Coupling of Zinc Porphyrin Dyes and Copper Electrolytes: A Springboard for Novel Sustainable Dye-Sensitized Solar Cells

Article abstract of DOI:10.1021/acs.inorgchem.7b02323

The combination of β-substituted Zn²⁺ porphyrin dyes and copper-based electrolytes represents a sustainable route for economic and environmentally friendly dye-sensitized solar cells. Remarkably, a new copper electrolyte, [Cu(2-mesityl-1,10-phe

Full text of DOI:10.1021/acs.inorgchem.7b02323

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
pubs.acs.org/IC
Coupling of Zinc Porphyrin Dyes and Copper Electrolytes: A
Springboard for Novel Sustainable Dye-Sensitized Solar Cells
Alessia Colombo,†,‡,§ Gabriele Di Carlo,
†,‡
Claudia Dragonetti,
†,‡,§
Mirko Magni,
†,‡
‡
,§
,†,‡,§
,†,‡,§
†,‡,§
Alessio Orbelli Biroli,
Maddalena Pizzotti,*
Dominique Roberto,*
Francesca Tessore,
∥
∥
∥
,∥
Elisabetta Benazzi, Carlo Alberto Bignozzi, Laura Casarin, and Stefano Caramori*
†
̀
Dipartimento di Chimica dell’Universita degli Studi di Milano, Via Golgi 19, 20133 Milano, Italy
‡
UdR INSTM di Milano, via Golgi 19, 20133 Milano, Italy
§
Istituto di Scienze e Tecnologie Molecolari del CNR (CNR-ISTM), SmartMatLab Centre, Via Golgi 19, 20133 Milano, Italy
∥Dipartimento Scienze Chimiche e Farmaceutiche, Universita
̀
di Ferrara, Via F. di Mortara, 44121 Ferrara, Italy
*
S Supporting Information
ABSTRACT: The combination of β-substituted Zn2+ porphyrin
dyes and copper-based electrolytes represents a sustainable route for
economic and environmentally friendly dye-sensitized solar cells.
Remarkably, a new copper electrolyte, [Cu(2-mesityl-1,10-phenan-
+
/2+
2+/3+
−
throline) ] , exceeds the performance reached by Co
and I /
2
−
I3 reference electrolytes.
1
9−22
INTRODUCTION
DSSCs efficiencies.
The use of long alkoxy chains on a
■
meso push−pull porphyrin with an ethyne-linked donor group
̈
With the milestone paper of Gratzel and O’Regan, Dye-
19
led to a PCE value over 10%. Efficiencies up to ca. 13%,
higher than those obtained with the most efficient Ru
complexes, were reached by combining meso-disubstituted
Sensitized Solar Cells (DSSCs) have emerged as a realistic
solution for harnessing the energy of the Sun and converting it
1
into electricity. Since then, an impressive amount of work has
2+
push−pull Zn diarylporphyrins with tris(bipyridine) cobalt
electrolytes, which allow significantly increasing the open-
been carried out in order to improve the photoconversion
efficiency (PCE), trying to optimize the sensitizer,
the past few years, the redox mediators.
Until recently, the most efficient DSSCs were based on
2
−7
and, in
−
−
7
−12
circuit voltage (V ) with respect to the traditional I /I
OC 3
23,24
couple.
Also for synthetically less demanding β-substituted
2+
Zn tetraaryl porphyrins, the presence of alkoxy chains
polypyridyl ruthenium complexes as dye and the iodide/
22
−
−
1−5
produces a better PCE, but in this case, their effect has
never been tested with a metal-complex redox mediator.
It is important to point out that one of the keys in boosting
the performance of DSSCs is the enhancement of the VOC
which stems from the quasi-Fermi level position in the
semiconductor photoanode, which can become more negative
by addition of electrolyte additives such as tert-butylpyridine
(tbp), and the level corresponding to the redox potential of the
electrolyte. The most often used redox couple is I /I , but it
has many problems such as a too negative redox potential,
complicated two-electron redox chemistry, corrosiveness, and
visible-light absorption. Co
some of these problems: they are outer-sphere one-electron
redox systems with more positive and easily tunable redox
triiodide couple (I /I ) as electrolyte.
However, the
3
relatively high cost and environmental issues of ruthenium
complexes encouraged the search for efficient noble-metal-free
6
,13
dyes.
By analogy with the process of solar energy collection
by bacteria and plants, which have a porphyrinic core as a light-
harvesting chromophore, porphyrins, with large absorption
coefficients in the visible region of sunlight, have drawn
attention as dyes for DSSCs.
porphyrin dyes suffered from aggregation by π−π stacking
interactions which reduced the electron injection efficiency in
14,15
2+
The first investigated Zn
−
−
3
the TiO electrode, leading to a detrimental effect on the
2
25
2+/3+
4
,16
2+
complexes have addressed
PCE.
Besides, the interaction between the Zn ion
−
coordinated inside the porphyrinic core and the I3 species of
the electrolyte promoted a fast recombination reaction with the
2
+/3+
1
7,18
potentials, providing large VOC. Nevertheless, Co
com-
injected electron.
The introduction of long alkoxy chains at
2+
the ortho-positions of phenyl rings of meso push−pull Zn
porphyrins, in order to envelop the porphyrinic core, allowed to
hinder these detrimental interactions, thus increasing the
Received: September 11, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02323
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
4,15,32−34
plexes bring some issues associated with their stability, slow
mass transport in the electrolyte solution and large internal
reorganization energy between the high-spin d and low-spin d
states, which costs additional driving force for dye regeneration.
