Towards efficient sustainable full-copper dye-sensitized solar cells

Article abstract of DOI:10.1039/c9dt00790c

Two new heteroleptic copper(i) sensitizers bearing 6,6′-dimethyl-2,2′-bipyridine-4,4′-dibenzoic acid, to anchor the dye on the titania surface, and a π-delocalized 2-(R-phenyl)-1H-phenanthro[9,10-d]imidazole (R = NPh₂ or O-hexyl) ancillary liga

Full text of DOI:10.1039/c9dt00790c

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MAGNI, A. Colombo, F. Fagnani, D. Roberto, F. Melchiorre, P. Biagini and S. Fantacci, Dalton Trans., 2019,
DOI: 10.1039/C9DT00790C.
Volume 45 Number
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7
January 2016 Pages 1–398
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DOI: 10.1039/C9DT00790C
Journal Name
ARTICLE
1
Received 00th January 20xx,
Towards efficient sustainable full-copper dye-sensitized solar cells
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
a,b
a,b
a,b*
a
Claudia Dragonetti, Mirko Magni, Alessia Colombo, Francesco Fagnani, Dominique
a,b
c
c
d
Roberto, Fabio Melchiorre, Paolo Biagini and Simona Fantacci
Two new heteroleptic copper(I) sensitizers bearing 6,6’-dimethyl-2,2’-bipyridine-4,4’-dibenzoic acid, to anchor the dye on
the titania surface, and a -delocalized 2-(R-phenyl)-1H-phenanthro[9,10-d]imidazole (R = NPh or OHexyl) ancillary ligand
were prepared and well characterized. Their performance as dyes in DSSCs is quite similar to that of the related complex
2
–
–
3
bearing 2,9-dimesityl-1,10-phenanthroline as ancillary ligand, when using the common I /I redox couple or homoleptic
copper complexes as electron shuttles. The experimental results along with theoretical calculations confirm the great
potential of full-copper DSSCs.
complex (N719) [13] are amongst the most performing
–
–
Introduction
photosensitizers with the iodide/triiodide couple (I /I ) as
3
electrolyte [14,15]. Cyclometalated ruthenium complexes,
To have enough clean and low cost energy for the future is one
of the world’s most important challenges. The sun could be
the answer to our energy needs, since more energy from
sunlight strikes the earth in one hour than all the energy
consumed on the planet in one year.
Following the discovery of Grätzel-type dye-sensitized solar
cells (DSSCs) as a cheap and convenient way for harnessing the
energy of the sun and converting it into electricity [1], there
has been a lot of effort to improve the photo-conversion
efficiency (PCE), trying to optimize every components of the
device. In particular, an impressive amount of work has been
devoted to optimize the sensitizer [2-7] and the redox
mediators [7-12].
characterized by a better stability, represent good alternatives
for application in DSSCs [16-23]. However, these sensitizers are
not ideal because ruthenium is one of the rarest and most
expensive metal. For this reason, there is currently significant
interest in the use of earth-abundant metals [5-7, 24]. Sauvage
and co-workers [25] were the first to use copper(I) complexes
as sensitizers. The rationale for their application is based on
similarities between the photophysical properties of copper(I)
and ruthenium(II) complexes [26-27].
In the last few years, an exponential amount of work has been
dedicated to the use of cheap and environmental-friendly
copper(I) complexes as sensitizers in DSSCs with the I /I
redox couple as electrolyte [5-7, 24, 28-32]. The actual 4.66%
record efficiency, corresponding to a remarkable 63% relative
efficiency (ηrel [33]) respect to a N719-sensitized control cell
set at 100%, was obtained with a copper(I) complex having as
anchoring ligand a 4,4’-dicarboxylic acid-6,6’dimesityl-2,2’-
–
–
3
Coordination compounds based on Ru with bipyridine ligands
such
as
cis-di(thiocyanato)bis(2,2’-bipyridine-4,4’-
dicarboxylate)Ru(II) (N3) and the related doubly deprotonated
bipyridine and, as ancillary ligand,
a
4,4’-bis(N,N
a. Dipartimento di Chimica dell’Università degli Studi di Milano, UdR-INSTM, via
Golgi 19, I-20133, Milano, Italy. E-mail: alessia.colombo@unimi.it.
ISTM-CNR and SmartMatLab dell’Università degli Studi di Milano, via Golgi 19, I-
diethylaminestyryl)-6,6’dimethyl-2,2’-bipyridine [34]. A lower
efficiency (1.46%) was reached by substitution of the latter
ligand with 4,4’-dimethyl-6,6’-dimethyl-2,2’-bipyridine due to a
lower light harvesting efficiency [34].
b.
2
0133, Milano, Italy.
c.
Research Center for Renewable Energy & Enviromental Istituto Donegani, Eni
S.p.A., via Fauser 4, I-28100, Novara, Italy. E-mail: paolo.biagini@eni.com
Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM,
Via Elce di Sotto 8, I-06213, Perugia, Italy. E-mail: simona@thch.unipg.it.
d.
Recently, the observed long term incompatibility of copper
1
13
complexes with I
–
/I3 , due to the low solubility of CuI [35,36],
–
†
Electronic supplementary information (ESI) available: [ H and C NMR, mass spectra,
fabrication and “current density-voltage” characteristics of DSSCs].
See DOI: 10.1039/x0xx00000X
and the fact that iodine is a non-abundant element [37] led to
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J. Name., 2013, 00, 1-3 | 1
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Journal Name
the preparation of iodine-free copper(I) solar cells. The use of semiconductor to the dye and to enhance electron transfer
View Article Online
2
+/3+
DOI: 10.1039/C9DT00790C
[
Co(2,2’-bipyridine)
complex bearing
diyl)bis(4,1-phenylene)bis(phosphonic acid) and
diphenylaminophenyl)-1H-phenanthro[9,10-d]imidazole
3
]
, as redox couple, and a copper(I) from the electrolyte [5a-7].
