Thiocyanate-free Ru(II) sensitizers with a 4,4′-dicarboxyvinyl-2, 2′-bipyridine anchor for dye-sensitized solar cells

Article abstract of DOI:10.1002/adfm.201201876

A new class of thiocyanate-free Ru(II) sensitizers with 4,4′-dicarboxyvinyl-2,2′-bipyridine anchor and two trans-oriented pyrid-2-yl pyrazolate (or triazolate) functional chromophores is synthesized, characterized, and evaluated in dye-sensitized solar cells (DSCs). Despite their enhanced red response and absorptivity when compared to the parent sensitizer TFRS-2 that possesses standard 4,4′-dicarboxyl-2,2′-bipyridine anchor and shows the best conversion efficiency of η = 9.82%, the newly synthesized carboxyvinyl-pyrazolate sensitizers, TFRS-11-TFRS-13, exhibit inferior performance characteristics in terms of short-circuit current density (J_SC), open-circuit voltage (V_OC), and power conversion efficiency (η), the latter being recorded to be in the range 5.60-7.62%. The reduction in device efficiencies is attributed to a combination of poor packing of these sensitizers on the TiO₂ surface and less positive ground-state oxidation potentials, which, respectively, increase charge recombination with I₃^- in electrolytes and impede the regeneration of sensitizers by I^- anions. The latter obstacle can be circumvented in part by the replacement of the pyrazolates with triazolates, forming the TFRS-14 sensitizer, which exhibits an improved J_SC, V_OC, and η of 16.4 mAcm^-2, 0.77 V, and 9.02%, respectively. Copyright

Full text of DOI:10.1002/adfm.201201876

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Thiocyanate-Free Ru(II) Sensitizers with a
4,4′-Dicarboxyvinyl-2,2′-bipyridine Anchor for
Dye-Sensitized Solar Cells
Kuan-Lin Wu, Wan-Ping Ku, Sheng-Wei Wang, Aswani Yella, Yun Chi,* Shih-Hung Liu,
Pi-Tai Chou,* Mohammad K. Nazeeruddin,* and Michael Grätzel*
power conversion efficiencies.[1] In recent
years, their overall performance has also
been improved substantially by fine-tuning
A new class of thiocyanate-free Ru(II) sensitizers with 4,4′-dicarboxyvinyl-2,2′-
bipyridine anchor and two trans-oriented pyrid-2-yl pyrazolate (or triazolate)
functional chromophores is synthesized, characterized, and evaluated in dye-
sensitized solar cells (DSCs). Despite their enhanced red response and absorp-
the morphology and hierarchical architec-
ture of TiO2 particles and photoanodes,[2]
blocking charge recombination at the
tivity when compared to the parent sensitizer TFRS-2 that possesses standard
4,4′-dicarboxyl-2,2′-bipyridine anchor and shows the best conversion efficiency
of η = 9.82%, the newly synthesized carboxyvinyl-pyrazolate sensitizers, TFRS-
11–TFRS-13, exhibit inferior performance characteristics in terms of short-circuit
current density (JSC), open-circuit voltage (VOC), and power conversion efficiency
(η), the latter being recorded to be in the range 5.60–7.62%. The reduction in
device efficiencies is attributed to a combination of poor packing of these sen-
sitizers on the TiO2 surface and less positive ground-state oxidation potentials,
which, respectively, increase charge recombination with I3− in electrolytes and
impede the regeneration of sensitizers by I− anions. The latter obstacle can be
circumvented in part by the replacement of the pyrazolates with triazolates,
forming the TFRS-14 sensitizer, which exhibits an improved JSC, VOC, and η of
16.4 mAcm−2 , 0.77 V, and 9.02%, respectively.
interface between TiO2 and electrolytes,
reducing the over-potential and increasing
the rate of dye regeneration[3] and, finally,
designing for more efficient and stable
sensitizers.[4] Among all sensitizers, it is
commonly accepted that Ru(II) complexes
endowed with thiocyanate ligands have
upheld a leading position in performance
(e.g., conversion efficiency) issues among
hundreds of dyes that have been scruti-
nized.[5] The studies on DSCs using func-
tionalized derivatives of N3 and N749 are
paradigms in this field, among which the
champion efficiency using mesoporous
−
TiO2 layer and I−/I3 redox couple under
standard air mass 1.5 G sunlight stands
presently at over 11%.[6]
Nonetheless, because of the possible dissociation of the thio-
cyanate ancillaries during device operation,[7] the use of the
above mentioned N3, N749, and relevant Ru(II)-based sensi-
tizers may only provide limited cell lifespans, and is thus con-
sidered as a major drawback of the current DSC embodiments.
To further improve the stability of DSCs, development of Ru(II)
sensitizers free of thiocyanate ancillaries is urgently demanded.
One successful example should be ascribed to the employment
of cyclometalating 4,6-difluorophenylpyridinato chelate for
substitution of the two thiocyanate ligands from the N3 coor-
dination framework. The resulting sensitizer, bis(4,4′-dicarboxy-
2,2′-bipyridine) 2-(2,4-difluorophenyl)pyridine ruthenium (II),
coded as YE05, exhibits remarkable performance with short-
circuit current density (JSC) = 17 mA cm−2 , open-circuit voltage
(VOC) = 0.80 V, FF (fill factor) = 0.74, and efficiency η = 10.1%.[8]
In a similar fashion, van Koten,[9] Berlinguette,[10] and our
team[11] have independently set out to explore the various viabili-
ties of Ru(II) based polypyridine complexes with either triden-
tate or bidentate cyclometalating chelate as the DSC sensitizers.