The limitations caused by the reorganization energy are
its photosensitization ability.
The presence of ortho-
dodecyloxy chains, which produces the best shielding effect on
the photoanode, enveloping the porphyrinic core was chosen
in order to reduce any back electron transfer to Cu species.
The D1 dye was combined with the new redox couple [Cu(2-
mesityl-1,10-phenanthroline)2]
designed to improve the VOC. For comparison, the known well
performing couple [Cu(2-mesityl-4,7-dimethyl-1,10-phenan-
7
6
34
2
+
+
/2+
+/2+
minimized by using well-designed Cu
complexes with
(1/2, Chart 1), specifically
25
distorted tetragonal geometry. These alternative electron
shuttles are of particular interest since copper is an economic,
environmentally friendly metal.
+/2+
28,29
throline) and the reference couple with 2,9-
dimethyl-1,10-phenanthroline (5/6) were also studied along
]
2
(3/4)
2
+
27
2+/3+
Recently, in combination with both Ru dyes and organic
+
/2+
−
−
sensitizers, Cu
complexes outperform not only iodine-based
with I /I
3
and [Co(dtb-bpy)
]
3
(dtb-bpy = 4,4′-di(tert-
2
6−29
2+/3+
electrolytes
but also the [Co(2,2′-bipyridyl) ]
ple. Fast dye regeneration and tunability of both redox
potential and electron transfer rate make copper mediators
prone to solve the thermodynamic and kinetic dichotomies of
cou-
butyl)-2,2′-bipyridyl) redox couples. The D2 dye bearing the
simpler −COOH group instead of CHC(CN)COOH was
then combined with the best redox shuttle.
3
3
0
31
the device operation. With the aim to synthesize mediators
characterized by intermediate values of both redox potential
and electron transfer kinetics to overcome this intrinsic conflict,
a few years ago, some of us first proposed a new approach in
designing copper-based electron mediators employing mono-
substituted chelating phenanthrolines functionalized with a
EXPERIMENTAL SECTION
General Comments. All reagents and solvents were purchased by
Sigma-Aldrich and used without further purification, except Et N
(freshly distilled over KOH). 4,7-(Dimethyl)-1,10-phenanthroline was
purchased by Alfa Aesar. Reactions requiring anhydrous conditions
were performed under argon. Glassware has been flame-dried under
vacuum before use when necessary. Microwave assisted reactions were
performed using a Milestone MicroSYNTH instrument.
■
3
28
single bulky group, as a valuable alternative to redox couples
based on highly sterically hindered 2,9-disubtituted analogues,
26
Thin layer chromatography, TLC, was carried out with precoated
Merck F254 neutral aluminum oxide 60 plates. Liquid chromatography
was carried out with neutral aluminum oxide (Brockmann grade II,
Alfa Aesar) or silica gel (Geduran Si 60, 63−200 μm, Merck).
first proposed by Fukuzumi et al. and to date largely adopted.
The above observations prompted us to couple β-substituted
Zn2 porphyrin dyes with Cu redox mediators as a novel route
to low cost, environmentally friendly DSSCs. We chose the
tetraaryl Zn2 porphyrin bearing the cyanoacrylic acid
anchoring group at the β-pyrrolic position via an ethynylphenyl
bridge (D1, Chart 1) instead of meso-disubstituted push−pull
sensitizers due to its easier accessibility that does not prejudice
+
1
13
19
31
H NMR, proton decoupled C NMR, F NMR, and P NMR
+
spectra were recorded at T = 300 K on a Bruker Avance DRX-400 or
300 instrument. Chemical shifts (δ) of spectra were expressed in ppm
1
13
19
relative to Me Si (for H and C), CFCl (for F), and H PO (for
4
3
3
4
31
P) reference compounds. Signals were abbreviated as s, singlet; d,
doublet; dd, doublet of doublets; t, triplet; q, quartet; sept, septet; m,
multiplet. Mass spectra, MS, were obtained with an FT-ICR Mass
Spectrometer APEX II & Xmass software (Bruker Daltonics) 4.7
Magnet and Autospec Fission Spectrometer (FAB ionization) or with
a Thermo-Finnigan apparatus with a Ion Trap analyzer (positive
mode) and an electrospray ionization source (ESI) using an LCQ-
Advantage instrument.
Chart 1. Chemical Structures of the Investigated Three
Couples of Copper-Based Redox Mediators (1−6) and of the
2
+
Two Tetraaryl Zn Porphyrin Dyes (D1, D2)
Elemental analysis was carried out with a PerkinElmer CHN 2400
instrument in the Analytical Laboratories of the Department of
Chemistry at the University of Milan.
Electronic absorption spectra were recorded at room temperature in
THF or acetonitrile solution, using a Shimadzu UV3600 spectropho-
tometer and quartz cuvettes with a 1 cm optical path length.
Photoluminescence experiments were carried out at room temper-
ature, in THF solution after N bubbling for 120 s. Photoluminescence
2
quantum yields were measured with a C11347 Quantaurus - QY
Absolute Photoluminescence Quantum Yield Spectrometer (Hama-
matsu Photonics K.K), equipped with a 150 W xenon lamp, an
integrating sphere and a multichannel detector. Steady state emission
and excitation spectra and photoluminescence lifetimes were obtained
with an FLS 980 spectrofluorimeter (Edinburg Instrument Ltd.).
