Stable Dye-Sensitized Solar Cells Based on Copper(II/I) Redox Mediators Bearing a Pentadentate Ligand

Article abstract of DOI:10.1002/anie.202104563

In recent years, copper redox mediators have attracted growing interest in dye-sensitized solar cells (DSCs). However, experiments revealed that ubiquitously used Lewis-base additives in the electrolytes coordinate to the Cu^II species, which restricts further enhancement of device performance and stability. We report the application of copper complexes endowed with diamine-tripyridine pentadentate ligands, [Cu(tpe)]^2+/+ (tpe=N-benzyl-N,N′,N′-tris(pyridin-2-ylmethyl)ethylenediamine) and [Cu(tme)]^2+/+ (tme=N-benzyl-N,N′,N′-tris(6-methylpyridin-2-ylmethyl)ethylenediamine), as redox mediators in DSCs. Experimental measurements demonstrate that the coordination environment of Cu(II) complexes with pentadentate ligands remains unchanged in the presence of TBP, which is in stark contrast to the state-of-the-art bipyridyl counterpart. DSCs based on [Cu(tme)]^2+/+ complexes exhibit an excellent long-term stability and maintain more than 90 % of the initial efficiency after 400 h under continuous illumination, which outperform the reference devices incorporating the bipyridyl counterpart (less than 80 %) under identical conditions.

Full text of DOI:10.1002/anie.202104563

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Stable Dye-Sensitized Solar Cells Based on Copper(II/I) Redox
Mediators Bearing a Pentadentate Ligand
Authors: Ze Yu, Hailong Rui, Junyu Shen, Lihua Li, Hongxian Han, and
Licheng Sun
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RESEARCH ARTICLE
Stable Dye-Sensitized Solar Cells Based on Copper(II/I) Redox
Mediators Bearing a Pentadentate Ligand
§
§
Hailong Rui, Junyu Shen, Ze Yu,* Lihua Li, Hongxian Han, and Licheng Sun*
H. Rui, Prof. Z. Yu, L. Li, and Prof. L. Sun
State Key Laboratory of Fine Chemicals, Dalian University of Technology (DUT), Dalian 116024, China.
E-mail: ze.yu@dlut.edu.cn
Dr. J. Shen
Jiangsu Laboratory of Advanced Functional Materials, School of Materials Engineering, Changshu Institute of Technology, Changshu 215500, China.
Prof. H. Han
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, Dalian,
116023, China.
Prof. L. Sun
Department of Chemistry, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, Stockholm 10044,
Sweden.
E-mail: lichengs@kth.se
Prof. L. Sun
Center of Artificial Photosynthesis for Solar Fuels, School of Science, Westlake University, 310024 Hangzhou, China.
[§] These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: In recent years, copper redox mediators have attracted
growing interest in dye-sensitized solar cells (DSCs), exhibiting high
photovoltage of over 1.0 V and outstanding performance under
ambient lighting. However, emerging experimental evidence reveals
that the ubiquitously used Lewis base additives in the electrolytes,
such as 4-tert-butylpyridine (TBP), coordinate to the Cu(II) species,
which restricts further enhancement of device performance and
stability for these electrolytes. Here, we report for the first time the
application of copper complexes endowed with diamine-tripyridine
DSCs, which have enabled high photovoltage of over 1.0 V and
astonishing performance under low light conditions.[13-25] The
majority of the studies for copper redox mediators has been
currently dominated by bidentate ligands, i.e. pyridine and
phenantroline derivatives.[
26-35]
The state-of-the-art systems
]2+/+ (tmby
based on [Cu(tmby) = 4,4,6,6’-tetramethyl-2,2’-
2
bipyridine) have shown excellent efficiencies of 13.5% and 11%
for liquid and solid-state DSCs, respectively, and 34.5% under
indoor lighting.[
14,17]
2+/+
pentadentate ligands, namely [Cu(tpe)]
(tpe = N-benzyl-N,N′,N′-
Copper-based redox couples are well-known to change the
coordination geometry depending on the metal oxidation
state.[25] Specifically, Cu(I) d complexes generally adopt 4–
2+/+
tris(pyridin-2-ylmethyl)ethylenediamine) and [Cu(tme)]
(tme = N-
10
benzyl-N,N′,N′-tris(6-methylpyridin-2-ylmethyl)ethylenediamine), as
redox mediators in DSCs. Cyclic voltammetry, absorption, and
electron paramagnetic resonance spectroscopies demonstrate that
the coordination environment of copper(II) complexes with
pentadentate ligands remains unchanged in the presence of TBP,
which is in stark contrast to the state-of-the-art bipyridyl counterpart.
