Influence of donor moiety in ruthenium sensitizers on the properties of dye-sensitized solar cells

Article abstract of DOI:10.1166/jnn.2010.2970

Heteroleptic ruthenium complexes c/s-[Ru(H₂dcbpy)(L)(NCS) ₂], where H₂dcbpy is 4,4'-dicarboxylic acid-2,2'-bipyridine and L is 4-(4-(N,N-di-(p-hexyloxyphenyl)-amino)styryl)-4'- methyl-2,2'-bipyridine (Rut-A) or 4-(4'-(3,6-

Full text of DOI:10.1166/jnn.2010.2970

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Journal of
Nanoscience and Nanotechnology
Vol. 10, 6811–6814, 2010
Influence of Donor Moiety in Ruthenium Sensitizers on
the Properties of Dye-Sensitized Solar Cells
Hong-Minh Nguyen1, Jongwan Choi1, Ji Hyun Heo1, Jin-Woo Oh1,
Duc-Nghia Nguyen2, and Nakjoong Kim1ꢀ∗
1Department of Chemistry and Research Institute for Natural Science, Hanyang University,
17-Haengdang-Dong, Seongdong-gu, Seoul 133-791, Korea
2Polymer Division, Institute of Chemistry Vietnam Academy of Science and Technology,
18-Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam
Heteroleptic ruthenium complexes cis-[Ru(H2dcbpy)(L)(NCS)2], where H2dcbpy is 4,4ꢀ-dicarboxylic
acid-2,2ꢀ-bipyridine and L is 4-(4-(N,N-di-(p-hexyloxyphenyl)-amino)styryl)-4ꢀ-methyl-2,2ꢀ-bipyridine
(Rut-A) or 4-(4ꢀ-(3,6-dihexyloxycarbazole-9-yl)-styryl)-4ꢀ-methyl-2,2ꢀ-bipyridine (Rut-B), have been
synthesized and characterized by NMR, UV-Vis spectroscopy, and cyclic voltammogram. The effect
of different electron donors on the properties of dye-sensitized solar cells has been studied. The
power conversion efficiency of DSSC based on Rut-B is 6.1% while Rut-A delivered a lower effi-
ciency of 4.52% under the same device fabrication and measuring conditions. The better pho-
tovoltaic performance of Rut-B is mainly associated with enhanced dye absorptivity and charge
recombination suppression.
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Keywords: Dye-Sensitized Solar Cells, Ruthenium Complexes, Conjugated Ligand,
IP: 91.216.3.191 On: Thu, 09 Jun 2016 10:05:55
HOMO–LUMO Energy Level, Substituted Carbazole.
Copyright: American Scientific Publishers
1. INTRODUCTION
functionalized by substituted carbazole moiety as a donor
to extend the ꢁ conjugated system. Another ligand was
synthesized where carbazole was replaced by a substituted
diphenylamine moiety. These two ligands were employed
as ancillary ligands in ruthenium complexes (Rut-A, Rut-
B). We also studied the effect of dye molecular structures,
especially the carbazole and diphenylamine moieties on
the photophysical, electrochemical properties of dyes as
well as solar cell performance.
Dye-sensitized solar cells (DSSCs) have attracted great
attention due to their low-cost production and high effi-
ciency (ꢀ).1 In a DSSC, the photosensitive dye plays a
crucial role because it can absorb visible light and injects
the photoexcited electrons to the conduction band of metal
oxide semiconductors (anatase TiO2, ZnO). Up to now,
DSSCs based on ruthenium polypyridyl complex photo-
sensitizers have exhibited the highest power conversion
efficiency (11%) and long-term stability.2–5
However, one of the main drawbacks of ruthenium sen-
sitizers is the relatively low molar extinction coefficient
in the visible and near-IR wavelengths. In order to solve
this problem, the ancillary ligand has been modified by
extending its conjugation with electron-donating groups
such as thiophene, bithiophene, and thienothiophene.6–8 As
an electron-rich molecule with a planar fused-ring hete-
rocycle, carbazole has been used as an electron donor in
organic dyes.9–11 The DSSCs based on these dyes showed
high photovoltaic performance (6–8.3%).