Continuous excitation for the steady state measurements was provided
by a 450 W xenon arc lamp. Photoluminescence lifetime measure-
ments were performed using an Edinburgh Picosecond Pulsed Diode
Laser EPL-445 (Edinburg Instrument Ltd.), with a central wavelength
of 442.2 nm and repetition rates of 20 MHz, by a time-correlated
single-photon counting method.
Synthesis. Redox Mediators. Synthesis of the Ligand 2-Mesityl-
3
5,36
1
,10-phenanthroline (L1). Ligand L1
was prepared adapting a
2
8
literature procedure. 2-Bromo-mesitylene (2.9 mL, 19.2 mmol) was
dissolved in degassed dry Et O (65 mL) and cooled at −78 °C under
2
an Ar atmosphere. tert-Butyl lithium (1.7 M in pentane) was carefully
dropped (24.8 mL, 42.2 mmol). When the addition was completed,
the stirred solution was maintained at −78 °C for 30 min, and then it
was let to warm up to r.t. over 30 min. The white solid product was
filtered under Ar, washed with some Et O, and dissolved, under Ar, in
2
B
DOI: 10.1021/acs.inorgchem.7b02323
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2
0 mL of dry toluene. The white solution was dropped into a flask,
filled with argon, containing a stirred solution of 1,10-phenanthroline
2.27 g, 12.6 mmol) dissolved in 260 mL of degassed dry toluene
Synthesis of the Known Copper Complexes 3, 5, 6, [Co(dtb-
3 2 3 6 3
bpy) ][OTf] , and [Co(dtb-bpy) ][PF ] . 3 and 5 were synthesized
(
according to the same procedure described for 1, using 2-mesityl-4,7-
28
cooled to 0 °C. The solution gradually turned dark. After 20 min, the
reaction mixture was allowed to warm at r.t. and stirred overnight. The
mixture was quenched with water, and the organic layer was separated.
The aqueous layer was washed with dichloromethane (2 × 140 mL).
The combined organic phases were dried with Na SO , filtered into a
flask containing activated manganese dioxide (35.0 g, 403 mmol), and
stirred at r.t. overnight. The mixture was filtrated on a Celite plug, and
the solvent was evaporated at reduced pressure. The whitish crude
dimethyl-1-10-phenanthroline, prepared as previously reported, or
commercially available 2,9-dimethyl-1,10-phenanthroline as ligand,
respectively. The red products were isolated in 86% and 87% yields,
1
respectively. H NMR spectra were in perfect agreement with the
2
8
literature.
Complex 6,
[PF6]3 were synthesized as previously reported.
Dyes. D1 was prepared as previously reported, whereas D2 ([2-
(4′-carboxyphenylethynyl)-5,10,15,20-tetrakis(2-dodecyloxyphenyl)-
porphyrinate]Zn ) was prepared as follows. In an anhydrous Schlenk
2
4
2
7,29
37
[Co(dtb-bpy) ][OTf] , and [Co(dtb-bpy) ]-
3 2 3
3
7
3
4
product was purified by liquid chromatography on neutral Al O3
through a gradient elution (from CH Cl /toluene 7:3 to pure
2
II
2
2
CH Cl ). Pure L1 was obtained as a white/yellow solid (1.5 g, 40%
tube, under a nitrogen atmosphere, 107.6 mg of 4-ethynylbenzoic acid
2
2
yield).
(736.3 μmol, 5 equiv), 17.0 mg of Pd(PPh ) (14.7 μmol, 0.1 equiv),
3
4
1
H NMR (400 MHz, CDCl ) δ(ppm): 9.28 (dd, J = 4.4 Hz, J =
and 220.1 mg of [2-bromo-5,10,15,20-tetrakis(2-dodecyloxyphenyl)-
3
2
3
II
1
.6 Hz, 1H), 8.39 (dd, J = 8.0 Hz, J = 1.6 Hz, 1H), 8.33 (d, J = 8.2
porphyrinate]Zn (147.3 μmol, 1 equiv), prepared as previously
2
3
2
3
4
Hz, 1H), 7.95 (d, J = 8.8 Hz, 1H), 7.88 (d, J = 8.8 Hz, 1H), 7.71 (dd,
reported, were dissolved in 5 mL of anhydrous DMF over molecular
2
2
J = 8.0 Hz, J = 4.4 Hz, 1H), 7.57 (d, J = 8.2 Hz, 1H), 6.89 (s, 2H),
2
3
2
sieves and 15 mL of Et N. The reaction mixture was deaerated with
3
2.36 (s, 3H), 1.97 (s, 6H).
three freeze−pump−thaw cycles at about −96 °C, using a bath of
liquid nitrogen and acetone. The solution was allowed to warm to
room temperature and transferred, under nitrogen flow, into a
microwave quartz vessel. Next, 4.2 mg of CuI (22.1 μmol, 0.15 equiv)
was added, and after an additional bubbling of nitrogen for 15 min, the
reaction was heated at 120 °C in a microwave cavity for 1 h. The
solvents were removed in vacuo, and the crude product was dissolved
in CH Cl again and washed with H O acidified with H PO . The
Synthesis of Cu(I) Complex 1. The precursor CuI (70 mg, 0.366
mmol) was dissolved under argon in 8 mL of acetonitrile and added
dropwise to a stirred solution of the ligand L1 (224 mg, 0.751 mmol)
dissolved in 20 mL of acetonitrile, under an inert atmosphere. The
solution instantaneously turned red, and it was stirred at room
temperature for ca. 30 min. The solvent was eliminated under reduced
pressure; the light red solid was dissolved in the minimum amount of a
2
2
2
3
4
1
2
:2 mixture of H O/EtOH (ca. 60 mL) under vigorous stirring. Next,
2
organic phase was dried over anhydrous Na SO , filtered, and
2 4
mL of an aqueous solution of NaPF (141 mg, 2.3 equiv) was added,
6
concentrated to dryness. The crude product was purified by column
and immediately a red precipitate was formed as a consequence of
chromatography on silica (CH Cl /MeOH 97.5:2.5 gradient to 95:5),
2
2
anion exchange. After 2 h, the solid was filtered off, washed with H O
2
obtaining 116.2 mg of product (yield 50.6%).
