A high efficiency ruthenium(II) tris-heteroleptic dye containing 4,7-dicarbazole-1,10-phenanthroline for nanocrystalline solar cells

Article abstract of DOI:10.1039/c6ra08666g

The tris-heteroleptic polypyridyl ruthenium(ii) dye, cis-[Ru(cbz₂-phen)(dcbH₂)(NCS)₂], cbz₂-phen = 4,7-dicarbazole-1,10-phenanthroline and dcbH₂ = 4,4′-dicarboxylic acid 2,2′-bipyridine, was designed, synthesized, purified via liquid chromatography and characterized using ¹H NMR, FTIR, cyclic voltammetry, absorption and emission spectroscopy. The compound exhibits broad and intense MLCT bands that overlap the visible spectrum and it is capable of sensitizing TiO₂ films. The energy levels of cis-[Ru(cbz₂-phen)(dcbH₂)(NCS)₂] are adequate for its use in DSSCs. The solar cells prepared using this dye achieved a performance that surpasses N3 dye under the same conditions. The introduction of the extended π-conjugated carbazole substituent at the 4 and 7 positions of 1,10-phenanthroline increases the IPCE and the overall solar cell efficiency. The high performance is ascribed to the HOMO stabilization and to the increase in electron delocalization of the triplet excited state, which favors the electron injection via singlet and triplet pathways.

Full text of DOI:10.1039/c6ra08666g

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ARTICLE
High efficiency ruthenium(II) tris-heteroleptic dye containing 4,7-
dicarbazole-1,10-phenanthroline for nanocrystalline solar cells
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
b
a
Andressa V. Müller , Luiz D. Ramos , Karina P. M. Frin , Kleber T. de Oliveira , André S. Polo*
DOI: 10.1039/x0xx00000x
2 2 2 2
The tris-heteroleptic polypyridyl ruthenium(II) dye, cis-[Ru(cbz -phen)(dcbH )(NCS) ], cbz -phen = 4,7-dicarbazole-1,10-
phenanthroline and dcbH
2
= 4,4’-dicarboxylic acid 2,2’-bipyridine, was designed, synthesized, purified via liquid
www.rsc.org/
1
chromatography and characterized using H NMR, FTIR, cyclic voltammetry, absorption and emission spectra. The
compound exhibits broad and intense MLCT bands that overlap the visible spectrum and it is capable of sensitizing TiO
films. The energy levels of cis-[Ru(cbz -phen)(dcbH )(NCS) ] are adequate for its use in DSSCs. The solar cells prepared
2
2
2
2
using this dye achieved a performance that surpasses the N3 one under the same conditions. The introduction of extended
π-conjugated carbazole substituent at the 4 and 7 positions of 1,10-phenanthroline increases the IPCE and the overall solar
cell efficiency. This high performance is ascribed to the HOMO stabilization and to the increase in electron delocalization
of the triplet excited state, which favors the electron injection via singlet and triplet pathways.
and metal-based dyes that aim to increase the performance of
2
1-30
INTRODUCTION
DSSCs.
However, just a few devices have achieved higher
efficiencies than those obtained by the Ru(II) complexes based
DSSCs.
Due to the outstanding performance of the N3 dye and its
double-deprotonated analogous N719, the efforts have been
concentrated on the design of new cis-[Ru(LL)(dcbH )(NCS) ]
Solar energy conversion is a meaningful alternative for
producing renewable energy to help solve the environmental
and energetic issues of the world because solar energy is
clean, plentiful and inexhaustible. A major harnessing strategy
of solar energy is its direct conversion into electrical output.
Among the photovoltaic devices that have been developed
over the past decades, Dye-Sensitized Solar Cells (DSSCs) stand
out due to their low cost, good power conversion efficiency,
2
2
dyes, dcbH = 2,2′-bipyridine-4,4′-dicarboxylic acid and LL =
2
polypyridyl ancillary ligands. Ruthenium(II) tris-heteroleptic
compounds with amphiphilic derivatives of 2,2’-bipyridine
-6 showed long-term stability. However, molar absorptivities
similar to those determined for N3 or N719 resulted in a lower
1
easy fabrication and low sensitivity to temperature changes.
