Tris(thiocyanate) ruthenium(II) sensitizers with functionalized dicarboxyterpyridine for dye-sensitized solar cells

Article abstract of DOI:10.1002/anie.201103515

Panchromatic sensitizers: A series of ruthenium(II) compounds was prepared that contain three thiocyanate ligands and a novel chelating terpyridine ligand. The latter (see picture: colors indicate orbital occupation) serves as an effective and versatile l

Full text of DOI:10.1002/anie.201103515

Communications
DOI: 10.1002/anie.201103515
Dye-Sensitized Solar Cells
Tris(thiocyanate) Ruthenium(II) Sensitizers with Functionalized
Dicarboxyterpyridine for Dye-Sensitized Solar Cells**
Shen-Han Yang, Kuan-Lin Wu, Yun Chi,* Yi-Ming Cheng, and Pi-Tai Chou*
Dedicated to Professor Christian Bruneau on the occasion of his 60th birthday
In recent years, there has been a swift increase of research
activity in ruthenium(II)-based coordination complexes as
effective sensitizers in making cost-effective, third-generation
photovoltaics, namely dye-sensitized solar cells (DSSCs). This
activity is largely due to their lower-lying metal–ligand charge
transfer (MLCT) transition that extends into the red and
near-infrared region of the solar spectrum.[1] Next to the
seminal N3 and N719 ruthenium(II) dyes, the second
generation of more effective system should be credited to
the anionic complex [Bu4N]3[Ru(Htctpy)(NCS)3] (H3tctpy =
4,4’,4“-tricarboxy-2,2’:6,2”-terpyridine), which is known as
the black dye or the N749 dye.[2] Despite its lower absorption
extinction coefficient, certified conversion efficiency of
11.1% was realized by employing a high-haze TiO2 elec-
trode.[3] Efforts have thus been made focusing on the
enhancement of absorptivity by conducting optimization of
molecular design of N749.[4] Parallel to this research, special
attention has been paid to relevant H3tctpy- (or Htctpy)-
containing ruthenium(II) complexes, which can be readily
prepared by substitution of only two thiocyanates with one
bidentate heteroaromatic chelate[5] or by replacing all three
thiocyanates with a single tridentate ancillary.[6] These sensi-
tizers provide access to several respectable dye-sensitized
solar cells (DSSCs), showing conversion efficiencies compa-
rable to the best ruthenium(II) sensitizers documented in
literature.
Scheme 1. Synthetic route to the functionalized RuII terpyridine sensi-
Realizing that only two carboxy groups are enough to
have stable adsorption on the TiO2 electrode,[7] herein we
tizers PRT-11–14; reagents and conditions: (i) Pd(PPh3)4, K2CO3, THF,
(ii) BunLi, SnBun Cl, À788C, (iii) PdCl2, PBut3, CuI, CsF, DMF, 808C,
3
functionalize H3tctpy by reducing the number of carboxy
anchors on terpyridine from three to two. We then strategi-
cally incorporate a highly conjugated p-electron-donating
(iv) RuCl3, ethanol, 908C, (v) [NBun ]NCS, DMF, 1458C, (vi) [N-
4
Bun ]OH, acetone/methanol, 608C, (vii) acidification to pH 2.
4
appendage, such as thiophene, 3,4-ethylenedioxythiophene
(EDOT), or functional triphenylamine, at the third carboxy-
free pyridine moiety to improve the light-harvesting capa-
bility.[8] The proof of concept was then carried out by the
syntheses of respective tris(thiocyanate) ruthenium(II) com-
plexes (PRT-11–14; Scheme 1).
The targeted tridentate ligands L1–L4 demand a key
starting material, namely diethyl 6-bromo-2,2’-bipyridine-
4,4’-dicarboxylate, which is synthesized using a modified
procedure that was originally designed for the preparation of
diethyl 6,6’-dibromo-2,2’-bipyridine-4,4’-dicarboxylate.[9] As
shown in Scheme 1, this diethyl 6-bromo-2,2’-bipyridine-4,4’-
dicarboxylate was then reacted with 2-tributylstannanyl
pyridine to afford the parent chelate L1, that is, diethyl 4,4’-
dicarboxy-2,2’:6,2“-terpyridine by Stille coupling.[10] Three
more 2-tributylstannanyl pyridines were also synthesized
using the rationalized procedures and subsequently applied
for the preparation of thiophene-, EDOT-, and triphenyl-
amine-functionalized chelates L2–L4 (see the Supporting
Information for experimental details). The ruthenium(II)
[*] S.-H. Yang, K.-L. Wu, Prof. Y. Chi, Dr. Y.-M. Cheng
Department of Chemistry, National Tsing Hua University
Hsinchu, Taiwan 30013 (R.O.C.)