9
coordinate tetrahedral geometries, while Cu(II) d complexes are
typically 5‒6 coordinate showing
a flattened Jahn‒Teller
distorted geometry.[25,36] Recently, several experimental
observations revealed that the most frequently used Lewis base
additives in the electrolytes, such as 4-tert-butylpyridine (TBP),
coordinate to the Cu(II) species, which have negative impacts on
the electrochemical behavior of the redox mediators and thus
the overall device performance.[13,30,36-40] It was shown that the
coordination and/or substitution of bipyridyl and phenantroline
ligands by TBP induces a more complicated electrochemistry of
the copper complexes. This results in multiple redox species
associated with a negative shift of the formal redox potentials by
up to hundreds of millivolts, which limits the maximum attainable
photovoltage of the devices.[30,36,37] Moreover, emerging
2+/+
Strikingly, DSCs based on [Cu(tme)]
excellent long-term stability and maintain more than 90% of the initial
efficiency after 400 under continuous illumination, which
complexes exhibit an
h
outperform the reference devices incorporating the bipyridyl
counterpart (less than 80%) under identical conditions. The present
work thus provides a guidance for future design of stable copper
redox mediators in DSCs.
evidence also indicated that the regeneration of TBP-
coordinated Cu(II) species is sluggish,[36,37,39] which may
Introduction
potentially deteriorate the device stability as observed recently
The ever-increasing energy demands combined with
climate and environmental concerns have motivated our society
to find alternative renewable energy sources.[1,2] Dye-sensitized
solar cells (DSCs) have attracted widespread research attention
as one of the most promising next-generation solar cell
technologies over the past three decades, because of their
simplicity of fabrication, aesthetic design characteristics, and
excellent performance especially for diffuse and low light
applications.[3-14]
by Kanankutty and co-workers.[37]
Here, we report a new class of copper complexes bearing
diamine-tripyridine pentadentate ligands, [Cu(tpe)]2+/+ and
2+/+
[
Cu(tme)] , as redox mediators in DSCs. The reasons for this
design are two-fold: first, a higher denticity leads to a higher
overall stability constant (β) of a metal complex due to the
chelating effect.[41,42] Second, the steric constraints of the
designed coordination sphere potentially will shield the copper
complexes, in particular the oxidized form, from TBP
coordination. Our motivation is to develop a stable DSC system
In the past few years, copper(II/I) complexes have gained
stimulating interest as redox mediators and hole conductors in
1
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RESEARCH ARTICLE
that incorporates copper complex-based electrolytes. Cyclic
voltammetry, absorption, and electron paramagnetic resonance
and angles are listed in Tables S1 and Table S2 in the
Supporting Information. Both copper complexes belong to
(
EPR) spectroscopy results prove that the coordination
1
monoclinic system with space group of P2 /c (Table S1,
geometry of copper(II) complexes with pentadentate ligands
remains essentially unaffected upon addition of TBP into the
electrolyte, which is in stark contrast to the case of its bipyridyl
counterpart [Cu(tmby)
2
]2+/+. As a consequence, DSCs based on
complexes exhibit an excellent long-term stability
2+/+
[
Cu(tme)]
and maintain ~91% of the initial efficiency after 400 h under
continuous irradiation, which outperform the devices
incorporating [Cu(tmby)
2
]2+/+ (less than 80%). The much
improved durability observed for the former devices should be
largely ascribed to a smaller fill factor loss after aging, which is
strongly associated with the relatively unaffected charge transfer
resistance at the counter electrode and diffusion resistance of
the electrolyte in the presence of TBP.
