Structural tuning of ancillary chelate in tri-carboxyterpyridine Ru(ii) sensitizers for dye sensitized solar cells

Article abstract of DOI:10.1039/c3ta14876a

Three distinct classes of ancillary chelates, namely: 2-(3- trifluoromethylpyrazol-5-yl)-6-(3-trifluoromethylphenyl)pyridine (L3, H ₂pzppy), 4-(3-trifluoromethylpyrazol-5-yl)-2-(3-trifluoromethyl) phenylpyrimidine (L5, H₂pzppm) and 4-(6-(3-trifluoromethylpyrazol-5- yl)pyridin-2-yl)-2-trifluoromethylpyrimidine (L6, H₂pzpypm), which showed an identical skeletal topology, but with the more electronegative nitrogen atom replacing the isoelectronic methine group at the selected skeletal position, were obtained to investigate the photophysical and electrochemical properties and hence the associated Ru(ii) sensitizers based DSCs. To increase the optical absorptivity we also strategically added thiophene (thienyl) or 3,4-ethylenedioxythiophene (EDOT) appendages to L6, for boosting the short-circuit photocurrent (J_SC) and the overall efficiency (η) of the fabricated DSC devices. Under AM 1.5G illumination, the best sensitizer showed performance data of J_SC = 18.11 mA cm^-2, V _OC = 0.66 V, FF = 0.729 and η = 8.72%, and a good cell stability at 60 °C for 1000 hours, being only decreased by ～5% in the η value.

Full text of DOI:10.1039/c3ta14876a

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Materials Chemistry A
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Structural tuning of ancillary chelate in
tri-carboxyterpyridine Ru(II) sensitizers for dye
Cite this: J. Mater. Chem. A, 2014, 2,
sensitized solar cells†
5418
a
a
a
a
Chun-Cheng Chou,‡ Pei-Hua Chen,‡ Fa-Chun Hu, Yun Chi, Shu-Te Ho,a
*
b
c
c
*
*
Ji-Jung Kai, Shih-Hung Liu and Pi-Tai Chou
Three distinct classes of ancillary chelates, namely: 2-(3-trifluoromethylpyrazol-5-yl)-6-(3-trifluoromethylphenyl)-
pyridine (L3, H2pzppy), 4-(3-trifluoromethylpyrazol-5-yl)-2-(3-trifluoromethyl)phenylpyrimidine (L5, H2pzppm)
and 4-(6-(3-trifluoromethylpyrazol-5-yl)pyridin-2-yl)-2-trifluoromethylpyrimidine (L6, H2pzpypm), which
showed an identical skeletal topology, but with the more electronegative nitrogen atom replacing the
isoelectronic methine group at the selected skeletal position, were obtained to investigate the
photophysical and electrochemical properties and hence the associated Ru(II) sensitizers based DSCs. To
increase the optical absorptivity we also strategically added thiophene (thienyl) or
Received 25th November 2013
Accepted 20th January 2014
3,4-ethylenedioxythiophene (EDOT) appendages to L6, for boosting the short-circuit photocurrent (JSC
)
and the overall efficiency (h) of the fabricated DSC devices. Under AM 1.5G illumination, the best
sensitizer showed performance data of JSC ¼ 18.11 mA cmꢀ2, VOC ¼ 0.66 V, FF ¼ 0.729 and h ¼ 8.72%,
and a good cell stability at 60 ꢁC for 1000 hours, being only decreased by ꢂ5% in the h value.
DOI: 10.1039/c3ta14876a
www.rsc.org/MaterialsA
(MLCT) transition. Among a variety of Ru(II) based photosensi-
tizers, the thiocyanate-containing sensitizers N749 (or black
dye),6–8 Z9079 and N71910 represent three of the best known
examples that have been tested for both fundamental and
commercial applications.
Introduction
The future prospect of a low-carbon society requires emerging
renewable energy sources, among which dye-sensitized solar
cells (DSCs) are considered to be one competitive candidate.
Especially, the low cost TiO2 photoanode in DSCs can poten-
tially be fabricated using printing technology, providing a viable
alternative to conventional photovoltaics. Despite the develop-
ment of a vast variety of different dyes to optimize the harvested
solar photons, up to the current stage, the Ru(II) based photo-
sensitizers remain the key component, because of the higher
cell performance, particularly in view of their stabilities versus
other sensitizers, such as organic donor-acceptor dyes and zinc
porphyrin/phthalocyanine dyes.1–5 Broadly speaking, the Ru(II)
sensitizers constitute at least one di- or tri-carboxy substituted
poly-pyridine chelate, to serve as the anchor to the TiO2 pho-
toanode, plus an ancillary ligand which would modify both the
light absorption capacity, ground state oxidation potential, and
relative peak position of the metal-to-ligand charge transfer
From the viewpoint of device stability, the robust skeletal
framework of sensitizers is considered as a major factor gov-
erning the lifespan. Thus, studies on thiocyanate-free Ru(II)
sensitizers with either a dicarboxy bipyridine, or tricarboxy
terpyridine anchor, have emerged in recent years.11–22 The
thiocyanate is expected to undergo dissociation from the
sensitizer in solution, due to its latent activity against ligand
exchange.23 To mitigate this problem, a new class of Ru(II)
sensitizers was synthesized, with the thiocyanate replaced by
either an electron-decient cyclometalate24–27 or a chelating
azolate.28–30 This approach was stimulated by the general belief
that the chelate should be less labile compared to the uni-
dentate bonding of thiocyanate, and the fact that the added
electron deciency is able to increase the ground state oxidation
ꢀ
potential of the sensitizers for faster dye regeneration, by Iꢀ/I3
redox couple.
