Three is not a crowd: Efficient sensitization of TiO2 by a bulky trichromic trisheteroleptic cycloruthenated dye

Article abstract of DOI:10.1039/c2cc00136e

A cyclometalated Ru dye bearing two triphenylamine groups to augment light absorption exhibits a power conversion efficiency (PCE) in excess of 7% in the dye-sensitized solar cell despite having a large molecular footprint on TiO ₂.

Full text of DOI:10.1039/c2cc00136e

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Three is not a crowd: efficient sensitization of TiO2 by a bulky
trichromic trisheteroleptic cycloruthenated dyew
Paolo G. Bomben, Javier Borau-Garcia and Curtis P. Berlinguette*
Received 8th January 2012, Accepted 28th February 2012
DOI: 10.1039/c2cc00136e
A cyclometalated Ru dye bearing two triphenylamine groups to
augment light absorption exhibits a power conversion efficiency
(PCE) in excess of 7% in the dye-sensitized solar cell despite
having a large molecular footprint on TiO2.
where one of the bidentate bpy ligands bears two TPA groups
to further enhance the light absorption relative to that of 2.
It is shown herein that the cyclometalating ligand and TPA
groups of 3 collaborate to produce a PCE of >7% in
the DSSC.
Complex 3 was formed by the addition of the cyclometalating
ligand to the metal prior to the synchronous addition of the
two polypyridyl ligands. The TPA-functionalized chelating
ligand P3 was formed using a Suzuki cross-coupling reaction
of 4,40-dibromo-2,20-bipyridine with proligand P2 (Scheme S1).
Addition of P3 and 4,40-diethylester-2,20-bipyridine (deeb) to
[Ru(CH3CN)4(ppy-(CF3)2)]PF6 (P4) in an ABS EtOH/CHCl3
(3 : 1) solvent mixture yielded the ester derivative of 3, P5.
P3 is soluble in CHCl3, but the formation of undesired
byproducts is observed in this solvent thus lowering the overall
reaction yields (B52.5%). The constitution of P5 was esta-
blished by 1H NMR spectroscopy, high-resolution mass spectro-
metry and elemental analysis. The deprotection of the ester
groups of P5 to produce 3 was achieved overnight in refluxing
DMF/H2O/NEt3 (3 : 1 : 1).
The dye-sensitized solar cell (DSSC) has attracted significant
attention as a potential alternative to crystalline-silicon and
thin-film devices for urban and indoor applications on account
of the low embodied energy and relative indifference to the
angle and intensity of the incident light.1–3 A particularly
attractive feature of the DSSC is that the light absorption and
charge-transport processes4–6 are reasonably well-understood
thereby rendering it possible to make efficiency and stability
gains through the rational design of molecular components.7–13
We14 and others15,16 have a longstanding interest in developing
organometallic Ru sensitizers devoid of labile NCSꢀ groups.
Replacing these labile17 monodentate ligands with chelating
cyclometalating ligands such as 2-phenylpyridine (e.g., 1)
provides the opportunity to utilize the chelate effect to poten-
tially enhance dye stability, and gain acute control of the
HOMO energy level through judicious chemical modification
of the anionic ring.14,18 This feature is particularly important
in view of the apparent need for DSSC dyes to have a HOMO
level positioned at ca. >0.9 V vs. NHE to be regenerated by
Iꢀ,20,21 which is consistent with the relevant redox couple for
Dye 3 is insoluble in most polar organic solvents (e.g. MeOH,
MeCN, DMSO)—even with the addition of NaOH—but is
readily soluble in DMF. Dye 3 can be easily constituted in
low-polarity organic solvents (e.g. CH2Cl2, CHCl3, acetone,
Et2O), thus indicating that the solubility of the complex is
governed by P3. The 1H NMR spectrum of 3 in d8-DMF
displayed broad, poorly resolved signals (Fig. S1, ESIw), which
we attribute to slow molecular tumbling on the NMR time-
scale arising from the large size of the complex. Elevated
temperatures (e.g., 373 K) yielded a nominal improvement in
spectral resolution. Notwithstanding, ESI-MS and elemental
analysis supported the identity of 3. The isomer shown in
Fig. 1 is based on the structural determination of a related
complex, [Ru(bpy)(deeb)(ppy)]PF6 (4; Fig S2, ESIw). Note
that this result provides the first structural confirmation of
the absolute configuration of a tris-heteroleptic Ru dye related
to 1; namely, the Ru–C bond is positioned trans to the
electron-deficient deeb.
ꢁ
the electrolyte being defined by E , I2 ꢀ/Iꢀ = 0.8ꢂ0.1 V vs.
NHE in MeCN.19 Importantly, the scaffold of 1 also lends
itself to the modification of one of the dcbpy ligands with
electron-rich aromatic substituents, such as alkylthiophenes
(e.g. 222), to render higher extinction coefficients and PCEs.23
We have also recently demonstrated that triphenylamine (TPA)
groups (which are effective light-harvesting units for metal24–28
and metal-free29,30 DSSC dyes) conjugated to tridentate chelating
ligands produce remarkably high efficiencies for bistridentate
complexes.31
o
0
We therefore set out to unite these concepts to furnish the first
trichromic trisbidentate cyclometalated Ru dye in the form of 3,
The UV-vis spectrum of 3 (Fig. 2) reveals a relatively intense
band centred at 580 nm (3.3 ꢃ 104 Mꢀ1 cmꢀ1) ascribed to a
‘‘metal’’-to-ligand charge transfer (MLCT) transition from the
Ru-aryl fragment to the dcbpy ligand.14 The large intensity of
the band centred at 465 nm (6.7 ꢃ 104 Mꢀ1 cmꢀ1) arises from
intraligand charge-transfer (ILCT) bands emanating from
Department of Chemistry, University of Calgary and the Institute for
Sustainable Energy, Environment & Economy, University of Calgary,
2500 University Drive N.W., Calgary, Canada T2N-1N4.
E-mail: cberling@ucalgary.ca; Tel: +1-403-220-3856
w Electronic supplementary information (ESI) available: Synthesis
and characterization details. See DOI: 10.1039/c2cc00136e
c
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Chem. Commun., 2012, 48, 5599–5601 5599