(6,6’-dimethyl-[2,2’-bipyridine]-4,4’- In the present work we study DSSCs based on two novel Cu(I)
a
2- photosensitizers, D2 and D3, having the anchoring ligand of
the previously studied D1 [48] but more -delocalized ancillary
(
derivative, as dye, afforded a DSSC with a 1.73% efficiency ligands in order to increase light harvesting. To define a
–
–
(
25% ηrel) similar to that obtained with the reference I /I
3
relationship between the structure and the electronic/optical
electrolyte [36]. This was an important step towards the properties of the investigated copper-based dyes, calculations
development of stable iodide-free copper(I) solar cells [38].
have been performed on D1-D3 sensitizers by employing
Besides, it was found that Cu+ mediators can outperform Density Functional Theory (DFT) and Time Dependent DFT
/2+
both iodine-based [39-42] and Co-based [43-44] electrolytes in methods.
2
+
combination with ruthenium(II) dyes, Zn porphyrin or organic
sensitizers. Fast dye regeneration and tunability of both redox
potential and electron transfer rate make Cu mediators able to
solve the thermodynamic and kinetic dichotomies of the
device operation [45-47].
These observations were the springboard for the development
of full-copper DSSCs in which a copper(I) dye is coupled with a
copper(I)/(II) electron shuttle [48, 49]. Thus, with the aim of
achieving sustainable energy conversion by solar cells with
earth-abundant components, Housecroft et al have
investigated the performances of DSSCs based on heteroleptic
bis(diimine)copper(I) sensitizers coupled with homoleptic
bis(diimine)copper(I)/(II) complexes; the best photoconversion
efficiency (2.06% corresponding to 38.1% relative to N719 set
at 100%) was achieved by using a phosphonic acid anchor, and
+
BF4-
COOH
N
N
N
N
Cu
COOH
D1
+
BF4-
COOH
N
N
N
N
N
N
Cu
N
4
,4′-dimethoxy-6,6′-dimethyl-2,2′-bipyridine as the ancillary
5
ligand in the dye and in the electrolyte [49]. In parallel we
found that the use of a simple heteroleptic copper dye (D1, Chart
COOH
D2
1
), bearing one 2,9-dimesityl-1,10-phenanthroline and a 6,6’-
+
BF4
-
dimethyl-2,2’-bipyridine-4,4’-dibenzoic acid anchoring ligand,
in combination with an homoleptic copper(I/II) complex bearing
COOH
HexO
two 2-n-butyl-1,10-phenanthrolines (E3/E4,
electron shuttle allows to reach a good photoconversion
efficiency (ca 2.0% corresponding to ca 22%
than that obtained with the conventional I /I
Chart 1) as
N
N
N
N
N
N
Cu
η
rel) although lower
–
–
5
3
couple as
electrolyte [48]. The use of the same copper dye with a copper
electron shuttle bearing two 2,9-dimethyl-1,10-phenanthroline
ligands (E1/E2) led to a lower efficiency [48].
COOH
D3

PF6]
+
[PF6]
[
These first examples of full-copper solar cells [48,49] open a
new route for cheap and environmentally friendly DSSCs. Now
work should be devoted to improve the photoconversion
efficiency by tuning the photophysical and redox properties of
the copper complexes. In the present work, we focus the
attention on the nature of the heteroleptic copper(I)
sensitizer.
In the design of such complexes, one keeps in mind that one
2
ligand should carry a group suitable for anchoring to TiO and
the second ligand be electronically and structurally tuned to
harvest light, to support efficient electron transfer from
electrolyte to dye and to provide directionality for electron
N
N
Cl
N
Cu
N
Cu
N
N
N
N
E1
E2

 [PF6]
2

[PF6]
3
3
N
N
N
Cu
N
Cu
N
N
N
N
3
3
E3
E4
injection in the TiO
on the ligand anchored on the TiO
2
conduction band. The LUMO should reside
surface so that the electron
2
is localized on this domain in the excited state (ideal for
injection into the semiconductor) whereas the HOMO of the
excited state should be localized on the ancillary ligand to
minimize back-migration of an electron from the
Chart 1. Chemical structures of the investigated dyes (D1, D2,
D3) and of the two couples of copper-based redox mediators
(E1/E2 and E3/E4).
2
| J. Name., 2012, 00, 1-3
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Synthesis of I3
Experimental
View Article Online
First the 3-(hexyloxy)benzaldehyde was prDeOpaI:r1e0d.10a3s9/fCo9llDowT0s0.7I9n0Ca
flask under argon were dissolved 3-hydroxybenzaldehyde (500 mg,
General comments
Solvents for the syntheses were dried by standard procedures: N,N-
dimethylformamide (DMF) was dried over activated molecular
sieves and toluene was distilled over Na/benzophenone. All
reagents were purchased from Sigma-Aldrich and were used
without further purification. Reactions requiring anhydrous
conditions were performed under argon or nitrogen. Thin layer
chromatography (TLC) was carried out with pre-coated Merck F254
silica gel plates. Flash chromatography (FC) was carried out with
Macherey-Nagel silica gel 60 (230-400 mesh).
4
2 3
.09 mmol) and K CO (5.7g, 10 mmol) in dry acetonitrile (15 mL).
After 5 min, n-hexylbromide was added and the mixture was heated
at reflux and stirred overnight. After cooling to r.t. the solvent was
removed and the residue dissolved in dichloromethane and washed
with water. The organic phase was dried on Na
the solvent removed under reduced pressure. The residue was
purified by flash chromatography on silica gel (Hex/Acetate/CH Cl
/2/1) to give the desired product in 67% yield (564 mg). H-NMR
400 MHz, CDCl ): δ ppm 10.00 (s, 1H, CHO) 7.47 (d, J = 0.8Hz, 1H,
2 4
SO , filtered, and
2
2
1
7
(
3
6
2
4
H ), 7.46-7.44 (m,1H, H ), 7.42-7.39 (m, 1H, H ), 7.22-1.17 (m, 1H,
H ), 4,04 (t, J = 6.6 Hz, 2H, OCH
Synthesis of new ligands (Scheme 1).
