The function of Chalcogenophene in the Cyclomatelated Ring of the Cycloruthenated Dyes applied in Dye-Sensitized Solar Cell

Article abstract of DOI:10.1021/acs.inorgchem.1c01293

Three thiocyanate-free cycloruthenated complexes, DUY24-O, DUY24, and DUY24-Se containing furan, thiophene, and selenophene, respectively, as a part of the cycloruthenated ring, were designed to reveal the function of the chalcogen atom on the physicochemical and photovoltaic performance of the cycloruthenated sensitizers applied in dye-sensitized solar cells (DSCs). The three sensitizers have a similar molecular size; therefore, the effect of molecular dimensions on their photovoltaic performance can be negligible. NMR data, electron-donating resonance effects, optical properties, and the energy levels of the frontier orbitals reveal that the physical/photovoltaic properties of the three sensitizers were affected significantly by the chalcogen atom on the cyclometalated chalcogenophene ring. The λmax (both in ethanol and adsorbed on TiO2), frontier orbital level, and dye loading of thiophene- and selenophene-containing dyes are very close. Nevertheless, DUY24-Se has a higher molar absorption coefficient compared to DUY24; therefore, the DSC based on DUY24-Se has higher efficiency (8.4% under AM1.5 G one-sun and 26% under T5-light at ca. 6000 lux) than that sensitized with the DUY24 dye. These efficiencies are also higher than those (7.9 and 21.6%, respectively) of the cell dyed with N719, fabricated using the same conditions. The better performance of the device sensitized with DUY24-Se compared to DUY24-based cells suggests that selenophene is as good as (or even better than) thiophene to be a part of the cyclometalated ring for thiocyanate-free cycloruthenated sensitizers applied in DSCs. Furan-containing DUY24-O has much worse photovoltaic performance compared to the other two dyes. This is not only because DUY24-O has the shortest λmax, the lowest molar absorption coefficient, and the highest HOMO level but also the lowest dye loading (because of the strong interaction between the oxygen in furan and TiO2, the array of DUY24-O occupies more surface when adsorbed on TiO2) and the fastest charge recombination. The physicochemical and photovoltaic properties as well as the adsorption behavior of the dye on the TiO2 anode for the cycloruthenated sensitizers affected significantly by the chalcogen atom of the chalcogenophene on the cyclometalated ring provide a new strategy to design high-efficiency NCS-free cyclometalated sensitizers for DSCs.

Full text of DOI:10.1021/acs.inorgchem.1c01293

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Article
The function of Chalcogenophene in the Cyclomatelated Ring of the
Cycloruthenated Dyes applied in Dye-Sensitized Solar Cell
The-Duy Nguyen, Yen-Po Lan, and Chun-Guey Wu*
Cite This: Inorg. Chem. 2021, 60, 11328−11337
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ABSTRACT: Three thiocyanate-free cycloruthenated complexes, DUY24-O, DUY24,
and DUY24-Se containing furan, thiophene, and selenophene, respectively, as a part of
the cycloruthenated ring, were designed to reveal the function of the chalcogen atom on
the physicochemical and photovoltaic performance of the cycloruthenated sensitizers
applied in dye-sensitized solar cells (DSCs). The three sensitizers have a similar
molecular size; therefore, the effect of molecular dimensions on their photovoltaic
performance can be negligible. NMR data, electron-donating resonance effects, optical
properties, and the energy levels of the frontier orbitals reveal that the physical/
photovoltaic properties of the three sensitizers were affected significantly by the
chalcogen atom on the cyclometalated chalcogenophene ring. The λmax (both in ethanol
and adsorbed on TiO2), frontier orbital level, and dye loading of thiophene- and
selenophene-containing dyes are very close. Nevertheless, DUY24-Se has a higher molar
absorption coefficient compared to DUY24; therefore, the DSC based on DUY24-Se has
higher efficiency (8.4% under AM1.5 G one-sun and 26% under T5-light at ca. 6000 lux)
than that sensitized with the DUY24 dye. These efficiencies are also higher than those (7.9 and 21.6%, respectively) of the cell dyed
with N719, fabricated using the same conditions. The better performance of the device sensitized with DUY24-Se compared to
DUY24-based cells suggests that selenophene is as good as (or even better than) thiophene to be a part of the cyclometalated ring
for thiocyanate-free cycloruthenated sensitizers applied in DSCs. Furan-containing DUY24-O has much worse photovoltaic
performance compared to the other two dyes. This is not only because DUY24-O has the shortest λmax, the lowest molar absorption
coefficient, and the highest HOMO level but also the lowest dye loading (because of the strong interaction between the oxygen in
furan and TiO2, the array of DUY24-O occupies more surface when adsorbed on TiO2) and the fastest charge recombination. The
physicochemical and photovoltaic properties as well as the adsorption behavior of the dye on the TiO2 anode for the
cycloruthenated sensitizers affected significantly by the chalcogen atom of the chalcogenophene on the cyclometalated ring provide a
new strategy to design high-efficiency NCS-free cyclometalated sensitizers for DSCs.
