Effect of Ligand Structures of Copper Redox Shuttles on Photovoltaic Performance of Dye-Sensitized Solar Cells

Article abstract of DOI:10.1021/acs.inorgchem.9b02740

In recent years, copper(I/II) complexes have emerged as alternative redox shuttles in dye-sensitized solar cells (DSSCs), exhibiting more positive redox potential than iodine- and cobalt-based redox shuttles. In particular, copper(I/II) complexes with 1,10-phenanthroline- or 2,2′-bipyridyl-based ligands attained moderate to high power conversion efficiencies (6-11%) with a high open-circuit voltage (V_OC) over 1.0 V due to the positive potentials. Although copper(I/II) complexes with 1,10-phenanthroline-based ligands with 2,9-substituents have been developed, the effect of their ligand structures on the photovoltaic performance of DSSCs have not been fully addressed due to limited synthetic access to 1,10-phenanthroline derivatives. In this study, we designed and synthesized a series of copper(I/II) complexes with 1,10-phenanthroline ligands with different substituents at the 2,9-positions: bis(2-n-butyl-1,10-phenanthroline)copper(I/II) ([Cu(bp)₂]^1+/2+), bis(2-ethyl-9-methyl-1,10-phenanthroline)copper(I/II) ([Cu(emp)₂]^1+/2+), bis(2,9-diethyl-1,10-phenanthroline)copper(I/II) ([Cu(dep)₂]^1+/2+), and bis(2,9-diphenyl-1,10-phenanthroline)copper(I/II) ([Cu(dpp)₂]^1+/2+). The more positive redox potentials of [Cu(emp)₂]^1+/2+ and [Cu(dep)₂]^1+/2+, compared to that of bis(2,9-dimethyl-1,10-phenanthroline)copper(I/II) ([Cu(dmp)₂]^1+/2+), originate from the larger steric hindrance of the ethyl group instead of the methyl group, whereas the redox potential of [Cu(bp)₂]^1+/2+ is significantly shifted to the negative direction because of the lower steric hindrance of the 2-monosubstituted 1,10-phenanthroline ligands. The efficiency of the DSSC with [Cu(bp)₂]^1+/2+ (5.90%) is almost comparable to the DSSC with [Cu(dmp)₂]^1+/2+ (6.29%). In contrast, the DSSCs with [Cu(emp)₂]^1+/2+ (3.25%), [Cu(dep)₂]^1+/2+ (2.56%), and [Cu(dpp)₂]^1+/2+ (2.21%) exhibited lower efficiencies than those with [Cu(dmp)₂]^1+/2+ and [Cu(bp)₂]^1+/2+. The difference can be rationalized by the electron collection efficiencies. Considering the similar photovoltaic properties of the DSSCs with [Cu(bp)₂]^1+/2+ and [Cu(dmp)₂]^1+/2+, the use of copper(I/II) complexes with 2-monosubstituted 1,10-phenanthroline ligands as the redox shuttle may be useful to improve the short-circuit current density while retaining the rather high V_OC value when dyes with a smaller bandgap (i.e., better light harvesting) are developed.

Full text of DOI:10.1021/acs.inorgchem.9b02740

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Effect of Ligand Structures of Copper Redox Shuttles on
Photovoltaic Performance of Dye-Sensitized Solar Cells
†
†
†
†
Tomohiro Higashino, , Hitomi Iiyama, Shimpei Nimura, Yuma Kurumisawa,
†,‡
and Hiroshi Imahori*
†
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan
Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
‡
*
S Supporting Information
ABSTRACT: In recent years, copper(I/II) complexes have
emerged as alternative redox shuttles in dye-sensitized solar
cells (DSSCs), exhibiting more positive redox potential than
iodine- and cobalt-based redox shuttles. In particular, copper(I/
II) complexes with 1,10-phenanthroline- or 2,2′-bipyridyl-
based ligands attained moderate to high power conversion
efficiencies (6−11%) with a high open-circuit voltage (VOC)
over 1.0 V due to the positive potentials. Although copper(I/
II) complexes with 1,10-phenanthroline-based ligands with 2,9-
substituents have been developed, the effect of their ligand
structures on the photovoltaic performance of DSSCs have not
been fully addressed due to limited synthetic access to 1,10-
phenanthroline derivatives. In this study, we designed and
synthesized a series of copper(I/II) complexes with 1,10-phenanthroline ligands with different substituents at the 2,9-positions:
1
+/2+
bis(2-n-butyl-1,10-phenanthroline)copper(I/II) ([Cu(bp) ]
), bis(2-ethyl-9-methyl-1,10-phenanthroline)copper(I/II)
2
1
+/2+
1+/2+
(
[Cu(emp)2]
phenanthroline)copper(I/II) ([Cu(dpp)2]
compared to that of bis(2,9-dimethyl-1,10-phenanthroline)copper(I/II) ([Cu(dmp)2]
hindrance of the ethyl group instead of the methyl group, whereas the redox potential of [Cu(bp)2]
to the negative direction because of the lower steric hindrance of the 2-monosubstituted 1,10-phenanthroline ligands. The
), bis(2,9-diethyl-1,10-phenanthroline)copper(I/II) ([Cu(dep) ]
), and bis(2,9-diphenyl-1,10-
2
1+/2+
1+/2+
1+/2+
). The more positive redox potentials of [Cu(emp) ]
and [Cu(dep)2]
,
2
1
+/2+
), originate from the larger steric
1
+/2+
is significantly shifted
1
+/2+
1+/2+
efficiency of the DSSC with [Cu(bp) ]
(5.90%) is almost comparable to the DSSC with [Cu(dmp) ]
(6.29%). In
(2.21%) exhibited lower
. The difference can be rationalized by the electron collection
2
2
1
+/2+
1+/2+
1+/2+
contrast, the DSSCs with [Cu(emp)2]
efficiencies than those with [Cu(dmp)2]1
efficiencies. Considering the similar photovoltaic properties of the DSSCs with [Cu(bp)2]
(3.25%), [Cu(dep) ]
(2.56%), and [Cu(dpp)2]
2
+/2+
1+/2+
and [Cu(bp)2]
1
+/2+
1+/2+
and [Cu(dmp)2]
, the use of
copper(I/II) complexes with 2-monosubstituted 1,10-phenanthroline ligands as the redox shuttle may be useful to improve the
short-circuit current density while retaining the rather high VOC value when dyes with a smaller bandgap (i.e., better light
harvesting) are developed.