5
hex
hex
2
), 1.89-1.73 (m, 2H, CH
2
), 1.55-
The synthesis of ligand L2 and L3 was performed by reaction of the
known 2,9-dimesityl-1,10-phenanthroline-5,6-dione [36, 50, 51]
with the suitable aldehyde affording the imidazole ring (I2 and I3)
which was then alkylated to afford in good yields L2 and L3. The
hex
hex), 0.93 (t, J
1
=
.43 (m, 2H, CH
2
), 1.37 (td, J
1
= 7.1 J = 3.6, 6H, CH
2
2
hex
7.0 Hz, 3H, CH
3
). The precursor I3 was then prepared as follows:
A solution of 2,9-dimesityl-1,10-phenanthroline-5,6-dione (37 mg,
.083 mmol), 3-(hexyloxy)benzaldehyde (29.5 mg, 0.108 mmol) and
NH OAc (160 mg, 2.07 mmol) in 6 mL of glacial acetic acid was
0
6
,6‘-dimethyl-2,2‘-bipyridine-4,4‘-dibenzoic acid was obtained as
4
previously reported [29].
stirred under reflux overnight. After cooling to room temperature,
water was added (15 mL) and the pale yellow precipitate was
collected by filtration and washed with water. The crude product
was dissolved in dichloromethane (30 mL) and washed twice with
water (2x40mL). The organic phase was dried on Na
solvent removed under reduced pressure. The residue was purified
by flash chromatography on silica gel (CH Cl /CH OH 99/1) affording
the pure product as a yellow solid (30 mg, 50%). H-NMR (300 MHz,
DMSO-d ): δ ppm 13.96 (s broad, 1H, NH ), 9.01 (d, J = 8.3 Hz, 2H,
6
H , H ), 7.92 (d, J = 7.8 Hz, 1H, H ), 7.85 (s, 1H, H ), 7.74 (d, J = 8.2
Hz, 2H, H , H ), 7.53 (t, J = 8.0 Hz, 1H, H ), 7.10 (d, J = 8.0 Hz, 1H,
H ), 6.96 (s, 4H, H ), 4.08 (t, J = 6.4 Hz, 2H, OCH
Synthesis of I2
A solution of 2,9-dimesityl-1,10-phenanthroline-5,6-dione (34 mg,
0
.076 mmol), 4-(diphenylamino)benzaldehyde (27.0 mg, 0.098
mmol) and NH OAc (147 mg, 1.9 mmol) in 6 mL of glacial acetic acid
4
2 4
SO and the
was stirred at reflux overnight. After cooling to room temperature,
water was added (15 mL) and the pale yellow precipitate was
collected by filtration and washed with water. The crude product
was dissolved in dichloromethane (20 mL) and washed twice with
2
2
3
1
imid
water (2x30mL). The organic phase was dried on Na
solvent removed under reduced pressure. The residue was purified
by flash chromatography on silica gel (CH Cl /CH OH 99/1) affording
2
SO
4
and the
4
7
13
14
3
8
12
11
mes
hex
2
2
3
), 2.54 – 2.46
2
1
the pure product as a yellow solid (34 mg, 64%). H-NMR (400 MHz,
hex
mes
mes
(m, 4H, CH
2
), 2.28 (s, 6H, p-CH
3
), 2.00 (s, 12H, o-CH
3
), 1.85 –
imid
DMSO-d
6
): δ ppm 13.64 (s broad, 1H, NH ), 8.98 (d, J = 8.4 Hz, 2H,
hex), 1.40 – 1.28 (m, 3H, CH
hex 13
1
.71 (m, 4H, CH
2
3
). C-NMR (75 MHz,
4
7
3
8
11
H , H ), 8.21 (d, J = 8.4Hz, 2H, H , H ), 7.73 (d, 2H, H ), 7.39 (t, J =7.8
DMSO) δ 160.0, 157.8, 151.3, 144.3, 138.9, 137.5, 136.1, 132.3,
1
2
1
3
17
12 14
15
16
Hz, 4H, H , H ), 7.22–7.08 (m,8H, H ,H , H , H ), 6.97 (s, 4H,
31.1, 130.9, 129.0, 125.3, 119.4, 116.2, 113.0, 68.5, 31.9, 29.8,
9.6, 26.1, 22.9, 21.6, 20.9, 14.8.
mes
mes
mes
H
), 2.30 (s,6H, p-CH
3
), 2.03 (s, 12H, o-CH
3
).