In parallel to these literature precedents, we have gained
more understanding from our previous research works that the
pyrid-2-yl azolate (both pyrazolate and triazolate) chelate, due
to their stronger electron deficient character,[12] can effectively
1. Introduction
Dye-sensitized solar cells (DSCs) belong to a class of modern
photovoltaic technologies that have attracted widespread atten-
tion due to their potentially lower production costs and higher
K.-L. Wu, W.-P. Ku, S.-W. Wang, Prof. Y. Chi
Department of Chemistry and Low Carbon
Energy Research Center
National Tsing Hua University
Hsinchu 30013, Taiwan
E-mail: ychi@mx.nthu.edu.tw
K.-L. Wu, Dr. A. Yella, Dr. M. K. Nazeeruddin, Prof. M. Grätzel
Laboratory of Photonics and Interfaces
Institute of Chemical Sciences and Engineering
Ecole Polytechnique Fédérale de Lausanne (EPFL)
CH-1015 Lausanne, Switzerland
E-mail: mdkhaja.nazeeruddin@epfl.ch; michael.graetzel@epfl.ch
S.-H. Liu, Prof. P.-T. Chou
Department of Chemistry and Center for Emerging Material and
Advanced Devices
National Taiwan University
Taipei 10617, Taiwan
E-mail: chop@ntu.edu.tw
DOI: 10.1002/adfm.201201876
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DOI: 10.1002/adfm.201201876
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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to give the carbonylvinyl intermediates;
the former is synthesized from addition
of
4,4′-bis(methoxycarbonylvinyl)-2,2′-
bipyridine to [Ru(p-cymene)Cl2]2 using an
approach described in the literature.[17] After
chromatographic purification on a silica gel
column to remove minor impurities, the
obtained Ru(II) intermediates were hydro-
lyzed with aqueous NaOH in acetone to yield
the Ru(II) sensitizers TFRS-11–TFRS-14.
These Ru(II) complexes also represent a frac-
tion of our current contribution aimed at
designing efficient and robust, thiocyanate-
free metal-based DSC sensitizers.[11,18]
Scheme 1. Synthetic route to the functional pyrazole and triazole chelates; conditions: i) diox-
aborolane, Pd(PPh3)4, K2CO3, reflux; ii) NaOEt, CF3CO2Et, THF; iii) N2H4, reflux; iv) NaOMe,
EtOH, RT; v) NH4Cl, reflux; vi) NaOH, trifluoroacetic acid hydrazide, reflux.
In theory, addition of asymmetric chelate
such as 5-(pyrid-2-yl) azole to the Ru(II)
p-cymene intermediates is expected to afford
substitute the class of pyridinato cyclometalate in the assembly
of potential high-efficiency Ru(II) sensitizers, for which their
oxidation potential should be sufficiently more positive than
up to three geometrical isomers if there involves limited or
even no stereoselectivity. However, in certain cases, upon elon-
gation of the reaction time and addition of alkali metal acetate
as promoter the trans-di-azolate complexes were isolated as the
dominant product. In accordance to this observation, the parent
pyrid-2-yl azole chelate (fppzH), i.e., 2-(3-trifluoromethyl-1H-
pyrazol-5-yl)pyridine, was known to react with Ru3(CO)12,
affording the mononuclear Ru(II) complex [Ru(fppz)2(CO)2]
with trans-di-azolate, cis-di-pyridyl, and cis-di-carbonyl coordina-
tion configuration.[19] Thus, its stereoselectivity is also identical
to that of the TFRS-11–TFRS-14 sensitizers reported herein.
−
that of I−/I3 redox couple and, thus, the oxidized sensitizers
can be easily regenerated back to the original state by electron
donation from the redox couple in the electrolyte.[13] One rep-
resentative sensitizer, coded TFRS-2, showed satisfactory cell
performances with JSC = 17.15 mA cm−2 , VOC = 0.82 V, and
FF = 0.678, corresponding to an overall conversion efficiency
η = 9.54%.[14] In this contribution, we now report an extension
by employing 4,4′-dicarbonylvinyl-2,2′-bipyridine as the TiO2
anchor to demonstrate the ligand π-conjugation trait of these
Ru(II) sensitizers, together with the studies of their intrinsic
properties associated with the fine-tuning of pyrid-2-yl azolate
chelates.
2.2. Optical Properties
The UV/Vis spectral data of these TFRS dyes in DMF are
depicted in Figure 1 and Table 1, together with those of TFRS-2
as references for better understanding the systematic variation
of the spectral pattern. As can be seen, all TFRS-11–TFRS-14
complexes exhibit two lower lying absorption bands covering
the visible region (400−700 nm), both of which are mainly
2. Results and Discussion
2.1. Synthesis and Characterization
The syntheses of 5-(pyrid-2-yl) pyrazole and 1,2,4-triazole
chelates, together with a variety of thiophene appendages, are
depicted in Scheme 1. The first step requires the preparation
of the 4-chloro substituted 2-acetyl and 2-cyano pyridines that
are not commercially available (see the Experimental Sec-
tion for details). These pyridine compounds were treated with
various dioxaborolane reagents under Suzuki cross-coupling
conditions, allowing the addition of thiophene pendent at the
4-position of 2-acetyl and 2-cyano pyridine in moderate yields.
The 2-acetyl or 2-cyano functional groups are then converted
to pyrazole or triazole moieties using established cyclization
protocols, among which the pyrazole is synthesized by initial
Claisen condensation with ethyl trifluoroacetate, followed by
treatment with hydrazine hydrate in refluxing ethanol,[15] while
formation of triazole is best achieved by a prior ammonolysis
of cyano group, followed by coupling with trifluoroacetic acid
hydrazide at higher temperature.[16]
70000
1.2
TFRS-2
TFRS-11
TFRS-12
TFRS-13
TFRS-14
1.0
60000
50000
40000
30000
20000
10000
0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
700
750
Wavelength / nm
300
400
500
600
700
Wavelength / nm
The 5-(pyrid-2-yl) azole ligands (i.e., either pyrazole or
triazole) were reacted with the starting material [Ru{4,4′-
bis(methoxycarbonylvinyl)-2,2′-bipyridine}(p-cymene)Cl]Cl
Figure 1. Absorption spectra of TFRS-2 and TFRS-11–TFRS-14 sensi-
tizers in DMF. Inset: absorption spectra for all corresponding samples
adsorbed on 6 μm of 20 nm TiO2 thin film.