Results and Discussion
The details of the synthesis and characterization of the
ligands and copper complexes are described in the Supporting
Information. Briefly, [Cu(tpe)]2+/+ and [Cu(tme)]2+/+ were prepared
by the reaction of copper salts with equivalent of tpe or tme
ligands in acetonitrile (ACN) at room temperature under nitrogen.
The molecular structures of the ligands and crystal
structures of [Cu(tpe)]2+ and [Cu(tme)]2+ are illustrated in Figure
Figure 1. Chemical structures of (a) tpe and (b) tme ligands. Crystal structures
of (c) [Cu(tpe)] (PF and (d) [Cu(tme)] (PF as ball-and-stick drawings.
Counterions, solvent molecule, and hydrogen atoms are omitted for clarity.
6
)
2
6 2
)
1. The crystallographic data, together with partial bond lengths
Figure 2. (a) Cyclic voltammograms of [Cu(tpe)]2+/+, [Cu(tme)]2+/+, tpe ligand and tme ligand (concentrations: 1 mM) as well as the blank electrolyte of a glassy
4 6
carbon electrode in acetonitrile containing 0.1 M nBu NPF
at a scan rate of 50 mV s–1. Cyclic voltammograms of (b) [Cu(tpe)] , (c) [Cu(tme)] , and (d)
2+
2+
2+
–1
[
Cu(tmby)
2
]
(concentrations: 1 mM) in the absence and presence of 0, 5, 10, and 15 molar eq. TBP in ACN at a scan rate of 50 mV s . Potentials are in unit of V
versus NHE after conversion with E1/2(Fc) = 0.64 V versus NHE.
2
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RESEARCH ARTICLE
Supporting Information). Although tpe and tme have similar
diamine-tripyridine ligand framework, three methyl groups
attached to the ortho-position of the pyridines for the latter result
in quite different spatial structures between [Cu(tpe)]2 and
than the redox peaks of the ligands. The more positive redox
potential (420 mV) observed for [Cu(tme)]2+ should be strongly
correlated with the longer Cu-N coordination bond lengths of that
complex as discussed above. This will result in a lower electron
density of the metal center. Thus, [Cu(tme)]2+ can be readily
reduced to [Cu(tme)]+.
The UV-vis absorption spectra of these copper complexes
are presented in Figure 3a and Figure S1 in the Supporting
Information. As shown in Figure 3a, the intense absorption
peaks at λmax ranging from 244 to 267 nm are caused by π→π*
absorptions of aromatic units in the ligands (Figure S1,
Supporting Information), while the shoulder peaks at 280 to 480
nm originate from metal-to-ligand charge transfer (MLCT)
+
[
Cu(tme)]2+ (Figure 1c and Figure 1d). Complex [Cu(tpe)]2+
displays a distorted square pyramidal geometry configuration (τ
0.47 < 0.5) with N3 at the apical site, and the basal square
plane is composed of the other four nitrogen atoms.However,
1
=
complex [Cu(tme)]2+ has
geometry configuration (τ
a
distorted trigonal bipyramidal
= 0.6 > 0.5),[ in which the copper(II)
43]
2
ion resides in the body-center, while N1 and N2 locate at the two
axial vertexes of trigonal bipyramid, and the remaining three
nitrogen atoms form a plane. In addition, the structural difference
is also reflected in the bond lengths. The Cu-N coordination
bond lengths of complex [Cu(tme)]2+ are ranged from 0.021 to
transitions of copper(I) complexes. The weak peaks at λmax
=
694 nm for [Cu(tpe)]2+ and 668 nm for [Cu(tme)] , respectively,
are characteristic absorptions of d-d transitions for Cu(II) ion
(Figure S1, Supporting Information). It is also worth mentioning
here that the absorptions of [Cu(tpe)]2+/+ and [Cu(tme)]2+/+ are
mainly distributed in UV-light region, which is in stark contrast
2+
0
.149
corresponding values in complex [Cu(tpe)]
Supporting Information), due to the steric-hindrance effect of
Å
(except Cu-N1), which are longer than the
2
+
(Table S2,
2
+
three methyl groups in [Cu(tme)] .