E-mail: ychi@mx.nthu.edu.tw
Prof. P.-T. Chou
Department of Chemistry, National Taiwan University
Taipei, Taiwan 10617 (R.O.C.)
E-mail: chop@ntu.edu.tw
[**] This research was supported by National Science Council of Taiwan,
R.O.C.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103515.
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complexes were then obtained from the reaction with
RuCl3·3H2O in ethanol, followed by treatment with tetrabu-
tylammonium thiocyanate for the anion metathesis.[11] The
crude ethoxycarbonyl ruthenium(II) products were then
purified by flash chromatography on silica gel. Finally,
ethoxycarbonyl groups were hydrolyzed in a mixture of
acetone and a 1m solution of tetrabutylammonium hydroxide
in methanol. The resulting panchromatic ruthenium(II)
terpyridine sensitizers PRT-11–14 were precipitated out of
the solution by adjusting the pH to 2, followed by rinsing with
diethyl ether and drying in vacuo. All isolated samples were
characterized by mass spectrometry and H NMR spectros-
copy (see the Supporting Information).
The absorption spectra of these PRT dyes are depicted in
Figure 1, together with that of N749 for a fair comparison.
Their pertinent photophysical and electrochemical properties
are summarized in Table 1. All PRT dyes have an apparent
lowest-lying absorption band at about 600 nm, which is
assigned to the MLCT transition to dicarboxy terpyridine,
as a similar transition was also observed for N749.[2] It is
notable that the intensity of MLCT band of PRT-11 is slightly
lower than that of N749, which is a result of the absence of the
third carboxy auxochrome. This MLCT band however regains
in intensity upon attachment of EDOT in PRT-13 and
especially triphenylamine in PRT-14, leading to a favorable
hyperchromic effect that is plausibly due to the increase of
donor–acceptor coupling and hence the corresponding tran-
sition dipole (see below). Moreover, except for PRT-11, the
rest of sensitizers exhibit an intense absorption peak ranging
from 350–500 nm, and the peak wavelengths are progressively
red-shifted upon increasing the electron donating capability
and elongating the p conjugation of the added appendage.
To gain more insight into their properties, calculations on
structural optimization as well as on the electronic transitions
were then carried out. Their structures were first optimized in
the S0 state with density functional theory (DFT) at B3LYP/
LANL2DZ (Ru) and 6-31G* (H, C, N, O, S) level, followed
by calculations of singlet electronic transitions using the time-
dependent (TD) DFT method. To mimic environmental
perturbation, a polarizable continuum model (PCM) was
applied using dimethylformamide (DMF) as solvent.[12] The
computing methods employed, together with the results of all
other titled dyes, are detailed in the Supporting Information.
The TDDFT results suggest that, apart for PRT-14, the PRT
dyes share almost identical lowest-lying electronic transitions,
of which the spin density propagation is mainly from RuII
dp orbital and occupied orbitals of thiocyanates to the
antibonding orbitals of the dicarboxy terpyridine chelate.
Figure 2a illustrates the calculated electronic transitions of
representative PRT-13. A localized intraligand p–p* transi-
tion appears, as evidenced by the frontier orbital analyses and
high oscillator strength, in the region of 350–480 nm, corre-
sponding to the S5 and/or higher
1
Figure 1. Absorption spectra of N749 and PRT-11–14 sensitizers
recorded in methanol.
excited states. For PRT-14, the
Table 1: Photophysical and electrochemical data of PRT sensitizers and respective DSSC parameters
EDOT and triphenylamine entities
play a more prominent role in the
intermediary transitions than that
of the rest PRT dyes, as indicated by
the greatly red-shifted and more
intense transition peaks. Most inter-
estingly, as showed in Figure 2b, the
lowest-energy transition band for
PRT-14 is dominated by a intrali-
gand p–p* transition with respect-
able oscillator strength (f ꢀ 0.1),
confirming its increased absorptiv-
ity versus the original lower-energy
MLCT. This reveals the key benefit
of adding the coupled EDOT–tri-
phenylamine chromophore.
obtained under global AM 1.5 irradiation.