and Et O, and, finally, dried under vacuum for some hours, until a red
1
2
H NMR (400.1 MHz, CDCl ) δ, ppm: 9.20 (1H, m), 8.78 (6H,
3
powder of compound 1 was obtained (252 mg, 85% yield). The
attempt to obtain crystals suitable for X-ray structural characterization
failed.
m), 8.01 (4H, m), 7.74 (3H, m), 7.54 (5H, m), 7.34 (7H, m), 7.23
(
1H, m), 3.89 (8H, m), 1.40−0.5 (92H, m).
Elemental analysis calcd (%) for C
H
N O Zn: C, 77.79; H,
4 6
1
101 128
H NMR (300 MHz, CD CN) δ(ppm): 9.03 (dd, J = 4.8 Hz, J =
3
2
3
8
.27; N, 3.59; found C, 77.56; H, 8.29; N, 3.58.
1
9
2
.3 Hz, 2H), 8.62−8.57 (m, 4H), 8.11 (d, J = 9 Hz, 2H), 8.06 (d, J =
2
2
MS-ESI(+) m/z: ca1cd for C101H128N O Zn 1556, found 1557 [M
4
6
Hz, 2H), 7.88 (dd, J = 8.2 Hz, J = 4.8 Hz, 2H), 7.59 (d, J = 8.2 Hz,
+
2
3
2
+
H] .
Characterization. Electrochemical Measurements. Cyclic vol-
H), 6.12 (s, 2H), 5.79 (s, 2H), 1.94 (s, 6H, partially hidden by
13
solvent signal), 1.69 (s, 6H), 1.00 (s, 6H); C NMR APT (75 MHz,
tammetry, CV, and electrochemical impedance spectroscopy, EIS, for
the electrochemical characterization of Zn-porphyrin dyes and of
copper- and cobalt-based complexes were performed in a three-
electrode customized minicell filled with 2−4 mL of a solution of a
suitable solvent with tetrabutylammonium hexafluorophosphate,
CD CN) δ(ppm): 158.7 (C), 149.4 (CH), 144.1 (C), 143.9 (C),
3
1
1
37.8 (CH), 137.5 (C), 136.5 (CH), 135.9 (C), 135.1 (C), 133.4 (C),
29.6 (C), 128.4 (CH), 127.9 (C), 126.9 (3CH), 126.7 (CH), 126.0
19
(CH), 20.3 (CH ), 19.7 (CH ), 19.1 (CH ); F NMR (282 MHz,
3 3 3
31
CD CN) δ(ppm): −73.2 (d, J = 704 Hz, 6F); P NMR (121 MHz,
3
F‑P
TBAPF (≥98.0%, Sigma-Aldrich), acting as supporting electrolyte.
6
CD CN) δ(ppm): −144.0 (sept, J = 704 Hz, 1P). The HSQC
3
P‑F
1
13
The working solution was well deareated by bubbling nitrogen before
each measure starts and blowing it over the surface of the solution
during the scans.
(
H− C) spectrum of complex 1 is reported in Figure S1 of the
Supporting Information.
Elemental analysis calcd (%) for C H CuF N P: C, 62.64; H,
42
36
6
4
Teflon-embedded glassy carbon electrodes, GC (geometric surface
4
.51; N, 6.96. Found C, 62.69; H, 4.52; N, 6.94.
MS (ESI ) m/z: 659.61 (659.22 calculated).
2
+
area 0.071 cm , purchased by Metrohm and Amel), were used as
working electrode in combination with a platinum wire as counter
electrode and a saturated calomel electrode, SCE, as the reference one.
To avoid water and chloride leakage into the working solution, the
SCE was inserted into a double bridge ending with a porous frit and
filled with the same blank electrolytic solution (i.e., solvent and
supporting electrolyte). The recorded potentials were subsequently
Synthesis of the New Cu(II) Complex 2 and the Known Complex
4
. The two complexes were prepared following a reported
29
procedure, here described for 2.
CuSO ·5H O (12.5 mg, 0.050 mmol) was dissolved in 1.5 mL of
4
2
water and added to a stirred solution of 2-mesityl-1,10-phenanthroline
30 mg, 0.100 mmol) in 6 mL of acetonitrile. The solution
(
+
referred to the intersolvental reference redox couple Fc |Fc
immediately turned light green, and it was stirred at room temperature
for 1 h. The solution was concentrated under reduced pressure to
eliminate acetonitrile, and about 1.5 mL of EtOH was added to
completely dissolve the green solid. A green precipitate was
3
(
ferricenium|ferrocene) added as external standard (ca. 1·10− M) at
the end of each daily measure.
The measures were performed with a PGSTAT302N potentiostat/
galvanostat (Metrohm Autolab, The Netherlands) controlled by a PC
through GPES and NOVA 1.10 software, equipped with a frequency
response analyzer module (FRA2) for EIS measurements. Staircase
immediately formed after the dropwise addition of NaPF (35 mg,
6
0
.210 mmol) in 1 mL of H O, and the suspension was allowed to stir
2
for 2 h. The product was collected by filtration, washed with H O and
2
−1
Et O, and, finally, dried under vacuum for some hours. Complex 2
CV were performed at different sweep potentials (0.02−2 V s ) with
a potential step of 0.001−0.002 V. An instrumental compensation of
the resistance (i.e., positive feedback) was carefully performed in order
to minimize the Ohmic drop between the working and reference
electrodes.