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Scheme 2), the MLCT/ππ∗ signals became even more broad-
ened and stronger in absorptivity, the result of which may be
attributed to the sulfur atom having higher polarizability. This
viewpoint is also supported by the computational approach, in
which the maximum oscillator strength (greater than 450 nm,
f = 0.47) for TFRS-13 is greater than that (0.41) of TFRS-12 (see
Tables S2 and S3 of the Supporting Information). Alternatively,
replacing pyrazolates of TFRS-12 with the more electron-with-
drawing triazolates, the sensitizer TFRS-14 is expected to have
stronger stabilization of the metal dπ orbitals, resulting in the
lowering of the HOMO (highest occupied molecular orbital)
energy level. However, such a lowering of energy is then coun-
terbalanced with the decrease of the LUMO (lowest unoccupied
molecular orbital) energy caused by the 4,4′-dicarboxyvinyl-
2,2′-bipyridine anchor (c.f. 4,4′-dicarboxyl-2,2′-bipyridine in
TFRS-2), such that TFRS-14 showed comparable visible spec-
tral coverage versus that of the TFRS-2 sensitizer, except for
the enhanced absorption cross section, i.e., the extinction coef-
ficient. Finally, the vinyl linkage of the carboxyvinyl fragment is
less efficient in terms of inducing both the bathochromic shift
and hyperchromic effect versus that of the thiophen-2-yl group
of the azolate ancillary. This delineation is confirmed by the less
intense MLCT absorptions of TFRS-11 versus those of TFRS-2.
In fact, the computational results show smallest f strength of
0.28 (492 nm) among all titled complexes, consistent with the
experimental UV/Vis spectra showed in Figure 1.
T a b le 1 . Photophysical and electrochemical data of the studied TFRS
sensitizers in DMF.
b)
c)
Dye
λabs
E°′ox
E0−0
E°′∗d)
−0.91
−0.93
−0.98
−0.85
−1.01
[nm] (ε [L mol−1 cm−1 ])a)
TFRS-11
TFRS-12
TFRS-13
TFRS-14
TFRS-2
321 (18 300), 419
0.83
0.81
0.81
1.03
0.90
1.74
1.74
1.79
1.88
1.91
(15 500), 545 (8 900)
324 (63 600), 456
(29 100), 551 (17 800)
324 (50 700), 467
(29 100), 543 (20 800)
323 (57 600), 448
(31 600), 521 (21 700)
310 (63 800), 426 (22 200),
477 (25 500), 516 (20 500)
a)Absorption spectra were measured in DMF solution; b)Oxidation potential of dyes
was measured in DMF with 0.1 M [TBA][PF6] and with a scan rate of 50 mV s−1 ; cali-
bration was performed using Fc/Fc+ as internal reference and converted to NHE by
c)
addition of 0.63 V;
E
was determined from the intersection of the absorption
0-0
and tangent of the emission peak in DMF; d)E°′∗ was calculated as E°′ox − E0−0
.
attributed to the metal-to-ligand charge transfer (MLCT) tran-
sitions, mixed, to a certain extent, with the ligand-to-ligand
π–π∗ charge transfer transitions (LLCT). This assignment is
firmly supported by computational approach (see the Sup-
porting Information for details). As shown in Figures S1−S4
and Tables S1−S4 of the Supporting Information, the frontier
analyses based on time-dependent DFT (TD-DFT) results sug-
2.3. Electrochemical Studies
gest that the TFRS dyes share almost identical character for The electrochemical properties of these TFRS sensitizers were
the lower electronic transitions, of which the optical excitation evaluated by cyclic voltammetry in DMF solution to give a pre-
is mainly from the Ru(II) dπ orbital and occupied orbitals of liminary assessment about the energetics of electron injection
pyrid-2-yl azolate chelates to the antibonding orbitals of the from the photoexcited sensitizers to the conduction band (CB)
4,4′-dicarboxyvinyl-2,2′-bipyridine chelate, denoted as MLCT of TiO2 and the subsequent regeneration of the oxidized sen-
and LLCT, respectively. Therefore, mixing of MLCT and ππ∗ sitizers by the I−/I3 redox couple. All potentials were meas-
−
(LLCT) characters in the lower-lying transitions (greater than ured in DMF solution with addition of 0.1 M [TBA][PF6] and
420 nm) is unambiguous. Note that for all titled complexes with a scan rate of 50 mV s−1 . The data was calibrated with
the localized intra-ligand ππ∗ transition (ILCT) appears with Fc/Fc+ internal reference and converted to value versus the
high oscillator strength in the region of 300−350 nm, corre- normal hydrogen electrode (NHE) by addition of 0.63 V. As
sponding to the S9 and/or higher excited states, consistent with showed in Table 1, the Ru+2 /Ru+3 oxidation potentials (E°′ox) of
the experimental observation. For clarity, Figure 2a,b illustrate TFRS-11 to TFRS-13 are within the narrow range 0.81–0.83 V
−
the calculated electronic transition and density plots for certain (vs. NHE), which are more positive than that of I−/I3 redox
transitions of representative TFRS-12 and TFRS-14 sensitizers, potential (approximately 0.35 V vs. NHE), but are slightly less
respectively, in which the vertical bars are pertinent optical positive than that of the TFRS-2 reference sample (ca. 0.90 V
transitions with the oscillator strength of larger than 0.01. The vs. NHE) under identical condition.[20] The cathodic shift of
suitability of this computational approach can be seen from the oxidation potentials versus TFRS-2 is apparently caused by
well fit of the major transition peaks to the absorption spectral the carboxyvinyl substituted bipyridine anchor, for which the
profile in terms of position and intensity.
greater π-conjugation is known to further destabilize the occu-
For a fair comparison, TFRS-12, a close analogue of TFRS-2, pied frontier orbitals associated with this chelate. Among the
exhibits two lower-lying absorption bands at 456 (ε ≈ 29 100) and titled Ru(II) complexes TFRS-14 showed the greatest oxidation
551 nm (ε ≈ 17 800). Noticeably, these bands are more red-shifted potential of 1.03 V, which is apparently caused by the influence
and intense than those observed in TFRS-2, for which the rele- of more electron-withdrawing triazolate groups. As these TFRS
vant absorptions appeared at 426, 477, and 516 nm with slightly sensitizer exhibit more positive oxidation potential versus
lower extinction coefficient of 22 200, 25 500 and 20 500 M−1 cm−1 , the I−/I3 couple, they are subject to efficient regeneration by
−
a result that is attributed to the reduced π-conjugation of accepting electrons from surrounding iodide ions of electrolytic
4,4′-dicarboxy-2,2′-bipyridine chelate in TFRS-2. Upon solution.
replacement of 5-hexylthiophen-2-yl in TFRS-12 by 5-(hexyl-
In a qualitative manner, the excited state oxidation potential
thio)thiophen-2-yl fragments, forming TFRS-13 sensitizer (see (E°′∗) of sensitizers can be extracted from the aforementioned
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which are all more negative than the CB edge
of the TiO2 anode (ca. −0.5 V vs. NHE) and
hence guarantee the efficient injection of
photoelectrons.
