Engineering of Ru(ii) dyes for interfacial and light-harvesting optimization

Article abstract of DOI:10.1039/c3dt53272k

A new Ru(ii) dye, Ru(L1)(L2) (NCS)₂, L1 = (4-(5-hexylthiophen-2- yl)-4′(4-carboxyl-phenyl 2,2′-bipyridine) and L2 = (4-4′-dicarboxy-2,2′-bipyridine), labelled MC112, based on a dissymmetric bipyridine ligand for improved interfacial and optical properties, was synthesized and used in DSCs, yielding photovoltaic efficiencies of 7.6% under standard AM 1.5 sunlight and an excellent device stability. Increased light harvesting and IPCE maximum were observed with MC112 compared to the prototypical homoleptic N719 dye, due to the functionalized bipyridyne ligand acting as an antenna. In addition, the mixed bipyridyne ligand allowed MC112 binding to TiO₂ to occur via three anchoring carboxylic groups, thus exhibiting similar interfacial properties to those of the N719 dye. DFT/TDDFT calculations were performed on the new dye, both in solution and adsorbed on a TiO₂ surface model, revealing that the peculiar photovoltaic properties of the MC112 dye are related to its anchoring mode. The new design rule thus allows us to engineer both light-harvesting and interfacial properties in the same dye.

Full text of DOI:10.1039/c3dt53272k

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Engineering of Ru(II) dyes for interfacial
and light-harvesting optimization†
Cite this: DOI: 10.1039/c3dt53272k
Maria Grazia Lobello,a Kuan-Lin Wu,c,d Marri Anil Reddy,b Gabriele Marotta,a
Received 20th November 2013,
Accepted 9th December 2013
Michael Grätzel,c Mohammad K. Nazeeruddin,c Yun Chi,d
DOI: 10.1039/c3dt53272k
Malapaka Chandrasekharam,*b Giuseppe Vitillaroa and Filippo De Angelis*a
www.rsc.org/dalton
A new Ru(II) dye, Ru(L1)(L2) (NCS)2, L1 = (4-(5-hexylthiophen-2-yl)- efficient C106 dye,8 have also been devised, in which one of
4’(4-carboxyl-phenyl 2,2’-bipyridine) and L2 = (4-4’-dicarboxy- the equivalent 2,2′-bipyridine-4,4′-dicarboxylate bipyridine
2,2’-bipyridine), labelled MC112, based on a dissymmetric bipyri- ligands of N3 is replaced by a functionalized bipyridine for
dine ligand for improved interfacial and optical properties, was improved light-harvesting or device stability, see Scheme 1.
synthesized and used in DSCs, yielding photovoltaic efficiencies of This type of dye delivered enhanced photocurrents (Jsc), but
7.6% under standard AM 1.5 sunlight and an excellent device stabi- their open circuit voltage (Voc) was lower than that observed
lity. Increased light harvesting and IPCE maximum were observed with N719-sensitized solar cells,10 eventually leading to com-
with MC112 compared to the prototypical homoleptic N719 dye, parable photovoltaic performance.
due to the functionalized bipyridyne ligand acting as an antenna.
While Jsc depends, among other factors, on the dye’s light-
In addition, the mixed bipyridyne ligand allowed MC112 binding to harvesting capability, the dye can indirectly modulate Voc by
TiO2 to occur via three anchoring carboxylic groups, thus exhibit- reducing recombination and/or by shifting the semiconductor
ing similar interfacial properties to those of the N719 dye. DFT/ conduction band (CB). The enhanced Voc observed for homo-
TDDFT calculations were performed on the new dye, both in solu- leptic dyes was traced by some of us to their adsorption mode
tion and adsorbed on a TiO2 surface model, revealing that the on TiO2, which may take place by up to three carboxylic
peculiar photovoltaic properties of the MC112 dye are related to its groups.11–13 Due to their different adsorption mode, hetero-
anchoring mode. The new design rule thus allows us to engineer leptic dyes may lead to an electrostatic-induced down-shift of
both light-harvesting and interfacial properties in the same dye.
the TiO2 CB energy compared to homoleptic dyes, leading to
reduced photovoltages.11,14,15 Homoleptic dyes may show
enhanced ground state charge transfer to TiO2, again shifting
the TiO2CB.16 Homoleptic dyes are also expected to form a
compact monolayer on the TiO2 surface, inhibiting recombina-
tion to oxidized species in the electrolyte.