INTRODUCTION
shuttles for DSSCs because their redox potentials can be tuned
■
22−24
by a selection of metal and ligand combinations.
The
It is important to develop sustainable energy systems because
of the increasing worldwide demand for energy. In this regard,
organic photovoltaics have attracted great attention as
promising technologies because of their potential advantages
including low cost, ease of fabrication, transparency and color,
representative redox shuttle is tris(bipyridyl)Co(II/III) ([Co-
2
+/3+
(
bpy)3]
), which can achieve a high VOC with a resultant
high efficiency as a consequence of its more positive redox
−
−
potential (+0.57 V vs NHE) compared to that of I /I (+0.40
3
1
−15
25
and high performance under diffused illumination.
Among
are particularly
attractive candidates owing to their low cost and relative ease
V vs NHE). However, the large reorganization energy (λ) of
1
−6
them, dye-sensitized solar cells (DSSCs)
2+/3+
2+/3+
[Co(bpy)3]
is a disadvantage because [Co(bpy)3]
still
needs a large driving force for efficient dye regeneration (0.2−
16−20
of high power conversion efficiency.
efficiency, it is a prerequisite to increase the open-circuit
voltage (VOC). Although iodide and triiodide (I /I ) have
been the most commonly used redox shuttles for DSSCs, the
To achieve a high
26−28
0.4 eV), lowering the VOC value.
At the same time, rather
−
−
fast charge recombination (CR) derived from the large λ value
should be suppressed by the use of bulky substituents around
3
high overpotential for efficient dye regeneration is a bottleneck
−
−
21
to the VOC value of DSSCs with I /I . In this context,
Received: September 14, 2019
3
transition-metal complexes are proposed as efficient redox
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02740
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1. Molecular structures of copper complexes [Cu(dmp)2]1+/2+, [Cu(dep)2]1+/2+, [Cu(emp) ]
1+/2+
, [Cu(bp) ]
1+/2+
, [Cu(dpp)2]1+/2+, and
2
2
−
1+
−
−
LEG4. The counterions for the copper complexes are trifluoromethanesulfonimide (TFSI ) for [CuL ] and TFSI and chloride (Cl ) for
2
2+
[
CuL ] .
2
2
+/3+
dyes to prevent [Co(bpy) ]
layer on TiO2.
from penetrating into the dye
photovoltaic properties of the DSSCs using an organic
35,45
3
2
9−32
donor−π−acceptor dye (LEG4)
and the copper redox
To overcome the issues, copper(I/II) complexes have
emerged as alternative redox shuttles exhibiting more positive
redox potential (0.8−1.1 V vs NHE) than [Co-
shuttles (Figure 1).
EXPERIMENTAL SECTION
■
2
+/3+ 33−40
(
bpy) ]
.
In particular, copper(I/II) complexes with
Instrumentation and Materials. Commercially available sol-
vents and reagents were used without further purification unless
otherwise mentioned. Silica-gel column chromatography, thin-layer
chromatography (TLC), and size-exclusion gel permeation chroma-
3
1
,10-phenanthroline- or 2,2′-bipyridyl-based ligands attained
moderate to high efficiencies (6−11%) with a high V value
OC
3
5−40
over 1.0 V.
These copper complexes have a distorted
19
tography (GPC) were performed according to our previous report.
tetragonal geometry with a small λ value. In this context, the λ
values of copper complexes have been estimated by DFT
calculations and the λ values were found to be affected by the
1
13
H and C NMR spectra were recorded with a JEOL EX-400
1
13
spectrometer (operating at 400 MHz for H and 100 MHz for C) by
using the residual solvent as the internal reference for H (CDCl (δ =
1
3
3
7,40−42
ligand structures.
phenanthroline)copper(I/II) ([Cu(dmp)2]
ate the oxidized dye efficiently in spite of the small driving
Importantly, bis(2,9-dimethyl-1,10-
7
.26 ppm), acetone-d (δ = 2.05 ppm), DMSO-d (δ = 2.50 ppm))
6
6
1+/2+
13
) can regener-
and C (CDCl (δ = 77.16 ppm), acetone-d (δ = 29.84 ppm)).
3
6
UV−vis/NIR absorption spectra, high-resolution mass spectra
37,38
(
(
HRMS), and attenuated total reflectance-Fourier transform infrared
force (ca. 0.1 eV),
which indicates that copper redox
ATR-FTIR) spectra were measured according to our previous
shuttles can reduce the loss through overpotential. Namely,
both high VOC and efficiencies can be obtained by using copper
redox shuttles. Nevertheless, the use of copper redox shuttles is
limited to sensitizers with low HOMO levels due to the
positive redox potentials. Along this line, several new
copper(I/II) complexes with 1,10-phenanthroline-based li-
19
report. Redox potentials were measured by a cyclic voltammetry
method on an ALS electrochemical analyzer model 660A with
ferrocene (+0.642 V vs NHE) as an external standard.