Dye labs [nm] (e [LmolÀ1 cmÀ1])[a]
lem
[nm][a] [V][b] [V][c]
E8ox ELUMO TiO2
JSC
VOC FF[e]
[mV]
h
[d]
[mAcmÀ2
]
[%]
N749 326 (21647), 339 (22247), 403 820
0.89 À0.77 10+5 13.9
720 0.688 6.89
(9633), 522 (4816), 600 (6766)
15+5 16.8
0.84 À0.85 10+5 13.5
720 0.707 8.54
680 0.733 6.75
PRT- 324 (19530), 393 (6375), 521
11 (4295), 579 (5704)
813
15+5 16.4
0.85 À0.81 10+5 14.7
680 0.722 8.04
710 0.693 7.29
PRT- 310 (13931), 368 (23697), 600 820
12 (3734)
15+5 17.0
0.85 À0.81 10+5 14.9
750 0.715 9.10
710 0.673 7.14
PRT- 316 (18904), 389 (29538), 535 819
13 (3987), 600 (5021)
15+5 19.7
0.86 À0.80 10+5 16.3
760 0.686 10.3
750 0.679 8.29
PRT- 309 (28192), 362 (28770), 475 817
14 (33276), 600 (7394)
Cyclic voltammetry was then
conducted in DMF solution to
verify whether their HOMO and
LUMO energy levels are suitable
for fabrication of DSSCs using a
TiO2 photoanode and iodide-con-
15+5 17.8
720 0.690 8.86
[a] Data were measured in MeOH solution. [b] The oxidation potential (vs. NHE) was measured in DMF
with 0.1m (nBu4N)PF6 and at a scan rate of 20 mVsÀ1. It was calibrated with Fc/Fc+ and converted to the
NHE scale by addition of 0.63 V. [c] ELUMO =Eox-E0À0, for which E0À0 was determined from the intersection
of the absorption and tangent of the emission peak in MeOH. [d] The first and second digits indicated
the thickness of 20 nm and 400 nm nanoporous TiO2 layer in mm, respectively. [e] FF=fill factor.
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Figure 3a,b shows the photocurrent action spectra for the
set of solar cells with TiO2 layer thicknesses of 10 + 5 and 15 +
5 mm. The onset of the IPCE spectra are close to about
900 nm, which is the optimal parameters for anticipated best
DSSCs.[15] For the first set of 10 + 5 mm devices, the PRT-14
dye revealed the highest IPCE of 78% at 500 nm and an
overall device efficiency of 8.29% (see Table 1), which is
obviously due to the greater absorptivity caused by combi-
nation of triphenylamine and EDOT moieties (see above).
Remarkably, upon increasing the TiO2 thickness to 15 + 5 mm,
Figure 2. UV/Vis spectra and spin density plots for the specified
transition of a) PRT-13 and b) PRT-14. The vertical bars are pertinent
optical transitions with an oscillator strength of more than 0.01, for
which occupied and unoccupied orbitals are represented in magenta
and green, respectively.
taining redox electrolyte. As shown in Table 1, all of their
oxidation potentials appeared at about 0.85 V (vs. NHE),
which are marginally lower than that of N749 at 0.89 V and
the I2 /IÀ redox potential (< 0.93 V), but are more positive
À
that of the IÀ/I3 redox couple (ca. 0.4 V vs. NHE).[13] The
À
small variation in Eox of all of the complexes could be
attributed to the reduced influence of the conjugated pendent
of terpyridine on the ruthenium(II) metal dominated oxida-
tion.[14] Lastly, the LUMO energy levels (ca. À0.81 V; see
Table 1), estimated from the difference in the HOMO and
onset of the optical energy gap E0À0, are sufficiently higher
than the conduction band edge of the TiO2 electrode (ca.
À0.5 V vs. NHE). Both electrochemical properties would
predict rapid dye regeneration and efficient electron injection
in current DSSC configurations.
The photovoltaic performance of N749 and PRT-11–14 on
the nanocrystalline TiO2 electrode was studied under stan-
dard AM 1.5 irradiation (100 mWcmÀ2) using electrolytes
composed of 0.6m DMPII (dimethylpropyl imidazolium
iodide), 0.5m tert-butylpyridine, 0.05m I2, and 0.1m LiI in
acetonitrile. The short-circuit photocurrent density JSC, open-
circuit voltage VOC, fill factors FF, and overall cell efficiencies
h for each dye-TiO2 electrode are summarized in Table 1, for
which two TiO2 layers with thicknesses of 10 + 5 and 15 +
5 mm were employed, so that their dye loading dependence on
cell performance can be fairly compared.