aDepartment of Chemistry and Low Carbon Energy Research Center, National Tsing
Hua University, Hsinchu 30013, Taiwan. E-mail: ychi@mx.nthu.edu.tw
bDepartment of Engineering and System Science, National Tsing Hua University,
Taiwan. E-mail: jjkai@ess.nthu.edu.tw
cDepartment of Chemistry and Center for Emerging Material and Advanced Devices,
National Taiwan University, Taipei 10617, Taiwan. E-mail: chop@ntu.edu.tw
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ta14876a
Recently, our interests have extended to bis-tridentate Ru(II)
sensitizers.31–35 We started with a series of multidentate ancillary
chelates, namely: 2,6-bis(3-triuoromethylpyrazol-5-yl)pyridine
(L1, H2pz2py), 2,6-bis(3-triuoromethyl-1,2,4-triazol-5-yl)pyridine
(L2, H2tz2py), 2-(3-triuoromethylpyrazol-5-yl)-6-(3-triuoro-
methylphenyl)pyridine (L3, H2pzppy) and 2-(3-triuoromethyl-
1,2,4-triazol-5-yl)-6-(3-triuoromethylphenyl)pyridine (L4, H2tzppy),
which are synthesized and employed in the construction of
‡ C.-C C. and P.-H C. contributed equally to this work.
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relevant Ru(II) sensitizers (see Scheme 1). Their outstanding DSC
performances were subsequently validated by showing an
optimal light harvesting capability, down to the near-IR region.
Motivated by this nding, we then launched a study on synthe-
sizing new multidentate ancillaries to further ne-tune their
electrochemical and spectroscopic properties, by the insertion of
a nitrogen atom at specic positions on the backbone, and to
explore the associated cell performances.
In this paper, we present two new classes of ancillary
chelates,
namely:
4-(3-triuoromethylpyrazol-5-yl)-2-(3-tri-
uoromethyl)phenylpyrimidine (L5, H2pzppm) and 4-(6-(3-tri-
uoromethylpyrazol-5-yl)pyridin-2-yl)-2-triuoromethylpyrimidine
(L6, H2pzpypm). Their structures differ by the incorporation of
one pyrimidine fragment, as opposed to the central pyridine
fragment, or at the terminal phenyl group in the parent L3
ancillary (Scheme 1). We then probe this subtle structural
change versus the critical effect on the DSC performance.
Moreover, substituted thiophene (thienyl) or 3,4-ethyl-
enedioxythiophene (EDOT) appendages were also added to the
L6 ancillary in an aim to increase the optical absorptivity and
hence boost the short-circuit photocurrent (JSC) and overall
efficiency (h) under AM 1.5G illumination.
Scheme
2
Synthetic protocols: (i) NaOMe, MeOH; (ii) 3-(tri-
fluoromethyl)phenyl boronic acid, [Pd(PPh3)4], Na2CO3, THF–H2O; (iii)
HCl(aq) (iv) POCl3, toluene; (v) KCN, DABCO, DMSO–H2O; (vi)
;
CH3MgBr, THF, HCl(aq); (vii) NaOEt, CF3CO2Et, THF; (viii) N2H4, EtOH.
acetyl group.40 Finally, this acetyl group was converted to the
pyrazole moiety using a Claisen condensation and hydrazine
cyclization, in sequence.41
On the other hand, the H2pzpypm chelate L6 and its thienyl
and EDOT functionalized derivatives, i.e. L6.1: R ¼ 5-hexyl-
2-thienyl and L6.2:
R
¼
7-hexyl-2,3-dihydrothieno[3,4-b]-
1,4-dioxin-5-yl, were prepared using another synthetic
sequence, starting from 2,6-diacetylpyridine and its deriva-
tives, see Scheme 3. One acetyl substituent was rstly
protected using a stoichiometric amount of ethylene glycol in
the presence of an acid catalyst;42 the second (unprotected)
acetyl group was sequentially reacted with N,N-dime-
thylformamide dimethyl acetal, triuoroacetamidine and
sodium ethoxide, to induce the formation of a CF3-substituted
pyrimidine fragment.43 Aer that, the dioxolane group was
hydrolyzed to release the acetyl group.44 It was then reacted
with ethyl triuoroacetate under conditions for a Claisen
condensation, followed by hydrazine cyclization, to afford the
anticipated pyrazole group.41
Results and discussion
Firstly, the required dianionic tridentate ancillaries were
prepared via the multi-step protocols depicted in Schemes 2 and
3. For the H2pzppm chelate L5, the rst step was to replace the
4-chloro substituent of 2,4-dichloropyrimidine with a meth-
oxide protecting group, by a simple nucleophilic substitution,36
followed by the replacement of the unprotected 2-chloro
substituent with the m-triuoromethylphenyl group, using a
Suzuki cross-coupling reaction.37 The subsequent conversion of
the 4-methoxy group back to the chloro substituent was ach-
ieved using the chlorination reagent POCl3.38 The resulting
chloride compound was subjected to cyanation using KCN,39
followed by methylation with a Grignard reagent, to afford the
Scheme 3 Synthetic protocols: (i) ethylene glycol; (ii) N,N-dime-
Scheme 1 Structures of the ancillary chelates in various bis-tridentate thylformamide dimethyl acetal; (iii) trifluoroacetamidine, NaOEt, EtOH;
Ru(II) sensitizers.