The DSSCs are photoelectrochemical devices that are based on
3
1-34
photoelectrochemical performance compared with N3.
a
wide band-gap semiconductor with
a
mesoporous
The complexes prepared with donor-antenna substituent
derivatives of 2,2’-bipyridine were designed to improve light
absorption and to enhance the hydrophobic character of the
nanocrystalline morphology, typically TiO . The semiconductor
is sensitized with a light-harvesting dye, in which optical
absorption is followed by a photoinduced electron injection
from the dye into the TiO conduction band.
2
3
5, 36
7
dye.
The use of aromatic substituents can promote this
2
8-14
15-20
effect because aromaticity increases light absorption.
Furthermore, the existence of a hydrophobic chain protects
the dye from desorbing by water.
Few papers have reported the use of 1,10-phenanthroline and
their derivatives as ancillary ligands.
conjugated structure and similarity to the 2,2’- bipyridine
Improvement of semiconductor
or redox mediator
has
been the focus of several publications. However, the
development of more efficient dye-sensitizers is also a key
aspect for advancing these devices because dye-sensitizers are
responsible for initiating the photoelectrochemical energy
conversion processes by absorbing photons and subsequently
transferring electrons. Several groups have developed organic
36
37-44
Its extended π-
ligand leads to favorable properties for the energy conversion
37
processes. The first compound developed for DSSCs that has
,10-phenanthroline as an ancillary ligand demonstrated its
potential as a new class of sensitizers. However, the devices
did not exhibit better performance than N3. The insertion of
different substituents at some positions of phenanthroline
cores can either improve or decrease the performance of
devices. This may be ascribed to interfacial electron
1
3
8
45, 46
recombination dynamics.
However, the YS5 complex,
which is based on 4,7-diphenyl-1,10-phenanthroline, exhibits
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ARTICLE
RSC Adv.
(
DMF-d ; Cambridge, 99.5%), ethanol (Merck, Lichrosolv),
7
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DOI: 10.1039/C6RA08666G
acetonitrile
hexafluorophosphate (TBAPF6
grade), ferrocene (Fc; Aldrich, 98%), isopropanol (Synth,
9.5%), hexachloroplatinic acid (H PtCl ; Acros), TiCl standard
(Merck,
Lichrosolv),
tetrabutylammonium
;
Fluka, ≥99.0%, electrochemical
9
2
6
4
-
1
aqueous solution (Sigma-Aldrich, 1.00 g L , standard solution),
transparent 20 nm TiO anatase nanoparticles paste (18NR-T,
2
Dyesol), fluorine-doped SnO -coated glass (FTO; Aldrich, 3.2
2
mm thick, 8 Ω/
Dyesol), valeronitrile (Aldrich, 99.5%), iodine (Sigma-Aldrich,
99.8%), guanidine thiocyanate (Sigma-Aldrich, ≥97%), 4-tert-
☐), low temperature sealant - Surlyn (30 μm -
≥
butylpyridine (Aldrich, 96%) and 1-butyl-3-methylimidazolium
iodide (Aldrich, 98%). Additionally, cis-[Ru(phen)(dcbH )(NCS) ]
2
2
and cis-[Ru(dcbH ) (NCS) ], N3, were available from a previous
study.
2
2
2
Figure 1. Structure of cis-[Ru(cbz
2 2
-phen)(dcbH )(NCS)
2
].
44
high molar absorptivity and
performance under the same experimental conditions. This
indicates that the high-conjugated substituents attached to
a
comparable to N719
Synthesis of 2,2’-bipyridine-4,4’-dicarboxylic acid, dcbH2
The compound 2,2’-bipyridine-4,4’-dicarboxylic acid (dcbH2)
was synthesized with slight modifications of the previously
1
,10-phenanthroline rings is a good strategy for improving
4
3, 51
described procedure.
dimethyl-2,2’-bipyridine was added to the solution of 11.80 g
39.6 mmol) of Na Cr O that was dissolved in 45 mL of
Briefly, 3.23 g (17.55 mmol) of 4,4’-
molar absorptivity of the complex. This makes these
compounds interesting to investigate.