2
resulted as a light green powder (29 mg, 72% yield).
Elemental analysis calcd (%) for C H CuF N P : C, 53.09; H,
42
36
12
4 2
3
.82; N, 5.92. Found C, 53.13; H, 3.83; N, 5.94.
MS (ESI ) m/z: 659.47 (659.22 calculated).
+
C
DOI: 10.1021/acs.inorgchem.7b02323
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
The determination of redox potential of electrolytes was carried out
with the same setup, recording CV of a Fc solution at 0.2 V s−1 (see
below for more details).
illumination (ABET sun simulator). EIS spectra were then properly
fitted using ZView software (Scribner Associates, Inc.).
IPCE spectra were determined in sandwich DSSCs under short-
38
EIS spectra of Cu complexes were recorded superimposing to a
continuous potential bias set at the half-wave potential, E1/2, a
sinusoidal alternating potential signal of 0.01 V amplitude, and
circuit conditions by using a previously described apparatus by
sampling the visible spectrum (400−800 nm) at 20 nm intervals with a
spectral bandwidth of 10 nm. Incident irradiance was measured with a
in-house calibrated silicon photodiode.
4
−1
frequency ranging from 1·10 to 1·10 Hz. Sixty logarithmically
distributed single sine frequencies were employed for recording each
spectrum. NOVA 1.10 software was used to validate EIS data (through
Kronig−Kramers analysis) and to fit them with a Randles-type
Transient Absorption Spectroscopy Measurements. Transient
absorption spectroscopy, TAS, was performed with a previously
3
9
described apparatus by using the 532 nm harmonic of a nanosecond
Nd:YAG laser (Continuum Surelite II). A typical pulse energy of 2 mJ
equivalent circuit to estimate the charge transfer resistance, R , at the
ct
−
2
electrolyte/electrode interface. Rct was related to the heterogeneous
electron transfer rate constant, kheter, according to the following
equation
cm was used. A 532 nm notch filter prevented laser light from
reaching the photomultiplier, whereas a 420 nm cutoff filter, placed in
front of the white light probe beam, prevented direct TiO excitation.
2
From 10 to 30 laser shots, at a frequency of 0.2 Hz, were averaged to
reach a good S/N ratio.
i0
FCRd
RT
kheter
=
=
2
The photoelectrodes were prepared by doctor-blading TiO 18NRT
F R C
2
ct Rd
paste on FTO substrates. After a programmed ramp heating up to 500
°
C, the TiClO treatment was applied, by immersing the electrodes in
where i is the exchange current density, F the Faraday constant, C
4
0
Rd
−3
a 0.4 M acqueous solution overnight, after which they were heated at
50 °C for 30 min. Then, the electrodes were put in a ethanol/
the concentration of the reduced complex (in mol cm ), R is the gas
constant, and T the absolute temperature. In our experimental
conditions, the temperature is about T = 295 ± 2 K.
Photoelectrochemical Measurements. Photoanodes were pre-
pared by a simple procedure. FTO glass plates were washed in 2-
propanol for 10 min using an ultrasonic bath, then were heated at 450
4
tetrahydrofuran 9:1 D1 solution overnight for the sensitization.
TAS measurements on transparent thin films in the presence of the
electron donor species in acetonitrile were performed by drawing the
electrolyte by capillarity inside the chamber (ca. 6−8 μm), constituted
by a glass slide pressed against the TiO2 photoanode. Thus, the
negligible optical path allowed avoiding interferences originated by the
mediators absorbance.
°
C for 20 min. A compact TiO blocking underlayer was prepared by
2
spin-coating a 0.3 M titanium tetraisopropoxide solution in 1-butanol
1000 rpm for 10 s, 2000 rpm for 20 s). Then, the substrates were
(
Regeneration efficiency was calculated by applying the formula
heated at 500 °C for 15 min. The photoelectrodes were prepared by
doctor blading, using a transparent anatase paste composed of 20 nm
nanoparticles (Dyesol 18NRT). The coated films were heated to 500
k′reg
η =
k′reg + k′rec
°
C for 10 min with programmed temperature ramping. After sintering,
the electrodes were treated with 0.4 M TiCl overnight and heated
4
where the first-order recombination, k′ , and pseudo-first-order
again at 450° for 30 min. Once cooled, the electrodes were immersed
in a 0.2 mM solution of the dye (D1 and D2) in ethanol/
tetrahydrofuran 9:1, containing chenodeoxycholic acid 1 mM.
The PEDOT counter electrodes were prepared by electrodeposition
through a cyclic voltammetry technique, scanning 2 times from 0 to
rec
regeneration, k′ , rate constants were obtained by fitting the
reg
experimental decay curves with a biexponential function
y = A exp(−x/t ) + A exp(−x/t ) + y
0
1
1
2
2
−1
1
.7 V vs SCE at 20 mV s in a 0.01 M EDOT acetonitrile solution
following the equation
with 0.1 M LiClO as supporting electrolyte. The PEDOT-modified
4
−
1
FTOs were then thoroughly washed with fresh acetonitrile and dried
in air.