1. Introduction
Dye sensitized solar cells (DSCs) represent a promising
approach to the direct conversion of light into electrical energy
at low cost and with high efficiency.1,2 Ru(II) complexes3–5 deli-
vered record photovoltaic efficiencies exceeding 11%.6–8 The
homoleptic N3 and N719 dyes, see Scheme 1, led to significant
advances in DSCs.7,9 Heteroleptic dyes, such as the highly
Altogether, these observations suggest that three carboxylic
anchoring to TiO2 is a possible design rule for high Voc dyes.
Indeed, the homoleptic YE-05 dye, showing the same adsorp-
tion mode as N719, delivered a comparable Voc.17 It is there-
fore highly desirable to design new dyes which combine the
“three anchoring groups” interfacial feature of homoleptic
dyes with the increased light-harvesting capability of heterolep-
tic complexes. In this paper, we report a combined experi-
mental and theoretical study of a new Ru(II) complex based on
a mixed bipyridine ligand, designed to achieve such a combi-
nation of desired properties. The new dye, of formula Ru(L1)-
(L2) (NCS)2, where L1 = (4-(5-hexylthiophen-2-yl)-4′(4-carboxyl-
phenyl 2,2′-bipyridine) and L2 = (4-4′-dicarboxy-2,2′-bipyri-
dine), hereafter MC112, Scheme 1, is based on the (4-(5-hexyl-
thiophen-2-yl)-4′(4-carboxyl-phenyl 2,2′-bipyridine) ligand,
where we introduce on the same bipyridyne ligand a 4-carboxy-
aComputational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM,
Via elce di Sotto 8, I-06213 Perugia, Italy. E-mail: filippo@thch.unipg.it
bInorganic & Physical Chemistry Laboratory, CSIR-Indian Institute of Chemical
Technology, Hyderabad-500007, India. E-mail: chandra@iict.res.in
cLaboratory of Photonics and Interfaces, Institute of Chemical Science and
Engineering, E P F L, CH-1015 Lausanne, Switzerland
dDepartment of Chemistry and Low Carbon Energy Research Center, National Tsing
Hua University, Hsinchu 30013, Taiwan. E-mail: ychi@mx.nthu.edu.tw
†Electronic
supplementary
information
(ESI)
available:
Synthesis,
characterization, device fabrication and computational details. See DOI:
10.1039/c3dt53272k
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Scheme 1 Top: Structure of the prototypical homoleptic and heteroleptic N3 and C106 dyes. Bottom: Structure of the new MC112 dye, obtained
as isomers A and B.
phenyl unit as an additional anchoring group and a hexyl- column chromatography using hexane–ethyl acetate (9 : 1) to
thiophene moiety to increase the dye’s molar extinction co- give 4-substituted-,4′-bromobipyridine.
2.1.1.1. 4-Bromo-4′-(5-hexylthiophen-2-yl)-2,2′-bipyridine (1a).
(55%). 1H NMR (300 MHz, CDCl3, δ): 8.65 (s, 1H), 8.55–8.56
(m, 2H), 8.45–8.47 (d, 1H), 7.38–7.46 (m, 3H), 6.76–6.78
(d, 1H), 2.83 (t, 2H), 1.66–1.76 (m, 2H), 1.29–1.37 (m, 6H), 0.9
(t, 3H); 13C NMR (300 MHz, CDCl3, δ): 157.15, 155.11, 149.73,
149.58, 148.80, 142.87, 133.91, 126.92, 125.61, 125.55, 124.60,
119.79, 116.94, 31.49, 30.34, 29.67, 28.71, 22.51, 14.05, ESI-MS
calcd for C20H21BrN2S 401.0687, found 401.0691.