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Fig. 1 ꢀ Structural drawing of 1, 2, 3 and P3. Counteranion for 1 and 3
Fig. 3 IPCE traces for 3 with (Entry 5, Table 1) and without
is PF6
.
chenodeoxycholic acid (Entry 6). Inset: J-V curves for 3 with (Entry 5)
and without chenodeoxycholic acid (Entry 6).
Table 1 Device characteristics for cells containing 2 and 3
Active area
Entry Dye (cm2)
Voc
V
/
Jsc/mA
cmꢀ2
Zb
FF (%)
Coadsorbenta
1
2
3
4
5
6
7
8
2
2
2
2
3
3
3
3
0.28
0.28
0.13
0.13
0.28
0.28
0.13
0.13
Yes
No
Yes
No
Yes
No
Yes
No
0.68 15.8
0.63 13.0
0.66 16.3
0.60 13.7
0.65 14.6
0.66 15.7
0.59 15.0
0.63 17.1
0.60 6.4
0.64 5.3
0.68 7.3
0.67 5.6
0.62 5.9
0.60 6.2
0.64 5.7
0.67 7.3
Fig. 2 UV-vis absorption spectra of P3, 2 and 3 recorded in DMF.
Inset: Cyclic voltammagrams of P3, 2 and 3 recorded in DMF at
200 mV sꢀ1 with 0.1 M NBu4BF4 supporting electrolyte.
a
b
Coadsorbent is chenodeoxycholic acid (0.0025 M). Performance of
DSSCs measured under AM1.5 light conditions at 1 sun intensity
using 12 mm active + 3 mm scattering TiO2 layers. All cells were filled
with an electrolyte consisting of 0.6 M 1-butyl-3-methylimidazolium
iodide (BMII), 0.06 M I2, 0.1 M NaI, 0.5 M 4-tertbutylpyridine and
0.1 M guanidinium thiocyanate.
the TPA groups to complement the MLCT bands.31 Excitation
of 3 at l = 580 nm in degassed DMF generates an emission
peak at l = 797 nm with a lifetime of 46 ns.
The cyclic voltammogram (CV) of 3 recorded in DMF
reveals a quasi-reversible oxidation process at +0.97 V vs. NHE
(Fig. 2, inset). The large peak separation (DEp B 170 mV)
is rooted in the nearly coincident Ru and TPA oxidation
processes; e.g., the first oxidation waves of P3 and 2 in
DMF are observed at +0.96 V and +0.99 V, respectively
(Fig. 2, inset). The oxidation potential of 3 is not anticipated
to deviate significantly from that of 2 because the HOMO is
confined to the metal and the aryl ring. (Efforts to resolve the
individual redox processes of 3 by square-wave voltammetry
were not successful.) An irreversible or quasi-reversible TPA
oxidation process superimposed with the ostensibly reversible
RuIII/RuII redox couple presumably causes the larger cathodic
signal in the CV of 3. The quasi-reversible reductive wave at
ꢀ1.17 V is assigned to the anchoring dcbpy ligand of 3. The
1-butyl-3-methylimidazolium iodide (BMII), 0.06
M I2,
0.1 M NaI, 0.5 M 4-tertbutylpyridine and 0.1 M guanidinium
thiocyanate. The photocurrent density-voltage diagrams for 3
are depicted in the inset of Fig. 3 along with performance
parameters listed in Table 1. The performance is effectively the
same at full active area (Entries 5 and 6) with marginal differ-
ences observed in Voc (D = 0.01 V) and FF (D = 0.02). A more
significant difference is evident with the Jsc (D = 1.1 mA cmꢀ2).
Overall, the performances with and without coadsorbent are
5.9 and 6.2% respectively. The incident-photon-to-current-
efficiency (IPCE) trace for 3 shows an onset of current at
nearly 790 nm and the curve plateaus between 65–70% in the
region of 450–600 nm (Fig. 3). A minimal enhancement is
observed from 350–600 nm for devices without CDCA
present, presumably a consequence of better dye loading in
these devices (vide infra). One possible cause of the low IPCE
values is a normalized diffusion length (Ln) that was calculated
to be less than the film thickness at Voc, both in the presence
(0.6Ln) and absence of coadsorbent (0.8Ln). This condition