Electron injection or dye regeneration should have at least
ca. 200 mV driving force to take place efficiently,5 indicating
that an optimal threshold energy gap of the light-harvesting
sensitizer would be around 1.45 eV (ca. 850 nm). Various
strategies were used to red-shift the absorption bands toward
850 nm and enhance the molar absorption coefficients of the
sensitizers, such as, the designing of push−pull metal-
containing porphyrin dyes6−9 and extending the π-conjugation
length of the ancillary10,11 and/or anchoring ligands.12−16
However, very few high molar extinction coefficient, long λmax
ruthenium sensitizers can achieve efficiencies higher than the
standard dye N719 (11.18%).17,18 This is probably due to the
INTRODUCTION
■
The dye-sensitized solar cell (DSC) developed by O’Regan
1
̈
and Gratzel in 1991 is a promising third generation
photovoltaic technology by virtue of its semitransparency,
flexibility, noteworthy stability, and low manufacturing cost. A
power conversion efficiency (PCE) of 13.6% has been
obtained very recently by Sun2 et al. via minimizing the
energy losses in the process of electron injection. In DSC
operation, charge transfer involves the excitation of an electron
from the highest occupied molecular orbital (HOMO) to the
lowest unoccupied molecular orbital (LUMO) of the dye
molecule and then injection into the conduction band of the
TiO2 mesoporous film. The HOMO of the oxidized dye
becomes a single occupied molecular orbital (SOMO) which
involves an α-highest occupied spin orbital (α-HOSO) and a
β-lowest unoccupied spin orbital (β-LUSO). Dye regeneration
can be regarded as the transfer of one electron from the
reductant in the electrolyte (i.e. iodide ion) to the β-LUSO of
the oxidized dye.3,4
Received: April 28, 2021
Published: July 19, 2021
https://doi.org/10.1021/acs.inorgchem.1c01293
© 2021 American Chemical Society
Inorg. Chem. 2021, 60, 11328−11337
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Figure 1. Structures of DUY24-O, DUY24, and DUY24-Se sensitizers (TBA: tetrabutylammonium).
extension of π-conjugation increasing the molecular size or
making the molecule less symmetric, which decreases dye
loading (resulting in low Jsc of the device) or may not be able
to cover the TiO2 surface fully, leading to charge
recombination at the TiO2/electrolyte interface, resulting in
low Jsc and Voc of the cell. Therefore, red-shifting the
absorption and increasing the light-harvesting ability without
enlarging the size or changing the structure of the dye
molecule is another strategy for searching high-efficiency
sensitizers for DSCs.
Thiophene is the most common heterocyclic moiety used to
extend the conjugation length of a sensitizer. However,
O’Regan et al.19 found that when the oxygen in the terminal
alkoxyl group in K19 dye is changed to softer sulfur (to be
TG6 dye), the VOC of the cell decreased. They proposed that
the softer sulfur on the thiophene ring has higher attraction
toward the triiodide ion in the electrolyte, promoting charge
the substitute in the bipyridine ancillary ligand also shows
higher absorption coefficient and lower MLCT energy
compared to its thiophene analogue.30 In this work, we
replaced the thiophene ring in DUY24 with a softer
selenophene (DUY24-Se) or a harder furan (DUY24-O)
analogue to prepare the other two NCS-free cycloruthenated
dyes with a molecular size similar to DUY24. The
physicochemical and photovoltaic properties of these three
cycloruthenated dyes were studied to unravel the functions of
the chalcogen on the chalcogenophene-containing cyclo-
ruthenated sensitizers to be used in DSCs.
EXPERIMENTAL SECTION
■
Materials and measurements. All reagents were obtained from
commercial resources and used as received unless specified. Nuclear
magnetic resonance (NMR) spectra were recorded on a Bruker 300
or 500 MHz NMR spectrometer. High-resolution mass spectrometry
(HRMS) was performed by using JMS-700 HRMS. Elemental
analyses (EA) were carried out with a PerkinElmer 2400 CHNS/O
analyzer. UV−vis spectra of the sensitizers dissolved in ethanol or
absorbed on the TiO2 film were measured using a Cary 300 Bio
spectrometer. Voltammetric measurements were performed in a
single-compartment, three-electrode cell with a platinum wire counter
electrode and a platinum disk working electrode. The reference
electrode is Ag+/Ag, and the supporting electrolyte is 0.1 M
tetrabutylammonium perchlorate in ethanol. Transient absorption
spectroscopy (TAS) was performed using a nanosecond kinetic
spectrometer (PROTEUS, Ultrafast Systems, USA) system at room
temperature (∼25 °C). The architecture of the cells for TAS
measurements is the same as that for measuring the cell efficiency,
except no TiO2 scattering layer was used. The excitation and probe
wavelengths are 480 and 820 nm, respectively. The square-wave
voltammograms (potential step increment: 5 mV; frequency: 25 Hz)
were recorded using a potentiostat/galvanostat (PGSTAT 30,
Autolab, Eco-Chemie, the Netherlands), and ferrocene was used as
a calibration internal standard. Electrochemical impedance spectros-
copy (EIS) was carried out in an Eco Chemie Autolab PGSTAT30
potentiostat/galvanostat at 50 mV voltage steps with a sinusoidal
potential perturbation of 10 mV in the dark and under 1 sun
illumination, following the model proposed by Longo et al.31 All the
EIS measurements and relative calculations can be found in our
previous papers.14,32
recombination, causing a deleterious effect on the device’s
19−22
VOC
.