2
+/+
The electrochemical properties of the two copper
complexes were studied by cyclic voltammetry measurements in
a three-electrode system. As depicted in Figure 2a, [Cu(tpe)]2
and [Cu(tme)]2+ show reversible redox peaks at E1/2 = 0.10 V
with its bipyridyl counterpart [Cu(tmby)
2
]
, the absorption
maxima of which is around 450 nm. Therefore, complexes
[Cu(tpe)]2+/+ and [Cu(tme)]2+/+ show almost neglected visible-light
absorption competition with dye Y123 (Figure S2, Supporting
Information) used in this work (Figure 3a), which is beneficial for
light harvesting in DSC devices.
+
and 0.52
V
versus normal hydrogen electrode (NHE),
respectively, which are assigned to the Cu(II/I) couples rather
+
+
+
Figure 3. (a) UV-vis absorption spectra of [Cu(tpe)] , [Cu(tme)] , and [Cu(tmby)
0
0
2
] , and dye Y123 in ACN. Concentrations: 0.1 mM for the copper complexes and
.02 mM for dye Y123. UV-vis absorption spectra of (b) [Cu(tpe)]2+, (c) [Cu(tme)]2+, and (d) [Cu(tmby)
.8, and 1.0 molar eq. TBP in ACN.
2+
(concentrations: 5.0 mM) in presence of 0, 0.2, 0.4, 0.6,
2
]
3
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RESEARCH ARTICLE
Lewis bases such as TBP in electrolytes of DSCs have
been known to regulate the conduction band (CB) edge of titania
and suppress charge recombination losses, which play an
important role for achieving optimal solar cell performance.[44]
However, as mentioned earlier, several studies reported that
TBP can coordinate to the Cu(II) complexes or substitute
2.240, g
y
= 2.130 and g
x
= 2.020. This is a strong indication that
TPB might be able to coordinate with Cu(II) that led to significant
changes of the electronic structures of the complex.
bipyridine and phenantroline ligands, which resulted in the
formation of new species.[
30,36,37,39]
For the designed two copper
complexes with pentadentate ligands, the copper centers are
located inside of the ligands completely due to the chelating
effect. This coordination geometry will likely protect the copper
complexes from TBP coordination. As expected, this hypothesis
is confirmed by the fact that no obvious changes are observed in
the CV curves (Figure 2b and Figure 2c) and UV-vis absorptions
(
Figure 3b and Figure 3c) of [Cu(tpe)]2+ and [Cu(tme)]2+ upon
addition of TBP into the electrolytes. In stark contrast, significant
shifts of redox peaks (Figure 2d) and spectral variations (Figure
2
+
3
d) are found for [Cu(tmby)
2
] , indicative of the structural
[
30,36,37,39]
change of such complex with addition of TBP.
For
comparison, we also examined the effect of another widely used
Lewis base additive N-methylbenzimidazole (NMBI) on the
electrochemical and absorption behavior for these three Cu(II)
complexes and the results are shown in Figure S3 in the
Supporting Information. Upon addition of NMBI to the
electrolytes, the trends obtained are similar to the cases with
TBP.
To deeper understand the coordination changes of
2
+
2+
complexes [Cu(tme)] and [Cu(tmby)
EPR spectroscopic measurements at 90
performed in non-coordinative solvent dichloromethane (CH
All the experimental EPR spectra show mainly 63Cu signal with
/3 nuclear spin momentum as shown in Figure 4. Note that the
2
]
upon addition of TBP ,
were further
Cl ).
K
2
2
2
corresponding simulated EPR spectra using Bruker WINEPR
SimFonia simulation software are shown in Figure S4 in the
Supporting Information. The EPR spectrum of the original
2+
[
Cu(tme)] can be well simulated with the following parameters:
hyperfine splitting constants of 63Cu A
= 168 G and A// = 20 G, g
values of g// = 2.055 and g = 2.220. Addition of TBP does not

Figure 4. Continuous wave X-band EPR spectra of [Cu(tme)]2+ and

]2+ (both at 50 mM) in the absence or presence of 1 molar eq. TBP.