Synthesis of L2
A solution of I2 (58 mg, 0.083 mmol) and Cs
Synthesis of L3
2
3
CO (173 mg, 0.53
2 3
A solution of I3 (33.7 mg, 0.0532 mmol) and Cs CO (110 mg, 0.340
mmol) in 17.5 mL of dry DMF was heated at 100 °C under argon for
0 minutes, and then n-hexyl iodide (0.038 mL, 3.1 mmol) was
mmol) in 4.8 mL of dry DMF was heated at 100 °C under argon for
0 minutes, and then n-hexyl iodide (0.024 mL, 0.165 mmol) was
3
3
injected into the flask and the mixture was further stirred at 100 °C
for 4 hours. After cooling to room temperature the solvent was
removed and the crude product was purified by flash
injected into the flask and the mixture was further stirred at 100 °C
for 4 hours. After cooling to room temperature the solvent was
removed and the crude product was purified by flash
chromatography on silica gel (CH
mg, 71%). H-NMR (400 MHz, CD
2
Cl
Cl
2
/CH
3
OH 99.5/0.5) to give L2 (46
chromatography on silica gel (CH
2
Cl
2
/CH
Cl
3
OH 99.5/0.5) to give L3
2
): δ ppm 9.10 (d, J = 8.2
1
2
): δ ppm 9.07 (d, J = 8.2 Hz, 1H,
1
2
(
29.4 mg, 76%). H-NMR (300 MHz, CD
2
7
4
3
8
H ), 8.70 (d, J = 8.6 Hz, 1H, H ), 7.67 (d, J = 8.2 Hz, 2H, H , H ), 7.64
7
4
3
8
Hz, 1H, H ), 8.69 (d, J =8,4 Hz, 1H, H ), 7.69-7.64 (m, 2H, H , H ),
7
1
1
13
17
(
d, J = 8.6 Hz, 2H, H ), 7.38 (t, J = 7.8 Hz, 4H, H , H ), 7.26 (t, J = 6.7
Hz, 6H, H , H , H ), 7.18 (t, J =7.3 Hz, 2H, H ), 7.01 (s, 2H, H ),
.99 (s, 2H, H ), 4.75 (t, J = 7.4 Hz, 2H, NCH
CH
13
11
12
.58 – 7.47 (m, 1H, H ), 7.39 – 7.29 (m, 2H, H , H ), 7.20 – 7.07
1
4
15
16
12
mes
14
mes
hex
(m, 1H, H ), 6.90 (s, 4H, H ) 4.72 (t, J = 7.4 Hz, 2H, NCH
2
), 4.12
mes
hex
6
2
), 2.37 (s, 6H, p-
hex
mes
(t, J = 6.5 Hz, 2H, OCH
2
), 2.28 (s, 6H, p-CH
3
), 2.18 – 1.97 (m,
mes
mes
mes
), 2.14 (s, 6 H, o-CH
3
), 2.11 (s, 6H, o-CH
3
), 2.05 (t, J = 7.4
mes
Ohex
Nhex
3
1
2
0
4H, p-CH
, CH
), 1.95 – 1.77 (m, 2H, CH
), 1.52-1.50 (m,
3
2
2
hex
Hz, 2H, CH
2
hex), 1.34–1.28 (m, 6H, CH
2
hex), 0.87-0.84 (m, 3H, CH
3
).
Ohex
Ohex), 1.31-1.25 (m, 6H, CH
Nhex
H, CH
.95 (t J = 6.9 Hz, 3H, CH
2
), 1.46 – 1.35 (m, 4H, CH
2
2
),
C
1
3
C-NMR (75 MHz, DMSO) δ 158.7, 157.5, 154.2, 149.6, 147.7,
45.1, 144.6, 138.9, 138.4, 137.7, 137.5, 136.4, 136.3, 131.1, 130.4,
29.9, 128.7, 128.6, 128.5, 125.6, 125.2, 124.3, 124.2, 124.1, 122.5,
18.9, 47.2, 31.4, 30.5, 26.3, 22.8, 21.2, 20.5, 20.4, 14.1. Elemental
Ohex
Nhex). 13
3
), 0.86 (t, J= 6.3 Hz, 3H, CH
3
1
1
1
3
NMR (75 MHz, CDCl ) δ 159.9, 158.9, 157.7, 154.1, 145.1, 144.6,
1
1
1
2
C
7
38.7, 138.2, 137.7, 137.5, 137.1, 136.2, 136.2, 132.4, 130.5, 130.2,
28.7, 128.6, 128.5, 125.5, 125.3, 124.4, 122.9, 122.2, 118.9, 116.4,
16.3, 68.7, 47.2, 32.0, 31.9, 31.4, 30.6, 29.6, 26.4, 26.1, 26.0, 23.0,
2.8, 21.1, 20.5, 20.5, , 14.2, 14.0. Elemental Analysis: Calcd. for
53 5
Analysis: Calcd. for C55H N : C 84.25, H6.81, N 8,93. Found. C 84.30,
H 6.82, N 8.91.
49 56
H N
4
O : C, 82.08; H, 7.87; N, 7.81 Found. C, 82.20; H, 7.89; N,
.80.
This journal is © The Royal Society of Chemistry 20xx
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Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
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ARTICLE
Journal Name
+
Synthesis of copper complexes
View Article Online
6 5
Found. C 69.78, H 5.95, N 6.52. MS (ESI+FTICR) C75H76CuN O
] was obtained as described m/z: 1203.5185
The copper precursor [Cu(CH
3
CN)
4
][BF
4
DOI: 10.1039/C9DT00790C
by Kubas G. et al [52] whereas the redox mediators (E1/E2 and
E3/E4) and D1 were prepared as we previously reported [48]. The
new copper complexes D2 and D3 were synthesized as D1,
introducing initially on copper the sterically hindered ligand.
DFT modeling
All the calculations were performed with Gaussian09 (G09), without
any symmetry constraints [53]. The molecular geometry of D1-D3
dyes has been optimized by using a 6-311G** basis set [54] and the
PBE0 exchange–correlation (xc) functional [55] integrated with the
Synthesis of D2
The 2,9-dimesityl-1,10-phenanthroline derivative L2 (23 mg, 0.029 D3-BJ model to include the dispersion interactions [56]. We
mmol) in 5mL of dry DMF was added under argon to a solution of optimized the geometries in dichloromethane solution including
the Cu(I) precursor [Cu(CH CN) ][BF ] (9.2 mg, 0.029 mmol) in dry solvation effects by means of the conductor-like polarizable
3 4 4
DMF (5mL). The mixture was stirred for 1h at room temperature continuum model (C-PCM) as implemented in G09 [57]. TDDFT
and then 6,6‘-dimethyl-2,2‘-bipyridine-4,4‘-dibenzoic acid (12.4 mg, calculations of the lowest singlet-singlet excitations were
0
.029 mmol) in dry DMF (3 mL) was added. The obtained red performed in dichloromethane solution using the PBE0 xc
mixture was stirred at room temperature for 1h and then the functional and the 6311g** basis set. The non equilibrium version
solvent was removed under reduced pressure affording the desired of C-PCM was employed for TDDFT calculations, as implemented in
product which was crystallized from CH
2
Cl
2
/MeOH (v/v 7/3) to give G09 [58]. To simulate the optical spectra, the 40 lowest spin-
allowed singlet-singlet transitions were computed on the ground
H-NMR (400 MHz; MeOD): δ ppm 9.38 (d, J = 8.2 Hz, 1H, H ), 9.23 state geometry. Transition energies and oscillator strengths were
a red complex which was dried under vacuum (37 mg, 93% yield).