2. EXPERIMENTAL DETAILS
2.1. Characterization of Dyes
1H-NMR spectra were recorded on a Varian INOVA
400 MHz NMR spectrometer in CDCl3 or (DMSO)-
d6. ESI-MS was performed on a HP 1100 LC/MS
with acetonitrile as solvent and FAB-MS on a JMS-
700 HRMS. The UV-Vis absorption spectra of the dye
solution were recorded on a Shimadzu UV-3101 PC spec-
trophotometer. Cyclic voltammogram (CV) was measured
with a three-electrode electrochemical cell on a CHI610B
electrochemical analyzer. A platinum working electrode,
platinum counter electrode, and Ag/AgCl reference elec-
trode were used. The supporting electrolyte was 0.1 M
According to the above mentioned data, we
have designed and synthesized a bipyridine ligand
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2010, Vol. 10, No. 10
1533-4880/2010/10/6811/004
doi:10.1166/jnn.2010.2970
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Influence of Donor Moiety in Ruthenium Sensitizers on the Properties of Dye-Sensitized Solar Cells
Nguyen et al.
OC6H13
tetrabutylammonium tetrafluoroborate (Bu4NBF4) in ace-
tonitrile and measurements were calibrated by the fer-
rocene/ferrocenium (Fc/Fc+) couple potential.
C6H13
O
C6H13
O
OC6H13
N
N
2.2. Preparation and Measurement of
Dye-Sensitized Solar Cells
HOOC
HOOC
N
HOOC
N
2+ N
2+
N
N
Photoanodes of the DSSCs were prepared from a colloidal
suspension of nanosized TiO2 particles (Nanoxide-T, Sola-
ronix, average size of 20 nm) by a doꢁctor blade technique.
The electrodes were sintered at 500 C for 30 min in air
to obtained TiO2 films with thickness of 8 ꢂm measured
by a ꢃ-Step Profiler. These electrodes were immersed into
solution containing 0.5 mM of ruthenium dyes for one
day while still hot (80 ꢁC). Solvents for the dye solu-
tion were CH3CN/t-BuOH (1:1 .v) for Rut-A and Rut-
B. Platinum-coated counter electrodes were obtained by
spreading a drop of 5×10−3 M H2PtCl6 (Fluka) solution in
isopropanol on theꢁsurface of a FTO glass and were treated
in an oven at 400 C for 15 min. The two electrodes were
assembled using 25 ꢂm thick Surlyn polymer (Dupont,
1702) and tightly held at 120–130 ꢁC to seal the cell.
The electrolyte was composed of 0.1 M lithium iodide,
0.1 M iodine, 0.5 M 4-tert-butylpyridine, and 0.6 M 1,2-
dimethyl-3-propyl imidazolium iodide in acetonitrile. The
photovoltaic characterization was carried out by illuminat-
N
N
N
N
Ru
Ru
N
N
C
S
N
C
S
C
S
C
HOOC
S
Rut-A
Rut-B
Scheme 1. Chemical structures of Rut-A and Rut-B sensitizers.
carboxaldehyde functional group. Two ligands were syn-
thesized by two steps: nucleophilic addition of the anion
of dimethylbipyridine with aldehyde intermediate, fol-
lower by dehydration of the corresponding alcohol in
refluxing acetic acid. The synthesis of Ru(II) complexes
was performed following a standard procedure developed
by Grätzel and co-workers.12 The target compounds were
obtained in moderate yields and characterized by 1H-NMR
spectroscopy, mass spectrometry and were found to be
consistent with the proposed structures as illustrated in
Scheme 1.
The UV-Vis spectra of Rut-A and Rut-B dissolved in
DMF (1 × 10−5 M) are shown in Figure 1 and the data
are collected in Table I. The Rut-A sensitizer shows three
maximum absorption peaks at 304 nm, 433 nm, and
526 nm. The band in UV region at 304 nm is assigned
to the ꢁ–ꢁ∗ transition of bipyridine units. The second
maximum at 433 nm is attributed to the ꢁ–ꢁ∗ transition
of ligand A and one of the metal-to-ligand-charge trans-
fer (MLCT) transition for Rut-A. The absorption band at
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ing the cell with a 1000 W Xenon lamp (Spectra-Physics)
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as a solar simulator. The light source was calibrated to
Copyright: American Scientific Publishers
100 mW/cm2 by a standard NREL Si solar cell equipped
with a KG-5 filter to approximate AM 1.5G one sun
light intensity. An active area of 0.32 cm2 was irradiated.