2
2
⎛
k′ = ⎜
⎝
⎞
⎟
⎠
A t + A t )
1
1
2 2
A
t
+ A t
DSSCs were finally assembled by sealing the sensitized photoanode
and a poly(3,4-ethylendioxythiophene)-modified FTO counter elec-
trode through a mask of 25 μm ionomer resin (Surlyn, DuPontTM),
properly pressed and warmed. Electrolyte was injected by a syringe
through one of two holes predrilled on the counter electrode, which
were finally sealed.
Electrolytes were all formulated using acetonitrile as solvent, and 0.1
M lithium triflate (LiOTf) and 0.25 M tert-butylpyridine (tbp) as
additives. A 10:1 molar ratio was chosen for reduced and oxidized
species of each redox couple, respectively. Thus, copper electrolytes
1
1
2 2
The following solutions were tested:
• 0.1 M LiOTf in acetonitrile (blank)
• 0.146 M 1-propyl-3-methylimidazolium iodide, 0.024 M LiI, in
−
acetonitrile (I )
•
0.17 M [Co(dtb-bpy) ](OTf) , 0.1 M LiOTf, in acetonitrile
3 2
(Co)
•
•
0.17 M 1, 0.1 M LiOTf, in acetonitrile (1)
0.17 M 3, 0.1 M LiOTf, in acetonitrile (3)
• 0.17 M 5, 0.1 M LiOTf, in acetonitrile (5)
(
1/2-El, 3/4-El and 5/6-El, where El means electrolyte), were
+
2+
prepared dissolving the redox couple (0.17 M Cu /0.017 M Cu ) in
+
CH CN with 0.1 M lithium trifluoromethanesulfonate as supporting
where LiOTf or LiI salts were employed as sources of Li ions.
Determination of the Redox Potential of Electrolytes. The redox
potential of the electrolytes, Eredox, was experimentally determined
through three-electrode voltammetric measurements recording a cyclic
voltammogram of a solution containing ca. 0.001 M Fc dissolved in
acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate as
supporting salt. A glassy carbon electrode was employed as working
electrode, a platinum wire as counter electrode, and an electrode filled
with the tested DSSC electrolyte as reference electrode. The redox
reference electrode was prepared by filling a small glass tube ending
with a porous frit (assuring the electric contact) with about 100 μL of
the solution, then employed in then DSSC, and tipping in it a well
polished platinum wire. After each measurement, the wire was rinsed
with fresh acetonitrile and deionized water, then tipped for some
3
salt and 0.25 M 4-tert-butylpyridine as additive to improve the V of
OC
−
−
the devices. Control electrolytes I /I -El and Cobalt-El were
3
prepared with a comparable amount of electroactive species: 0.17 M
−
I (0.1 M LiI + 0.07 1-propyl-3-methylimidazolium iodide)/0.017 M
2
+
3+
I2 and 0.17 M [Co(dtb-bpy) ] /0.017 M [Co(dtb-bpy) ] ,
3
3
respectively.
−
1
Current−voltage measurements (scan rate of 10 mV s ), chopped
chronoamperometric measurements (at short circuit, i.e., E = 0),
bias
open-circuit voltage decay OCVD (at 50 mV sampling resolution),
and electrochemical impedance spectroscopy EIS (50 single sine
frequencies logarithmically distributed between 100 kHz and 10 mHz,
1
0 mV amplitude) were performed with a PGSTAT302N potentio-
stat/galvanostat equipped with a frequency response analyzer module
and controlled by NOVA or GPES software (Metrohm Autolab). Cell
performances were evaluated under 100 mW cm AM 1.5G
minutes in concentrated HNO , abundantly rinsed with water, then
sonicated in water, and finely dried.
3
−
2
D
DOI: 10.1021/acs.inorgchem.7b02323
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
+
+
In this way, the half-wave potential of Fc |Fc, E
, was
1
/2,Fc /Fc
Table 1. Absorption and Emission Data of D1 and D2
detected with respect to the Eredox of the tested electrolyte. Therefore,
+
a
a
the redox potential of the electrolyte (referred to E of Fc |Fc)
dye
λ
B
/nm (log ε)
λ
Q
/nm (log ε)
λ
ex/nm
λ
em/nm Φ/%
τ/ns
1/2
+
b
corresponds to Eredox = −E
. It is correlated to the energy of its
1/2,Fc /Fc
D1
438 (5.17)
566 (4.58)
604 (4.35)
566 (4.32)
439
616
671
617
672
3.8
2.38
2.16
Fermi level, UF,redox, by the equation
D2
438 (5.48)
438
4.0
UF,redox = −e(Eredox + 4.8)
6
04 (4.00)
a
Values recorded at 10−6 M. Reference 34.
b
where e = 1 C and 4.8 stands for the recommended redox potential (in
volts) of the ferrocenium/ferrocene, Fc |Fc, couple versus vacuum.
The knowledge of Eredox is also useful as a “reference potential”, to
+
40
supporting electrolyte, they exhibited a metal-centered process
properly compare results obtained from two-electrode cells when a
bias potential is applied. In a DSSC, the applied potential is Ebias
Ephotoanode − Ecathode, where Ephotoanode is the potential of the quasi-
Fermi level of electron in TiO , and E
referring Ephotoanode = EFn to the proper Eredox, it is possible to compare
at a fixed potential (and hence energy) the DSSC features, evidencing
intrinsic differences in behavior due to different electrolytes. In doing
so, experimental Ebias is previously corrected for Ohmic drop between
=
with half-wave potential, E1/2, of −0.21, −0.02, 0.06, and 0.30 V
+
2+
vs Fc |Fc for [Co(dtb-bpy) ] , 3, 1, and 5, respectively
3
is actually Eredox. By
2
cathode
(Supporting Information; Table S1 and Figures S6 and S7). As
expected for the absence of electron releasing methyl groups in
31
the 4,7 positions of the phenanthrolines, the new complex 1
exhibits a 0.08 V positively shifted E1 with respect to related 3.