35
37
Synthesis. [Cu(dmp) ](Cl)(TFSI), [Cu(dmp) ](TFSI), 2-
2
2
45
46
methyl-1,10-phenanthroline, and LEG4 were prepared according
to the literature. The details of syntheses of phenanthroline ligands
are described in the Supporting Information.
4
1−44
gands with 2,9-substituents have been developed.
[
Cu(dep) ](Cl)(TFSI). CuCl (27 mg, 0.20 mmol) was added to a
However, the effect of their ligand structures on the
photovoltaic performance of DSSCs has not been fully
elucidated due to limited synthetic access to 1,10-phenanthro-
line derivatives.
In this study, we designed and synthesized a series of
copper(I/II) complexes with 1,10-phenanthroline ligands with
different substituents at the 2,9-positions: bis(2-n-butyl-1,10-
phenanthroline)copper(I/II) ([Cu(bp)2]1 ), bis(2-ethyl-9-
2
2
solution of 2,9-diethyl-1,10-phenanthroline (189 mg, 0.80 mmol, 4.0
equiv) in ethanol (6 mL) under an argon atmosphere. After the
solution was stirred for 9 h at room temperature, water (12 mL) and
LiTFSI (287 mg, 1.0 mmol, 5.0 equiv) were added and the mixture
was stirred for 1 h. The reaction mixture was extracted with CH Cl .
2
2
The crude product was collected by filtration and washed with diethyl
ether to give the target complex as an orange solid (76 mg, 0.089
mmol, 43%).
+/2+
−
1
1
+/2+
FT-IR (neat): ν (cm ) = 1590, 1500, 1460, 1340, 1179, 1131,
057, 857, 783, 737, 649, 595, 568, 509. Mp: 192−193 °C.
methyl-1,10-phenanthroline)copper(I/II) ([Cu(emp)2]
bis(2,9-diethyl-1,10-phenanthroline)copper(I/II) ([Cu-
),
1
1
+/2+
[Cu(dep) ](TFSI). A mixture of CuI (70 mg, 0.38 mmol, 1.0 equiv)
2
(
dep)2]
), and bis(2,9-diphenyl-1,10-phenanthroline)-
and 2,9-diethyl-1,10-phenanthroline (354 mg, 1.5 mmol, 4.0 equiv) in
ethanol (40 mL) was stirred for 2 h at room temperature under an
argon atmosphere. Then, LiTFSI (546 mg, 1.9 mmol, 5.0 equiv) was
added and the mixture was stirred for 1.5 h. The solvent was removed
under reduced pressure, and the crude product was washed with
1
+/2+
copper(I/II) ([Cu(dpp)2]
) (Figure 1). We examined the
electrochemical properties of copper complexes together with
the well-studied copper complex [Cu(dmp)2]1 . To further
evaluate the effect of the ligand structures, we investigated the
+/2+
B
DOI: 10.1021/acs.inorgchem.9b02740
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
diethyl ether to give the target complex as an orange solid (232 mg,
using THF as eluent. Reprecipitation from CH Cl /n-hexane gave the
2
2
0
.28 mmol, 74%).
target complex as a brown solid (79.4 mg, 0.076 mmol, 38%).
1
−1
H NMR (acetone-d , 400 MHz, 25 °C): δ = 8.82 (d, J = 8.4 Hz,
H), 8.53 (s, 4H), 8.04 (d, J = 8.4 Hz, 4H), 2.88−2.79 (m, 8H), 0.96
FT-IR (neat): ν (cm ) = 1583, 1548, 1506, 1486, 1446, 1424,
6
4
1360, 1155, 866, 740, 699. Mp: 209−211 °C.
(
t, J = 7.8 Hz, 12H). ESI-MS: m/z calcd for C H CuN [M −
[Cu(dpp) ](TFSI). A mixture of CuI (64.3 mg, 0.34 mmol, 1.0
3
2
32
4
2
+
TFSI] : 535.1917; found 535.1911. Mp: 211 °C. FT-IR (neat): ν
equiv) and 2,9-diphenyl-1,10-phenanthroline (450 mg, 1.4 mmol, 4.0
equiv) in ethanol (40 mL) was stirred for 2 h at room temperature
under an argon atmosphere. Then, LiTFSI (488 mg, 1.7 mmol, 5.0
equiv) was added and the mixture was stirred for 2 h. The solvent was
removed under reduced pressure, and diethyl ether was added to the
residue to dissolve the unreacted ligand. The crude product was
collected by filtration and purified by GPC using THF as eluent.
Reprecipitation from CH Cl /n-hexane gave the target complex as a
−
1
(
cm ) = 1590, 1558, 1498, 1450, 1366, 1342, 1181, 1143, 1050, 978,
1
3
8
53, 687, 647, 597. Due to low solubility, we could not obtain the C
NMR spectrum in a sufficient S/N ratio.
Cu(emp) ](Cl)(TFSI). CuCl (32 mg, 0.24 mmol) was added to a
[
2
2
solution of 2-ethyl-9-methyl-1,10-phenanthroline (210 mg, 0.94
mmol, 4.0 equiv) in ethanol (7.7 mL) under an argon atmosphere.
After the solution was stirred for 2 h at room temperature, water (15
mL) and LiTFSI (345 mg, 1.2 mmol, 5.0 equiv) were added and the
mixture was stirred for 5 h. The reaction mixture was extracted with
CH Cl . The solvent was removed under reduced pressure, and the
2
2
brown solid (170 mg, 0.17 mol, 50%).