(iv) HCl(aq); (v) NaOEt, CF3CO2Et, THF; (vi) N2H4, EtOH.
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With these ancillary chelates in hand, the required bis-tri-
dentate Ru(II) sensitizers could be synthesized from the
coupling of Ru(tectpy)Cl3 (tectpy ¼ 4,40,400-triethoxycarboxy-
2,20:60,200-terpyridine) and the ancillary chelate in the presence
of KOAc. Specically, the designated Ru(II) sensitizers TF-25, 26,
27 and 28 were synthesized using the aforementioned chelates
L5, L6, L6.1 and L6.2, respectively. Their molecular structures,
together with that of the reference sensitizer TF-21, are depicted
in Scheme 4.
The UV-vis absorption spectra of the sensitizers TF-25–TF-28
in DMF (N,N-dimethylformamide) at a concentration of 1 ꢃ
10ꢀ5 M are depicted in Fig. 1, together with the spectrum of TF-
21 as a reference, while the photophysical and cyclic voltam-
metric data are listed in Table 1. Generally speaking, TF-21
exhibited two sharp MLCT bands at 404 nm and 521 nm,
together with a longer wavelength shoulder that showed a slow
decrease in the intensity and extended to the near infrared
(>800 nm). In accordance with this pattern, both TF-25 and TF-
26 depicted two MLCT absorptions at ꢂ400 and ꢂ520 nm, with
the extinction coefficient of the lower energy MLCT band (1.2 ꢃ
104 L molꢀ1 cmꢀ1) being slightly lower than that of the higher
energy one (1.5 ꢃ 104 L molꢀ1 cmꢀ1), with a much faster decline
in the intensity for the lower energy MLCT shoulder versus that
of TF-21. Furthermore, upon the addition of the thienyl and
EDOT groups to TF-26, the resulting sensitizers TF-27 and TF-28
gave the lower energy MLCT peak maxima at around 530 and
533 nm, i.e. with a bathochromic shi of at least 13 nm versus
that of TF-26, and a substantial gain in the absorptivity to 3 ¼
2.1–2.3 ꢃ 104 L molꢀ1 cmꢀ1. This could imply a better light
harvesting capability for TF-27 and TF-28 over that of the parent
TF-26, which is consistent with the inuence of the attached
electron donating and p-conjugating appendage observed in
many organic push–pull and Ru(II) based sensitizers.45–52
Calculations based on density functional theory (DFT) and
time-dependent DFT in DMF were performed for the title
complexes. Fig. 2 and 3 depict the simulation of the absorption
wavelengths (vertical line) and the relative transition probability
(magnitude of the vertical line) of TF-25–TF-28 (see the results of
Fig. 1 UV-vis absorption spectra of Ru(II) sensitizers TF-25–TF-28 and
reference TF-21 at 1 ꢃ 10ꢀ5 M in DMF at RT.
TF-21 in ref. 33). Also depicted in these gures are the frontier
orbitals contributing to the major electronic transition recor-
ded. All the numerical data for the vertical transition and
comprehensive frontier orbital analyses are listed in Tables
S1–S4 and Fig. S1–S4 of the ESI.† As a result, the peak wave-
length over 500 nm was calculated to be 529, 521, 522, and
525 nm for TF-25–TF-28, respectively, for which both the value
and trend are consistent with the experimental absorption
maximum for TF-25 (523 nm), TF-26 (517 nm), TF-27 (530 nm)
and TF-28 (533 nm) in DMF (see Table 1), supporting the validity
of the computational approaches. A careful examination of the
optical transition and its associated frontier orbitals indicates
that the lower lying singlet transitions over 500 nm are mainly
contributed by a metal-to-ligand (tricarboxy-terpyridine) charge
transfer (MLCT), together with a minor part of an ancillary to
anchor (terpyridine) charge transfer (LLCT). In the higher lying
transition, around 400–550 nm, the unoccupied orbitals of the
thienyl (TF-27) and EDOT (TF-28) appendages have imposed an
appreciable contribution (see Fig. 3) due to an elongation of the
p-conjugation,53–56 which rationalizes the substantial gain of the
absorptivity as well as the red shi of the peak wavelength
observed experimentally for TF-27 and TF-28 in this region.