The current–voltage curves were obtained by measuring
the photocurrent of the cell using a Keithley model 2400
digital source meter (Keithley, USA) under an applied
external potential scan. Electrochemical impedance spectra
(EIS) were measured by a frequency resonance analyzer
(Solartron, SI 1260) connected to a potentiostat (Solartron
1287) with an oscillation level of 10 mV at the open-circuit
voltage under 100 mVcm−2 broadband illumination.
3. RESULTS AND DISCUSSION
The synthesis of 4-(4-(N,N-di(p-hexyloxyphenyl)amino)-
styryl)-4ꢀ-methyl-2,2ꢀ-bipyridine (ligand A) is based
on the multi-steps synthetic pathway, starting from
N,N-di-p-(hexyloxy)phenylamine. The carboxaldehyde
intermediate was prepared by N-arylation of this amine
with 4-bromobenzaldehyde. The synthesis of 4-(4ꢀ-(3,6-
dihexyloxycarbazole-9-yl)styryl)-4ꢀmethyl-2,2ꢀ-bipyridine
(ligand B) is started from N-phenylcarbazole. This com-
pound was converted to 3,6-dibromo-N-phenylcarbazole
by bromination reaction with N-bromosuccinimide. This
intermediate was followed by treatment with sodium and
n-hexanol to afford 3,6-disubstituted phenylcarbazole.
The Vilsmeier-Haack reaction was carried out to obtain
Fig. 1. Electronic absorption spectra of Rut-A and Rut-B in DMF.
J. Nanosci. Nanotechnol. 10, 6811–6814, 2010
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Nguyen et al.
Influence of Donor Moiety in Ruthenium Sensitizers on the Properties of Dye-Sensitized Solar Cells
Table I. Photophysical property and energy levels of Rut-A and Rut-B.
Absorption
ꢄMLCT/nm
(ꢅ×103 M−1cm−1
VOX
(V vs. Ag/Ag+ꢆ
HOMOa LUMO Ebgap
[eV] [eV] [eV]
ꢆ
Rut-A
Rut-B
526 (20.56)
535 (20.76)
0.78
0.87
−5.40 −3.32 2.08
−5.49 −3.41 2.08
aHOMO was calculated from cyclic voltammogram. bBand gap was estimated from
intersection of absorption and emission spectra in DMF.
526 nm corresponds to low energy MLCT caused by NCS
ligands. Similar absorption bands can be observed for Rut-
B with three bands at 294 nm, 384 nm, and 535 nm. The
low energy MLCT absorption band of Rut-B exhibits high
molar extinction coefficient of 20760 M−1cm−1, which is
similar to that of Rut-A dye (20560 M−1cm−1ꢆ but the
maximum is red shift by 9 nm compared to Rut-A, respec-
tively. The enhanced molar extinction coefficient and the
red-shift of MLCT band in Rut-B are due to the extension
of the ꢁ-conjugation in hybrid sensitizer.
The cyclic voltammograms of Rut-A and Rut-B in
DMF with 0.1 M tetrabutylammonium tetrafluoroborate
show oxidation potentials at 0.78 V and 0.87 V versus
Ag/Ag+ (Fig. 2), providing HOMO levels of −5.4 eV and
−5.49 eV with respect to zero vacuum level. As summa-
rized in Table I, the HOMO levels of Rut-A and Rut-B
Fig. 3. Current density–voltage characteristics of DSSC devices with
Rut-A and Rut-B as photosensitizers under AM 1.5G (100 mWcm−2
)
illumination. (Thickness of TiO2 film: 8 ꢂm, active area: 0.32 cm2ꢆ.
The current density–voltage characteristics of the Rut-A
and Rut-B sensitized cells are shown in Figure 3. Under
standard global AM 1.5 solar irradiation, the short-circuit
photocurrent density (JSC), open-circuit voltage (VOC) and
fill factor (ff) of the device with Rut-A sensitizer are
10.04 mA/cm2, 0.662 V, and 0.68, respectively, corre-
sponding to an overall conversion efficiency of (ꢀ) of
are calculated to be −3.32 eV anDde−liv3e.r4e1debVy, rIensgpeecnttiavetloy:, Purdue University Libraries
4.52%. In contrast, the device based on Rut-B has a fea-
IP: 91.216.3.191 On: Thu, 09 Jun 2016 10:05:55
which are higher than the conduction band edge of TiO2
ture of higher JSC (13.6 mA/cm2) and VOC (0.71 V) with a
Copyright: American Scientific Publishers
(−4 eV, −0.5 V vs. NHE), ensuring the electron injec-
fill factor of 0.63, resulting in a higher ꢀ (6.1%) compared
to Rut-A. This indicates that Rut-B with carbazole moi-
ety as electron donor shows better performance compared
to Rut-A. The higher JSC value is contributed by Rut-B
molar extinction coefficient with the enhanced absorptivity
at long wavelengths. The HOMO level of Rut-A (−5.4 eV)
is 0.09 eV higher than that of Rut-B (−5.49 eV) due to its
lower oxidation potential, as illustrated in Scheme 2. This
would lower the driving force for reduction of the oxidized
dye by iodide anion. The lower dye regeneration efficiency
is one of reasons for the lower conversion efficiency of
Rut-A dye.