/2
+
Interestingly, all investigated Cu complexes present higher
anode and cathode (Edrop = IRdrop) according to Ecorr = Ebias + Edrop
,
2+
oxidation potentials than [Co(dtb-bpy) ] , an intriguing
being Ebias negative.
Determination of the Conduction Band Edge Energy. Con-
3
feature for maximization of the open-circuit potential, V , of
oc
duction band edge energy, U , at open circuit was estimated according
photoelectrochemical devices. They also present a faster
cb
41
to the following equation
heterogeneous electron transfer constant (Table S1, k , 6.9
heter
−3
−2
−3
−1
×
10 to 2.0 × 10 compared to 1.1 × 10 cm s )
⎛
⎞
⎟
⎠
UFn − Ucb
reasonably attributable to the hindered phenanthrolines that
limit the geometry modification around the Cu center during
the electron transfer and so lower the energy barrier. In the
same conditions, apparent diffusion coefficients, D, are similar
for all the investigated copper complexes (between 7 and 10·
ncb = N exp⎜
cb
⎝
kbT
with k the Boltzmann constant, T the absolute temperature, N the
b
cb
3
/2
m*k T
e
b
density of states calculated as N = 2( 2
(where m* = 2.3m
e e
cb
2
)
πℏ
−6
2
−1
2+
−6
1
0
cm s ) but twice that for the Co compound (3.9·10
was selected as the effective mass of an electron), and ncb the
2
−1
cm s , Table S1), coherently with the lower coordination
number of Cu and the absence of six bulky t-butyl substituents
in the ligand sphere. In particular, the apparent diffusion
concentration of conduction band electrons calculated according to n
cb
2
=
k TC /e (with e the elementary charge) from the chemical
b
μ
capacitance of the TiO2 film (C ) at V under full sunlight
μ
oc
−5
2
−1
illumination, obtained by EIS fitting and considering 10 μm as film
thickness and a 50% porosity.
coefficient of complex 5 (1.04·10 cm s ) is in very good
25,30,42
agreement with reported data,
especially considering
Due to uncertainty on m* (in the literature, it varies from 1 to 10-
differences in both composition of working medium and
electrochemical setup.
e
fold m ), the reported values have a systematic error of about ±40
e
meV.
The D2 dye is characterized by two subsequent reversible
oxidation processes centered on the porphyrinic core, the first
RESULTS AND DISCUSSION
+
■
at E1/2 = 0.31 V vs Fc |Fc in N,N′-dimethylformamide, similar
+
34
The 2-mesityl-1,10-phenanthroline was synthesized as reported
to that reported for D1 (0.29 V vs Fc |Fc) (Supporting
Information, Figure S8, Table S2).
28
for related ligands, whereas copper complexes were obtained
by reacting the adequate phenanthroline with CuI (1, 3, 5),
CuSO ·5H O (2, 4), or CuCl ·2H O (6), as described in the
Copper electrolytes (1/2-El, 3/4-El, and 5/6-El, where El
means electrolyte) were prepared dissolving the redox couple
4
2
2
2
+
2+
Experimental Section. The reference couple [Co(dtb-
(0.17 M Cu /0.017 M Cu ) in CH CN with 0.1 M lithium
3
2
+/3+
37
bpy)3]
was prepared according to the literature. The
trifluoromethanesulfonate as supporting salt and 0.25 M 4-tert-
new Zn2 porphyrin dye D2 was prepared adapting the
procedure developed for D1 (see the Experimental Section).
The absorption spectrum of the new complex 1 in
acetonitrile shows a broad band centered at 451 nm (log ε =
+
butylpyridine as additive to improve the V of the devices.
OC
34
−
−
Control electrolytes I /I -El and Cobalt-El were prepared
3
with the same amount of electroactive species (see the
Experimental Section).
3
.62) due to a metal-to-ligand charge transfer transition, in
Results of the investigated thin film (ca. 6 μm) transparent
DSSCs are presented in Table 2.
agreement with related 3 and 5 exhibiting λmax at 445 nm (log ε
=
oxidized form 2 exhibits less intense transitions centered
around 696 nm (log ε = 2.18), like 4 and 6 with λmax at 697 nm
28,29
3.64) and 455 nm (log ε = 3.90), respectively.
Its
Notwithstanding, the lower VOC 3/4-El outperforms the Cu
28,29
reference electrolyte 5/6-El, confirming previous reports
and extending the potentiality of this copper redox couple
toward a new class of dyes. Interestingly, the new electrolyte 1/
2-El gives the best PCE (3.7%), with 28% and 75% increase of
efficiency with respect to 3/4-El and 5/6-El, respectively. A
deeper characterization of the device performance and
knowledge of the kinetics for the copper electron donation to
the dye are mandatory to clarify the reasons for such a behavior.
In general, all the monoelectronic couples based on
coordination metal complexes exhibit, at comparable concen-
tration, nearly unitary dye regeneration efficiencies (91−85%,
Supporting Information, Table S3) and are superior to the
29
(
log ε = 2.0) and 741 nm (log ε = 2.3), respectively (Figure
S2). The absorption and emission properties of the new
porphyrin dye D2 are similar to those previously reported for
D1 (Table 1; Figures S3−S5). The 100-fold higher absorption
of dyes with respect to redox mediators minimizes the light
harvesting competition.