Table 1 Photophysical and electrochemical data for the studied Ru(II)
sensitizers
labs [nm] (3 [103 L molꢀ1 cmꢀ1]) Eoꢁxb (V) E0–0 (eV) Eꢁ0* (V)
a
0
Dye
TF-21 404 (15), 520 (13), 703 (2.2)
TF-25 405 (15), 523 (12), 555 (10)
TF-26 393 (15), 517 (12), 548 (11)
TF-27 415 (21), 530 (22), 558 (20)
TF-28 415 (22), 533 (23), 558 (21)
0.84
0.93
0.96
0.96
0.94
1.62
1.72
1.75
1.72
1.74
ꢀ0.78
ꢀ0.79
ꢀ0.79
ꢀ0.76
ꢀ0.80
a
Absorption and emission spectra were measured at 1 ꢃ 10ꢀ5 M in
b
DMF. Oxidation potential of dyes was measured in DMF with 0.1 M
[TBA][PF6] and a scan rate of 50 mV sꢀ1. It was calibrated with Fc/Fc+
as internal standard and converted to normal hydrogen electrode
(NHE) by addition of 0.63 V.
Scheme 4 Structures of Ru(II) sensitizers TF-25–TF-28 and TF-21
reference.
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Fig. 3 The absorption spectra of (a) TF-27 and (b) TF-28 sensitizers.
Also depicted are the TD-DFT calculated absorption wavelengths
(vertical line) and the relative transition probability (magnitude of
vertical line). Inset: the selected frontier orbitals contributed to the
major transitions. The occupied and unoccupied orbitals are repre-
sented in pink and yellow, respectively.
Fig. 2 The absorption spectra of (a) TF-25 and (b) TF-26 sensitizers.
Also depicted are the TD-DFT calculated absorption wavelengths
(vertical line) and the relative transition probability (magnitude of
vertical line). Inset: the selected frontier orbitals contributed to the
major transitions. The occupied and unoccupied orbitals are repre-
sented in pink and yellow, respectively.
Cyclic voltammetry was conducted to reveal the ground state aforementioned viewpoint that the added thienyl (TF-27) and
0
0
oxidation potential (Eꢁox). With the Eoꢁx data in hand, the EDOT (TF-28) appendages only affect the higher lying excited
corresponding excited state oxidation potentials (Eꢁ0*) were state, while the lowest lying excited state is dominated by the
0
estimated using the equation Eꢁ0* ¼ Eoꢁx ꢀ E0–0, for which E0–0 Ru(II)-to-anchor (tricarboxy-terpyridine) MLCT transition. The
0
stands for the optical energy gap, i.e. at a 5% onset of their invariance of Eꢁox and E0–0 leads to a similar Eꢁ0* for TF-26–28.
lowest energy absorption. In general, the absorption onset of For all these newly developed Ru(II) complexes, evidently, the
0
TF-25 and TF-26 show
a
signicant hypsochromic shi obtained Eoꢁx and Eꢁ0* are sufficiently more positive and negative
compared to TF-21, resulting in the higher onset energy, by 0.1 than that of the redox potential of an Iꢀ/I3ꢀ couple (ca. 0.35 V vs.
eV. However, this increase in E0–0 was offset by the more positive NHE) as well as the corresponding conduction band edge of the
0
Eꢁox of TF-25 and TF-26 recorded (0.93 and 0.96 V), leaving the TiO2 electrode (ca. ꢀ0.7 V vs. NHE)27,57 respectively, conrming
respective Eꢁ0* essentially unchanged. Moreover, the TF-27 and their suitability to serve as suitable DSC sensitizers.
TF-28 sensitizers were modied by the addition of the thienyl
DSCs were next fabricated using these sensitizers absorbed
and EDOT appendages to TF-26. Although this functionaliza- on a TiO2 photoanode, that consisted of a 15 mm layer of 20 nm
tion red shis the higher lying MLCT/pp* absorptions, as well absorbing particles and a 7 mm layer of 400 nm light scattering
0
as simultaneously increasing the extinction coefficient, their Eꢁox particles, deposited with multiple screen-printing manipula-
values are virtually unaltered versus that of TF-26 (see Table 1). tion.33 Moreover, the sensitizers were dissolved in a mixture of
Moreover, as shown in Fig. 1, the onset of the absorption, i.e., EtOH and DMSO (v/v 4 : 1) to afford a 0.3 mM solution, together
E0–0, is nearly the same among TF-26, TF-27 and TF-28. Support with the addition of 1 mM chenodeoxycholic acid (CDCA) as a
of this concept was also provided by the computational results, coadsorbent to reduce dye aggregation. The electrolyte solution,
in which the calculated S0 / S1 transitions are virtually iden- coded A, consisted of 2.0 M 1,3-dimethylimidazolium iodide
tical, being 668, 666 and 671 nm for TF-26, TF-27 and TF-28, (DMII), 0.1 M guanidinium thiocyanate (GuNCS), 0.05 M LiI,
respectively (see Tables S2–S4†). The result also reconrms the 0.03 M I2, and 0.5 M 4-tert-butylpyridine (tBP) in acetonitrile and
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Table 3 The performance characteristics of DSCs using electrolyte B
and under AM 1.5G illumination
valeronitrile (v/v, 85 : 15). Prior to the measurement, the solar
simulator (Sun 3000, ABET Technologies) was calibrated with a
certicated c-Si solar cell equipped with a KG-3 lter.58 The
device performances were measured using a metallic shadow