would effectively reduce the charge collection efficiency, as well
as Voc and Jsc.
E
0ꢀ0 energy of 1.76 eV determined from the intersection of the
absorption and emission curves corresponds to an E(S+/S*) of
ca. ꢀ0.79 V (assuming the metal-based HOMO resides at
+0.97 V), which is suitable for efficient electron injection into
TiO2.4,32
The absorption properties and ground- and excited-state
energy levels of 3 were poised for sensitizing TiO2 in the
DSSC. Cells were therefore constructed by immersing TiO2
photoelectrodes (12 mm active + 3 mm scattering layer) in
absolute ethanol solutions of 3 with and without chenodeoxycholic
acid (CDCA) present (Supporting Information). Prior to sealing,
the cells were filled with an electrolyte containing 0.6 M
The larger photocurrent observed in the absence of CDCA
is attributed to a higher dye loading on the surface of TiO2. This
claim is supported by dye desorption studies indicating a surface
concentration of 3 on TiO2 as 5.7 and 4.2 ꢃ 10ꢀ8 mol cmꢀ2 in
the absence and presence of the coabsorbent, respectively.
c
5600 Chem. Commun., 2012, 48, 5599–5601
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Surprisingly, an opposite trend was observed for 2: a higher
dye loading (8.1 ꢃ 10ꢀ8 mol cmꢀ2) was found for substrates
with CDCA compared to those without (5.4 ꢃ 10ꢀ8 mol cmꢀ2).
Despite these differences, the disparities in cell performance for
2 and 3 were nominal (Entries 1 and 5).
Notes and references
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Further insight into the performance of DSSCs composed
of 2 and 3 was obtained from electrochemical impedance
spectroscopy (EIS) data (Fig. S3, ESIw), which was fit to
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coadsorbent (Entry 6) compared to that of 2 without CDCA
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improve Voc and/or Jsc. A similar observation for the organic
dye D35 reported by Jiang et al.34 was rationalized by a highly
ordered dye arrangement on the surface limiting the uptake
and effect of CDCA. This scenario could also be the case for 3,
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achieved in the absence of CDCA. A PCE of 6.2% at full
active area (0.28 cm2) and 7.3% with a reduced mask size
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This work provides the first example of a trichromic cyclo-
metalated Ru dye—a strategy inspired by the precedent for
TPA-functionalized Ru dyes bearing two or three NCSꢀ
ligands in the literature.24,25 A particularly relevant example
to this work is dye IJ-1 reported by Ko et al., which has two
NCSꢀ ligands in place of the cyclometalating ligand and yields
a PCE of 10.3%.26 The inferior performance of 3 may be due
to the lower driving force for intramolecular electron-transfer
(i.e. reduction of the photo-oxidized Ru site by the TPA)
relative to IJ-1 because the two NCSꢀ groups do not raise the
HOMO energy to the same extent as a ppyꢀ ligand (although
differences in cell fabrication cannot be excluded). Studies are
underway to directly examine these intramolecular and inter-
facial electron-transfer processes.
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We gratefully acknowledge Qiao Wu, Dr Michelle Forgeron,
Dr Roland Roesler and Dr Zhipan Zhang for experimental
assistance and helpful discussions. This work was funded by
NSERC and the Canada School of Sustainable Energy and
Environment.
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5599–5601 5601