1
7
4
A
D,
F
C
(
d, J = 8.5 Hz, 1H, H ), 8.49 (s, 2H, H ), 8.11-8.04 (m, 8H, H H , H
interpolated by a Gaussian convolution with a σ value of 0.12 eV.
E
3,
8
B
H ), 7.75 (d, J = 8.4 Hz, 2H, H H ), 7.68 (s, 2H, H ), 7.42-7.39 (m, J =
.7 Hz, 5H, H H , H ), 7.32 – 7.13 (m, 9H, H H , H , H , H ),
.45 (s, 2H, H ), 6.43 (s, 2H, H ), 4.96-4.94 (m, 2H, NCH
1
1, 17
13
12 14
15
16
13
7
6
Results and discussion
mes
mes
hex
2
), 3.01
), 2.21 – 2.01 (m, 12H, o-
bip), 1.43 – copper(I) dyes D2 and D3 (Chart 1) bearing one -delocalized 2,9-
), 1.85 (s, 3H, CH
3
mes
mes
We prepared, with the HETPHEN synthetic method [59], the new
(s, 3H, o-CH
3
), 2.87 (s, 2H, o-CH
3
), 1.86 (s, 3H, CH
), 0.98 – 0.78 (m, 5H, CH
mes
mes
bipy
CH
1
3
, p-CH
3
3
hex
hex,
hex 13
.17 (m, 6H, CH
2
2
CH
3
). C NMR (100 dimesityl-1,10-phenanthroline (L2 and L3), where the mesityl
3
MHz, CDCl ) δ 157.9, 157.2, 156.9, 152.3, 150.1, 148.8, 146.9, 142.2,
groups provide enough steric hindrance to avoid the formation of
homoleptic complexes and prevent geometric changes, and a 6,6’-
dimethyl-2,2’-bipyridine-4,4’-dibenzoic acid, chosen as anchoring
ligand because the related homoleptic copper complex had a high
1
1
1
4
41.9, 138.1, 138.0, 137.3, 136.9, 135.8, 134.8, 131.4, 130.8, 130.4,
29.4, 127.3, 126.8, 125.3, 124.1, 123.3, 123.1, 121.6, 121.2, 119.7,
16.9, 53.6, 53.3, 53.0, 52.7, 52.5, 48.3, 48.1, 47.9, 47.7, 47.5, 47.3,
7.1, 46.6, 31.3, 30.6, 29.6, 25.4, 25.1, 22.1, 19.8, 19.2, 12.9.
efficiency as dye [29].
R
Elemental Analysis: Calcd. for C81
H73BCuF
4
N
7
O
4
: C 71.60, H 5.41, N
+
7
7 4
.22. Found. C 71.77. H 5.41, N 7.29. MS (ESI+FTICR) C81H73CuN O
m/z: 1270.5052
O
N
O
N
HN
N
N
R
Synthesis of D3
CHO
The 2,9-dimesityl-1,10-phenanthroline derivative L3 (20.5 mg,
N
0
.028 mmol) in 5mL of dry DMF was added under agon to a solution
NH4OAc; CH3COOH
118 °C, 18 h
3 4 4
of Cu(I) precursor [Cu(CH CN) ][BF ] (9.3 mg, 0.028 mmol) in dry
DMF (5mL). The mixture was stirred for 1h at room temperature
and then 6,6‘-dimethyl-2,2‘-bipyridine-4,4‘-dibenzoic acid (12.2 mg,
I2 : R=
I3 : R= m-OHex
p-NPh2
n-HexI; Cs2CO3
100 °C, 4 h
0
.028 mmol) in dry DMF (8 mL) was added. The obtained red
mixture was stirred at room temperature for 1h and then the
solvent was removed under reduced pressure affording the
8
8
7
7
desidered product which was crystallized from CH
2
Cl
2
/MeOH (v/v
HexO
1
2
11
14
11
N
N
N
N
N
N
7
9
9
/3) to give a red complex which was dried under vacuum (36 mg,
N
13
1
7
17
8% yield). H NMR (400 MHz, CD
2
Cl
2
) δ 9.43 (d, J = 8.2 Hz, 1H, H ),
N
N
1
2
4
A
D
16
13
.01 (d, J = 8.6 Hz, 1H, H ), 8.14 (s, 2H, H ), 8.06 – 7.86 (m, 6H, H
4
5
5
4
F
3
8
13
14
11
12
B
15
14
3
3
H , H , H ), 7.63-7.54 (m, 4H, H H , H , H ), 7.39 (s, 2H, H ),
7
NCH
C
mes
L3
.22 (d, J = 8.3 Hz, 2H, H ), 6.44 (d, J = 4.4 Hz, 4H, H ), 4.84 (t, 2H,
L2
hex
hex
mes
2
), 4.14 (t, J = 6.3 Hz, 2H, OCH
2
), 2.16 (s 6H, o-CH
3
),
mes
mes,
bipy
1
1
.89 (s, 6H, o-CH
7H, p-CH
3
), 1.77 (s, 9H, p-CH
3
CH
), 1.06 – 0.67 (m, 8H, CH
Nhex, CH
Cl ): δ 200.4, 160.0, 158.1, 157.8, 157.0,
3
), 1.63 – 1.20 (m,
mes
hex
Ohex
hex
3 4 4
1) [Cu(CH CN) ]BF ; DMF, r.t. 1h
3
,CH
2
3
3
,CH
2
).