[
Cu(tmby)
2
change the main feature of the EPR spectrum, as it can be well
simulated using the following parameters: hyperfine splitting
The samples were dissolved in CH Cl , and the spectra were acquired at 90 K.
2 2
constants of 63Cu A
.050 and g
= 168 G and A// = 15G, g values of g//

= 2.240. The slight differences of these parameters
=

Two new copper redox mediators [Cu(tpe)]2+/+ and
[Cu(tme)]2+/+ were further evaluated in DSCs, in combination
with an organic sensitizer Y123 and a poly(3,4-
ethylenedioxythiophene) (PEDOT) counter electrode. The
energy level diagram is depicted in Figure S6 and fabrication
details of DSCs is described in the Supporting Information. The
photocurrent density-voltage (J−V) characteristics of the
champion devices for each electrolyte recorded at one sun
irradiation (100 mW cm–2, AM 1.5G) and in the dark are
displayed in Figure 5a, and the corresponding parameters are
collected in Table S3 in the Supporting Information. The devices
based on [Cu(tme)]2+/+ redox system show a higher power
conversion efficiency (PCE) of 9.4%, with an open-circuit voltage
(V ) of 0.84 V, a short-circuit current density (J ) of 15.9 mA
2
are within the experimental error and might be due to the solvent
effect of TPB on Cu(II) coordination environment rather than
direct coordination of TPB with Cu(II) species. Similar
phenomenon of no significant changes of EPR parameters was
also observed for [Cu(tpe)]2+ as presented in Figure S5 in the
Supporting Information. Combining cyclic voltammetry,
absorption spectra and EPR results, we can conclude that
addition of TBP into the electrolyte does not change the ligand
2
+/+
coordination environment of the complexes [Cu(tpe)]
and
2+/+
[
Cu(tme)]
On the contrary, significant changes of the EPR parameters
of [Cu(tmby)
]2+ upon addition of TPB were observed. The EPR
spectrum of the original [Cu(tmby)
]2+ complex can be simulated
using the following parameters: hyperfine splitting constants of
63Cu A
= 120 G and A// = 18 G, g values of g// = 2.072 and g
.270. After addition of TBP, the obtained EPR spectrum
.
2
2
oc
sc
cm–2, and a fill factor (FF) of 0.70, respectively. All the
photovoltaic parameters for the [Cu(tpe)]2+/+ electrolyte are


=
2
observed to be lower, in particular V (0.51 V) and Jsc (6.6 mA
oc
–2
becomes strongly anisotropic as it can be also well simulated
using the following parameters: hyperfine splitting constants of
cm ), which give rise to a rather low PCE of only 2.1%. V is
oc
typically governed by the potential difference between the quasi-
Fermi energy level of TiO2 and the redox potential of redox
63Cu A
z y x z
= 142 G, A = 58 G and A = 10 G, g values of g =
4
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RESEARCH ARTICLE
Figure 5. (a) J−V curves of the best-performing DSCs based on [Cu(tpe)]2+/+ and [Cu(tme)] redox mediators measured under illumination (100 mW cm–2, AM
2+/+
+/+
2+/+
1
.5 G) and in the dark. (b) IPCE spectra of DSC devices employing [Cu(tpe)]2 and [Cu(tme)]
electrolytes. (c) Nanosecond laser transient absorption
film with an inert electrolyte (0.1 M LiTFSI and 0.6 M TBP in ACN), [Cu(tpe)]2 , and [Cu(tme)]2+/+ electrolytes (0.25
+/+
spectroscopy measurements of Y123 on TiO
2
M Cu(I) complex, 0.1 M LiTFSI and 0.6 M TBP in ACN). The excitation wavelength was 532 nm and the probe wavelength was 690 nm. (d) Electron lifetime as a
function of varied bias potentials for DSCs based on [Cu(tpe)]2+/+ and [Cu(tme)]2+/+ electrolytes.
mediator in the electrolyte.[24] Therefore, the more positively
shifted redox potential of [Cu(tme)]2+/+ should mainly account for
the significant enhancement of Voc (330 mV) observed for such
the absorption of [Cu(tmby)
2
]+ species in this region. It highlights
once again that these newly developed copper complexes are
beneficial for light harvesting and thus the photocurrent.