Nevertheless, a softer atom may have a stronger
attraction toward I− ions (due to the soft−soft interaction) in
the electrolyte, enhancing dye regeneration to increase the Jsc
of the cell,23 although this phenomenon was not observed in
the case of K19 and TG6 dyes reported by O’Regan.20 We did
DFT calculations of the oxidized K19 and TG6 dyes (the
results are displayed in Figure S1, Supporting Information) and
found that there is no β-LUSO character on the oxygen or
sulfur atoms in [K19]+ or [TG6]+. The DFT results suggest
that the heteroatoms (O and S) in the ancillary ligand of K19
and TG6 are not involved in the dye regeneration process (no
β-LUSO distribution). Therefore, the stronger interaction of
the sulfur with the triiodide ion could be the major reason that
the TG6 cell has lower Voc compared to the K19-sensitized
device.
On the other hand, NCS-free cycloruthenated sensitizers
have the unique advantage (compared to NCS-containing
ruthenium dye) of being able to acutely control the HOMO
energy through the modification of the substituents on the
anionic cyclometalated ring and are more stable due to the lack
of monodentate NCS ligands.24−26 Recently, we have reported
a good-efficiency NCS-free cycloruthenated dye (DUY24)
with thiophene as a part of the cyclometalated ring in the
ancillary ligand.21 It was also shown that an organic dye
containing a selenophene π-bridge has higher absorption
coefficient and longer λmax compared to that containing the
thiophene ring.27−29 NCS-containing ruthenium (not the
NCS-free cyclometalated structure) dye with selenophene as
Synthesis of the Target Sensitizers. The new complexes
DUY24-O and DUY24-Se were synthesized by a cyclometalation
reaction similar to the procedure for preparing DUY24 reported
previously.21 The detailed synthesis steps and structure character-
ization can be found in Scheme S1, Supporting Information.
Density Functional Theory Calculation. All computational
calculations were performed using the Gaussian 09 program similar to
our previous report.22 For ground-state geometry optimization, the
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1
Figure 2. H NMR spectra of DUY24-O, DUY24, and DUY24-Se.
Figure 3. Three possible resonance structures of each chalcogenophene-containing cycloruthenated sensitizer.
B3LYP (Becke, three-parameter, Lee−Yang−Parr) hybrid func-
tional33,34 was used. The LanL2DZ effective core potential35 was
used for the ruthenium atom, and the split-valence 6-31G(d,p) basis
set36 was applied for the hydrogen, sulfur, carbon, oxygen, nitrogen,
and selenium atoms. The time dependent-DFT (TD-DFT) excited
state calculation of the lowest singlet−singlet electronic transitions
was also performed using the same basic sets. The solvent effects were
included in the calculations using the conductor-like polarizable
continuum model (C-PCM) similar to what we reported before.22
Device Fabrication and Photovoltaic Performance Charac-
terization. The preparation of TiO2 paste, fabrication of the TiO2
anode, dye loading, assembly of the cell, and photovoltaic parameter
measurements are similar to what we reported previously14 unless
specified. For photovoltaic characterization under indoor lighting, the
T5 (ADATA Inc, Taiwan, AF-TBT5-2-14W65) or LED (INNOTEC
Inc. Taiwan, LUX-T14401AL/NT8) light source was used. The setup
and process for measuring the photovoltaic performance of the DSCs
under room lighting are the same as what we reported recently.37 The
dye loading of the devices was determined as the following. A
(concentration vs absorption intensity at λmax) calibration curve of the
dye solutions [1 × 10−6 ∼ 2 × 10−5 M in 20 wt %
tetrabutylammonium hydroxide/CH3OH (TBAOH/MeOH)] was
build first. The dye molecules were then desorbed from the practical
TiO2 photoanode with 5 mL of 20 wt % TBAOH/MeOH, then, the
absorption intensity at λmax was measured. The concentration of the
desorbed dyes [therefore the number (in mole) of the dye molecules]
was determined by the interpolation of the calibration curve, similar
to our previous reports.10,12,21,22
RESULTS AND DISCUSSION
■
Structural Characterization. The structures of DUY24-O
and DUY24-Se as well as DUY2422 dyes are displayed in
Figure 1. These sensitizers exist as zwitterions containing one
free carboxylate (−COO−) and three n-tetrabutylammonium
carboxylate (−COONBu4) in the two anchoring ligands. The
+
presence of NBu4 cations makes the sensitizers soluble in
many common solvents (such as CH2Cl2, CHCl3, EtOAc,
dioxane, MeCN, DMF, DMSO, acetone, EtOH, MeOH, and
H2O), which facilitates the preparation of dye solutions for
various spectroscopic measurements and device fabrication.
1
Figure 2 displays the aromatic region of the H NMR spectra
for the three sensitizers (the detail peak assignment can be
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Figure 4. UV−Vis absorption spectra of DUY24-O, DUY24, and DUY24-Se. (A) Dissolved in EtOH and (B) anchored on transparent TiO2 thin
films.