2.4. Photovoltaic Performance
DSCs were assessed by employing TiO2 pho-
toanodes with size 4 × 4 mm2 and of two
distinct thicknesses, namely 8 + 6 and 12 +
6 μm, consisting of mesoporous (20 nm) plus
light-scattering (400 nm) layers. The double-
layered architecture was employed to increase
the light absorbing capability at the longer
wavelength region.[22] The photoanodes were
then stained in the respective dye solutions
(0.3 mM) in ethanol, together with DMSO
(20%, v/v) and CDCA (1 mM) for 18 h. An
electrolyte solution consists of 0.6 M 1-methyl-
3-propylimidazolium iodide (PMII), 0.03
M
of iodine, 0.5 M 4-tert-butylpyridine (TBP),
0.1 M guanidinium thiocyanate, and 0.05 M
LiI in a mixture of acetonitrile and valeroni-
trile (85:15, v/v). The cell performances were
measured using a 6 × 6 mm2 shadow mask
to prevent excessive exposure to the simu-
lated solar radiation.[23] As shown in Table 2,
all of the carboxyvinyl-pyrazolate sensitizers,
namely: TFRS-11 to TFRS-13, showed higher
JSC and slightly lower VOC upon using thicker
TiO2 photoanodes, yielding a better power
conversion efficiency (η) compared to that of
those fabricated using a thinner TiO2 layer.
The higher JSC is attributed to the build-up
of sufficient amount of dye molecules that
adsorbed on the thicker TiO2 layer, while the
slightly lower VOC is due to the increased
surface area of the TiO2 layer that provides
additional charge recombination sites. The
additional charge recombination sites are
caused at the interface between TiO2 and
electrolyte, enhancing the dark current and
Figure 2. UV/Vis spectra and spin density plots for the specified transition of: a) TFRS-12,
and, b) TFRS-14. The vertical bars are pertinent optical transitions with the oscillator strength
of more than 0.01, for which occupied and unoccupied orbitals are represented in pink and
yellow, respectively.
ground state oxidation potential (E°′ox) and
the zero-zero-excited energy (E0-0) according
to the equation:
ꢀ
ꢀ
E◦ ∗ = Eo◦x − E0−0
where E0-0 was determined from the intersec-
tion of the absorption onset and tangent of
the emission profile. It is believed that the
E°′∗ value should be at least 0.2–0.3 V greater
than the CB energy of TiO2 photoanode to
warrant favorable electron injection.[21] As
can be seen in Table 1, the excited state oxi-
dation potential (E°′∗) of these studied TFRS
sensitizers fell in the range −0.85 to −0.98 V,
Scheme 2. Structural drawings of the Ru(II) sensitizers TFRS-2 and TFRS-11–TFRS-14.
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T a b le 2 . The performances for DSCs measured under AM 1.5G one sun simulated irradiation.a)
Dye
TiO2
[μm]
JSC
[mA cm−2
VOC
[V]
FF
Dye loadingb)
[mol cm−2
]
η
[%]
]
2.01 × 10−7
1.72 × 10−7
1.63 × 10−7
1.67 × 10−7
1.62 × 10−7
TFRS-11
12.0
13.7
15.9
14.7
14.1
14.7
15.2
16.4
14.9
19.4
0.67
0.65
0.70
0.69
0.70
0.67
0.78
0.77
0.78
0.75
0.696
0.695
0.683
0.690
0.679
0.664
0.713
0.698
0.690
0.661
5.60
6.30
7.62
7.28
6.71
6.68
8.44
9.02
8.29
9.82
8 + 6
12 + 6
8 + 6
TFRS-12
TFRS-13
TFRS-14
TFRS-2
12 + 6
8 + 6
12 + 6
8 + 6
12 + 6
8 + 6
12 + 6
a)All devices were fabricated using double-layered TiO2 anode and with 4 × 4 mm2 working area, an electrolyte that consists of 0.6 M PMII, 0.03 M I2, 0.5 M TBP, 0.1 M GNCS,
and 0.05 M LiI in the mixed valeronitrile/acetonitrile solution (15:85, v/v); also, all as - prepared devices were covered by a 6 × 6 mm2 shadow mask for the performance
measurement; b)The dye loading on 12 + 6 μm TiO2 films was conducted by desorbing the dye into 0.1 M tetrabutylammonium hydroxide solution in MeOH and H2O
(volume ratio: 1:1) and then measured using the UV/Vis spectral analysis.
lowering the photovoltage.[24] The only exception is TFRS-12, for
which the efficiency (η) of 8 + 6 μm cell is recorded to be 7.62%,
which is slightly higher than that of η = 7.28% obtained for the
12 + 6 μm cell, a result of the higher JSC of 15.9 mA cm−2 . This
observation implies that the TFRS-12 sensitizer, with 5-hexylthio-
phen-2-yl pendants, has the best light harvesting ability among
the carboxyvinyl-pyrazolate sensitizers. Furthermore, the small
decrease in the cell performances for TFRS-13 versus TFRS-12
may be attributed to the slightly stronger binding of its 5-hexy-
analogues, coded K8 and K9, all showed relatively more positive
ground state oxidation potential at the vicinity of 0.9 V versus
NHE.[28] Even the recently reported amphiphilic Ru(II) com-
plex BTC-2, in which the bipyridine anchor is incorporated with
the unique 5-carboxythiophen-2-yl units, has shown a slightly
more positive oxidation potential of 0.84 V versus NHE.[29] As
a result, the driving force for dye regeneration is expected to
be more efficient. Thus, as the VOC turned smaller for DSCs
with K8, K9, and BTC-2, their increased absorption extinction
coefficient of the carboxyl anchor enables notable enhancement
in JSC versus the DSCs fabricated with N3 and Z907. The net
result gives a better performance, particularly for those fabri-
cated with thinner TiO2 layer. In good agreement with this
finding, the carboxyvinyl-triazolate sensitizer TFRS-14 exhibits
the most positive oxidation potential E°′ox of 1.03 V (see Table 2)
due to the reduced electron density at the Ru(II) metal center
that exerted by the better electron accepting triazolate moieties.