2.1.2. General procedure for the synthesis of an un-
symmetrically substituted ancillary bipyridine ligand. A 50 mL
Schlenk tube was charged with aryl boronic acid (140 mg,
0.35 mmol) and Pd(PPh3)4 (40.39 mg, 0.035 mmol). Dimethoxy
ethane (8 mL) and 2 M aqueous sodium carbonate (2 mL) were
added, and the tube was purged with argon gas with 5 evacuate/
efficient. While several Ru(II)-heteroleptic complexes based on
two different symmetric bipyridyne have been reported, only a
few Ru(II)-complexes based on dissymmetric bipyridine ligands
were previously reported,18–20,21 but to the best of our knowl-
edge their application in DSCs is limited to a homoleptic
[Ru(L)3]2+ complex.19 A related terpydidine ligand substituted
by a thiophene and a carboxylic group was also reported,22
though the additional carboxylic group was probably not
exploited for dye binding to TiO2.
2. Experimental section
2.1. Synthesis
2.1.1. General procedure for the synthesis of 4-substituted- refill cycles. 4-Substituted-,4′-bromobipyridine (95.5 mg,
4′-bromobipyridine. To a degassed solution of 4,4′-dibromobi- 0.385 mmol) was subsequently added as a neat liquid. The
pyridine (500 mg, 1.592 mmol) in DMF (10 mL) were added tube was sealed and heated at 90 °C vigorously for 18 h. Upon
the corresponding hexylthiophene boronic acid pinacol ester cooling to ambient temperature, the organics were extracted
(1.592 mmol) dissolved in DMF (5 mL), Pd(dppf)2Cl2 (35 mg, into dichloromethane (3 × 30 mL) from 30 mL water. The com-
0.047 mmol), KOAc (1.56 g, 15.92 mmol), and KF (923 mg, bined organics were washed with water (1 × 30 mL) and brine
15.92 mmol) under an argon atmosphere. The reaction (1 × 30 mL), dried over Na2SO4, filtered and the solvent was
mixture was heated at 85 °C for 7 h. The reaction mixture was removed under reduced pressure. The crude product was pre-
filtered off and the solvent was removed under reduced adsorbed onto silica gel and chromatographed (9 : 1 hexane–
pressure. The crude compound was purified by silica gel ethyl acetate) to give unsymmetrically substituted bipyridine.
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2.1.2.1. 4-(2-(4-(5-Hexylthiophen-2-yl)pyridin-2-yl)pyridin-4-yl)- 2.3. Photovoltaic characterization
benzoic acid (1b). (55%). 1H NMR (300 MHz, CDCl3 + CD3OD, δ):
Current–voltage characteristics were recorded by applying an
external potential bias to the cell while recording the generated
photocurrent with a Keithley model 2400 digital source meter.
The light source was a 450 W xenon lamp (Oriel) equipped
with a Schott K113 Tempax sunlight filter (Praezisions Glas &
Optik GmbH) in order to match the emission spectrum of the
lamp to the AM1.5G standard. Incident photon-to-electron
conversion efficiency (IPCE) spectra were recorded with a
Keithley 2400 Source meter (Keithley) as a function of wave-
length under a constant white light bias of approximately
5 mW cm−2 supplied by a white LED array. The excitation
beam coming from a 300 W xenon lamp (ILC Technology) was
focused through a Gemini-180 double monochromator (Jobin
Yvon Ltd) and chopped at approximately 4 Hz.