R
N
N
N
1
3
D2, D3
C-NMR (100 MHz; CD
2
2
N
2) HOOC
D
F
COOH
1
1
1
5
2
55.6, 152.4, 148.9, 142.7, 142.2, 138.6, 138.5, 137.7, 137.3, 137.0,
35.4, 132.3, 131.4, 130.5, 130.4, 129.3, 127. 3, 127.1, 125.6, 124.3,
23.7, 122.1, 120.2, 117.2, 116.8, 116.5, 68.8, 54.5, 54.2, 53.8, 53.4,
3.1, 51.2, 50.4, 47.4, 37.2, 36.8, 32.3, 32.0, 31.4, 30.6, 30.1, 29.6,
6.4, 26.2, 26.1, 25.1, 23.0, 22.8, 21.0, 20.3, 14.2, 14.0. Elemental
E
5
C
B
A
; DMF, r.t. 1h
N
N
L2 : R= p-NPh2
L3 :
R= m-OHex
Scheme 1: Synthesis of ligands L2-L3 and related complexes D2-D3.
4 6 5
Analysis: Calcd. for C75H76BCuF N O : C 69.73, H 5.93, N 6.51
4
| J. Name., 2012, 00, 1-3
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The alkylation of the imidazole is important to improve the plots are reported. The HOMO-LUMO gap enerVgieyw AisrticlaelOmnolinset
solubility and to avoid the easy deprotonation of the NH group; in coincident in the three dyes (D1=2.94 eV, D2D=O2I.:9180.1e0V3;9D/C39=D2T.90907e9V0)C.
addition, the presence of a long alkyl chain inhibits the dye The HOMO/HOMO-2 of D1 and D3 are mainly d-states of the Cu
aggregation on the semiconductor surface [24] and protects the centre, while in D2, the triphenylamino substituent orbital insert as
surface of the semi-conductor preventing the reaction between the HOMO-2 among the d-type Cu orbitals. In D1-D3 the LUMO is
oxidized form of the redox mediator and the electrons injected in delocalized on the bipyridine ligand, while the LUMO+1 is a π*
the TiO
dyes can be easily prepared following the pathways shown in show substantial contribution from the COOH groups which is a
Scheme 1 (see Experimental). prerequisite for an effective charge injection from the dye excited
2
conduction band. The new ligands and related copper(I) orbital of the phenanthroline ligand. The LUMOs of the three dyes
2
The performance of D1, D2 and D3 in DSSCs was investigated state to the TiO conduction band.
in the presence of three different electrolytes, namely the
–
–
common I /I
3
electrolyte and two couples of copper electron
shuttles (E1/E2 and E3/E4, Chart 1), prepared as previously
reported [48].
The UV-visible absorption spectrum of D1, D2 and D3 in CH
2 2
Cl
are shown in Figure 1 along with that of E3, for comparison.
As expected for the increase of the conjugation of the ancillary
ligands, the band at 478 nm of D1 becomes more intense and is
red-shifted on going to D2 and D3 (Table 1), justifying their use in
the design of dyes for DSSCs.
Figure 2. Left: top and side views of D1 optimized geometry. Right:
schematic representation of the energy levels of the frontier
molecular orbitals of D1-D3 dyes along with the isodensity surface
plots of D1 HOMO-1, HOMO, LUMO and LUMO+1 (isodensity
contour=0.3).
The absorption spectra of D1-D3 sensitizers have been simulated to
assign the lowest absorption bands in the visible region. In Figure 3
the simulated absorption spectra for the D1-D3 dyes are reported.
The computed spectra satisfactorily reproduce the main visible
absorption band of the experimental spectra. The maxima of the
simulated spectra have been computed at 495 nm, 508 nm and 501
nm from D1 to D3, in good agreement with the experimental λmax,
see Table 1. Also the experimental shoulder at lower energies is
well reproduced even though this feature is slightly red shifted and
its intensity partly overestimated. In the three dyes the main
absorption band is originated by an electronic transition having as
starting state HOMO-1 (Cu d orbital) and as final states both LUMO
and LUMO+1 (delocalized on bipyridine and phenanthroline,
respectively). This intense transition has a MLL’CT transition from
Cu to bypiridine and phenanthroline. Also the shoulder shows a
MLL’CT character involving the same final state of the main
absorption band but having as starting state the HOMO of Cu
character.
2 2
Figure 1. UV-Vis absorption spectra for D1-D3 Cu(I) dyes in CH Cl .
Inset: magnification of the visible window with, for sake of
comparison, absorption spectrum of Cu(I)-based redox mediator E3.
To understand the effect of different ligands in D1-D3 dyes, we
investigated the structural, electronic and optical properties of the
experimentally characterized complexes by means of DFT. The
optimized molecular geometry of D1-D3 complexes core is quite
similar (Figure 2), with Cu-N distances in between 2.04-2.05 Å. The
Cu centre shows a distorted octahedral coordination related to the
presence of the mesityl substituents of the phenanthroline, that are
oriented in such a way to π-stack to the bipypidyl rings, thus
inducing a strain in the bipyridine. We analysed the frontier
molecular orbitals in terms of energy levels and electron density
delocalization. In Figure 2 the energy level of frontier molecular
orbitals and the HOMO (Highest Occupied Molecular Orbital) and
LUMO (Lowest Unoccupied Molecular Orbital) isodensity surface
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Table 2. Main PV parameters of the DSSCs based on copper
View Article Online
a
dyes D1 – D3 in comparison with the ben
D
c
O
h
I
m
: 1
a
0
r
.1
k
03
N
9
7
/C
1
9
9
D
.
