( ) during visible-light soaking (AM 1.5G, 100 mWcmꢀ2) at 608C. A
&
420-nm cut-off filter was placed on the cell surface during illumination.
Ionic-liquid electrolyte: 0.2m iodine, 0.5m NMBI, 0.1m GuNCS in
PMII/EMINCS (13/7).
Experimental Section
Electron-transport measurements: The electron-diffusion coefficients
and lifetimes were measured by means of stepped light-induced
transient measurements of photocurrent and voltages (SLIM-
PCV).[12] The transients were induced by a stepwise change in the
laser intensity. A diode laser (l = 635 nm), which served as light
electron-injection properties of JK-45 and JK-46 are good
enough for application in DSSCs. Differences in the te values
Angew. Chem. Int. Ed. 2008, 47, 327 –330
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www.angewandte.org
329

Communications
Table 1: Optical, redox, and DSSC performance parameters of the dyes.
Dye
labs[a] [nm] (e [mꢀ1 cmꢀ1])
Eredox[b] [V]
E0-0[c] [V]
ELUMO[d] [V]
Jsc [mAcmꢀ2
]
Voc [V]
FF
h
[e] [%]
JK-45
JK-46
N719
369 (60 200), 430 (34 800)
372 (35 000), 430 (29 200)
1.02
0.99
2.50
2.35
ꢀ1.48
ꢀ1.36
16.13
17.45
18.19
0.641
0.664
0.787
0.718
0.742
0.720
7.42
8.60
10.31
[a] The absorption spectra were measured in THF solution. [b] The redox potentials of the dyes on TiO2 were measured in CH3CN with 0.1m
(n-C4H9)4NPF6 at a scan rate of 50 mVsꢀ1 (vs. NHE). [c] E0ꢀ0 was determined from the intersection of the absorption and the emission spectra in THF.
[d] ELUMO was calculated by EoxꢀE0-0. [e] The performances of the DSSCs were measured using a working area of 0.18 cm2. Electrolyte: 0.6m DMPImI,
0.05m I2, 0.1m LiI, and 0.5m tert-butylpyridine in acetonitrile.
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approximately 10 mWcmꢀ2
) by using an ND filter which was
positioned at the front side of the fabricated samples (0.04 cm2). For
the SLIM-PCV measurements, the TiO2 thickness was controlled at
approximately 8 mm. The photocurrent and photovoltage transients
were monitored using a digital oscilloscope through an amplifier. A
total of five points were measured to determine the electron-diffusion
coefficients and lifetimes.
Dye-sensitized solar cells: Fluorine-doped tin oxide (FTO) glass
plates (Pilkington TEC Glass-TEC 8, Solar 2.3 mm thickness) were
cleaned in a detergent solution using an ultrasonic bath for 30 min and
then rinsed with water and ethanol. Then, the plates were immersed
in 40mm TiCl4 (aqueous) at 708C for 30 min and washed with water
and ethanol. A transparent nanocrystalline layer was prepared on the
FTO glass plates by using a doctor blade printing TiO2 paste
(Solaronix, Ti-Nanoxide T/SP), which was then dried for 2 h at
258C. The TiO2 electrodes were gradually heated under an air flow at
3258C for 5 min, at 3758C for 5 min, at 4508C for 15 min, and at
5008C for 15 min. The thickness of the transparent layer was
measured by using an Alpha-step 250 surface profilometer (Tencor
Instruments, San Jose, CA). A paste containing 400-nm-sized anatase
particles (CCIC, PST-400C) was deposited by means of doctor blade
printing, to obtain the scattering layer, and then dried for 2 h at 258C.
The TiO2 electrodes were gradually heated under an air flow at 3258C
for 5 min, at 3758C for 5 min, at 4508C for 15 min, and at 5008C for
15 min. The resulting film was composed of a 20-mm-thick transparent
layer and a 4-mm-thick scattering layer. The TiO2 electrodes were
treated again with TiCl4 at 708C for 30 min and sintered at 5008C for
30 min. Then, they were immersed in JK-45 and JK-46 solutions
(0.3mm in tetrahydrofuran, THF) and kept at room temperature for
24 h. FTO plates for the counter electrodes were cleaned in an
ultrasonic bath in H2O, acetone, and 0.1m aqueous HCl, subsequently.
The counter electrodes were prepared by placing a drop of an H2PtCl6
solution (2 mg Pt in 1 mL ethanol) on an FTO plate and heating it (at
4008C) for 15 min. The dye-adsorbed TiO2 electrodes and the Pt
counter electrodes were assembled into a sealed sandwich-type cell
by heating at 808C using a hot-melt ionomer film (Surlyn) as a spacer
between the electrodes. A drop of the electrolyte solution was placed
in the drilled hole of the counter electrode and was driven into the cell
via vacuum backfilling. Finally, the hole was sealed using additional
Surlyn and a cover glass (0.1 mm thickness).
Received: August 22, 2007
Revised: September 27, 2007
Published online: November 16, 2007
Keywords: dyes/pigments · electrochemistry · ionic liquids ·
.
long-term stability · solar cells
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330
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Angew. Chem. Int. Ed. 2008, 47, 327 –330