The increase of dye regeneration efficiency (from electrolyte)
then makes this triazolate sensitizer TFRS-14 superior in DSCs
performance versus all carboxyvinyl-pyrazolate sensitizers
TFRS-11 to TFRS-13 elaborated in this study.
The J–V characteristics and incident photon-to-current
conversion efficiency (IPCE) action diagrams are depicted in
Figure 3. It is notable that TFRS-2, which possesses the parent
4,4′-dicarboxy-2,2′-bipyridine anchor, exhibits the best J–V char-
acteristics and IPCE at all wavelengths among these series of
Ru(II) sensitizers, c.f. TFRS-11 to TFRS-14. On the other hand,
the IPCE of the second-best sensitizer TFRS-14 only maintains
a plateau of over 70% from 420 to 570 nm, with a maximum
of ca. 77% at ca. 510 nm, which is about 7% lower than that of
TFRS-2 at the same position, showing inferior light-harvesting
efficiency. Moreover, the J–V characteristics and the IPCE data
of TFRS-11 to TFRS-13 are all inferior than TFRS-14; the origin
of this can be traced back to the previous discussion.
−
thiothiophen-2-yl group with I3 ions in electrolyte, which then
increased its local concentration near the TiO2 surface and, con-
currently, the rate of electron recombination.[25]
Under identical conditions, the TFRS-2 reference sensitizer
displays JSC of 14.9 and 19.4 mA cm−2 , VOC of 0.78 and 0.75 V,
and overall power conversion efficiency of 8.29% and 9.82% for
DSCs that employed 8 + 6 μm and 12 + 6 μm of TiO2 pho-
toanode, respectively. These performance characteristics rep-
resent the best data ever recorded for this class of sensitizers
and are superior to those of TFRS-12, which is a close analogue
to TFRS-2 that differs only by the carboxyvinyl anchor. Thus,
despite the increase of dye loading (see Table 2) and more
intense absorptivity, TFRS-12 is still not able to boost the DSCs’
efficiencies (cf. TFRS-2). On the one hand, this inferior per-
formance could be a result of less efficient blocking of the TiO2
surface due to the increased spatial separation from the Ru(II)
core framework,[26] which allows the I3− anion to penetrate into
the TiO2/sensitizer interface, resulting in a faster charge recom-
bination. On the other hand, the less positive ground-state oxi-
dation potential of TFRS-12 (E°′ox = 0.81 V vs. NHE) would also
reduce the thermodynamic driving force for the dye regenera-
tion by I− ions, giving relatively poor device performances. In
fact, all of the carboxyvinyl-pyrazolate sensitizers TFRS-11 to
•−
TFRS-13 showed potentials that are smaller than that of I−/I2
couple (ca. 0.79
0.1 V vs. NHE)),[27] implying the occur-
rence of inefficient dye regeneration. In contrast, the pristine
Ru(II)-based sensitizers N3 and Z907 and their carboxyvinyl
To gain more insight into the intrinsic DSCs property, elec-
trochemical impedance spectroscopy was performed to analyze
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a)
Transient photocurrent and photovoltage decay measure-
ments were carried out in order to investigate the rates of
interfacial recombination of electrons from the TiO2 conduc-
tion band to electrolyte. The density of state (DOS) of the films
loaded with sensitizer was determined from transient photo-
current decay measurements. The chemical capacitance Cμ for
the TFRS sensitizer films as a function of VOC are compiled in
Figure 5a. Cμ is directly proportional to the DOS: Cμ = q(e)DOS,
where q(e) is the charge of one electron. The Cμ values are very
similar for the TFRS sensitizers, with small differences being
20
15
10
TFRS-2
TFRS-11
TFRS-12
TFRS-13
TFRS-14
noted only at VOC > 0.6 V This observation rules out any sub-
.
5
0
stantial contribution of a conduction band shift to the observed
decrease in the back-electron transfer rate. Figure 5b illustrates
electron lifetime as a function of VOC for the different sensi-
tizers. The VOC was adjusted by varying the intensity of the bias
light impinging on the cell. The trend correlates rather well
with the VOC values obtained for these devices, i.e., as electron
lifetime gets shorter, VOC decreases. Longer electron lifetimes
were observed for TFRS-12, compared to the TFRS-13. This
difference in charge separation lifetime can be explained by
various degrees of electrolyte/dye interactions. Several authors
have reported previously that atoms such as sulfur and oxygen
can bind I3− and I2 species, forming charge-transfer complexes.
This can lead to a higher concentration of these species at TiO2
surface, catalyzing e−TiO2/electrolyte recombination.[30] There-
fore, TFRS-13, which contains one additional sulfur atom com-
0.0
0.2
0.4
0.6
0.8
Voltage / V
b)
100
80
60
40
20
0
TFRS-2
TFRS-11
TFRS-12
TFRS-13
TFRS-14
−
pared to TFRS-12, would be expected to bind I3 and I2 species
leading to faster recombination.
In view of the long-term stability, a high-performance elec-
trolyte based on butyronitrile (BN) was selected.[31] Initially,
the fabricated devices showed maximum performance charac-
teristics of JSC = 18.3 mA cm−2 , VOC = 0.70 V, FF = 0.637 and
η = 8.16% for TFRS-2 and of 15.8 mA cm−2 , 0.70 V, 0.686 and
7.59% for TFRS-14 at 200 h (Figure 6). After 1000 h of light
soaking at 60 °C, the device parameters of TFRS-2 vary only
slightly from the initial values, affording JSC = 17.3 mA cm−2 ,
VOC = 0.70 mV, FF = 0.621 and η = 7.52% at the end of the
400
500
600
700
800
Wavelength / nm
Figure 3. a ) J–V characteristics and b) IPCE spectra for DSCs with
12 + 6 μm TiO2 layer and measured under AM 1.5G simulated solar
irradiation.