4
1
(
2
2 7
concentrated H SO , and the mixture was stirred for 30 min in
2
4
Specifically, the introduction of two aromatic groups at the 4
and 7 positions of the 1,10-phenanthroline ring can increase
molar absorptivity of the sensitizer and directly result in higher
photocurrent generation and in the overall solar cell efficiency.
Furthermore, insertion of a bulky substituent decreases the
an ice bath. The resulting solution was added to 410 mL of cold
water, the solid was removed, and 50 mL of water was added.
The solid was dissolved by adding 10% NaOH until pH 10. The
-1
final product was precipitated by adding 2.0 mol L HCl to the
solution until it reached pH 2, and the white solid was
collected. Yield: 93% Anal. Calc. for C H N O : C, 59.02; H,
mediator concentration in the vicinity of TiO , which harms the
2
4
7
12 8 2 4
semiconductor-electrolyte recombination dynamics. In this
context, carbazole is a potential substituent that has already
demonstrated improvements in the stability and light-
3
.30; N, 11.47%. Found: C, 58.21; H, 3.45; N, 11.09%.
4
8-50 Synthesis of 4,7-di(9H-carbazol-9-yl)-1,10-phenanthroline, cbz2-
harvesting capacity of 2,2’-bipyridine-based Ru(II) dyes.
phen
Herein, we report the synthesis, characterization and
photoelectrochemical performance of the novel tris- The compound was prepared with modifications of the
52-54
heteroleptic ruthenium(II) cis-[Ru(cbz -phen)(dcbH )(NCS) ], previously described procedure.
In a round bottom flask,
2
2
2
cbz -phen = 4,7-dicarbazole-1,10-phenanthroline and dcbH = 2.73 g (16.0 mmol) of carbazole and 1.95 g (16.0 mmol) of
2
2
4
,4’-dicarboxylic acid 2,2’-bipyridine, Figure 1, as a dye- potassium tert-butoxide was added to dry THF (40 mL) under
sensitizer for solar cells.
argon atmosphere, and the mixture was heated to reflux. After
h, 1.0 g (4.00 mmol) of 4,7-dichloro-1,10-phenantroline was
1
added, and the reflux was maintained for 24 h. The reaction
was quenched with water (100 mL) and was extracted with
toluene (3 × 100 mL). Organic layers were dried over
anhydrous sodium sulfate, and the solvent was distilled off
under reduced pressure. Purification was performed using
column chromatography. Specifically, silica was used as a
stationary phase, and toluene:ethyl acetate (gradient from 9:1
to 6:4) was used as an eluent. The fraction of interest was
collected, the solvent was distilled off, and the solid was
recrystallized from MeOH. Yield: (0.57 g, 1.1 mmol, 55%). Anal.
EXPERIMENTAL
Materials
The materials and reagents were used as received from the
indicated commercial suppliers: 4,4’-dimethyl-2,2’-bipyridine
(
Aldrich, 99%), H SO (Sigma-Aldrich, 97%), Na Cr O (Synth,
2 4 2 2 7
9
9.5%), NaOH (Sigma-Aldrich, >98%), HCl (Synth, 36.5%),
carbazole (Aldrich, ≥95%), potassium tert-butoxide (Aldrich,
98%), 4,7-dichloro-1,10-phenantroline (Aldrich, 97%),
≥
Calc. for C H N : C, 84.68; H, 4.34; N, 10.97% Found: C, 84.55;
tetrahydrofuran (THF; Synth, 99.0%), toluene (Synth, 99.5%),
ethyl acetate (Synth, 99.5%), silica (Sigma-Aldrich, ≥98%),
36 22 4
1
H, 4.55; N, 10.76%. H NMR (CDCl , 500 MHz,

/ppm): 9.48 (d,
3
2
H); 8.15 (d, 4H); 7.85 (d, 2H); 7.28-7.37 (m, 10H); 7.07 (d, 4H).