1
H NMR (acetone-d , 400 MHz, 25 °C): δ = 8.75 (d, J = 8.4 Hz,
6
4H), 8.22 (s, 4H), 8.07 (d, J = 8.0 Hz, 4H), 7.56 (d, J = 8.4 Hz, 8H),
2
2
1
3
crude product was purified by GPC, using THF as the eluent, and
6.83 (t, J = 7.6 Hz, 4H), 6.61 (t, J = 7.8 Hz, 8H). C NMR (acetone-
d6, 100 MHz, 25 °C): δ = 157.7, 144.4, 140.1, 138.6, 129.5, 128.7,
128.0, 127.5, 125.7. ESI-MS: m/z calcd for C H CuN [M −
reprecipitation (CH Cl /hexane) to give the target complex as a
2
2
brown solid (39 mg, 0.047 mmol, 20%).
4
8
32
4
−
1
+
−1
FT-IR (neat): ν (cm ) = 1713, 1628, 1509, 1347, 1176, 1131,
053, 856, 737, 599. Mp: 160 °C.
TFSI] : 727.1917; found 727.1917. FT-IR (neat): ν (cm ) = 1582,
1486, 1418, 1354, 1181, 1134, 1053, 857, 737, 694. Mp: 120−121 °C.
1
[
Cu(emp) ](TFSI). CuI (11 mg, 0.058 mmol) was mixed with 2-
Preparation of LEG4-Sensitized TiO
2
Electrode and Photo-
voltaic Measurements. The preparation of TiO electrodes, and
2
2
ethyl-9-methyl-1,10-phenanthroline (51 mg, 0.23 mmol, 4.0 equiv) in
ethanol (2.7 mL) under an argon atmosphere. After the solution was
stirred for 2 h at room temperature, LiTFSI (83 mg, 0.29 mmol, 5.0
equiv) was added and the mixture was stirred for 1.5 h. The reaction
mixture was filtered and washed with diethyl ether to give the target
the counter Pt electrode, and the fabrication of the sealed cells for
photovoltaic measurements were performed according to the
4
7,48
literature.
A 2 μm nanocrystalline TiO layer (20 nm,
2
PST18NR-T, GreatCell Solar) was coated on the fluorine-doped tin
oxide (FTO) glass plate by a screen-printing method as a transparent
complex as a red solid (18 mg, 0.023 mmol, 40%).
1
H NMR (DMSO-d , 400 MHz): δ = 8.87 (d, J = 8.4 Hz, 2H), 8.77
TiO
(400 nm, CCIC:PST400C, JGC-C&C) was further deposited as a
light-scattering TiO film on the transparent TiO film. The TiO
2
2
film. Then, a layer of the 2 μm submicrocrystalline TiO layer
2
6
(
d, J = 8.8 Hz, 2H), 8.23 (s, 4H), 8.00 (d, J = 8.8 Hz, 2H), 7.98 (d, J =
8
6
.4 Hz, 2H), 2.69−2.65 (m, 4H), 2.41 (s, 6H), 0.82 (t, J = 7.6 Hz,
2
2
+
H). ESI-MS: m/z calcd for C H CuN [M − TFSI] : 507.1604;
electrode was immersed into an ethanol solution of LEG4 (0.20 mM)
3
0
28
4
−
1
found 507.1607. FT-IR (neat): ν (cm ) = 1559, 1496, 1349, 1186,
for 5 h at 25 °C.
1
052, 853, 739, 614, 570. Mp: 241 °C. Due to low solubility, we could
A sandwich cell was prepared by using the dye-anchored TiO film
2
13
as a working electrode and a counter Pt electrode, which were
assembled with a hot-melt ionomer film Surlyn polymer gasket
(DuPont, 50 μm). The electrolyte solution was composed of 0.20 M
not obtain the C NMR spectrum in a sufficient S/N ratio.
Cu(bp) ](Cl)(TFSI). A mixture of CuCl (40 mg, 0.30 mmol, 1.0
[
2
2
equiv) and 2-butyl-1,10-phenanthroline (284 mg, 1.2 mmol, 4.0
equiv) in ethanol (10 mL) was stirred for 2.5 h at room temperature
under an argon atmosphere. Then, water (20 mL) and LiTFSI (431
mg, 1.5 mmol, 5 equiv) were added and the mixture was stirred for
[
4
CuL ](TFSI), 0.05 M [CuL ](Cl)(TFSI), 0.1 M LiTFSI, and 0.5 M
-tert-butylpyridine in acetonitrile. It is noted that all copper
2
2
complexes were dried under a vacuum at 50 °C prior to use.
Incident photon-to-current efficiency (IPCE) and photocurrent−
voltage (I−V) performance were measured with a simulated sunlight
2
.5 h. The target complex was collected by filtration and washed with
diethyl ether to give the target complex as a yellow solid (222 mg,
−
2
19
of AM1.5 (100 mW cm ) according to our previous report. The
convolution of the spectral response in the photocurrent action
spectrum with the photon flux of the AM1.5G spectrum provided the
0
.26 mmol, 87%).
FT-IR (neat): ν (cm ) = 2931, 2873, 1513, 1499, 1346, 1176,
−
1
1
133, 1050, 857, 785, 739, 723, 648. Mp: 114−116 °C.
estimated short-circuit current (J ), which is in good agreement with
[
Cu(bp) ](TFSI). A mixture of CuI (87 mg, 0.46 mmol, 1.0 equiv)
SC
2
the JSC value obtained from the I−V performance.
and 2-butyl-1,10-phenanthroline (424 mg, 1.8 mmol, 4.0 equiv) in
ethanol (50 mL) was stirred for 2 h at room temperature under an
argon atmosphere. Then, LiTFSI (660 mg, 2.3 mmol, 5.0 equiv) was
added and the mixture was stirred for 2 h. The solvent was removed
under reduced pressure, and the crude product was dissolved in
CH Cl . After removal of the solvent, diethyl ether was added to the
Microsecond Time-Resolved Transient Absorption Spec-
troscopy. The measurements were carried out with the laser system
19
provided by UNISOKU according to our previous report. A sample
was excited at λ = 610 nm, and the photodynamics were monitored by
continuous exposure to a Xe lamp as a probe light. All the samples
were made by the same method used for preparing the dye-anchored
2
2
residue to dissolve the unreacted ligand. The crude product was
TiO electrodes.
collected by filtration and purified by reprecipitation from CH Cl /n-
2
2
2
hexane to give the target complex as an orange solid (239 mg, 0.29
mmol, 63%).