mask with a square aperture of 4 ꢃ 4 mm2. The obtained
photovoltaic parameters are listed in Table 2. It is notable that
the recorded cell efficiency of TF-21 is similar to the previously
reported data,33 while obviously all the newly developed
complexes TF-25–TF-28 show improved efficiencies. This
JSC
Dye loading
Sensitizers [mA cmꢀ2
]
VOC [V] FF
h [%] [ꢃ10ꢀ7 mol cmꢀ2
]
TF-21
TF-25
TF-26
TF-27
TF-28
N749
15.41
16.92
17.12
17.43
18.11
16.92
0.64
0.654 6.44
0.683 7.86
0.728 7.85
0.733 7.92
0.729 8.72
0.727 8.86
1.69 ꢄ 0.08
2.11 ꢄ 0.05
1.89 ꢄ 0.07
1.04 ꢄ 0.05
1.05 ꢄ 0.04
0.68
0.63
0.62
0.66
0.72
0
appears to be due to the more positive Eꢁox, i.e., a higher oxida-
tion potential versus TF-21, so that faster dye regeneration is
expected, giving a better cell efficiency.59–61
The device was further improved by adding tetrabutyl-
ammonium deoxycholate [TBA][DOC] to the dye solution, and
switching to a different electrolyte system. The addition of the
co-adsorbent [TBA][DOC] was expected to conduct the proton-
to-TBA exchange at the sensitizer, which in turn could improve
the solubility and shi the Eꢁ0* to a more negative value.62,63 In
this approach the alternative electrolyte, now coded as B, con-
Upon further modication of TF-26, by decoration of either
the thienyl (TF-27) or EDOT (TF-28) substituent, the device
efficiency increased to 7.92% for TF-27 and more prominently to
8.72% for TF-28. These improvements were attributed to both a
better dye loading and light harvesting effect, but the increment
was smaller for the thienyl derivative TF-27 than that of the
EDOT functionalized TF-28. While the exact cause is unclear, we
suspect it could be related to the faster charge recombination,68
which reduced both the JSC and VOC, as shown by the
inferior device performance of TF-27 (h ¼ 5.32%) vs. that of
TF-26 (h ¼ 7.08%) and TF-28 (h ¼ 6.00%) recorded under a
lower dye loading and with electrolyte A (cf. Table 2).
sisted of 0.6
M 1,2-dimethyl-3-propylimidazolium iodide
(DMPII), 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine
(tBP) in acetonitrile and valeronitrile (v/v, 85 : 15). The higher
concentration of LiI in electrolyte B was expected to lower the
conduction band edge of the TiO2 and lead to a faster electron
injection and higher JSC for the DSC devices.64–67
The corresponding J–V characteristics and incident photon-
to-current conversion efficiency (IPCE) action spectra are shown
in Fig. 4. The IPCE proles of all the sensitizers (i.e. TF-25–TF-
28) clearly reveal a signicant improvement over TF-21 in the
region 400–700 nm, and we attributed this to the more positive
The resulting photovoltaic parameters are listed in Table 3.
To conrm the increase of both the solubility and dye loading
by the addition of [TBA][DOC], we re-measured the dye loading
under this new condition. As expected, the loading of TF-21 was
notably increased, by 19%, and the reference TF-21 device now
afforded an improved conversion efficiency (h) of 6.44%. In the
meantime, DSCs of the respective TF-25 and TF-26 under the
same conditions also demonstrated a higher loading and much
improved efficiencies; namely: h of 7.86 and 7.85%, JSC of 16.92
and 17.12 mA cmꢀ2, VOC of 0.68 and 0.63 V, and a ll factor (FF)
of 0.683 and 0.728, as shown in Table 3. The superiority of TF-25
and TF-26 over TF-21 was also attributed to the electron with-
drawing property of the extra nitrogen atoms in the ancillary,
0
Eꢁox and the higher driving force for dye regeneration with the
iodide-electrolyte. In Fig. 4(b), the TF-28 sensitizer, with an
EDOT appendage, showed the highest JSC of 18.11 mA cmꢀ2
,
which is also in agreement with the higher IPCE (by a deviation
of <10%) over the solar spectral region. Thus, coupled with the
VOC of 0.66 V and FF of 0.729, TF-28 gave the highest conversion
efficiency of 8.72%. On the other hand, the TF-27 device did not
perform with an equivalent efficiency, despite showing a similar
spectrum prole and absorptivity versus that of TF-28.
For an intimate comparison to the pristine sensitizer N749,
we also fabricated a reference DSC device using identical cell
parameters and the electrolyte B (cf. Table 3). This reference
device showed an essentially identical value for the overall
efficiency, but with a slightly lower JSC and higher VOC versus our
best sensitizer TF-28 in this study. The variations of the JSC and
0
that shied the Eꢁox to the more positive value for a faster dye
regeneration (Table 1).59–61 Moreover, since both TF-25 and
TF-26 show nearly identical efficiencies, the result implies that
the location of the pyrimidinyl substituent imposes an insig-
nicant variation in the device efficiency, due to the retention of
both the spatial and electrochemical properties.