Table 1. Optical and Electronic Properties of DUY24-Series Sensitizers
dye
λmax (nm), in EtOH/on TiO2
ε (M−1 cm−1) at λmax, in EtOH
HOMO (V vs NHE)
Eg (V)
LUMO (V vs NHE)
DUY24-O
DUY24
DUY24-Se
549/500
556/575
557/575
13,875
16,800
18,000
0.68
0.80
0.83
1.99
2.00
2.01
−1.31
−1.20
−1.18
1
found in Figure S2, Supporting Information), in which the red-
marked sharp singlets were assigned to the protons on the
cycloruthenated chalcogenophene segments (named ortho
(3) compared to DUY24 and DUY24-Se. The H NMR data
suggest that the resonance effect is more important than the
inductive effect for the three chalcogenophenes. In general, the
HOMOs of high-efficiency Ru-based dye primarily locate on
the ruthenium center;21 therefore, the gathering of electrons
on the cycloruthenated carbons indicates that the resonance
effect of the chalcogens has more influence on the molecular
properties compared to the inductive effect. Therefore, one can
expect that the physical and photovoltaic properties of these
three cycloruthenated sensitizers were affected more signifi-
cantly by the chalcogen atom on the cyclometalated ring
compared to that on the other site of the ancillary ligand such
as K19 and TG6 dyes.19
1
proton). The H NMR data for DUY24-Se are quite similar
to those of DUY24 [the difference in the chemical shift (Δδ)
for related signals is less than 0.1 ppm], while the NMR data
for DUY24-O are very different from those of DUY24. This
result indicates that S and Se have a similar effect on the
electronic properties of the molecule but they are much
different from O. Chemical shifts of the ortho protons indicate
that the electron densities around the protons on the
chalcogenophene segments are in a descending order of
DUY24-O ≫ DUY24 > DUY24-Se, which can be explained by
the difference in the distribution of the three resonance
structures of chalcogenophene displayed in Figure 3.
The three possible resonance structures of chalcogenophene
displayed in Figure 3 show that the distribution of the three
structures depends on the inductive effect of the chalcogen and
the strength of the resonance effect. The inductive effect
suggests that DUY24-O has its structure more toward structure
2 compared to DUY24 and DUY24-Se because oxygen with
high electronegativity38 does not favor a positive charge on it.
On the other hand, the strength of the resonance effect
depends on the overlap between the lone-pair electrons in the
p-orbital (2p for O, 3p for S, and 4p for Se) of the chalcogens
and the 2p orbitals of the carbon-based π-system.39 The
efficiency of the resonance effect of the chalcogens is in the
descending order of O > S ≫ Se because the orbital at higher
(>2) principle quantum number is more diffuse and has higher
energy, therefore having less overlap with 2p orbitals of the
carbon-based π-system. Consequently, the structure of
DUY24-O moves more toward structure (1) and (3)
compared to that of DUY24 and DUY24-Se. In other words,
the electron densities at the cycloruthenated carbon and the
hydrogenated carbon are much higher for DUY24-O than
those for DUY24 and DUY24-Se. The chemical shift of the
ortho protons in DUY24-O has a significant up-field shift
which supports that DUY24-O moves more toward structure
Theoretical Calculation of the Frontier Orbital
Distributions. The distribution of the first 10 DFT-calculated
frontier orbitals of DUY24-O and DUY24-Se (displayed in
Figures S3 and S4, Supporting Information) is very similar to
those of DUY24 we reported previously.21 The HOMOs of all
dyes distribute primarily on both the ruthenium center and the
cycloruthenated ligands while the LUMOs virtually locate on
the anchoring ligands, indicating that the lowest-energy
absorption band arise from the M/LLCT (mixed metal/ligand
to anchoring ligand charge transfer) transitions. The significant
contribution of the cyclometalated chalcogenophene segments
to the HOMOs implies that the HOMO energy levels are very
sensitive to these segments which can be modified effectively
by using a different atom or suitable substituent(s) on the
segments. One can expect that the atoms in the chalcogeno-
phene ring can affect significantly the electronic properties of
the corresponding sensitizer, which will be discussed in detail
in the later paragraphs. The unoccupied molecular orbitals
(except LUMO+4 mainly contributing from the cyclometalat-
ing ligands) distributed on the anchoring ligands can provide
an efficient pathway for electron injection from the excited
dyes into the TiO2 conduction band. Noticeably, the 5-
hexylthiophen-2-yl substituent on the pyridine ring of the
cycloruthenated ligands contributes insignificantly to all
HOMOs and LUMOs (except LUMO+4), suggesting that
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Figure 5. (A) Square-wave voltammograms and (B) frontier orbital energy level diagram of DUY24-series sensitizers. [HOMO energy levels
(EHOMO) were obtained from the square-wave voltammograms. LUMO energy levels (ELUMO) were calculated by the formula of ELUMO = EHOMO
−
Egap, where Egap (energy gap) values are calculated from the onsets of the absorption spectra illustrated in Figure 4A].
modifying the substituent on the pyridine of the cyclo-
ruthenated ligand to change the molecular, optical, and
electronic properties is not as efficient as altering the
cyclometalated chalcogenophene segments.
carboxylate group and Ti4+ ion (acting as a Lewis acid).41,42
The blue-shift of the adsorbed DUY24-O may be due to an
interaction between the highly electronegative oxygen atom in
the furan of the cyclometalated ligand and TiO2 or inter/intra-
dye molecular interaction causing a change in the electronic
structure or a serious molecular aggregation when anchored on
TiO2. Furthermore, DUY24 and DUY24-Se adsorbed on TiO2
films exhibit two distinct absorption bands. DUY24-O-coated
TiO2 films show two indistinguishable absorption bands
centered at 490 nm which results from the overlap of a few
electronic transitions at wavelengths between 450−600 nm,
even though the absorption profile of DUY24-O is similar to
those of DUY24 and DUY24-Se in ethanol (see Figure 4A).