8.747(s, 1H), 8.523–8.598(m, 3H), 8.194(d, 2H), 7.855(d, 2H),
7.551–7.678(m, 3H), 6.871(d, 1H), 2.881(t, 2H), 1.740(q, 2H),
1.337–1.459(m, 6H), 0.910(t, 3H). 13C NMR (300 MHz,
CDCl3
+
CD3OD, δ): 155.99, 155.77, 149.28, 149.15,
148.85, 148.68, 140.70, 137.72, 130.09, 126.53, 125.69, 125.47,
121.58, 119.36, 119.23, 116.92, 31.20, 30.02, 29.32, 28.39,
22.20, 13.57. ESI-MS calcd for C27H26N2S2O2 442.57, found
443.33.
2.1.3. General procedure for the synthesis of a ruthenium
complex (MC112). A solution of ligand L1 (100 mg,
0.225 mmol) and dichloro(p-cymene)-ruthenium dimer
(69.1 mg, 0.112 mmol) dissolved in dry DMF (100 mL) was
heated at 60 °C for 4 h under a nitrogen atmosphere in the
dark. Subsequently, 4,4′-dicarboxylic acid-2,2′-bipyridine
(55.08 mg, 0.225 mmol) was added and the reaction mixture
was heated to 140 °C for another 4 h. To the resulting dark
green solution was added solid NH4NCS (515.62 mg,
6.772 mmol) and the reaction mixture was further heated for
4 h at 140 °C. After completion of the reaction (monitored by
absorption) the reaction mixture was cooled to room tempera-
ture and the solvent was removed under reduced pressure and
water (200 mL) was added to get the precipitate. The purple
solid was filtered off and washed with distilled water, ether
and dried under vacuum. The crude compound was dissolved
2.4. Cyclic voltammetry
Ered and Eox were measured in DMF solution with 0.1 M
n-Bu4NPF6 as an electrolyte. The scanning rate was 100 mV s−1
.
A Pt wire working electrode and counter electrode were
employed along with a SCE reference electrode.
2.5. Computational details
All the calculations have been performed by the Gaussian 09
program package.23 We optimized the molecular structure of
MC112_xH, with x = 3–0, under vacuum using the B3LYP
exchange–correlation functional24 and a 3-21G* basis set.25
TDDFT calculations of the lowest singlet–singlet excitations
were performed in acetonitrile and in ethanol solution, on the
structure optimized under vacuum and using a DGDZVP basis
set.26 The non-equilibrium version of C-PCM27–29 was
employed for TDDFT calculations, as implemented in G09. To
simulate the optical spectra, the 70 lowest spin-allowed
singlet–singlet transitions were computed on the ground state
geometry. Transition energies and oscillator strengths were
interpolated by a Gaussian convolution with an σ value of 0.14 eV,
corresponding to an FWHM of ∼0.35 eV. We optimized the
geometries of the bare TiO2 models and of the corresponding
dye-adsorbed structures in various configurations using a DZ
basis set and a dispersion-corrected D3-PBE functional,30,31 as
implemented in the ADF code.32 On the optimized geometries,
we performed single-point energy evaluations and time-depen-
dent DFT (TDDFT) excited-state calculations using the B3LYP
functional and a 3-21G* basis set, including solvation effects
by the C-PCM model, as implemented in the Gaussian 09
program package.
in
methanol
and
further
purified
on sephadex
LH-20 methanol as an eluent. The main band was collected
and concentrated to give MC112 (65%).
2.1.3.1. MC112. 1H NMR (300 MHz, CDCl3 + CD3OD, δ):
9.38–9.44(m, 1H), 9.12–9.31(dd, 1H), 8.89(s, 1H), 8.45–8.73(m,
3H), 8.09–8.19(m, 2H), 7.987(d, 2H), 7.87–7.90(m, 1H),
7.71–7.75(m, 2H), 7.43–7.61(m, 2H), 6.84–7.36(m, 3H),
2.74–2.90(dt, 2H), 1.56–1.76(dq, 2H), 1.18–1.38(m, 6H),
0.77–0.86(m, 3H). ESI-MS calcd for C41H34N6O6S3Ru 904.01,
found 903.4444.