Table 2 The performance characteristics of DSCs using electrolyte A and under AM 1.5G illumination
Sensitizers
JSC [mA cmꢀ2
]
VOC [V]
FF
h [%]
Dye loadinga [ꢃ10ꢀ7 mol cmꢀ2
]
TF-21
TF-25
TF-26
TF-27
TF-28
10.54
15.25
15.34
12.04
13.19
0.58
0.64
0.63
0.59
0.61
0.704
0.709
0.733
0.748
0.746
4.31
6.92
7.08
5.32
6.00
1.42 ꢄ 0.08
1.55 ꢄ 0.08
1.42 ꢄ 0.06
0.88 ꢄ 0.01
0.79 ꢄ 0.01
a
The value was calculated from the MLCT band ratio for desorbed dye solution versus (0.01 mM) reference solution in mixed MeOH and water (v/v,
1 : 1) with 0.1 M of added TBAOH.
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Fig.
4 IPCE action diagram and J–V characteristics for devices
fabricated using the electrolyte B.
VOC between TF-28 to N749 could be due to the better light
harvesting property and faster charge recombination, a result of
the introduction of the EDOT appendage at the ancillary chelate
of TF-28.
Fig. 5 (a) TiO2 electron density versus voltage deduced from charge
extraction measurements and (b) electron lifetime versus TiO2 elec-
tron density deduced from transient photovoltage measurements for
DSC devices containing TF-21 and TF-25–TF-28 dyes. The cell voltage
was controlled via tuning the illumination from a halogen lamp.
To further probe the device performances, measurements of
the charge extraction (CE) and transient photovoltage (TPV)
decay were carried out, for which the relevant data are shown in
Fig. 5(a) and (b). The differences in the VOC between the cells
can generally be explained by shis in the TiO2 conduction
band edge (manifested by the shi of the exponential distri-
bution of the experimental data measured by CE) and/or by the
TiO2 recombination lifetimes (investigated via TPV measure-
ments).69,70 As can be seen, except for TF-21 which has a slightly
lower VOC at a xed photo-induced charge density due to a
downward shi of the band edge, all the other DSC devices
show a very similar VOC, indicating that the conduction band
potentials are similar in the cells TF-25–TF-28. In Fig. 5(b), the
transient photovoltage measurements at a xed charge density
indicate that the device made with TF-25 has the longest elec-
tron lifetime, while TF-27 is the shortest among all the others.
The trend of the electron lifetime of these TF sensitizers seems
to be in good agreement with the VOC of the devices (Table 2),
suggesting that the conduction band edge of TiO2 is less
dark at a forward bias, the same as the respective VOC tested
under a one-sun illumination. The impedance spectra are
composed of two semicircles, in which the le (smaller cycle)
one depicts the electrochemical reaction at the Pt–electrolyte
interface, and the right (larger cycle) one represent the
inuential to the observed VOC
.
Then, electrochemical impedance spectroscopy was also
utilized to analyze the resistance to charge recombination in
Fig. 6 Electrochemical impedance spectra of DSC devices tested in
dark with an external bias as each corresponding VOC under one-sun
these devices. Fig. 6 shows the Nyquist plots measured in the illumination.
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Journal of Materials Chemistry A
Paper
showed an improved efficiency, up to 7.92%, while TF-28
demonstrated the highest efficiency of 8.72%, with an average
IPCE of 80% over 400–700 nm and an onset wavelength of
860 nm, conrming their usefulness as a guideline for ne-
tuning the functional DSC sensitizers.
Experimental section
General procedures
All the reactions were performed under a nitrogen atmosphere.
The solvents were distilled from the appropriate drying agents,
prior to use. Commercially available reagents were used,
without further purication. All the reactions were monitored
by TLC with pre-coated silica gel plates (Merck, 0.20 mm with a
uorescent indicator UV254). The compounds were visualized
with UV irradiation at 254 or 365 nm. Flash column chroma-
tography was carried out using silica gel obtained from Merck
(230–400 mesh). The mass spectra were obtained on a JEOL
SX-102A instrument, operating in the electron impact (EI) or
fast atom bombardment (FAB) modes. The 1H and 19F NMR
spectra were recorded on a Bruker-400 or INOVA-500 instru-
ment; the chemical shis are quoted with respect to the internal
standard, tetramethylsilane. The elemental analysis was carried
out with a Heraeus CHN–O Rapid Elementary Analyzer. The
photophysical data were obtained using an Edinburgh uores-
cence spectrometer FLS928P. Details of the synthetic protocols
for the tri-dentate ancillary chelates and the procedures for the
DSC cell fabrication and measurement are all given in the ESI.†
Fig. 7 Device performances of TF-28 under a one-sun light soaking,
at 60 ꢁC, for 1000 h.
impedance characteristics of the charge recombination (Rr) at
the TiO2-dye–electrolyte interface.71–73 The radii of the right
series of semicircles indicates Rr to be in the order TF-25 > TF-28
> TF-21 > TF-26 z TF-27 and, thus, the dark currents (Jdark
)
appear as an inverse trend, which are consistent with the
previous TPV results.