[
Ru(p-cymene)Cl ] (Strem, 98%), NaNCS (Merck, 98.5%), N,N-
2 2
13
C NMR (CDCl , 125 MHz,

/ppm): 151.5 (CH); 148.7 (C); 143.3
C); 141.2 (C); 126.5 (C); 126.4 (CH); 123.9 (C); 122.8 (CH);
20.6 (CH); 110.0 (CH).
dimethylformamide (DMF; Synth, 99.8%), methanolic solution
of tetrabutylammonium hydroxide (TBAOH; Acros Organics,
3
(
-
1
1
1
6
.0 mol L ), Sephadex LH-20 (Aldrich), HNO (Sigma-Aldrich,
3
5%), methanol (Synth, 99.8%), N,N’- dimethylformamide-d7,
2
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Synthesis of cis-[Ru(cbz -phen)(dcbH )(NCS) ]
ARTICLE
-
1
L ) was added as the supporting electrolyte. The
2
2
2
View Article Online
DOI: 10.1039/C6RA08666G
ferrocene/ferrocenium pair was used as the internal standard.
The cis-[Ru(cbz -phen)(dcbH )(NCS) ] complex was prepared
2
2
2
Potentials were reported versus the normal hydrogen
using one-pot procedure that was previously reported for
+
55,
43, 44
electrode (NHE) using E (Fc/Fc ) = 0.67 V vs. NHE at 298 K.
1/2
similar compounds.
Briefly, the ruthenium p-cymene
5
6
dimer, [Ru(p-cymene)Cl ] (0.17 g, 0.27 mmol), was dissolved
2
2
in 50 mL of N,N’-dimethylformamide and 0.28 g of cbz -phen
2
Fabrication of the dye-sensitized solar cells
(0.54 mmol) was added. The mixture was kept at 80 °C for 2 h
under argon atmosphere. After this period, 0.13 g of dcbH2 Solar cells were prepared in a sandwich-type arrangement
5
7
(
0.54 mmol) was added to the mixture, and the temperature using a modified previously described procedure. The 3.2
was increased to 160 °C and was kept at this level for 4 h. mm FTO conducting glass plates were cleaned, immersed in a
-1
Finally, 0.44 g of NaNCS (5.4 mmol) was added, the 40 mmol L aqueous TiCl solution at 70 °C for 30 min and
4
temperature was decreased to 140 °C, and the reaction was washed with water and ethanol. TiO paste was deposited by
2
2
kept under these conditions for 4 h, which allowed it to screen-printing spots (A = 0.196 cm ) on the FTO glass, and
proceed to completion. The compound was purified by heating at 125 °C for 6 min. This procedure was repeated 4
concentrating the obtained solution, washed and filtered with times. After the deposition, sintering process was performed
ultrapure H O, dissolved in a methanolic TBAOH solution (1.0 by heating the TiO films: 5 min at 325 °C, 5 min at 375 °C, 15
2
2
−
1
mol L ), centrifuged to remove any residual particles and min at 450 °C and 15 min at 500 °C. This resulted in TiO films
2
loaded on a liquid column chromatography containing with a 13.0 ± 0.7 μm thickness, as measured using a KLA-
Sephadex LH-20 as the stationary phase and methanol as the Tencor P-7 Stylus profiler. The films were sensitized by
eluent. The purification process was repeated 3 times. The immersing TiO electrodes into the ethanolic solution of the
2
-1
main fraction was concentrated, precipitated by adding H SO
dye (~0.3 mmol L ) for 24 h. The counter-electrodes were
2
4
-1
(0.5 mol L ) to reach pH ~ 1.0 and, finally, filtered. The prepared by depositing a hexachloroplatinic acid layer on the
obtained solid was dried in a vacuum oven and weighed 0.21 conductive side of a drilled conductive glass, followed by
mg. Yield = 39%. Anal. Calc. for C H N O RuS : C, 58.53; H, heating at 450 °C for 30 min. Solar cells were sealed using a
5
0
36
8
7
2
1
3
.54; N, 10.92%. Found: C, 58.87
; H, 3.37; N, 10.44%. H NMR film of sealant between the photoanode and counter-
(
DMF-d , 500 MHz, /ppm): 10.08 (d, J = 5.8 Hz; 1H); 9.90 (d, J electrode, followed by heating to 110 °C using a custom-built

7
=
8
8
6.0 Hz; 1H); 9.36 (s, 1H); 9.19 (s, 1H); 8.79 (d, J = 5.8 Hz; 1H); sealing equipment. The mediator was placed between the
.60 (d, J = 5.8 Hz; 1H); 8.54 (d, J = 6.0 Hz; 1H); 8.41 (s, 1H); photoanode and counter-electrode through the hole drilled on
.40 (s, 1H); 8.28-8.32 (m, 4H); 8.01 (d, J = 5.8 Hz; 1H); 7.83 (d, the counter-electrodes. The mediator was prepared by
-1
J = 9.5 Hz; 1H); 7.71 (d, J = 9.5 Hz; 1H); 7.63-7.67 (m, 2H); 7.49- dissolving iodine (38 mg, 0.03 mol L ), guanidinium
7
-
1
.55 (m, 2H); 7.42-7.46 (m, 3H); 7.26-7.38 (m, 5H).