RESULTS AND DISCUSSION
1
■
H NMR (acetone-d , 400 MHz, 25 °C): δ = 9.20 (d, J = 3.6 Hz,
6
Synthesis of Copper Complexes. The synthetic schemes
2
8
H), 8.87 (d, J = 8.0 Hz, 2H), 8.78 (d, J = 8.4 Hz, 2H), 8.28 (s, 4H),
.08 (dd, J = 8.0 Hz, J = 4.8 Hz, 2H), 8.02 (d, J = 8.4 Hz, 2H), 2.80
of phenanthroline ligands and copper complexes are shown in
Scheme 1. The reaction of 1,10-phenanthroline with various
organolithium reagents afforded the corresponding 2-mono-
substituted or 2,9-disubstituted 1,10-phenanthrolines (see the
Supporting Information). Then, the reaction of the ligands
+
(
m, 18H). MALDI-MS: m/z calcd for C H CuN [M − TFSI] :
3
2
32
4
−
1
5
1
1
35.1917; found 535.1906. FT-IR (neat): ν (cm ) = 2952, 2866,
508, 1492, 1348, 1328, 1192, 1134, 1062, 849, 828, 741. Mp: 164−
66 °C. Due to low solubility, we could not obtain the C NMR
1
3
spectrum in a sufficient S/N ratio.
Cu(dpp) ](Cl)(TFSI). A mixture of CuCl (26 mg, 0.2 mmol, 1.0
with CuCl and the subsequent counterion exchange with
2
[
2
2
lithium trifluoromethanesulfonimide (LiTFSI) gave copper(II)
equiv) and 2,9-diphenyl-1,10-phenanthroline (266 mg, 0.8 mmol, 4.0
equiv) in ethanol (8 mL) was stirred for 1.5 h at room temperature
under an argon atmosphere. Then, water (16 mL) and LiTFSI (574
mg, 2.0 mmol, 10 equiv) were added and the mixture was stirred for 3
h. The crude product was collected by filtration and purified by GPC
−
complexes with chloride (Cl ) and trifluoromethanesulfoni-
−
mide ions (TFSI ) as counterions ([Cu(L) ](Cl)(TFSI)).
2
The reaction with CuI in the presence of LiTFSI also provided
−
copper(I) complexes with TFSI as a counterion ([Cu(L) ]-
2
C
DOI: 10.1021/acs.inorgchem.9b02740
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Synthesis of 1,10-Phenanthroline Ligands and
Copper Complexes
complexes exhibit reversible or quasi-reversible oxidation peaks
at 0.84−1.08 V vs NHE. The steric bulkiness of substituents at
the 2,9-positions affects the redox potentials because copper(I)
complexes prefer a tetrahedral geometry with a dihedral angle
of ca. 90°, but copper(II) complexes prefer a distorted
tetrahedral geometry. Upon oxidation of copper(I) to
copper(II), the bulky substituents at the 2,9-positions hamper
the conformational change. Consequently, the bulky sub-
stituents destabilize the copper(II) complex and the redox
49,50
potential should be shifted to the positive direction.
In fact,
and
1
+/2+
the more positive E
values of [Cu(emp)2]
redox
1
+/2+
1+/2+
[Cu(dep)2]
compared to that of [Cu(dmp)2]
are
consistent with the larger steric hindrance of the ethyl group
than of the methyl group in the coordination sphere geometry
1
+/2+
(
Figure 2). On the contrary, the Eredox value of [Cu(bp)2]
is significantly shifted to the negative direction because of the
lower steric hindrance of the 2-monosubstituted 1,10-
phenanthroline ligands. In contrast, the E
value of the
redox
1
+/2+
phenyl-substituted [Cu(dpp) ]
is comparable to that of
2
1
+/2+
[Cu(dmp)2]
. Considering the oxidation potential (E ) of
ox
51
LEG4 on TiO (1.07 V vs NHE), we calculated the driving
2
forces for dye regeneration (ΔG ) (Figure 2). The ΔG
reg
reg
1
+/2+
1+/2+
values for [Cu(dmp) ]
, [Cu(bp) ]
, and [Cu-
2
2
1
+/2+
(
dpp)2]
are more negative than −0.1 eV, ensuring that
the dye-regeneration processes with these copper redox
shuttles should occur smoothly. The less negative ΔG values
reg
1
+/2+
1+/2+
for [Cu(emp) ]
(−0.05 eV) and [Cu(dep) ]
(+0.01
2
2
(
TFSI)). These ligands and copper complexes were charac-
1
13
eV) may inhibit the efficient dye regeneration.
terized by H and C NMR, FT-IR spectroscopies, and high-
resolution mass spectrometry (Figures S1−S9). We conducted
DOSY experiments for [Cu(dmp) ](TFSI) and [Cu(bp) ]-
Photovoltaic Properties. We fabricated LEG4-sensitized
DSSCs by using the copper redox shuttles and investigated
their photovoltaic properties under standard AM1.5 illumina-
tions to evaluate the effect of the ligand structures. The
electrolyte solution was composed of 0.2 M [CuL ](TFSI),
.05 M [CuL ](Cl)(TFSI), 0.1 M LiTFSI, and 0.5 M 4-tert-
butylpyridine in acetonitrile. The photocurrent−voltage
characteristics are shown in Figure 3a, and the photovoltaic
parameters are summarized in Table 1. The DSSC with
2
2
(
TFSI) in CD CN, and the diffusion coefficients were
3
−
5
2
−1
determined to be 1.41 × 10 cm s , which is consistent
with that determined by a rotating disk electrode measurement
2
−
5
2
−1 37
0
for [Cu(dmp) ](TFSI) (1.26 × 10 cm s ). Thus, the
ligand modification would have little impact on the mass
transport limitation.