The absorption profiles of the anchored DUY24-O dyes
suggest that there is a complicated interaction with TiO2 inter/
intramolecularly due to the presence of the highly electro-
negative oxygen in furan, which will be discussed more in the
later paragraphs. The electronic properties are similar between
selenophene- and thiophene-containing dyes but obviously
differ from the furan-containing sensitizer even though the
chalcogen atom is just a very small part of the whole molecule.
The result suggests that the design of the dye for the DSC can
be as subtle as altering an atom on a single position in a metal
complex.
Optical Properties. The UV−Vis absorption spectra of
DUY24-O, DUY24, and DUY24-Se in ethanolic solution are
shown in Figure 4A, and the related data are listed in Table 1.
The profiles of their absorption spectra did not show much
difference. Molar absorption coefficients of the three dyes
increased slightly with an increase in the atomic number of the
chalcogen at most of the wavelengths due to the flexibility of
lone pair electrons on the heavy chalcogen atom. Furthermore,
the flexibility of the lone pair electrons also contributes to the
slight red-shift of the lowest-energy absorption band (M/
LLCT band, see Table 1) when the atomic number of the
chalcogens increased. Similar effects were observed when the
chalcogenophenes were used as π-conjugation substituents on
the ancillary bipyridine ligands for thiocyanate ruthenium
sensitizers.30 The absorption profiles of all sensitizers are quite
broad and complicated due to the low symmetric molecular
structure which results in relatively complicated orbital
splitting and increases the number of allowed electronic
transitions.40 All sensitizers show broad absorption bands at
longer wavelengths (350−600 nm) primarily from the mixed
metal/ligand to ligand (mainly anchoring ligand) charge-
transfer transitions.21
Electrochemical Properties. The square-wave voltammo-
grams (SWV) as well as the HOMO (EHOMO, obtained from
The absorption spectra of DUY24-O, DUY24, and DUY24-
Se anchored on thin TiO2 films are shown in Figure 4B. The
absorption profiles of the dyes adsorbed on TiO2 are quite
different from those of the dyes in EtOH, and the absorption
intensity of adsorbed DUY24 and DUY24-Se is stronger than
that of the adsorbed DUY24-O at the wavelength of 450−600
nm. Moreover, the absorption maxima of the M/LLCT band
for DUY24 (575 nm) and DUY24-Se (575 nm) adsorbed on
the TiO2 film red-shift slightly compared to those (556 and
557 nm, respectively) in EtOH, whereas DUY24-O adsorbed
on the TiO2 film shows a blue-shift (compared to that in
EtOH) in the absorption spectrum. For a ruthenium sensitizer
containing no acidic proton, the red-shift of the absorption
spectrum when adsorbed on TiO2, compared to that in
solution, could be attributed to the change of the conformation
of the dye molecules that occurs or the interaction between the
the SWV) and LUMO energy levels (ELUMO = EHOMO − Egap
,
E
gap, energy gaps determined from the absorption onset of the
absorption spectra illustrated in Figure 4A) of DUY24-O,
DUY24, and DUY24-Se are illustrated in Figure 5. The
HOMO levels of DUY24 and DUY24-Se dyes are 0.80 and
0.83 V (vs NHE), respectively, which are more positive than
the threshold level (∼0.79 V vs NHE) for sufficient dye
regeneration by the iodide electrolyte.43 Nevertheless, the
HOMO of DUY24-O is 0.68 V (vs NHE), not low enough to
ensure the thermodynamic driving force for dye regeneration
by the iodide electrolyte. Therefore, the oxidized DUY24-O
was expected to be more sluggish in dye regeneration
compared to the oxidized DUY24 and DUY24-Se dyes. The
LUMO energy levels of DUY24-O, DUY24, and DUY24-Se
are −131, −1.19, and −1.18 V (vs NHE), respectively, which
are at least 680 mV higher than the TiO2 conduction band
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a
Table 2. Photovoltaic Parameter and Interface Resistance of DUY24-Series-Sensitized Cells
b
b
b
c
Dye
JSC (mA/cm2)
VOC (V)
FF (%)
ηmax (%)
average η (%)
dye-loading (mole/cell)
DUY24-O
DUY24
DUY24-Se
N719
8.10
16.93
19.04
16.41
0.556
0.625
0.634
0.727
68
71
70
67
3.06
7.51
8.45
7.99
3.01 0.04
7.48 0.05
8.35 0.07
7.95 0.03
4.7 × 10−8
6.8 × 10−8
6.8 × 10−8
a
Cell fabrication parameters: dye: 0.2 mM (DUY24-Se/DUY24/DUY24-O) in ACN/IPA/DMSO (V/V/V = 1/1/3) and 40 mM CDCA; TiO2:
b
absorb layer 3; scattering layer 2, total 18 μm thick; electrolyte: 0.3 M LiI + 1.0 M BMII + 30 mM I2 + 0.8 M TBP in ACN. Jsc, Voc, and FF listed
in Table 2 are the photovoltaic parameters for the cell with maximum power conversion efficiency (ηmax). η is the average power conversion
c
efficiency of six cells.