thiocyanate (59 mg, 0.1 mol L ), 4-tert-butylpyridine (0.38 mL,
-1
0
.5 mol L ) and 1-butyl-3-methylimidazolium iodide (0.80 g,
-1
Methods
0.6 mol L ) in a mixture of acetonitrile:valeronitrile (85:15) to
yield a 5 mL solution.
NMR spectra were recorded at 298 K using a DRX-500 Bruker
Avance spectrometer at 500.13 MHz using DMF-d as the
solvent. The residual DMF peaks were used as the internal
7
Photoelectrochemical measurements
standard. All NMR spectra are presented in the supporting Photocurrent action spectra and current-voltage curves were
4
3
information. FTIR spectrum of the solid was recorded at 298 K measured using a previously described Newport system. The
-
1
in KBr pellets using a Bomem MB 100 spectrometer (4 cm
results of the presented photoelectrochemical experiments
resolution). The adsorbed TiO film spectrum was registered are the averages of, at least, four individual experiments.
2
using attenuated total reflectance (ATR) FTIR that was
recorded at 298 K using a Perkin Elmer Spectrum Two
-
1
RESULTS AND DISCUSSION
The FTIR spectrum of cis-[Ru(cbz -phen)(dcbH )(NCS) ] as a
spectrometer (4 cm resolution). Electronic absorption spectra
were recorded in quartz cuvettes (1.000 cm path length) using
an Agilent 8453 diode-array spectrophotometer. Emission
spectra were recorded in quartz cuvettes (1.000 cm path
length) using a Cary Eclipse spectrofluorimeter after the
samples were purged with argon for 2 min. The spectra were
not corrected for the photomultiplier spectral response.
2
2
2
neat solid exhibits peaks in the aromatic region, from 1650 to
-1
1
430 cm , that are ascribed to C=C and C-N of the aromatic
ligands, Figure 2. Additionally, aromatic C-H bending peaks are
-
1 58, 59
.
observed between 1275 and 1000 cm
Intense bands at
-1
-1
2
107 cm and 754 cm are, respectively, ascribed to N‒C and
-
C‒S and are characteristic of the NCS ligands that are
coordinated through the nitrogen atom.
isothiocyanate isomer is obtained after purification. The
60, 61
Cyclic voltammetry was performed using a μAutolab III
potentiostat/galvanostat (Autolab) using a standard three-
electrode arrangement comprised of a glassy carbon working
electrode (Metrohm), a Pt rod counter electrode (Metrohm)
and a Ag wire as a pseudo-reference electrode. The
Thus, only the
distinct C=O of the dcbH protonated carboxylic groups can be
2
-1
observed at 1725 cm . When adsorbed on the nanocrystalline
mesoscopic TiO film, cis-[Ru(cbz -phen)(dcbH )(NCS) ] exhibits
2
2
2
2
experiments were performed in acetonitrile. TBAPF (0.1 mol
6
similar FTIR spectra, Figure 2, except for the absence of the
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Figure 2. FTIR spectrum of cis-[Ru(cbz
2
-phen)(dcbH
)(NCS) ] that is adsorbed on the TiO
indicates the peak ascribed to the ‒NCS ligand at 754 cm ; Inset: binding modes of the
dye on the TiO surface).