180
160
140
120
100
80
the effects on charge generation, transport,
and collection. Figure 4 depicts the imped-
ance spectra of DSCs fabricated with TFRS-
11 to TFRS-14 sensitizers and TFRS-2 ref-
erence, in the dark under a forward bias of
0.65 V. As a result, the radius of these semi-
circles revealed a descending order of TFRS-
14 (342.7 Ω) > TFRS-2 (225.7 Ω) > TFRS-13
(109.9 Ω) > TFRS-11 (78.6 Ω) ≈ TFRS-12
(77.2 Ω), indicating that the recombination
process from the as prepared TFRS-14 cell is
the slowest of all, including that of TFRS-2,
leading to the highest photovoltage. On the
other hand, all other sensitizers TFRS-11 to
TFRS-13 showed much inferior performance
compared to that of TFRS-2 standard that
without the carboxyvinyl groups. The results
are also consistent with the measured VOC
values listed in Table 2.
TFRS-2
TFRS-11
TFRS-12
TFRS-13
TFRS-14
60
40
20
0
0
50
100
150
200
250
300
350
400
450
Z' / ohm
Figure 4. Electrochemical impedance spectra measured under dark at a forward bias of 0.65 V
for the cells employing different TFRS dyes.
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a)
20
18
16
14
12
1
0.8
0.6
0.8
0.7
0.6
0.5
10
TFRS-2
TFRS-11
TFRS-12
TFRS-13
TFRS-14
0.1
8
6
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
Voltage / V
0
200
400
600
800
1000
b)
Time / h
TFRS-2
TFRS-11
TFRS-12
TFRS-13
TFRS-14
Figure 6. Evolution of solar cell parameters of TFRS-2 (open squares)
and TFRS-14 (closed squares) measured under the irradiance of AM
1.5 G sunlight-soaking at 60 °C. A 370 nm cut-off long pass filter was
put on the cell surface during illumination. Electrolyte: 1.0 M DMII,
0.15 M iodine, 0.1 M GNCS, 0.1 M LiI, and 0.5 M NBB (N-butyl-1H-benz-
imidazole) in butyronitrile.
0.1
NHE, caused by increasing π-conjugation, the best DSC devices
among the pyrazolate sensitizers TFRS-11 to TFRS-13 afforded
an overall conversion efficiency of 7.62%, which is obtained by
TFRS-12 and is still lower than that of the reference TFRS-2
that possesses only typical carboxyl anchors. The poor match in
oxidation potential versus that of I−/I3− couple can be corrected
using the carboxyvinyl-triazolate TFRS-14, for which the more
positive E°′ox of 1.03 V improves the dye regeneration efficiency
and showing performances superior to all other carboxyvinyl
sensitizers elaborated in this study.
0.01
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
Voltage / V
Figure 5. Electron capacitance (a) and lifetime (b) determined with
photocurrent and photovoltage decay measurements of devices with
TFRS dyes.
Finally, as showed by the light soaking experiments con-
ducted at 60 °C for 1000 hours, the TFRS-14 device showed
higher degradation versus those of the TFRS-2 reference cell.
Such an observation may reflect the higher reactivity between
C=C double bond (vinyl group) and halogen element (both I−
experiment. As the result, the overall efficiency retained 92% of
its initial value, confirming its excellent stability. On the other
hand, the photovoltaic performance of the carboxyvinyl sensi-
tizer TFRS-14 suffers a much severe decline from the initial
values, giving JSC = 14.4 mA cm−2 , VOC = 0.64 mV, FF = 0.668
and η = 6.16%, and afforded total reduction of overall efficiency
by 18% after 1000 hours of light soaking. Since none of these
sensitizers possess the thiocyanate ancillary, the instability is
most probably caused by the carboxyvinyl anchors that existed
in the TFRS-14 sensitizer.
−
and I3 ) present in the electrolytes. Clearly, such a structural
modification should be avoided in future designs.
4. Experimental Section
General Procedures: All reactions were performed under argon
atmosphere and solvents were distilled from appropriate drying agents
prior to use. Commercially available reagents were used without further
purification unless otherwise stated. 4,4′-Bis(methoxycarbonylvinyl)-
2,2′-bipyridine and the corresponding Ru(II) complex [Ru{4,4′-
3. Conclusions
In summary, we have reported a new series of tris-bidentate
Ru(II) sensitizers (TFRS-11 to TFRS-14), in which a pair of
carboxyvinyl fragments are strategically added to replace the
carboxyl group of typical 4,4′-dicarboxy-2,2′-bipyridine anchor.
These new carboxyvinyl anchors would increase the light-har-
vesting as confirmed by their absorption spectra and verified
using DFT calculations. However, due to the concomitant reduc-
tion of their ground state oxidation potentials, of 0.81–0.83 V vs.
bis(methoxycarbonylvinyl)-2,2′-bipyridine}(p-cymene)Cl]Cl
were
synthesized according to the literature procedures.[28a,32] 2-Acetyl-
4-chloropyridine was prepared from a two-step process involving
combination of picolinic acid, NaBr, and thionyl chloride to afford
first 4-chloro-2-methoxycarbonylpyridine;[33] next, it was converted
to the desired acetyl compound by sequential Claisen condensation
and decarboxylation,[34] while the respective 2-cyano-4-chloro-pyridine
was obtained in high yield from treatment of 4-cholropyridine N-oxide
with dimethylcarbamyl chloride and trimethylsilanecarbonitrile.[35] All
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reactions were monitored using pre-coated TLC plates (0.20 mm with
fluorescent indicator UV254). Mass spectra were obtained on a JEOL
SX-102A instrument operating in electron impact (EI) or fast atom
bombardment (FAB) mode. 1H spectra were recorded on a Varian
Mercury-400 instrument. Elemental analysis was carried out with a
Heraeus CHN-O Rapid Elementary Analyzer.
the realistic environmental perturbation, a polarizable continuum model
(PCM) was applied using dimethylformamide (DMF) as solvent.