Generally, IPCE at a certain wavelength is determined by the
light-harvesting efficiency (LHE), electron injection yield (φinj),
dye regeneration yield (φreg), and charge collection efficiency
(ηcol).[24,45] If we assume LHE and φinj are analogous for these
two systems (same dye and working electrode), the difference of
IPCE values observed should be attributed to an improved φreg
devices. The devices based on [Cu(tmby)
also tested, showing a best PCE of 10.3% with a Voc of 1.04 V, a
2
]2+/+ electrolyte were
–2
J
(
sc of 13.7 mA cm , and a fill factor (FF) of 0.73, respectively
Figure S7a and Table S3, Supporting Information). The redox
potential of [Cu(tmby)
]2+/+ is reported to be 0.87 V vs. NHE,
2
which corresponds to a theoretical maximum Voc of 1.37 V if the
CB edge is taken as –0.5 V vs. NHE.[ Thus, a 330 mV Voc
13]
and/or ηcol for [Cu(tme)] -based electrolyte.
2+/+
TiO
2
deficit is observed for such a redox system. In contrast, a
Nanosecond transient absorption spectroscopy (TAS) was
implemented to gain an understanding of the dye regeneration
kinetics for the two redox systems as presented in Figure 5c.
With an inert electrolyte (0.1 M lithium bis(trifluoromethane-
sulfonyl)imide (LiTFSI) and 0.6 M TBP in ACN), the decay signal
relating to electron transfer from the titania to the oxidized dye
molecule, depicts a half-time (t1/2) of 8.27 µs. In the presence of
both of the copper complexes, the decay of the signals are
found to be accelerated, suggesting that the dye cations are
regenerated by the Cu(I) species. The regeneration half-time
substantially smaller Voc deficit of only 180 mV is present in
[
Cu(tme)]2+/+-based devices.
The incident photon-to-current conversion efficiency (IPCE)
spectra of DSCs incorporating these two electrolytes are
presented in Figure 5b. The devices based on [Cu(tme)]2+/+
electrolyte illustrate higher IPCE values over the entire region
tested, with a broader IPCE plateau of over 85% in the range of
4
00–600 nm. This result is consistent with the current density
obtained from the J–V measurements. The IPCE spectrum of
Cu(tmby)
]2+/+-based devices was also evaluated as shown in
2+/+
[
2
(t1/2) values are measured to be 1.0 and 0.5 µs for [Cu(tpe)]
and [Cu(tme)]2 , respectively. According to the equation: φreg
=
+/+
Figure S7b in the Supporting Information. Notably, a dramatic
decrease of IPCE values around 450 nm can be found for such
an electrolyte as compared to that of [Cu(tme)]2+/+, largely due to
k
reg/(kreg + krec), the dye regeneration yield of [Cu(tme)]2+/+-based
electrolyte is calculated to be 94.0%, which exceeds that of
5
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RESEARCH ARTICLE
Figure 6. Evolution of (a) normalized PCE and (b) photovoltaic parameters for DSCs based on [Cu(tme)]2+/+ and [Cu(tmby)
]2+/+ electrolytes measured over 400 h
2
under continuous full sun illumination with a UV cut-off filter at temperature of 25 ℃ and humidity around 50%. (c) Nyquist plots of fresh and aged devices with
2
+/+
2+/+
2+/+
[
[
Cu(tme)]
Cu(tmby)
and [Cu(tmby)
2
]
electrolytes measured in the dark at a bias potential of 0.9 V. (d) Nyquist plots of symmetrical devices containing [Cu(tme)]
and
]2+/+ electrolytes with and without TBP measured in the dark at a bias potential of 0 V.