2
)(NCS)
2
] in KBr (─) and the ATR-FTIR
Figure 3. Absorption spectrum of cis-[Ru(cbz
(∙∙∙) in ethanol and the emission spectrum of cis-[Ru(cbz
acetonitrile at T = 298 K (---). (λex = 500 nm; v = 30 nm min ).
2
-phen)(dcbH
2
)(NCS)
-phen)(dcbH
2
] (─) and cbz
2
-phen
] in
spectrum of cis-[Ru(cbz
2
-phen)(dcbH
2
2
2
film (---). (*
2
2
)(NCS)
2
-
1
−1
2
maxima, which indicates that a dye monolayer was adsorbed
C=O peak near 1700 cm due to the formation of ester on TiO2.
-
1
48
bonding. This indicates that the two carboxylic acid groups are The emission spectrum of cis-[Ru(cbz -phen)(dcbH )(NCS) ] in
2
2
2
completely anchored on the TiO surface with a carboxylate acetonitrile, Figure 3, is broad and non-structured. It is typical
2
-
-1
3
COO bidentate coordination. The bands at 1620 cm and 1374 of the MLCT lowest lying excited state of ruthenium
-
1
60
cm are ascribed to asymmetric and symmetric vibrations of polypyridyl compounds and exhibits an emission maximum
the carboxylate groups, respectively. This confirms the at 790 nm. The estimated HOMO-LUMO energy gap for the
bidentate or bridging chelating mode of adsorption of the compound, E0-0, is 1.86 eV. The gap is determined at the
emission spectrum onset. Its emission maximum has a
In the UV region, cis-[Ru(cbz -phen)(dcbH )(NCS) ] exhibits hypsochromic shift of 10 nm in comparison to cis-
6
2
complexes on the TiO surface.
2
2
2
2
4
4
intense absorption bands at approximately 330 and 310 nm, [Ru(phen)(dcbH )(NCS) ]. It is known that in cis-[Ru(L-
2
2
Figure 3, which are ascribed to intraligand π-π* transitions of phen)(dcbH )(NCS) ] dyes, the shifts in emission maxima are
2
2
the cbz -phen and dcbH ligands. The electronic spectrum of primarily ascribed to the differences in HOMO energies.
2
2
cis-[Ru(cbz -phen)(dcbH )(NCS) ] exhibits two broad and Because cis-[Ru(Ph -phen)(dcbH )(NCS) ] exhibits an emission
2
2
2
2
2
2
intense MLCT absorption bands with a maxima at 375 nm (1.4 energy higher than cis-[Ru(phen)(dcbH )(NCS) ] due its lower
2
2
4
-1
-1
4
-1
-1
43
×
10 L mol cm ) and 525 nm (1.8 × 10 L mol cm ) assigned HOMO energy, the same trend on HOMO energy is expected
to MLCT transitions and are responsible for the high sunlight for
cis-[Ru(cbz -phen)(dcbH )(NCS) ]. Electrochemical
harvesting by this compound. This has been previously experiments were performed to evaluate the HOMO energy of
2
2
2
41, 42
described in the literature for similar compounds.
the compound.
Particularly, the 375 nm absorption band have a contribution The cyclic voltammogram of cis-[Ru(cbz -phen)(dcbH )(NCS) ],
2
2
2
of the cbz -phen ligand, since the cbz -phen compound itself Figure 4, exhibits
a
quasi-reversible ground state
2
2
also absorbs in this region.
electrochemical process for the ruthenium(II/III) wave at E1/2
=
Molar absorptivities determined for this compound are higher 1.07 V vs. NHE. This positive shift in comparison to cis-
4
4
than those determined for cis-[Ru(phen)(dcbH )(NCS) ], Figure [Ru(phen)(dcbH )(NCS) ] (1.05 V vs. NHE) suggests that the
2
2
2
2
4
-1
-1
4
-1
S6 (ε
= 1.1 × 10 L mol cm and ε = 1.0 × 10 L mol
presence of carbazole at the 4 and 7 positions of the
4
25nm
525nm
-
1
cm ) at almost all wavelengths in the 250-800 nm range due phenanthroline ring decreases the electronic density at the
to the existence of the π-conjugated carbazole group at the 4 metal center. In addition, two reduction peaks are observed at
and 7 positions of the phenanthroline ring. This increases the -1.09 V and -1.56 V vs. NHE and are ascribed to
molar absorptivity of this compound, as was observed for electrochemical processes on the ligands. The more positive
2
3, 48-50
similar π conjugated groups.