2.2. Device fabrication
The cells consisted of a mesoscopic TiO2 film composed of a
10 μm thick transparent layer of 20 nm sized TiO2 anatase
nanoparticles onto which a second 4 μm thick scattering layer
of 400 nm sized TiO2 was superimposed. The double layer film
was heated to 520 °C and sintered for 30 min, then cooled to
80 °C and immersed into the dye solution (0.3 mM) containing
10% DMSO in ethanol for 18 h. The W40 electrolyte contains
0.6 M PMII, 0.03 M I2, 0.05 M LiI, 0.1 M GNCS, 0.5 M TBP in
the mixed solvent of acetonitrile and valeronitrile (85/15, v/v).
The cell was sealed with 25 mm thick transparent Surlyn ring
3. Results and discussion
at 130 °C for 15 s to the counter electrode (FTO glass, 15 Ω per The MC112 dye was characterized by cyclic voltammetry, UV-
square, coated with a platinum solution chemically deposited vis spectroscopy, NMR, HMRS, elemental analysis and FT-IR
at 450 °C for 15 min). The cells were filled with an electrolyte spectroscopy. Due to the asymmetry of the L1 ligand, the Ru(II)-
solution through a predrilled hole in the counter electrode. complex is obtained as a mixture of isomers, A and B in
The hole was then sealed with a Bynel disk and a thin glass to Scheme 1, corresponding to the 4-carboxy-phenyl moiety lying
avoid leakage of the electrolyte.
trans or cis to one of the NCS ligands, respectively, ESI.† This
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can be inferred from the H-NMR in the aliphatic region dye (−0.97 V) is sufficiently more negative than the conduction
showing two distinct signals, ESI,† and it is due to the similar band of nanocrystalline TiO2 electrodes, indicating that the
stability of the two isomers, as calculated by DFT, with A only electron injection process from the excited dye molecule to
0.5 kcal mol−1 more stable than B.
TiO2 conduction band is energetically permitted.
The HRMS measurement provided high resolution accurate
The UV-vis and TDDFT-simulated absorption spectra of
mass of the new ruthenium complex along with elemental MC112 in ethanol solution are reported in Fig. 1. The experi-
composition. The M + 1 peak at 905.0832 and the corres- mental spectrum shows bands at 524, 387 and 307 nm with
ponding elemental analysis of C41H35O6N6RuS3 (905.0818) ε = 18 300, 20 950 and 56 100 mol−1 L cm−1, respectively. Thus
confirm the formation of the desired MC112 dye, ESI.†
the main visible transition has an increased molar extinction
A survey of the electrochemical and optical properties of coefficient compared to N719,33 although with a slightly blue-
MC112 is shown in Table 1, where they are compared to those shifted absorption, which is possibly due to the different dye
of N719 under the same conditions. The cyclic voltammetry protonation, ESI.† The TDDFT-simulated absorption spectrum
and UV-vis spectra are reported in ESI† and in Fig. 1, of the fully deprotonated dyes (average of A and B isomers) is
respectively.