Electrochemical Properties of Copper Complexes.
The electrochemical properties of the copper(I) complexes
were examined by cyclic voltammetry (CV) techniques in
acetonitrile versus NHE with LiTFSI as the electrolyte (Figure
S10). We estimated the redox potentials (Eredox) of the copper
redox shuttles from one electron oxidation potential of the
2
2
1
+/2+
[Cu(dmp)
]
2
exhibited the highest VOC (1.03 V) and a
−
2
high short-circuit current density (JSC = 10.0 mA cm ) and
thus achieved the highest efficiency (6.29%). Although the
1
+/2+
DSSC with [Cu(bp)
]
exhibited the slightly higher JSC
2
−
2
1+/2+
value (10.6 mA cm ) compared to that with [Cu(dmp) ]
,
2
copper(I) complexes because the oxidized dye on TiO should
be regenerated by the copper(I) complex. The copper(I)
the former VOC value (0.842 V) was smaller than the latter one
2
1
+/2+
because of the less positive redox potential of [Cu(bp)2]
.
Figure 2. Energy level diagrams for LEG4 and copper redox shuttles.
D
DOI: 10.1021/acs.inorgchem.9b02740
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. (a) Photocurrent−voltage characteristics and (b) photocurrent action spectra of the DSSCs based on LEG4 with the copper redox
1+/2+
shuttles [CuL ]
: L = bp (red), dmp (blue), emp (green), dep (orange), and dpp (purple).
2
a
Table 1. Photovoltaic Performances of the DSSCs Based on LEG4 with Copper Redox Shuttles
CuL ]1
+/2+
JSC/mA cm−2
VOC/V
ff
η/%
[
2
[
[
[
[
[
Cu(bp) ]1
+/2+
10.6 (10.4 ± 0.6)
10.0 (10.1 ± 0.6)
7.52 (7.82 ± 0.75)
6.77 (6.45 ± 0.23)
4.18 (4.30 ± 0.16)
0.842 (0.836 ± 0.007)
1.03 (1.03 ± 0.01)
0.823 (0.794 ± 0.002)
0.813 (0.803 ± 0.006)
0.848 (0.799 ± 0.036)
0.664 (0.634 ± 0.037)
0.609 (0.592 ± 0.034)
0.526 (0.505 ± 0.064)
0.464 (0.449 ± 0.021)
0.624 (0.602 ± 0.025)
5.90 (5.53 ± 0.23)
6.29 (6.12 ± 0.11)
3.25 (3.09 ± 0.11)
2.56 (2.33 ± 0.17)
2.21 (2.06 ± 0.11)
2
Cu(dmp)2]1+/2+
Cu(emp)2]1+/2+
Cu(dep)2]1+/2+
Cu(dpp)2]1+/2+
a
Photovoltaic parameters are derived from the highest efficiency. The average values from four independent experiments are denoted in
parentheses. Error bars represent a standard error of the mean.
1
+/2+
Overall, the efficiency of the DSSC with [Cu(bp)2]
1
(
5.90%) is almost comparable to that with [Cu(dmp) ] +/2+.
2
1+/2+
, [Cu(dep)2]1+/2+,
Although the DSSCs with [Cu(emp) ]
2
1
+/2+
1+/2+
[Cu(dpp)2]
, and [Cu(bp)2]
revealed rather similar
VOC values (0.813−0.848 V), the J values of the DSSCs with
SC
1+/2+
1
+/2+
1+/2+
[
Cu(emp) ]
, [Cu(dep) ]
, and [Cu(dpp)2]
are
2
2
1+/2+
lower than those with [Cu(bp)2]
the efficiencies of the DSSCs with [Cu(emp) ]
, leading to a decrease in
1
+/2+
, [Cu-
2
1
+/2+
1+/2+
1+/2+
(
dep)2]
, and [Cu(dpp)2]
relative to [Cu(bp) ]
. It
2
1+/2+
is notable that the fill factors for [Cu(emp) ]
(0.526) and
2
1
+/2+
[Cu(dep)2]
(0.464) are very low, which can be ascribed to
the insufficient ΔG values.
reg
Figure 4. Time absorption profiles of the LEG4-sensitized TiO films
The photocurrent action spectra are illustrated in Figure 3b.
The integrated incident photon-to-current efficiency (IPCE)
values are in good agreement with the JSC values. The
2
in the absence (black) and presence of copper redox shuttles
1
+/2+
[CuL2]
in acetonitrile: L = bp (red), dmp (blue), emp (green),
1
+/2+
dep (orange), and dpp (purple). The samples were excited at 610 nm,
and the absorption profiles were monitored at 715 nm.
maximum IPCE value of the DSSC with [Cu(bp) ]
is
2
slightly higher than that with [Cu(dmp)2]1 , whereas the
+/2+
1
+/2+
maximum IPCE values of the DSSCs with [Cu(emp) ]
2
(
(
[
ca. 50%), [Cu(dep)2]1
+/2+
(ca. 40%), and [Cu(dpp) ]
1+/2+
the copper redox shuttles were also determined to be 13 μs for
2
1
+/2+
1+/2+
1+/2+
ca. 30%) are lower than those with [Cu(bp) ]
and
[Cu(bp)2]
, 2.9 μs for [Cu(dmp) ]
, 5.8 μs [Cu(dep)2]
, 3.9 μs for
, and 5.8 μs for
(Table S1). From these results, the dye-
regeneration efficiencies (φ ) were estimated to be 81−96%.