reduction potential is ascribed to the dcbH ligand due its
2
TiO2 sensitization process was monitored up to 24 h by lower LUMO levels. 41
recording absorption changes on the semiconductor films, The excited state oxidation potential of this compound,
+
Figure S7. These absorption changes occur due to the increase E(S /S*) = -0.79 V vs. NHE was determined using the equation
+
+
41
+
in dye loading, which is responsible for sensitization. It is E(S /S
important to notice the absence of shifts on the absorption
*
) = E1/2(S/S ) – E
,
where E1/2(S/S ) is the ground
0-0
state oxidation potential determined by cyclic voltammetry.
4
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DOI: 10.1039/C6RA08666G
Figure 4. Cyclic voltammogram of cis-[Ru(cbz
2
-phen)(dcbH
2
)(NCS)
2
] in acetonitrile at
-
1
-1
298 K (v = 100 mV s ; [TBAPF
6
] = 0.1 mol L ).
Figure 6. Photocurrent action spectra of solar cells that were sensitized by cis-[Ru(cbz
phen)(dcbH )(NCS) ] (─●─) or N3 (─●─).
2
-
2
2
The determined emission and electrochemical parameters are
summarized in Table 1 and are compared to those determined
Dye-sensitized solar cells were characterized using the
photocurrent action spectra and current-voltage curves. The
44
for cis-[Ru(phen)(dcbH )(NCS) ].
2
2
+
photocurrent
action
spectrum
of
cis-[Ru(cbz2-
The E(S /S*) of cis-[Ru(cbz -phen)(dcbH )(NCS) ] is more
2
2
2
phen)(dcbH )(NCS) ], Figure 6, resembles the absorption
2
2
negative than the conduction band edge of TiO anatase (E
=
2
CB
63, 64
spectrum of its photoanode and shows that this compound
−0.4 V vs. NHE at pH=4),
and its ground state oxidation
−
•
−
65 has high efficiency for harvesting sunlight and converting it
into electrical energy for the major part of visible spectrum. Its
spectral features are similar to those determined for N3.
However, its IPCE values are slightly higher in all the spectrum.
Over the 370-800 nm spectral range, the integrated IPCE
potential is more positive than the I2 /I (+0.93 V vs. NHE)
redox pair. These energies are adequate for using the dye in
dye-sensitized solar cells because the energies allow electron
injection from the excited state of the complex into the
conduction band of TiO . Thermodynamically, its subsequent
2
response
for
cis-[Ru(cbz -phen)(dcbH )(NCS) ]
was
2
2
2
regeneration by the redox pair is favorable, as seen in the
proposed energy diagram, Figure 5.
approximately 5.2% higher compared with the values for the
sensitized by N3 solar cells under the same experimental
conditions. This suggests that these solar cells have a more
efficient photocurrent generation.
Table 1. Emission maxima wavelength (λem), estimated HOMO-LUMO energy gaps (E0-0),
+
ground state oxidation potentials [E1/2(S /S)], and excited state oxidation potentials
+
44
[
E(S /S*)] for cis-[Ru(cbz
2
-phen)(dcbH
2
)(NCS)
2 2 2
] and cis-[Ru(phen)(dcbH )(NCS) ].