Figure 3. a) Incident photon-to-current action (IPCE) spectra of the
10+5 mm devices, b) IPCE spectra of the 15+5 mm devices, and
c) current–voltage characteristics of the 15+5 mm devices sensitized
with N749 and various PRT dyes.
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the PRT-13 dye was the most efficient and showed the best
IPCE of 82% at 420 nm and comparable IPCE values
between 550 and 630 nm, which then produced the highest
JSC of 19.7 mAcmÀ2 among all DSSC devices studied herein.
Figure 3c shows the photocurrent density–voltage curve
of the 15 + 5 mm devices recorded under AM 1.5G simulated
sunlight at a light intensity of 100 mWcmÀ2. In this case, the
N749 cells had JSC = 16.8 mAcmÀ2, VOC = 720 mV, FF = 0.707,
and h = 8.54%. It is notable that, under similar condition,
PRT-13 dye gives JSC = 19.7 mAcmÀ2, VOC = 760 mV, and
FF = 0.686, corresponding to an overall conversion efficiency
of h = 10.3%. Changing from parent PRT-11 to thiophene-
anchored PRT-12 or to EDOT-attached PRT-13 and PRT-14
was found to induce an increase in VOC as large as 50–70 mV,
indicating that the thiophene (or EDOT) unit and the
associated hydrophobic carbon chain, in part, may inhibit
the dark current (see below).
To gain more insight into the cell characteristics, alternat-
ing current (AC) electrochemical impedance spectroscopy
was performed to analyze the effects on charge generation,
transport, and collection. Typical Nyquist plots of DSSCs
fabricated with all PRT cells in the dark under forward bias
(À0.73 V) are shown in the Supporting Information, Fig-
ure S1. Two semicircles from left to right in the Nyquist plot
represent the impedances of the charge transfer (Rct) on the
Pt counter electrode and the charge recombination (Rr) on
the interface of the TiO2/dye/electrolyte. As a result, the
radius of these semicircles reveals a descending order of PRT-
13 > PRT-12 > PRT-14 ꢀ N749 > PRT-11, thus indicating that
the recombination rate increases in the order PRT-13 < PRT-
12 < PRT-14 ꢀ N749 < PRT-11 in the dark. This AC impe-
dance data coincides with the trends of dark current and VOC
values.
We have presented a series of newly developed tris(thio-
cyanate) ruthenium(II) sensitizers that possess a functional-
ized dicarboxy terpyridine chelate. These complexes not only
show better light-harvesting capabilities relative to that
observed for parent N749 in the shorter wavelength region,
but also retain the characteristic MLCT transition at about
600 nm, which is necessary for improving the overall efficien-
cies of DSSCs. Despite the fact that the molecular design still
retains all three thiocyanates, which, by conventional wisdom,
were thought to be the latent photolabile moiety, the cell
stability test using PRT-13 is promising. To test this, evolution
of photovoltaic parameters of PRT-13 was measured under
irradiance of AM 1.5G sunlight during visible-light soaking at
608C (see Supporting Information). The resulting data shown
in Figure S2 reveals good stability. The device efficiency
changed only from the highest recorded value of 7.96% (after
40 h) to 7.12% after 1000 h illumination, that is, a factor of
10.5% decrease. In comparison, N749 was subject to more
significant drop in JSC (15%) and overall efficiency (22.2%)
during the same period of irradiation. Though pending
decisive explanation, one possibility of superior device
stability is due to the replacement of the third carboxy
group by the bulky and hydrophobic pendant, a situation
relevant to the case study between N719 and Z907,[16] which
then prohibits the electrolyte solution to approach the
sensitizer and TiO2 surface, increasing the lifespan of the
solar cells. Along this line, RPT dyes provide exquisite models
for the comparative studies with N749 in terms of photo-
chemistry.
In summary, the successful preparation of sensitizers using
these dicarboxy chelate opens the gateway to a brand new
class of ruthenium(II)-based sensitizers that are attractive for
harvesting solar irradiation up to near IR region. To further
increase the photostability, the replacement of three thiocya-
nates with other tridentate ligands, such as 2,6-bis(5-pyrazo-
lyl)pyridine, may offer[6] further improvement. Research
focused on this is currently in progress in our laboratory.
Received: May 23, 2011
Published online: July 21, 2011
Keywords: dye-sensitized solar cells · N ligands · ruthenium ·
.
thiocyanate · thiophene
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