Device Fabrication: The fluorine-doped tin oxide (FTO) glass (3.2 mm
thickness, sheet resistance of 9 Ω cm−2 , Pilkington) was first cleaned in a
detergent solution using an ultrasonic bath for 30 min, and then rinsed
with water and ethanol in sequence. After treated with ozone generated
from PSD series UV-ozone cleaning system for 15 min (Novascan
Technologies, Inc.), the FTO glasses were immersed into a 40 mM
aqueous TiCl4 solution at 70 °C for 30 min and washed with water
and ethanol. The photoanodes composed of nanocrystalline TiO2 were
prepared using a previously reported procedure. The TiO2 electrodes
of 12 μm thickness were deposited on transparent conducting glass,
over which a scattering layer of 6 μm thickness containing 400 nm TiO2
particles (PST-400, JGC Catalysts and Chemicals, Japan) was screen-
printed (active area, 0.16 cm2). TiO2 film thickness was measured by
alpha-step IQ surface profile (KLA Tencor). The TiO2 electrodes were
heated under an air flow at 325 °C for 30 min, followed by heating
at 375 °C for 5 min, 450 °C for 15 min, and 500 °C for 15 min. The
TiO2 electrodes were treated with a 40 mM aqueous solution of TiCl4,
sintered at 70 °C for 30 min and then washed with water and ethanol.
The electrodes were heated again at 500 °C for 30 min and left to cool
to 80 °C before dipping into the dye solution (0.3 mM) containing 20%
DMSO in ethanol for 18 h at 25 °C. The Pt counter electrodes were next
prepared by spin-coating employing an H2PtCl6 solution (2 mg of Pt in
1 mL isopropyl alcohol) on FTO plates, followed by sintering at 400 °C
for 15 min. The dye-sensitized TiO2 electrodes were assembled with Pt
counter electrodes by heating with a hot-melt Surlyn film (Meltonix 1170-
25, 25 μm, Solaronix) as spacer between the electrodes, and then heated
at 130 °C. The electrolyte containing 0.6 M 1-methyl-3-propylimidazolium
Synthesis
of
TFRS-11:
2-(3-(Trifluoromethyl)-1H-pyrazol-5-yl)
pyridine (90.0 mg, 0.422 mmol), [Ru{4,4′-bis(methoxycarbonylvinyl)-
2,2′-bipyridine}(p-cymene)Cl]Cl (125 mg, 0.211 mmol) and potassium
acetate (206 mg, 2.11 mmol) were dissolved in xylenes (20 mL). The
mixture was refluxed for 5 h. After removing the solvent, the residue was
extracted with CH2Cl2 (3 × 25 mL), washed with water and concentrated
to dryness. The mixture of products was purified by silica gel column
chromatography (CH3CN/CH2Cl2 = 1:5). Next, the solid product was
dissolved in a mixture of acetone (20 mL) and 1 M NaOH solution
(1.2 mL). The solution was heated to 60 °C under nitrogen for 2 h.
After then, the solvent was removed under vacuum and the residue was
dissolved in 10 mL of H2O. The solution was titrated with 2 M HCl to
pH 3 to afford a dark brown precipitate. The dark brown precipitate was
washed with deionized water, acetone and diethyl ether in sequence,
giving dark brown solid (47.0 mg, 28%).
1
Spectral Data of TFRS-11: MS (FAB, 102Ru): m/z 822 (M)+. H NMR
(400 MHz, d6-DMSO, 298 K): δ 9.05 (s, 2H), 8.19 (d, J = 7.6 Hz, 2H),
7.94 (t, J = 8.2 Hz, 2H), 7.81 (d, J = 4.8 Hz, 2H), 7.71–7.47 (m, 6H),
7.38–7.25 (m, 4H), 7.05 (d, J = 16 Hz, 2H). 19F NMR (376 MHz, d6-
DMSO, 298K): δ −58.00 (s, 6F). Anal. Calcd. for C34H22F6N8O4Ru·H2O:
C, 48.63; N, 13.35; H, 2.88; found: C, 48.70; N, 13.06; H, 3.33.
Synthesis of TFRS-12, TFRS-13, and TFRS-14: A similar procedure was
used as described for TFRS-11, employing 4-(5-hexylthiophen-2-yl)-2-
(3-trifluoromethyl-1H-pyrazol-5-yl)pyridine (144 mg, 0.380 mmol, and
respective Ru(II) source reagent. The residue was purified by silica gel
column chromatography (ethyl acetate/CH2Cl2 = 1:10). After hydrolysis,
the sensitizer TFRS-12 was obtained as a greenish-brown solid (69.6 mg,
31%). Other derivatives, namely: TFRS-13 and TFRS-14, were
synthesized in approx. 25% yield using 4-(5-(hexylthio)thiophen-2-yl)-
2-(3-trifluoromethyl-1H-pyrazol-5-yl)pyridine and 4-(5-hexylthiophen-
2-yl)-2-(3-trifluoromethyl-1H-1,2,4-triazol-5-yl)pyridine as the starting
material.
iodide (PMII), 0.03 M of iodine, 0.5 M tert-butylpyridine (TBP), 0.1
M
guanidinium thiocyanate, and 0.05 M of LiI in the mixed valeronitrile/
acetonitrile solution with volume ratio of 15:85, which was then injected
into the cell through a drilled hole at the counter electrode. Finally, the
hole was sealed using a hot-melt Surlyn film and a cover glass. In order
to reduce scattered light from the edge of glass electrodes, a light-
shading mask (0.36 cm2) was used to cover the device assembly. The
dye loading on 12 + 6 μm TiO2 films was measured by desorbing the dye
into a 0.5 M tetrabutylammonium hydroxide solution in MeOH and H2O
mixture (volume ratio: 1:1), followed by detecting their absorbance using
the UV/Vis spectrometer.
Photovoltaic Characterization: Photovoltaic measurements were
recorded with a Newport Oriel Class A Solar Simulator (Model 91159)
equipped with a class A 150 W xenon light source powered by a Newport
power supply (Model 69907). The light output (area: 2 × 2 in.) was
calibrated to AM 1.5 using a Newport Oriel correction filter to reduce
the spectra mismatch in the region of 350–750 nm to less than 4%.
The power output of the lamp was calibrated to 1 Sun (100 mWcm−2 )
using a certified Si reference cell (SRC-1000-TC-QZ, VLSI standard S/N:
10510-0031). The current voltage characteristic of each cell was obtained
by applying an external potential bias to the cell and measuring the
generated photocurrent with a Keithley digital source meter (Model 2400).