The overall performance of DSSCs under the simulated solar
+
λ
em
/
E
eV
0-0
/
E
1/2 (S /S) /
E(S/S*) / spectrum was evaluated using the current-voltage curves,
V vs. NHE Figure 7, and using the photovoltaic performance data for cis-
[Ru(cbz -phen)(dcbH )(NCS) ] and N3 as sensitizers on
Sensitizer
nm
90
800
V vs. NHE
cis-[Ru(cbz
phen)(dcbH )(NCS)
cis-[Ru(phen)(dcbH )(NCS)
2
-
2
2
2
7
1.86
1.90
+1.07
-0.79
-0.85
2
2
]
nanocrystalline TiO solar cells, Table 2. As expected, based on
2
2
2
]
+1.05
the photocurrent action spectra, a higher JSC was determined
for
the
devices
sensitized
with
cis-[Ru(cbz2-
phen)(dcbH )(NCS) ] in comparison with the N3-sensitized
2
2
devices. In addition, the higher VOC determined also
contributes for enhancing device efficiencies in comparison
with those sensitized by N3. These differences cause cis-
[
Ru(cbz -phen)(dcbH )(NCS) ] to surpass the efficiency values
2 2 2
both of N3 as a dye-sensitizer for nanocrystalline TiO cells and
2
other ruthenium(II) tris-heteroleptic dyes that contain ligands
4
1-45
of phenanthroline derivatives.
Figure 5. Energy diagram and schematic representation of some electron transfer
processes that occur in a DSSC that uses a cis-[Ru(cbz -phen)(dcbH )(NCS) ] dye-
2 2 2
sensitizer.
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CONCLUSIONS
View Article Online
DOI: 10.1039/C6RA08666G
The novel tris-heteroleptic ruthenium(II) dye, cis-[Ru(cbz2-
phen)(dcbH )(NCS) ], was synthesized, characterized and
2
2
applied as a sensitizer in DSSCs. Its absorption spectrum
overlaps with the visible spectrum and also has high molar
absorptivity MLCT bands. The adequate energy levels make cis-
[
Ru(cbz -phen)(dcbH )(NCS) ] suitable for the use as a dye-
2 2 2
sensitizer in photovoltaic devices. The sensitization of TiO2
with cis-[Ru(cbz -phen)(dcbH )(NCS) ] led to solar cells that
2
2
2
surpassed the efficiency of both N3 and other published
ruthenium(II) tris-heteroleptic that contain phenanthroline
derivative ligands, as a dye-sensitizers for nanocrystalline TiO2
solar cells. The improved observed efficiency is attributed to
carbazole groups that are capable of changing energy levels of
the dye, which results in thermodynamically favorable electron
injection and dye regeneration processes. Additionally, these
groups can increase the triplet lifetime due to higher π
electron delocalization on this excited state, which favors a
higher contribution for electron injection via the triplet
pathway. The use of electron delocalized groups at the 4 and 7
positions of phenanthroline is a promising alternative in the
molecular design of new ruthenium(II) polypyridyl dye-
sensitizers.
Figure 7. Current-voltage curves measured for solar cells sensitized using cis-[Ru(cbz
2
-
-
-
2
phen)(dcbH
2
)(NCS)
2
] (─) or N3 (─) under A.M. 1.5G irradiation (Pirr = 100 mW cm ).
Table
2.
Photoelectrochemical parameters
determined for
cis-[Ru(R
2
phen)(dcbH
2
)(NCS)
2
], R = cbz or H and for N3.
4
3
R = cbz
0.76 ± 0.01
15.6 ± 0.7
8.6 ± 0.2
R = H
N3
VOC / Volts
0.733
8.67
3.78
3.88
0.75 ± 0.01
14.4 ± 0.3
7.9 ± 0.2
-
2
JSC / mA cm
η / %
-
2
P
max / mW cm
8.5 ± 0.2
7.8 ± 0.2
Acknowledgements
The authors are grateful to CAPES and to Fundação de Amparo
à Pesquisa do Estado de São Paulo (FAPESP) for the financial
support (2013/25173-5, 2015/00605-5, 2015/13149-8 and
Fill factor (ff)
IPCE (λmax) / %
0.716 ± 0.004
79 ± 3 (500)
0.72 ± 0.01
77 ± 3 (520)
56 (485)
The devices sensitized using cis-[Ru(cbz -phen)(dcbH )(NCS) ]
2
2
2
2
015/21110-4).
exhibited
unsubstituted
Ru(phen)(dcbH )(NCS) ].
a
higher performance compared with the
phenanthroline dye, cis-
This can be explained using the
3
8, 43
[
2
2
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