edge (ECB, −0.5 V vs NHE). The energy difference provides an
ample driving force for efficient electron injection.4 The
resonance structures illustrated in Figure 3 showed that the
distribution of the three resonance structures depends on the
overlap between the lone pairs in p-orbitals (2p for O, 3p for S,
and 4p for Se) of the chalcogens and the 2p orbitals of the
carbon-based π-system. In general, orbitals with a closer energy
level overlap more efficiently. Therefore, the electron-donating
ability (toward the metal center) of the chalcogenophenes is in
a descending order of furan ≫ thiophene > selenophene. In
other words, the electron-donating resonance effect causing
DUY24-O moves more toward structure (1) compared to
DUY24 and DUY24-Se and raises the HOMO energy levels of
the sensitizers as shown in Figure 5B. The electrochemical data
prove that the energy of the frontier orbitals is affected
effectively by the resonance effect more than the inductive
effect of the cyclometalated ligand, consistent with the
though these three dyes have a similar molecular size. This may
be due to the difference in the array of the dyes adsorbed on
TiO2. There may also be a strong inter/intramolecular
hydrogen bonding of DUY24-O, causing serious aggregation;
therefore, dye molecules may not able to penetrate into the
bottom of the TiO2 film. The blue-shift of DUY24-O versus the
red-shift of DUY24 and DUY24-Se after adsorbing on the
TiO2 surface (see Figure 4) is another indication that the
behavior of DUY24-O anchored on TiO2 is different from that
of DUY24 and DUY24-Se. The incident photon-to-current
efficiency (IPCE) spectra of DUY24-O-, DUY24-, and
DUY24-Se-sensitized devices illustrated in Figure S5B are
consistent with the JSC values obtained from the I−V curves.
The IPCE value of DUY24-O is limited in the spectral region
from 370 to 750 nm, reaching its maximum of ∼30% at 500
nm. IPCE profiles of DUY24-Se and DUY24-based cells are
quite similar (both IPCE values extend to 800 nm) except the
IPCE values reach the maxima (at ∼580 nm) of ∼71 and 66%,
respectively.
Interfacial Charge Transfer. The dynamics of the
interfacial charge-transfer processes within the devices was
probed with electrochemical impedance (EIS) measurements.
EIS spectra (Nyquist plots) of the devices obtained under
illumination and in the dark are displayed in Figure S6,
Supporting Information. The series (Rs) and interface (R2)
resistances, typically representing the resistance of the whole
device and TiO2/dye/electrolyte interfacial resistance, respec-
tively,44 are tabulated in Table 3. The series resistances (Rs) of
1
chemical shift assignments of the H NMR spectra displayed
in Figure 2. Interestingly, the frontier orbital distribution for
the three sensitizers is very similar (see Figures S3 and S4,
Supporting Information and the data in ref 22) in which the
LUMO distributed mainly on the anchoring ligand which may
not be affected significantly by the chalcogenophene in the
ancillary ligand. Nevertheless, compared to the other two dyes,
furan-containing DUY24-O has larger portion of equilibrium
structure 1 (see Figure 3) and makes the Ru metal center more
electron-rich. Therefore, the metal center-anchoring ligand
bonding destabilized, resulting in raising both the HOMO and
LUMO energy levels.
Photovoltaic Performance under Standard 1 Sun
Lighting. The current density−voltage characteristics (J−V)
of the three dyes as well as N719 sensitized cells under an
irradiance of AM 1.5G full sunlight are displayed in Figure
S5A, Supporting Information, and the corresponding photo-
voltaic parameters as well as dye loading are summarized in
Table 2. The power conversion efficiencies (η) of DUY24-O,
DUY24, and DUY24-Se dyed cells are 3.06, 7.51, and 8.35%,
respectively. At the same device fabrication parameters, the
N719-sensitized cell exhibits a η of 7.95%. The Voc, FF, and
dye loading (similar molecular size) of DUY24-Se sensitized
cells are close to those of the DUY24-based cell; therefore, the
difference in the efficiency is mainly from the Jsc values, which
is due to DUY24-Se having higher absorption coefficient than
DUY24. The much lower efficiency (3.06%) of the cell based
on DUY24-O compared to those (7.51 and 8.35%) of DUY24
and DUY24-Se dyed cells could be because of several reasons.