T00790C
j
/
SC
V
OC
/mV
802
592
(571)
602
(607)
597
(599)
694
647
370
593
570
573
Dye
N719
D1
Electrolyte
FF
%

rel %b
–
2
mA cm
I–/I
– c
15.4
.1
d
71
71
(74)
75
(75)
74
(72)
36
35
37
8.9
2.5
(2.0)
2.5
(2.3)
2.6
(2.4)
0.7
0.6
0.1
100 %
28 %
(22 %)
28 %
(26 %)
29 %
(27 %)
8.0 %
6.5 %
1.3 %
16 %
3
6
I–/I
–
c
3
d
(
(
(
4.7)
5
.6
I–/I
I–/I
– c
D2
D3
3
d
d
5.2)
5
.8
–
c
3
5.6)
2.9
2.6
0.9
3.8
3.2
2.8
e
D1
D2
D3
D1
D2
D3
E1/E2
E1/E2
E1/E2
E3/E4
E3/E4
E3/E4
e
e
e
e
e
61
67
62
b
1.4
1.2
1.0
13 %
11 %
Figure 3. TDDFT computed absorption spectra and computed
energy and oscillator strengths excitations of D1 (red), D2 (blue)
and D3 (green).
a
2
In the presence of a 16mm black mask. Relative efficiency
c
respect to a N719-sensitized control cell set at 100%. 0.28M
tert-butylpyridine in 15/85 (v/v) mixture of
valeronitrile/acetonitrile; 0.65M N-methyl-N-butylimidazolium
d
A cyclic voltammetry study (CV) on glassy carbon electrode (GC) iodide, 0.025M LiI, 0.04M iodine. 0.05M guanidinium iodide is
was performed to clarify the electrochemical behaviour of the dyes present. 0.17M Cu(I) : 0.017M Cu(II) + 0.1M LiTFSI in CH
e
3
CN.
(
see ESI) and the redox couples (Table 1). Clearly the nature of the
–
–
It turned out that the dye-sensitized solar cells containing I /I
3
as
ancillary ligand in the investigated dyes has a negligible effect on
the oxidation potential of the copper(I) center.
redox shuttle and the novel heteroleptic copper complexes D2 and
D3 as dyes have a photo-conversion efficiency similar to that
reached with D1 despite the higher delocalization of their
ancillary ligand. Their slightly larger fill factor (FF) is
Table 1. Key optical absorption and cyclovoltammetric data for
the investigated complexes.
λ
max /nm
E
1/2,Ox /V
HOMO
/eV
counterbalanced by a slightly lower photocurrent density (jsc)
3
–1
–1
+
a
b
(
10 ε /M cm )
478 (7.6)d
491 (9.3)
490 (12)
455 (8.0)e
741 (0.23)
452 (6.2)e
756 (0.096)e
vs Fc |Fc
whereas their VOC is similar (Table 2). When guanidinium iodide
was added in the electrolyte solution, the photo-conversion
efficiency decreases slightly due to a lower jsc, especially in the case
of D1 (Table 2), confirming its negative effect in the optimization of
the electrolyte solution for efficient Cu(I) based DSSCs [29].
Interestingly, the three dyes are much more performing than the
related heteroleptic complex bearing 2,2’-biquinoline-4,4’-
D1c
D2
D3
0.39f
0.39g
-5.19
-5.19
-5.19
-5.10
-4.84
-4.91
d
d
h
h
h
h
h
0.39
0.30
0.04
0.11
0.11
c
E1
c
e
E2
c
E3
c
E4
-4.91
a
–1
b
With [NBu
4
][PF
6
] 0.1 M, on GC electrode at 0.2 V s . HOMO =
+
–
e[E1/2,Ox+4.8]. e=unitary charge; 4.8 = potential of Fc |Fc couple dicarboxylic acid as anchoring ligand and the same ancillary ligand
c
d
e
f
versus vacuum [60]. Reference 48. In CH
2
Cl
2
. In CH
3
CN. In
of D1 (2,9-dimesityl-1,10-phenanthroline) ( = 0.49%; rel = 7.5%) or
an ancillary ligand quite similar to that of D2 but with a
dianisylamino group instead of the diphenylamino substituent ( =
+
g
CH
2
3
CN/CH
2
Cl
2
25/1, E1/2 Fc |Fc = 0.40 V vs SCE. In CH
3
CN/CH Cl
2
2
+
.4:1, E1/2(Fc /Fc)=0.427V vs SCE; there is a second oxidation
2
process at 0.62 V probably associated with oxidation of the Ph N
h
unit, as previously reported for a similar complex [36]. In
0
.22%; rel = 3.4%) [24], mainly due to a much higher jsc, confirming
+
3
CH CN, E1/2 Fc |Fc = 0.39 V vs SCE.
the particular goodness of 6,6’-dimethyl-2,2’-bipyridine-4,4’-
dibenzoic acid to anchor the dye on the titania surface [29, 48].
–
–
3
The level energy matching between all the investigated dyes and Substitution of the common I /I redox couple by the E1/E2 couple
the Cu-based redox mediator E3/E4 is very promising, being the leads to a significant drop of the DSSCs performance efficiency,
driving force for dye regeneration 0.28 eV [61]. On the contrary, in mainly due to a drop of jSC and FF, the effect being more
the case of E1/E2 the driving force could be too low, being 0.09 eV pronounced for D3 (Table 2). However, the use of the E3/E4 couple
only (Table 1).
gives good efficiencies for the three dyes (1.0-1.4 %; rel = 11-16 %)
Dye-sensitized solar cells were fabricated using FTO glass coated due to much better jSC and FF, although the Voc is lower than that
-
-. It is worth pointing out that the redox potential of copper
TiO
2
sensitized with D1, D2 or D3 as photoanode, a platinized FTO with I /I
3
–
–, complexes is controlled by steric factors and it is dependent from
as counter electrode and an electrolyte solution containing I /I
3
E1/E2 or E3/E4 as redox couple (see ESI). They were fully masked internal substituents [45], sometimes leading to a relatively low Voc
62]. Results of the investigated thin film DSSCs are presented in [48]. The lower short-circuit photocurrent observed with the E1/E2
[
Table 2 together with those obtained with the Ru(II) benchmark couple can be attributed to a more competitive light harvesting
N719. In addition to the absolute photo-conversion efficiency (), with the dyes, as indicated by a higher molar extinction coefficient
Table 2 reports the efficiency relative to a cell based on N719 dye of the absorption band around 450 nm (Table 1) [48] and by a too
–
–
3
and I /I electrolyte set at 100% (rel).