The spectra of incident photon-to-current conversion efficiency (IPCE)
were calculated with the equation 1240JSC(λ)/(λPin(λ)), where JSC is the
short-circuit current density under each monochromatic illumination in A
cm−2 , λ is the wavelength of incident monochromatic light in nm, and Pin
is the monochromatic light intensity in W cm−2 , and plotted as a function
of incident wavelength with increments of 10 nm. It should be noted
that 20 values of JSC (interval 50 ms) were collected after a device was
illuminated monochromatically 3 s later and were averaged for calculation
of IPCE.[37] A 300 W Xe lamp (Model 6258, Newport Oriel) combined with
an Oriel cornerstone 260 1/4 m monochromator (Model 74100) provided
an unchopped monochromatic beam onto a photovoltaic cell. The beam
intensity was calibrated with a power meter (Model 1936-C, Newport)
equipped with a Newport 818-UV photodetector.
Spectral Data of TFRS-12: MS (FAB, 102Ru): m/z 1155 (M+1)+. 1H
NMR (400 MHz, d6-DMSO, 298 K): δ 9.08 (s, 2H), 8.53 (s, 2H), 8.02–
7.88 (m, 4H), 7.82 (d, J = 3.4 Hz, 2H), 7.71–7.56 (m, 4H), 7.50 (d, J =
5.6 Hz, 2H), 7.24 (s, 2H), 7.06 (d, J = 16 Hz, 2H), 6.96 (d, J = 3.4 Hz,
2H), 2.81 (t, J = 7.4 Hz, 4H), 1.66–1.54 (m, 4H), 1.35–1.16 (m, 12H),
0.81 (t, J = 6.4 Hz, 6H). 19F NMR (376 MHz, d6-DMSO, 298K): δ −58.00
(s, 6F). Anal. Calcd. for C54H50F6N8O4RuS2·5H2O: C, 52.12; N, 9.01; H,
4.80; found: C, 51.82; N, 8.93; H, 4.51.
Spectral Data of TFRS-13: MS (FAB, 102Ru): m/z 1219 (M+1)+. 1H
NMR (400 MHz, d6-DMSO, 298K): δ 9.01 (s, 2H), 8.24 (s, 2H), 8.02 (d,
J = 6.0 Hz, 2H), 7.81 (d, J = 3.6 Hz, 2H), 7.69–7.53 (m, 4H), 7.50–7.39
(m, 4H), 7.24 (d, J = 3.6 Hz, 2H), 7.09 (d, J = 4.8 Hz, 2H), 7.01 (d, J =
16 Hz, 2H), 2.91 (t, J = 7.0 Hz, 4H), 1.63–1.50 (m, 4H), 1.42–1.20 (m,
12H), 0.82 (t, J = 6.6 Hz, 6H). 19F NMR (376 MHz, d6-DMSO, 298K):
δ −58.21 (s, 6F). Anal. Calcd. for C54H50F6N8O4RuS4·2H2O: C, 51.74; N,
8.93; H, 4.34; found: C, 51.65; N, 8.82; H, 4.27.
Spectral Data of TFRS-14: MS (FAB, 102Ru): m/z 1157 (M+1)+. 1H
NMR (400 MHz, d6-DMSO, 298 K): δ 9.03 (s, 2H), 8.17 (s, 2H), 8.01
(d, J = 5.6 Hz, 2H), 7.81 (d, J = 3.6 Hz, 2H), 7.70–7.55 (m, 6H), 7.27 (d,
J = 6.0 Hz, 2H), 7.02 (d, J = 16 Hz, 2H), 6.96 (d, J = 3.2 Hz, 2H), 2.82
(t, J = 7.2 Hz, 4H), 1.68–1.55 (m, 4H), 1.36–1.18 (m, 12H), 0.83 (t, J =
6.4 Hz, 6H). 19F NMR (376 MHz, d6-DMSO, 298K): δ −61.77 (s, 6F).
Anal. Calcd. for C52H48F6N10O4RuS2·H2O: C, 53.19; N, 11.93; H, 4.29;
found: C, 52.87; N, 12.10; H, 4.75.
DFT Calculations: All calculations on were performed with the
Gaussian 09 program package.[36] Their ground state structures were first
optimized with density functional theory (DFT) at B3LYP/LANL2DZ (Ru)
and 6-31G∗ (H, C, N, O, F, S) level. The optimized structures were then
used to calculate 60 lowest singlet energy optical excitations using the
time-dependent density functional theory (TD-DFT) method. To mimic
Electrical Impedance Measurements: Electrical impedance experiments
were carried out with a PARSTAT 2273 (AMETEK Princeton Applied
Research, U.S.A.) electrochemical workstation, with a frequency range of
0.05−106 Hz and a potential modulation of 5 mV.
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Transient Photocurrent/ Photovoltage Measurements: Transient decays
were measured under a varying white-light bias with superimposed red-
light perturbation pulses (both light sources were light-emitting diodes
(LEDs)). The voltage dynamics were recorded by using a Keithley 2400
source meter. Varying the intensity of white-light bias allowed the estimate
of recombination rate constant (and thus apparent electron lifetime) at
different open-circuit potentials by controlling the concentration of the
free charges in TiO2. Red perturbation pulses were controlled not to
exceed 5% of the signal resulting from the white bias (either current or
voltage) in order to maintain single-exponential voltage decay.
Stability Test: The photoanodes of the device employed in this study
are composed of a 12 μm transparent TiO2 thin film and a 6 μm thick
layer of 400 nm TiO2 particles. A 370 nm cut-off long pass filter film was
attached on the cell surface during illumination. The cell was irradiated
under a Suntest CPS plus lamp (ATLAS GmbH, 100 mW cm−2 ) during
visible-light soaking at 60 °C. The electrolyte consisted of 1.0 M DMII,
0.15 M iodine, 0.1 M GNCS, 0.1 M LiI, and 0.5 M NBB (N-butyl-1H-
benzimidazole) in butyronitrile (BN).
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Acknowledgements
K.-L.W. and W.-P.K. contributed equally to this work. Y.C. thanks the
National Science Council of Taiwan (NSC-101-3113-E-007-006) for
financial support. K.-L.W. is a recipient of the Graduate Students Study
Abroad Program sponsored by National Science Council of Taiwan. A.Y.
thanks the Balzan foundation for financial support under the Balzan
Prize 2009 awarded to M.G.
Received: July 6, 2012
Revised: September 9, 2012
Published online:
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