DUY24-O has a lower absorption coefficient, more narrow
absorption profile, smaller dye loading, blue-shift in absorption
when adsorbed on TiO2, and higher HOMO level, resulting in
much lower Jsc of the corresponding cell. Surprisingly, DUY24-
O has only a 67% dye loading of DUY24 and DUY24-Se even
Table 3. Series Resistances (Rs) and TiO2/Dye/Electrolyte
Interfacial Resistances (R2) of DUY24-Series-Sensitized
DSCs Obtained From the EIS Measurements Under One-
Sun Illumination and in the Dark
Rs (Ω) in
R2 (Ω) 1
R2 (Ω) in
Dye
Rs (Ω) 1 sun
dark
sun
dark
DUY24-O
DUY24
DUY24-Se
17.7
17.8
18.0
16.3
17.0
16.6
22.7
13.3
12.2
127
156
181
all dye-sensitized cells are very close to each other both in the
dark and under illumination, suggesting that dye molecules did
not affect significantly the resistances of the DSCs and the cell
fabrication processes are reproducible. Under illumination of 1
sun light, the R2 trend is DUY24-Se < DUY24 ≪ DUY24-O,
consistent with the trend of JSC listed in Table 2. The
significantly high R2 value of the DUY24-O-dyed cell is due to
the low dye absorption coefficient and low dye loading and
therefore low photocurrent under illumination. In the dark, the
order of the R2 (related to the coverage of the TiO2 surface)
values is consistent with the Voc of the cells listed in Table 2.
The dye loading of DUY24-Se and DUY24 is very close. The
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Figure 6. Efficiency of DUY24-series and N719-sensitized cells under room lighting at various intensity. (A) T5 (B) LED.
slightly higher TiO2/dye/electrolyte interface resistance of the
DUY24-Se-dyed cell (compared to the DUY24 based device)
maybe due to the low interaction inter/intramolecularly or
between Se and TiO2, therefore covering the TiO2 surface well.
To unravel the poor photovoltaic performance of DUY24-O,
several studies were carried out. The UV−Vis absorption
spectra of the dye-adsorbed TiO2 at various dye-loading times
were taken (and displayed in Figure S7, Supporting
Information) to see the degree of dye aggregation when
adsorbed on TiO2. The intensity of the absorption for the
three dyes on TiO2 increased as the dye-loading time
increased; nevertheless, the profile did not change. The blue-
shift behavior of DUY24-O-adsorbed TiO2 was observed even
at very low dye loading suggesting that the physical properties
of DUY24-O change as soon as it is adsorbed on TiO2.
Therefore, the aggregation of DUY24-O dye molecules when
adsorbed on TiO2 is unlikely. This is probably due to the
strong interaction between the oxygen in furan with TiO2 (as
revealed in Figure S8, Supporting Information) to change the
electronic property of adsorbed DUY24-O. The array of the
sensitizers (see Figure S8a, Supporting Information) also
explains why DUY24-O has lower dye loading since it occupies
more TiO2 surface compared to DUY24 and DUY24-Se
(Figure S8b, Supporting Information). Furthermore, there
exists a possibility that DUY24-O decomposes when anchored
on TiO2. Furan groups are sensitive to reversible hydrolysis
reactions, and it is conceivable that TiO2 could catalyze these
reactions. To prove that DUY24-O was stable on TiO2, the
absorption spectra of DUY24-O dissolved in 20 wt %
tetrabutyl ammonium hydroxide/methanol solution
(TBAOH/MeOH) and that desorbed from the TiO2 photo-
anode with 20 wt % TBAOH/MeOH were measured and are
illustrated in Figure S9a, Supporting Information. The
similarity in the absorption patterns of the two solutions
suggests that furan in DUY24-O did not react with TiO2 and/
or decompose. The photostability of DUY24-O on TiO2 was
also tested by exposing the DUY24-O-stained TiO2 under AM
1.5G one-sun light for 10 mins. The absorption spectrum is
very similar to the as-prepared sample (see Figure S9b). It
seems that DUY24-O is also stable under one-sun irradiation at
least for 10 mins.
partially due to the slow dye regeneration as well as the low
absorption coefficient and dye loading. To reveal the
regeneration kinetics of the oxidized dye, transient absorption
decay of the cells dyed with the three sensitizers with/without
−
(I2/I3 ) a redox couple was observed and is shown in Figure
S10, Supporting Information. The TAS traces were fitted by a
stretched exponential decay function to afford the half-life time
(τx). Ruling out the photodecomposition of the dye molecules,
in the absence of the electrolyte, the decay of the absorption
signal reflects the dynamics of charge recombination of the
injected electrons (in TiO2) with the oxidized dye (S+). On the
other hand, both dye regeneration by the electrolyte and
charge recombination can be revealed from the TAS decay
traces taken when the cell contains the iodide redox
couple.45,46 As expected, without the electrolyte, the τ1 value
of the DUY24-O-sensitized cell is smaller than those of
DUY24 and DUY24-Se dyed cells due to the lower dye loading
and therefore worse TiO2 coverage causing more serious
charge recombination between the injected electrons in TiO2
and the oxidized dye (Figure S10a, Supporting Information).
Surprisingly, with the electrolyte, both the dye regeneration
and charge recombination rates of the DUY24-O dyed cell are
all faster than those of DUY24 and DUY24 Se-based cells
(Figure S10b, Supporting Information).