in excellent agreement with the experimental one, Fig. 1, both
The first oxidation potential (Eox) of MC112, measured in in terms of band positions (524, 376 and 313 nm) and molar
DMF solution, is found at 1.02 V. This value is sufficiently extinction coefficient (ε = 16 187, 36 345 and 54 426 mol−1
more positive than the iodine/triiodide redox potential, indi- L cm−1). Our results seem to suggest that the deprotonated dye
cating a thermodynamically favored regeneration of the oxi- species could dominate the solution equilibrium, due to the
dized dye by the redox shuttle. The excited-state oxidation expectedly low pKa of the dye carboxylic groups.34 The highest
potential (E*ox) of the MC112 dye was estimated from the occupied orbitals of MC112 are mainly contributed by the
ground state oxidation potential and the lowest adiabatic tran- Ru-NCS character. A slight mixing of this Ru-NCS states with
sition energy, E0–0, which is obtained from the intersection of the thiophene π framework can be observed, see isodensity
the absorption and emission spectra. The E*ox of the MC112 plots for the representative HOMO and HOMO−2, involved in
the main visible absorption band, in Fig. 1. The LUMO and
LUMO+1 are localized on the two bipyridine ligands, Fig. 1,
with the orbital localization depending on the protonation of
Table 1 Optical and electrochemical properties of the MC112 dye
a
b
c
λmax a [nm]
λmax
Eox
[V]
E0–0
[eV]
the carboxylic groups. The 524 nm band, of mixed MLCT
character, gains additional intensity from the small but sizable
thiophene character in the HOMO−2, Fig. 1, as previously
found for related heteroleptic complexes.35
d
Dye
(ε 104 M−1 cm−1
)
[nm]
E*ox
MC112
N719
524 (1.8)
524 (1.4)
726
760
1.02
0.82
1.99
1.85
−0.97
−1.03
The MC112 dye was tested in DSCs made by a 10 + 4 μm
transparent + scattering TiO2 layer with an iodine-based elec-
trolyte. The TiO2 films were immersed into a 10% DMSO
ethanol dye solution (0.3 mM) for 18 h, with or without 1 : 2
CDCA. The results are reported in Table 2 and in Fig. 2, where
they are compared to the standard N719 dye.
a Absorption and emission spectra were recorded in an ethanol
b
solution at 298 K. Eox measured in DMF with 0.1 M n-Bu4NPF6 as an
electrolyte (scanning rate: 100 mV s−1, working electrode and counter
electrode: Pt wires, and reference electrode: SCE). c The E0–0 was
derived from the intersection of the absorption and emission spectra.
d E*ox was calculated using Eox − E0–0
.
The MC112 dye was very sensitive to the presence of CDCA,
which almost doubled the photovoltaic performances. At 1
sun, MC112-based solar cells delivered a 7.59% efficiency.
Under exactly the same dyeing and fabrication conditions, the
N719 dye exhibited higher performances (8.94%) mainly due
to the increased FF and Jsc. Although the employed conditions
slightly penalize the overall performance of N719, the two dyes
showed the same Voc, consistent with the desired design strat-
egy. The IPCE spectra, Fig. 2, mirror the UV-vis spectra and
show a slightly red-shifted photocurrent onset for N719 com-
pared to MC112, related to the increased Jsc measured for the
latter. MC112 showed, however, an enhanced IPCE in the
absorption maximum region, consistent with the enhanced
molar extinction coefficient of this dye due to the new dissym-
metric bipyridyne ligand.
The same Voc measured for the two dyes is suggestive of
similar interfacial properties. Voc depends statically on the
TiO2 CB energy and dynamically on the charge density accu-
mulated in the semiconductor.16 Thus similar recombination
dynamics and CB energy can lead to the observed Voc. As
Fig. 1 Comparison between the simulated UV-vis spectrum of the
deprotonated MC112 dye (average of the A and B isomers, red line) and
the experimental spectrum (blue line) in ethanol solution. Selected
orbitals for the more stable A isomer are also reported.
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Table 2 Photovoltaic performance of the MC112 (with and without
CDCA) and N719 complexes. The electrolyte composition is 0.6 M PMII,
0.03 M I2, 0.05 M LiI, 0.1 M GNCS, 0.5 M TBP in acetonitrile–valeronitrile
(85/15, v/v)
Dye
JSC [mA cm−2
]
VOC [mV]
FF
η [%]
MC112
MC112 + CDCA
N719 + CDCA
7.16
15.31
17.57
701
665
669
0.770
0.745
0.761
3.86
7.59
8.94
Fig. 3 Evolution of solar cell parameters of MC112 based DSCs
measured under one sunlight-soaking at 60 °C. Electrolyte: 1.0 M DMII,
0.05 M I2, 0.1 M GuNCS, and 0.5 MNBB (N-butyl-1H-benzimidazole) in
butyronitrile.