tion from excited dye molecules into the conduction band
of TiO2 is thermodynamically favorable. Both Rut-A and
Rut-B show the same band gap (2.08 eV) but energy lev-
els of Rut-B are shifted to more negative levels than those
of Rut-A. This indicates that Rut-B dye incorporated car-
bazole moiety has lower energy levels than those for Rut-A
with diphenylamine moiety.
Electrochemical impedance spectroscopy (EIS) was per-
formed under open-circuit-conditions and AM 1.5 sim-
ulated sunlight illumination (100 mW/cm2). The results
are displayed in the form of Nyquist plot, as shown in
Figure 4. The Zreal and –Zimg are the real part and imag-
inary part of the impedance, corresponding to the charge
transfer resistant formed between interfaces and the resis-
tant formed by the existence of electrochemical capaci-
tance. In both DSSCs, total impedance of Rut-B/DSSC
is higher than that of Rut-A/DSSC. The large semicir-
cle at intermediate frequencies (1–5 × 102 Hz) represents
the electron transfer impedance at the TiO2/dye/electrolyte
interface. The charge transfer resistance at the TiO2 surface
(RCT2ꢆ can be calculated by fitting curves from the range
of intermediate frequencies using Z-view software. The
Fig. 2. Cyclic voltammograms of Rut-A and Rut-B in DMF: scanning
rate is 50 mV/s; working electrode and counter electrode, Pt wires; ref-
erence electrode, Ag/AgCl.
J. Nanosci. Nanotechnol. 10, 6811–6814, 2010
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Influence of Donor Moiety in Ruthenium Sensitizers on the Properties of Dye-Sensitized Solar Cells
Nguyen et al.
effectively the probability of injected electron capture by
electron acceptor (I3−) in the electrolyte.
4. CONCLUSION
In conclusion, two high molar extinction coefficient
Ruthenium (II) complexes (Rut-A and Rut-B) carrying
diphenylamine and carbazole moieties were synthesized
and characterized. The introduction of carbazole moi-
ety leads to red-shift of MLCT band and lower HOMO
level of Rut-B dye compared to Rut-A. Incorporation of
a substituted carbazole moiety in bipyridine ligand was
demonstrated to be beneficial for retarding the electron
transfer from TiO2 to the oxidized dye or electrolyte and
to enhance the charge transfer efficiency in the excited
state, resulting in improved power conversion efficiency.
Our future work is focused on the introduction of a new
ancillary ligand containing one more substituted carbazole
moiety to bathochromic shift the absorption and to control
the energy levels of ruthenium complex.
Scheme 2. Energy level diagram for Rut-A and Rut-B dyes.
Acknowledgment: This work was supported by the
Korea Science and Engineering Foundation (KOSEF),
Grant No. R11-2007-050-01003-0.
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References and Notes
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Fig. 4. Electrochemical impedance spectra (Nyquist plot) of DSSCs
based on Rut-A and Rut-B dyes measured under open circuit conditions
and AM 1.5 simulated sunlight illumination (100 mWcm−2ꢆ.
charge transfer resistance is related to the charge recom-
bination rate. The RCT2 value for Rut-B was estimated to
be 76 ohm that is larger than that for Rut-A (49 ohm),
respectively, corresponding reduction in the radius of the
semicircle. This impedance from the Rut-A dye-sensitized
solar cell is evidence that the recombination process is
faster than the Rut-B sensitizer, leading to the lower pho-
tovoltage of Rut-A dye cell. This indicates that incorpora-
tion of the substituted carbazole moiety in Rut-B decreases
Received: 31 October 2008. Accepted: 30 June 2009.
6814
J. Nanosci. Nanotechnol. 10, 6811–6814, 2010