2
2
1
+/2+
1+/2+
1+/2+
Cu(dmp)2]
(ca. 70%). The IPCE value is calculated by
[Cu(emp)2]
[Cu(dpp)2]
1
+/2+
the product of light-harvesting efficiency (LHE), electron
injection efficiency (ϕ ), and charge collection efficiency
inj
reg
1
+/2+
(ηcol). Because all the DSSCs are fabricated with LEG4 in this
However, the smaller φ value for [Cu(bp) ]
(81%) than
reg
2
1
+/2+
study, the LHE and ϕinj values should be identical. Thus, the
difference in the IPCE values is rationalized by the ηcol values.
The ηcol value is associated with the dye-regeneration and CR
processes between the electron in the CB of TiO and the dye
radical cation or the oxidized redox shuttle in the electrolyte
solution.
To assess the dye-regeneration process, we performed
microsecond time-resolved transient absorption (TA) experi-
ments for the LEG4-sensitized TiO films (Figures 4 and S11).
for [Cu(dmp)2]
driving force for [Cu(bp) ]
(96%) is inconsistent with the larger
1
+/2+
(0.23 V) than for [Cu-
(0.11 V) and similar high IPCE and J values.
2
1
+/2+
(dmp)2]
Moreover, the high φ values for [Cu(dep)2]
SC
1
+/2+
(91%) and
(94%) contradict the insufficient driving
2
reg
1
+/2+
[Cu(emp)2]
force for the dye regeneration and the low IPCE and JSC values.
To address these inconsistencies, we compared the initial
intensities of the TA of LEG4• (ΔOD) in the presence of the
copper redox shuttles. It increases in the following order:
+
2
1
+/2+
1+/2+
We monitored the decay profiles at 715 nm because the LEG4
[Cu(dpp) ]
(0.002) < [Cu(dep)2]
(0.005) ≈ [Cu-
2
•
+
1+/2+
1+/2+
radical cation (LEG4 ) possesses characteristic absorption at
(emp)2]
(0.005) < [Cu(dmp)2]
(0.007) < [Cu-
•
+
1+/2+
•+
6
70−800 nm (Figure S12). The lifetime (τ ) of LEG4 in the
(bp)2]
(0.009). If the intensity (i.e., LEG4 concen-
1
absence of the copper redox shuttle was determined to be 68
tration) corresponds to the ηcol value, this trend is in good
agreement with those on the IPCE and JSC values. The reason
•
+
μs (Table S1). The lifetimes (τ ) of LEG4 in the presence of
2
E
DOI: 10.1021/acs.inorgchem.9b02740
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
is not clear at this stage; this suggests at least the partial
CONCLUSION
■
•
+
occurrence of a fast CR process between LEG4 and TiO2
<100 ns), involving the copper redox shuttles, which would
lower the IPCE and J values significantly in the cases of
We designed and synthesized a series of copper(I/II)
complexes with 1,10-phenanthroline ligands with different
substituents at the 2,9-positions to examine the effect of the
ligand structures on their electrochemical and photovoltaic
properties. For the copper complexes with alkyl-substituted
ligands, the redox potentials correlate with the steric hindrance
of the ligands on the coordination sphere geometry. The redox
(
SC
, [Cu(dep)2]
1
+/2+
1+/2+
1+/2+
[
Cu(emp)2]
, and [Cu(dpp)2]
with
the small driving forces. Considering rather similar high φ
reg
values, the LEG4• remaining from the fast CR processes is
quenched effectively by the redox shuttles, generating the
photocurrent corresponding to the initial intensity of the TA of
+
1
+/2+
1+/2+
potentials of [Cu(emp)
]
2
and [Cu(dep)
]
are shifted
2
•
+
1+/2+
LEG4 . In particular, for [Cu(bp) ]
the large initial
to the positive direction because of the larger steric hindrance
of the ethyl than of the methyl groups, whereas that of
2
•+
intensity of the TA of LEG4 suggests that the fast CR
process (<100 ns) would be suppressed significantly, probably
due to the large driving force. Thus, the DSSC with
1
+/2+
[
Cu(bp)2]
is shifted to the negative direction because of
the lower steric hindrance of the 2-monosubstituted 1,10-
phenanthroline ligands. In contrast, the redox potential of the
1
+/2+
[
Cu(bp)2]
exhibited the high IPCE and JSC values in
1+/2+
phenyl-substituted [Cu(dpp) ]
is comparable to that of
2
spite of the somewhat smaller φreg value. Although our
microsecond time-resolved TA experiments cannot fully
evaluate the CR process, the ηcol value and resultant IPCE
and JSC values are affected by both the fast CR process beyond
the time resolution (<100 ns) and the slow regeneration
process occurring in the microsecond region.