To test the stability of the Ru(II) sensitizers, the highest
efficiency device, TF-28, was subjected to a light soaking test at
60 ꢁC, for 1000 h, aer adopting a low-volatility electrolyte
composed of 0.6 M PMII, 0.15 M I2, 0.1 M GuNCS, and 0.5 M
NBB (N-butyl-1H-benzimidazole) in butyronitrile (BN).74 With a
duration of 1000 h, the JSC, VOC, FF, and h were steady until
600 h and then decreased (Fig. 7). The hmax of 7.87% was located
Synthesis of TF-25
A
mixture of 4-(3-(triuoromethyl)-1H-pyrazol-5-yl)-2-(3-(tri-
uoromethyl)phenyl) pyrimidine (114 mg, 0.32 mmol),
Ru(tectpy)Cl3 (204 mg, 0.32 mmol) and KOAc (156 mg, 1.60
mmol) in 30 mL of xylenes was heated at 140 ꢁC, under stirrin,g
for 15 h. Aer the removal of solvent, the crude product of TF-
25-Et was puried by silica gel column chromatography (ethyl
acetate–hexane ¼ 1 : 2). Aer that, the resulting solid was dis-
solved in a mixture of acetone (20 mL) and 1 N NaOH solution
(1.0 mL). For the hydrolysis, the mixture was stirred at room
temperature under nitrogen for 5 h. The solvent was removed,
and the residue was dissolved in H2O (5 mL). This solution was
titrated with 2 N HCl(aq) to pH ¼ 3, to afford a brown precipitate.
This brown product was washed with acetone and ethyl acetate
in sequence, to yield the nal product. Yield: 187 mg, 72%. All
the other Ru(II) derivatives, e.g. TF-26–TF-28, were synthesized
from Ru(tectpy)Cl3 and the respective ancillary chelates, using
identical procedures.
around 500 h. The decline of the efficiency, dened as (hmax
ꢀ
h1000h)/hmax, was 5.2% which is close to other data reported by
our group.33 This result reconrmed that the bis-tridentate
architecture was stable to the applied thermal stress, under
simulated solar irradiation.
Conclusion
In summary, two new types of terdentate ancillary ligands,
namely: H2pzppm (L5) and H2pzpypm (L6) and the derivatives
(L6.1 and L6.2), were synthesized and employed to construct
four bis-tridentate, thiocyanate-free Ru(II) complexes TF-25–TF-
28. The substitution of the pyrimidine for pyridinyl and the
phenyl group in the reference TF-21, yielded TF-25 and TF-26,
Selected spectral data of TF-25
respectively, which successfully increased the ground state MS (FAB, 102Ru): m/z 824 (823) [M + 1]+. 1H NMR (400 MHz, d6-
oxidation potentials to a level of 0.93–0.96 V (vs. NHE). With the DMSO,298 K): d 9.39 (s, 2H), 9.15 (s, 2H), 8.14–8.16 (m, 2H),
addition of the thienyl and EDOT decorations on the H2pzpypm 7.60–7.63 (m, 4H), 7.47 (s, 1H), 6.76 (d, JHH ¼ 7.6 Hz, 1H), 5.68
ancillary, the resulting TF-27 and TF-28 sensitizers exhibited (d, JHH ¼ 8 Hz, 1H); 19F NMR (376 MHz, d6-DMSO, 298 K): d
expected hyperchromic and bathochromic effects, respectively, ꢀ58.54 (s, 3F), ꢀ60.64 (s, 3F). Anal. calcd for C33H17F6N7O6R-
on their UV-vis absorption spectra. The devices made with u$2H2O: C, 46.16; N, 11.42; H, 2.47. Found: C, 46.09; N, 11.31; H,
TF-25–28 all had superior performance over TF-21. TF-27 2.15%.
5424 | J. Mater. Chem. A, 2014, 2, 5418–5426
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Journal of Materials Chemistry A
¨
C. A. Bignozzi and M. Gratzel, J. Am. Chem. Soc., 2001, 123,
Selected spectral data of TF-26
1613.
MS (FAB, 102Ru): m/z 825 (M + 1)+. 1H NMR (400 MHz, d6-DMSO,
298 K): d 9.31 (s, 2H), 9.11 (s, 2H), 8.33–8.28 (m, 2H), 8.22 (t, JHH
¼ 8 Hz, 1H), 7.64 (dd, JHH ¼ 6 Hz, 2H), 7.59 (d, JHH ¼ 6 Hz, 2H),
7.26 (s, 1H), 7.13 (s, 1H). 19F NMR (376 MHz, d6-DMSO, 298 K): d
ꢀ58.36 (s, 3F), ꢀ68.33 (s, 3F). Anal. calcd for C32H16F6N8O6-
Ru$2H2O: C, 44.71; N, 13.04; H, 2.35. Found: C, 44.79; N, 12.70;
H, 2.46%.
7 Y. Chiba, A. Islam, R. Komiya, N. Koide and L. Han, Appl.
Phys. Lett., 2006, 88, 223505.
8 H. Ozawa, R. Shimizu and H. Arakawa, RSC Adv., 2012, 2,
3198.