investigated the MC112 dye adsorption on TiO2. We calculated
the dye interaction with a (TiO2)82 model considering both
three and two carboxylic anchoring, respectively, representative
of isomer A and B, Fig. 4. Geometry optimization followed by
electronic structure calculations in acetonitrile solution (ESI)
showed adsorption of isomer A to be favoured by 5 kcal mol−1
over adsorption of isomer B, suggesting that three carboxylic
anchoring is the more stable adsorption mode. The FT-IR for
the TiO2-adsorbed dye is consistent with a dissociative binding
of the three dye carboxylic groups, see ESI.† Thus, even though
the dye is obtained as a mixture of isomers, TiO2 binding
could preferentially lead to adsorption of isomer A. The TiO2
DOS calculated for adsorbed MC112 isomers A and B, Fig. 3,
shows that isomer A induces a CB energy up-shift of 0.03 eV
compared to B. These data are in line with earlier studies per-
formed for the N719 dye, which showed a CB upshift for dye
binding through three anchoring groups.11
We also notice that, based on the calculated adsorption
structure, MC112 isomer A occupies ∼17% more surface than
N719, in agreement with the measured surface coverage ratio.
The matching of the calculated surface occupations with the
measured coverage ratio further supports the preferential
adsorption mode of isomer A under the employed dyeing con-
ditions (Fig. 5).
Fig. 2 I–V curves of DSCs based on MC112 (magenta) and N719 (light
blue) with 1 : 2 CDCA. Inset: IPCE spectra.
mentioned in the Introduction, a TiO2 CB shift can be origi-
nated by both electrostatic (EL) and charge-transfer (CT)
effects. EL-induced CB shifts are due to the electrostatic poten-
tial generated by the dye on the semiconductor surface, and
are mainly related to the dye dipolar field. CT effects are
related to the dye → semiconductor charge transfer occurring
in the ground state upon formation of the chemical bonds
between the dye and the semiconductor. Notably, a possible
EL and/or CT TiO2 CB shift should scale almost linearly with
the amount of adsorbed dye.16 From the UV-vis spectra of the
dyes on TiO2, ESI,† we can estimate that about 18% more
N719 is adsorbed on TiO2 compared to MC112. Thus, the
similar Voc is not due to a higher MC112 coverage, which may
induce a favourable CB shift, as discussed above.
While we cannot rule out a different recombination behav-
iour of the two dyes, it has to be noticed that usually a more
compact dye monolayer inhibits recombination, though the
long alkyl chains introduced in MC112 could also partly block
recombination with oxidized species in the electrolyte.
To assess the long-term stability of DSCs made by the
MC112 dye, we performed accelerated stability tests at 60 °C,
employing a low-volatility butyronitrile based electrolyte. The
results are reported in Fig. 3, showing that over the 1000 hours
testing period, the photovoltaic parameters declined by 3.8%
from the highest values, which is a remarkable stability.
We can further speculate that the three carboxylic anchor-
ing of isomer A could partly contribute to the high DSCs stabi-
lity observed for MC112, possibly by inhibiting dye desorption
by virtue of the stronger binding to TiO2.
4. Conclusions
To gain insight into the possible MC112 interfacial pro- A new heteroleptic Ru(II) dye based on a dissymmetric functio-
perties, in relation to the observed photovoltaic behaviour, we nalized bipyridine ligand has been computationally designed,
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ruthenium dyes for highly efficient and long durable DSCs.
Further work is in progress to obtain a single dye isomer.
Acknowledgements
The authors thank FP7-ENERGY-2010 (contract 261920) and
the DST project “ESCORT” for financial support.
Notes and references
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Fig. 4 Adsorption geometry of MC112, isomers A and B, on the TiO2
and TiO2 partial density of states in the CB region for the A (red line) and
B (blue line) adsorption modes. Inset: zoom of the CB edge.
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