1+/2+
[
Cu(dmp)2]
. The driving force for the dye regeneration
1
+/2+
increases in the order [Cu(dep) ]
(+0.01 V) < [Cu-
2
1+/2+
1+/2+
(
[
emp)2]
(−0.05 V) < [Cu(dpp) ]
(−0.11 V) ≈
2
1+/2+
1+/2+
Cu(dmp)2]
(−0.11 V) < [Cu(bp) ]
(−0.23 V). The
2
1+/2+
efficiency of the DSSC with [Cu(bp)2]
(5.90%) is almost
1+/2+
comparable to that with [Cu(dmp) ]
the DSSCs with [Cu(emp)2]
2.56%), and [Cu(dpp)2]
(6.29%). In contrast,
(3.25%), [Cu(dep)2]
2
We also conducted electrical impedance spectroscopy (EIS)
to evaluate the effect of the ligand structures on the CR
processes. The EIS Nyquist plots under standard AM1.5
illumination and open-circuit conditions are shown in Figure
S13. The electron transfer (ET) process at the interface of the
TiO /dye/electrolyte configuration is represented by R , which
1+/2+
1+/2+
1+/2+
(
(2.21%) exhibited lower
1
+/2+
efficiencies than those with [Cu(dmp) ]
and [Cu-
2
1
+/2+
(bp)2]
. The difference can be rationalized by the fact
that the electron collection efficiencies associated with both the
fast CR process (<100 ns) and slow regeneration process occur
in the microsecond region. Considering the similar photo-
2
p
corresponds to a CR resistance between the electrolyte and
1
+/2+
TiO . The R values increase in the following order:
voltaic properties of the DSSCs with [Cu(bp)
2
]
and
2
p
1
+/2+
1
+/2+
1+/2+
1+/2+
[
Cu(dmp)2]
, the use of copper(I/II) complexes with 2-
[
[
[
Cu(bp)2]
Cu(dpp)2]
Cu(dep)2]
(29 Ω) < [Cu(dmp) ]
(73 Ω) <
2
1
+/2+
monosubstituted 1,10-phenanthroline ligands as the redox
shuttle may be useful to improve the short-circuit current
density while retaining the rather high VOC value when dyes
with a smaller bandgap (i.e., better light harvesting) are
developed. Therefore, we believe that exploration of new
ligands for copper(I/II) complexes would contribute to
achieving a high photovoltaic performance of the DSSCs
with copper-based redox shuttles.
(131 Ω) < [Cu(emp) ]
(181 Ω) <
2
1
+/2+
(322 Ω). This tendency is consistent with the
order of the redox potentials for the copper redox shuttles.
This is reasonable considering that the CR process from the
electron in the CB of TiO to the oxidized redox shuttle occurs
2
in the Marcus inverted region, and thus the larger driving force
leads to the slower CR. Although the larger R values for
p
1
+/2+
1+/2+
[
Cu(dmp)2]
than for [Cu(bp) ]
match with the larger
2
VOC for [Cu(dmp)2]1+/2+ than for [Cu(bp) ]
1+/2+
, the large Rp
, and [Cu-
values in
2
ASSOCIATED CONTENT
1
+/2+
1+/2+
■
values for [Cu(dpp) ]
, [Cu(emp)2]
2
1
+/2+
*
S
Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b02740.
(
dep)2]
contradict with the small V
OC
1+/2+
comparison with those of [Cu(dmp)2]
. The contradiction
may be explained by the plausible fast CR process between
LEG4• and TiO involving the redox shuttle, decreasing the
+
2
Details of DSSC preparations and photovoltaic measure-
ments, synthesis of phenanthroline ligands, high-
resolution mass spectra, NMR spectra, cyclic voltammo-
grams, details of photovoltaic properties, figures of
absorption profiles, UV−vis/NIR absorption spectrum
of LEG4 radical cation, EIS Nyquist plots, and table of
the lifetime of LEG4 radical cation and dye-regeneration
efficiency (PDF)
1+/2+
VOC values (vide supra). The DSSC with [Cu(bp)2]
owns
the slowest dye regeneration and lowest recombination
resistance, then achieving the largest photocurrent and
moderate photovoltage. Although the small recombination
resistance indicates a fast CR of electrons in TiO with
2
copper(II) in the electrolyte, the further fast CR process
involving the copper redox shuttle (<100 ns) would not be
included in the recombination resistance. Accordingly, the
photocurrent and voltage are influenced by the fast CR process
AUTHOR INFORMATION
■
*
(
<100 ns) rather than the slow CR process. Consequently, the
Corresponding Author
E-mail: imahori@scl.kyoto-u.ac.jp.
1+/2+
DSSC with [Cu(bp)2]
attained the slightly higher J
SC
value than the DSSC with [Cu(dmp)2]1 , which is
+/2+
ORCID
consistent with other copper redox shuttles with 2-mono-
4
1−43
Tomohiro Higashino: 0000-0002-9531-8569
Hiroshi Imahori: 0000-0003-3506-5608
substituted phenanthroline ligands.
The use of [Cu-
1
+/2+
(
bp)2]
may be useful to improve the JSC value by retaining
the rather high VOC value (>0.8 V) when dyes with a smaller
Notes
bandgap (i.e., better light harvesting) are developed.
The authors declare no competing financial interest.
F
DOI: 10.1021/acs.inorgchem.9b02740
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
20) Zhang, L.; Yang, X.; Wang, W.; Gurzadyan, G. G.; Li, J.; Li, X.;
ACKNOWLEDGMENTS
■
An, J.; Yu, Z.; Wang, H.; Cai, B.; Hagfeldt, A.; Sun, L. ACS Energy
Lett. 2019, 4, 943−951.
This work was supported by the JSPS (KAKENHI Grant
Numbers JP18K14198 (T.H.) and JP18H03898 (H.I.)). We
thank Dr. Kenji Sugase and Professor Masahiro Shirakawa for
assistance with the DOSY-NMR experiments.
(21) Boschloo, G.; Hagfeldt, A. Characteristics of the Iodide/
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(22) Cong, J.; Yang, X.; Kloo, L.; Sun, L. Iodine/iodide-Free Redox
Shuttles for Liquid Electrolyte-Based Dye-Sensitized Solar Cells.
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