9 P. Wang, S. M. Zakeeruddin, J. E. Moser, M. K. Nazeeruddin,
T. Sekiguchi and M. Gratzel, Nat. Mater., 2003, 2, 402.
¨
10 M. K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni,
¨
G. Viscardi, P. Liska, S. Ito, B. Takeru and M. Gratzel,
J. Am. Chem. Soc., 2005, 127, 16835.
Selected spectral data of TF-27
MS (FAB, 102Ru): m/z 991 (M + 1)+. 1H NMR (400 MHz, d6-DMSO, 11 T. Bessho, E. Yoneda, J.-H. Yum, M. Guglielmi, I. Tavernelli,
¨
298 K): d 9.32 (s, 2H), 9.12 (s, 2H), 8.64 (s, 1H), 8.27 (s, 1H), 8.02
(d, JHH ¼ 3.6 Hz, 1H), 7.70 (d, JHH ¼ 6 Hz, 2H), 7.63 (d, JHH ¼ 6
H. Imai, U. Rothlisberger, M. K. Nazeeruddin and M. Gratzel,
J. Am. Chem. Soc., 2009, 131, 5930.
Hz, 2H), 7.41 (s, 1H), 7.15 (s, 1H), 7.11 (d, JHH ¼ 3.6 Hz, 1H), 2.95 12 P. G. Bomben, T. J. Gordon, E. Schott and C. P. Berlinguette,
(t, JHH ¼ 8 Hz, 2H), 1.74 (quin, JHH ¼ 8 Hz, 2H), 1.40–1.31 (m,
Angew. Chem., Int. Ed., 2011, 50, 10682.
6H), 0.89 (t, JHH ¼ 8 Hz, 3H). 19F NMR (376 MHz, d6-DMSO, 298 13 F. Gajardo, M. Barrera, R. Vargas, I. Crivelli and B. Loeb,
K):
C
d
ꢀ58.38 (s, 3F), ꢀ68.22 (s, 3F). Anal. calcd for
Inorg. Chem., 2011, 50, 5910.
42H30F6N8O6RuS$H2O: C, 50.05; N, 11.12; H, 3.20. Found: C, 14 K. C. D. Robson, B. D. Koivisto, A. Yella, B. Sporinova,
¨
50.28; N, 11.01; H, 3.10%.
M. K. Nazeeruddin, T. Baumgartner, M. Gratzel and
C. P. Berlinguette, Inorg. Chem., 2011, 50, 5494.
15 Y. Qin and Q. Peng, Int. J. Photoenergy, 2012, 2012, 291579.
16 S. P. Singh, K. S. V. Gupta, G. D. Sharma, A. Islam and
L. Han, Dalton Trans., 2012, 41, 7604.
17 K. C. D. Robson, P. G. Bomben and C. P. Berlinguette, Dalton
Trans., 2012, 41, 7814.
18 T. Funaki, H. Funakoshi, N. Onozawa-Komatsuzaki,
K. Kasuga, K. Sayama and H. Sugihara, Chem. Lett., 2012,
41, 647.
19 L. E. Polander, A. Yella, B. F. E. Curchod, A. N. Ashari,
J. Teuscher, R. Scopelliti, P. Gao, S. Mathew, J.-E. Moser,
I. Tavernelli, U. Rothlisberger, M. Gratzel, M. K. Nazeeruddin
and J. Frey, Angew. Chem., Int. Ed., 2013, 52, 8731.
20 T. Funaki, N. Onozawa-Komatsuzaki, K. Kasuga, K. Sayama
and H. Sugihara, Inorg. Chem. Commun., 2013, 35, 281.
21 C. Dragonetti, A. Colombo, M. Magni, P. Mussini, F. Nisic,
D. Roberto, R. Ugo, A. Valore, A. Valsecchi, P. Salvatori,
M. G. Lobello and F. D. Angelis, Inorg. Chem., 2013, 52,
10723.
Selected spectral data of TF-28
MS (FAB, 102Ru): m/z 1049 (M + 1)+. 1H NMR (400 MHz, d6-
DMSO, 298 K): d 9.31 (s, 2H), 9.12 (s, 2H), 8.49 (s, 1H), 8.39 (s,
1H), 7.68 (d, JHH ¼ 5.6 Hz, 2H), 7.64 (d, JHH ¼ 5.6 Hz, 2H), 7.36
(s, 1H), 7.12 (s, 1H), 4.56–4.51 (m, 2H), 4.41–4.35 (m, 2H), 2.76 (t,
JHH ¼ 8 Hz, 2H), 1.67 (quin, JHH ¼ 8 Hz, 2H), 1.45–1.36 (m, 6H),
1.07 (t, JHH ¼ 8 Hz, 3H). 19F NMR (376 MHz, d6-DMSO, 298 K): d
ꢀ58.33 (s, 3F), ꢀ68.24 (s, 3F). Anal. calcd for C44H32F6N8O8-
RuS$2H2O: C, 48.76; N, 10.34; H, 3.35. Found: C, 48.63; N, 10.02;
H, 3.04%.
Acknowledgements
This work was supported by the National Science Council of
Taiwan. The computations were conducted at the National
Center for High-Performance Computing (NCHC) and we are
grateful to the NCHC for their computer time and facilities.
22 G. D. Sharma, S. P. Singh, R. Kurchania and R. J. Ball, RSC
Adv., 2013, 3, 6036.
23 P. T. Nguyen, A. R. Andersen, E. M. Skou and T. Lund, Sol.
Energy Mater. Sol. Cells, 2010, 94, 1582.
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