6
| J. Name., 2012, 00, 1-3
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low driving force, as observed above from cyclovoltammetric data TDDFT calculations assign the main absorption bandVieawsAarticMle LOLn’lCinTe
Table 1).
involving not only the bipyridine but also thDeOpI:h1e0n.1a0n3t9h/rCo9lDinTe00lig79a0nCd
The incident monochromatic photon-to-current conversion which upon TiO sensitization should point in the opposite way of
efficiency of some investigated DSSCs are shown in Figure 4. The the TiO surface. Such phenanthroline involvement in the excited
IPCE data agree with the current/voltage measurements (Table 2). state originating visible light absorption becomes therefore
detrimental implying reduction in the charge transfer
(
2
2
a
directionality. Further investigation on the tuning of this class of
compounds should point to up-shift the phenanthroline-based
LUMOs by electron donating ligands, for instance, to avoid interfere
on the low-lying MLCT transitions.
Nevertheless, this study confirms the potential of full-copper DSSCs,
consolidating the way to DSSCs with earth-abundant components.
Future work should be devoted also to increase the solar light
harvesting of the copper(I)-based DSSCs, may be by co-sensitization
b
a
D1 (E3/E4)
D2 (E3/E4)
D3 (E3/E4)
4
0
0
0
0
0
40
3
2
1
3
0
20
10
0
[
63], and to check the effect of the concentration of the copper-
based electrolytes and of additives such as tert-butylpyridine.
D2 (E3/E4)
D2 (iodine)
D2 (E1/E2)
D2 (iod.+ guan.)
Acknowledgements
350
500
650
A. C. thanks Università degli Studi di Milano (Piano Sostegno alla
Ricerca 2015–17-LINEA 2 Azione A – Giovani Ricercatori) for
financial support. We thank Dr Stefano Chiaberge (Eni) for mass
spectra, and both Regione Lombardia and Fondazione Cariplo for
3
50
500
650
Wavelength (nm)
Figure 4. IPCE of DSSCs: a) sensitized by dyes D1 – D3 in the
presence of copper based electrolyte E3/E4; b) sensitized by D2 in the use of instrumentation purchased through the SmartMatLab
the presence of different electrolytes.
Centre project (2014). CNR and the “Ministero degli Affari Esteri e
della Cooperazione Internazionale” is also acknowledged (bilateral
project Italy-India, Prot. nr. MAE0104617) for financial support. Part
of this work has been performed under research contract No.
The D1 based device showed the most intense external quantum
efficiency (~30% in the 450 – 500 nm region), when the copper
based redox couple E3/E4 was employed (Figure 4a). In these
conditions the IPCE results do not reproduce the trend of the UV
spectra (Figure 1) suggesting that when copper based electrolytes
are used the light harvesting capability of the dye is not the most
important feature in order to improve the photo-current. Regarding
the D2 based devices (Figure 4b), the best IPCE was reached in the
3
500005452 between University of Milano and Eni S.p.A, Rome,
Italy.
Notes and references
1
2
3
B. O’Regan and M. Grätzel, Nature, 1991, 353, 737.
M. Grätzel, Accounts Chem. Res., 2009, 42, 1788.
-
-
presence of a classic I /I
3
redox shuttle, with a maximum value of
~50% in the 450 – 500 nm region. All the other electrolytes tested
G. C. Vougioukalakis, A. I. Philippopoulos, T. Stergiopoulos
and P. Falaras, Coord. Chem. Rev., 2011, 255, 2602.
L.-L. Li and E. W.-G. Diau, Chem. Soc. Rev., 2013, 42, 291.
a) C. E. Housecroft, E. C. Constable, Chem. Soc. Rev., 2015,
produced a significant decrease of the performances.
4
5
4
4, 8386; b) F. J. Malzner, C. E. Housecroft, and E. C.
Conclusions
Constable, Inorganics, 2018, 6, 57; c) A. Büttner, S. Y.
Brauchli, E. C. Constable and C. E. Housecroft, Inorganics,
In this work two new heteroleptic copper(I) sensitizers bearing a -
delocalized 2-(R-phenyl)-1H-phenanthro[9,10-d]imidazole (R = NPh
2
018, 6, 40.
a) M. Sandroni, Y. Pellegrin and F. Odobel, C. R. Chimie, 2016,
9, 79; b) Y. Liu, S-C. Yiu, C-L Ho, W-Y. Wong Coord. Chem.
Rev., 2018, 375, 514; c) G.Boschloo, Frontiers in Chemistry
019, 7, article 77.
2
6
or OHexyl) ancillary ligand were prepared and well characterized.
Both of them, like the related complex bearing 2,9-dimesityl-1,10-
phenanthroline as ancillary ligand, can be coupled with the
1
2
–
–
common I /I
3
redox couple to prepare DSSCs with a good
7
8
9
M. Magni, P. Biagini, A. Colombo, C. Dragonetti, D. Roberto
and A. Valore, Coord. Chem. Rev., 2016, 322, 69.
T. W. Hamann, R. A. Jensen, A. B. F. Martinson, H. Van
Ryswykac and J. T. Hupp, Energy Environ. Sci., 2008, 1, 66.
T. W. Hamann, Dalton Trans., 2012, 41, 3111.
efficiency. They can also be combined with homoleptic copper
redox couples to afford full-copper DSSCs with fair to good
performance, depending on the nature of the electron shuttle.
It appears that, in copper(I) dyes, the use of -delocalized ancillary
ligands is not sufficient to improve drastically light harvesting,
electron transfer enhancement from electrolyte to dye and
1
0 M. Wang, Graetzel, S. M. Zakeeruddin and M. Grätzel,
Energy Environ. Sci., 2012, 5, 9394.
1
1 J. Wu, Z. Lan, J. Lin, M. Huang, Y. Huang, L. Fan and G. Luo,
Chem. Rev., 2015, 115, 2136.
2
directionality for electron injection in the TiO conduction band.
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Journal Name
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Journal Name
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DOI: 10.1039/C9DT00790C
Two new heteroleptic copper(I) sensitizers with -delocalized ancillary ligands for full copper
DSSCs