To understand why DUY24-O with a higher HOMO level
has faster dye regeneration dynamics, DFT calculations were
also performed on the oxidized DUY24-O, DUY24, and
DUY24-Se to show the β-LUSO distributions (see Figure S11,
Supporting Information) which are the sites on the oxidized
dyes that interact directly with the electron donor.4 The β-
LUSO of all oxidized sensitizers mainly delocalizes on the
ruthenium center and the cyclometalated chalcogenophene
segments. Due to the nonexistence of the seven-coordinated
ruthenium center and dye regeneration which involves the
approaching of I− to the β-LUSO site,47 the β-LUSO
distribution on the ligands plays a major role in dye+−I−
interaction and therefore the dye-regeneration efficiency.48
The small τ1 value displayed in Figure S10b, Supporting
Information, may come from the dye regeneration by both the
iodine electrolyte and back charge transfer from TiO2 via
anchoring ligand and oxygen in furan. Certainly, lowering the
HOMO level of DUY24-O upon adsorption on TiO2 to
facilitate dye regeneration needs to be considered. Never-
theless, the small Jsc could rule out the small τ1 value of the
DUY24-O dyed cell which is due to fast dye regeneration by
the electrolyte because of the lowering of the DUY24-O
HOMO when adsorbed on TiO2. TAS data suggest that the
Data in Figure 5 show that the HOMO level (0.68 V vs
NHE, estimated from the dye in EtOH solution) of DUY24-O
is slightly negative to the threshold level (∼0.79 V vs NHE) for
sufficient dye regeneration by the iodide electrolyte.43 The
much lower Jsc (compared to DUY24 and DUY24-Se-based
cells, see Table 2) for DUY24-O-dyed cells was expected to be
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fast charge recombination of the DUY24-O-dyed cell most
probably is the reason for the poor photovoltaic performance.
Photovoltaic Performance under Room Lighting. The
irradiation spectra of indoor lighting are very different to the
standard 1 sun irradiance. The photons of the indoor lighting
are mainly visible light (the irradiation spectra of fluorescent
lamp T5 and LED are illustrated in Figure S12, Supporting
Information), whereas a full solar irradiation has ca. 45% NIR
and IR photons.49 The DSC sensitized with a ruthenium-based
dye generally has large IPCE values at 400−700 nm, which is
very suitable to be driven by the indoor light to produce
electricity. The absorption spectra of DUY24 series dyes
immobilized on TiO2 (Figure 4B) have good overlap with the
emission spectra of T5 and LED lights. One can expect that
the cells based on the three dyes will have higher efficiency
under room lighting compared to that under standard 1 sun
illumination. The efficiencies of the DUY24-series- and N719-
sensitized cells under T5 and LED light illumination at various
intensities are illustrated in Figure 6 (the detailed photovoltaic
parameters are listed in Tables S1−S6, Supporting Informa-
tion). Cells that have the highest efficiency under standard 1
sun illumination also show the highest PCE under all studied
light intensities because the absorption of all three dyes falls in
the visible-light range. DUY24-Se-based cells has PCE as high
as 26% at ca. 6000 lux which our T5-light can provide.
spectra; frontier orbitals distribution; I-V curves; IPCE
curves; EIS diagrams; UV/Vis absorption spectra;
schematic representation of the layout of the sensitizers;
absorption spectra; transient absorption spectra; DFT
models; and emission spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Chun-Guey Wu − Research Center for New Generation Light
Driven Photovoltaic Modules and Department of Chemistry,
National Central University, 32001 Taiwan, Republic of
China; orcid.org/0000-0001-8540-5602;
Email: t610002@cc.ncu.edu.tw
Authors
The-Duy Nguyen − Research Center for New Generation
Light Driven Photovoltaic Modules and Department of
Chemistry, National Central University, 32001 Taiwan,
Republic of China
Yen-Po Lan − Research Center for New Generation Light
Driven Photovoltaic Modules and Department of Chemistry,
National Central University, 32001 Taiwan, Republic of
China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c01293
CONCLUSIONS
■
Notes
Three dyes (DUY24-O, DUY24, and DUY24-Se) were
prepared to systematically study the influence of the
chalcogenophenes (furan, thiophene, and selenophene) in
the cyclometalated ring on the photophysical, physicochemical,
electrochemical, and, therefore, photovoltaic properties of the
thiocyanate-free cyclometalated ruthenium dyes applied in
DSCs. Heavier chalcogens (such as Se and S) in the
cyclometalated ring increase the molecular size insignificantly,
can red-shift the λmax, and increase the absorption coefficients
of the corresponding dye molecules and dye loading compared
to O. Overall, DSCs based on DUY24-Se (containing
selenophene) dye have the highest efficiency of 8.4% which
is higher than the efficiency (7.9%) of the cell dyed with N719
fabricated using the same conditions. Moreover, the better
performance of the device sensitized with DUY24-Se
compared to DUY24-based cells suggests that the selenophene
ring is as good as (or even better than) thiophene to be a
cyclometalated segment for thiocyanate-free cycloruthenated
sensitizers applied in DSCs. The absorption and TAS data
suggested that the much poorer photovoltaic performance of
the DUY24-O dye (compared to DUY24 and DUY24-Se) is
due to the strong interaction between oxygen (in furan) and
TiO2, resulting in low dye loading and serious charge
recombination. The physical/photovoltaic properties of the
cycloruthenated sensitizers were affected significantly by the
chalcogenophenes on the cyclometalated ring, which provides
a new strategy to design high-efficiency cyclometalated
sensitizers for DSCs.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the Ministry of Science and
Technology (MOST), Taiwan, ROC (grand number: MOST
107-2113-M-008-006-MY3) is greatly acknowledged. The
device fabrication was carried out in the Advanced Laboratory
of Accommodation and Research for Organic Photovoltaics,
MOST, Taiwan, ROC.
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