2
[
Cu(tpe)]2+/+ complexes (87.9%). Electrochemical impedance
these two systems (Figure 6b), whereas a more pronounced FF
loss from 0.71 to 0.58 was observed for DSCs containing
spectroscopy (EIS) measurements were further carried out to
gain sights into the charge recombination kinetics of these
devices. The obtained Nyquist plots together with an equivalent
circuit are presented in Figure S8a and Figure S8b and the
fitting results from mid-frequency semicircle are listed in Table
S4 in the Supporting Information. Figure 5d illustrates the
electron lifetime (τ ) derived from EIS measured in the dark as a
e
function of varied bias potentials. A longer electron lifetime (a
factor of ~10) is observed at a given potential for the devices
2+/+
[Cu(tmby)
2
]
. The significantly reduced slope near the open-
circuit voltage point of the J−V curve after aging suggests a
higher serious resistance for the bipyridyl-based devices as
shown in Figure S9 in the Supporting Information.
To verify this hypothesis, EIS measurements were
conducted for the devices before and after aging. As illustrated
in Figure 6c, the charge transfer resistance at counter electrode
(Rct) for [Cu(tmby)
2
]2+/+-based devices is more than two times
2
+/+
with [Cu(tme)] , which is most likely correlated with an
improved ηcol
The long-term durability of DSCs based on [Cu(tme)]2
was further evaluated under continuous one sun irradiation in
comparison to the reference devices incorporating
Cu(tmby)
]2+/+. A mixture solvent of ACN and 3-methoxy-
higher (from 2.9 to 7.2 Ω) after aging, whereas the
corresponding value for [Cu(tme)]2+/+ remains essentially
unperturbed (~1.7 Ω). The discrepancy observed in Rct should
be most likely due to the fact that the coordination/dissociation of
.
+/+
2
+
TBP retards the regeneration of the [Cu(tmby)
2
]
species at the
[
2
counter electrode.[36,37] To gain deeper insights into the effect of
TBP on Rct, impedance spectra were recorded for these two
electrolytes with and without TBP in symmetrical cells consisting
of two identical PEDOT films deposited on fluorine-doped tin
oxide conducting glasses. The spectra were fitted to an
equivalent circuit as illustrated in Figure S8c in the Supporting
propionitrile (MPN) (4:1, vol:vol) was employed to reduce the
volatility of the electrolyte. Figure 6a and Figure 6b presents the
evolution of normalized PCEs and photovoltaic parameters for a
period of over 400 h, and corresponding data recorded at
different stages are collected in Table S5 in the Supporting
Information. As shown in Figure 6a, the devices constructed with
Information, except for the Rrec–C
μ
element. Apparently, Rct for
2+/+
2+/+
2
[
Cu(tme)]
after aging, which surpassed that of its bipyridyl counterpart
~79%). Notably, both Voc and Jsc remained relatively stable for
electrolyte maintained ~91% of the initial efficiency
[Cu(tme)]
is only slightly changed from 2.7 to 3.6 Ω cm in the
presence of TBP (Figure 6d). By stark contrast, the value of Rct
for [Cu(tmby)
]2+/+ is markedly increased by three times (from 2.7
(
2
6
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
2
to 11.7 Ω cm ) upon addition of TBP. This result agrees well with
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the complicated redox behavior observed for the latter
electrolyte with additional TBP as manifested by the CV
measurements (Figure 2d). Besides, it is also notable that the
diffusion resistance (R
also considerably enlarged from 36.7 to 102.6 Ω cm when TBP
[
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2
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[7]
[8]
2
On the contrast, only a marginal augment of R
d
(6.6 Ω cm ) is
found for [Cu(tme)]2+/+, which might originate from the increased
viscosity of the solution induced by addition of TBP. Overall, the
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the minor changes of Rct and R in the presence of TBP. Here, it
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Entry for the Table of Contents
For the first time, copper complexes bearing pentadentate ligands, namely [Cu(tpe)]2 and [Cu(tme)]2+/+, have been reported as
redox mediators in dye-sensitized solar cells. Strikingly, devices based on [Cu(tme)]2 complexes exhibit an excellent long-term
stability and maintain more than 90% of the initial efficiency under continuous illumination, which outperform the reference devices
employing the state-of-the-art bipyridyl counterpart.
+/+
+/+
9
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