Cyclic voltammetry, CV, and electrochemical impedance
spectroscopy, EIS, revealed a quite different electrochemical
behavior of the redox mediator metal complexes. In acetonitrile
with 0.1 M tetrabutylammonium hexafluorophosphate as
E
DOI: 10.1021/acs.inorgchem.7b02323
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Table 2. Photoelectrochemical Performances of DSSCs
jSC/mA cm−
2
VOC/V
FF
PCE %
dye
electrolyte
D1
D1
D1
D1
D1
D2
1/2-El
5.9 ± 0.1
5.6 ± 0.5
3.5 ± 0.2
8.0 ± 0.2
8.6 ± 0.1
4.8 ± 0.3
0.81 ± 0.01
0.68 ± 0.04
0.86 ± 0.05
0.58 ± 0.02
0.60 ± 0.01
0.75 ± 0.03
0.77 ± 0.01
0.77 ± 0.05
0.70 ± 0.03
0.63 ± 0.05
0.66 ± 0.02
0.74 ± 0.04
3.7 ± 0.1
2.9 ± 0.4
2.1 ± 0.2
2.9 ± 0.2
3.4 ± 0.1
2.7 ± 0.3
3/4-El
5/6-El
Cobalt-El
I−/I3−-El
1/2-El
a
Under 100 mW cm−2 AM 1.5G irradiation; active area 0.20−0.25 cm ; data are referred to at least two replicates.
2
equimolar iodide based electrolyte (71%). The best regener-
ation, monitored at 650 nm, diagnostic for the formation of the
3
3,43
oxidized dye,
is found with 3/4-El and 5/6-El. The latter is
characterized by a lower incident photon-to-current conversion
with respect to the other redox mediators (Figure 1), but it has
Figure 2. Energy diagram showing the energy levels useful to
understand the cell operation.
drawback that could explain the lowest photocurrent and the
lower fill factor observed for the DSSC based on such
electrolyte (Table 2).
The extremely high mass transport resistance seems to
contrast with the comparable apparent diffusion coefficients of
Cu(I) complexes (Table S1). However, two important
differences must be mentioned: (i) in DSSC, the diffusion-
limiting species is actually the Cu(II) complex, being 10-fold
less concentrated than Cu(I); (ii) the presence of tbp seems to
negatively affect the diffusion properties of copper complexes,
both increasing the solution viscosity and directly interacting
Figure 1. Top: polarization curves under 100 mW cm−2 AM 1.5G
illumination (solid lines) and in the dark (dashed lines) of D1-
sensitized DSSCs. Bottom: photoaction spectra of the devices.
25
with Cu(II) species.
the strongest oxidative power of all the investigated electrolytes,
On the other hand, the 28% increase in PCE over 3/4-El by
1/2-El could be mainly attributed to its more positive redox
with a related Fermi-level energy (UF,redox) of −4.76 eV (Table
3
and Figure 2), and the longest lifetime (τ) for electrons in
potential (ΔE
= 0.10 V, Table 3), which leads to an increase
of the V from 0.68 to 0.81 V. In agreement with this
redox
TiO (Figure S9), resulting in the narrowest gap between the
2
OC
conduction band energy (U ) and the quasi-Fermi level of
thermodynamic explanation, comparable electron lifetime τ
were obtained by EIS when reported as a function of UFn
cb
TiO (U ) within the explored series. This affords the highest
2
Fn
percentage of photopotential conversion (Table 3). The
(Figure S9), along with a similar Ucb − U that suggests
Fn
highest total resistance of 5/6-El (Figure S9), probably due
comparable charges in the two cells and hence comparable rate
29
to mass-transport limitations as previously reported, is a
of recombination (Table 3). This last finding is in good
a
Table 3. Eredox, UF,redox, U , and U , at Open-Circuit Potential of D1 DSSCs
Fn
cb
+
b
c
d
e
f
f
electrolyte
Eredox vs Fc |Fc/V
UF,redox/eV
U /eV
Fn
U /eV
cb
Ucb − U /eV
photopotential conversion %
Fn
1
3
5
/2-El
/4-El
/6-El
−0.10
−0.20
−0.04
−0.28
−0.32
−4.70
−4.60
−4.76
−4.52
−4.48
−3.89
−3.92
−3.90
−3.94
−3.88
−3.38
−3.42
−3.45
−3.44
−3.40
0.51
0.50
0.45
0.50
0.48
61
58
66
54
55
Cobalt-El
I−/I3−-El
a
Under 100 mW cm−2 AM 1.5 G irradiation. Determined through cyclic voltammetry as Eredox = −E
b
+
, measured as described in the
d
1/2,Fc /Fc
c
Supporting Information. Energy of the Fermi level of electrolyte, U
TiO at open circuit, evaluated as U = UF,redox + eV . Conduction band edge energy at open circuit, calculated as reported in the Supporting
Information. Calculated comparing the generated photopotential V with respect to the theoretical maximum photopotential (Ucb − UF,redox)/e.
= −e(E
+ 4.8), with e the positive unit charge. Quasi-Fermi level of
F,redox
redox
e
2
Fn
oc
f
oc
F
DOI: 10.1021/acs.inorgchem.7b02323
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
agreement with the already mentioned comparable kheter
determined by EIS in a classical three-electrode cell (Table S1).
The great potential of the new 1/2 redox shuttle is confirmed
by the PCE value (3.7%) of the related DSSC, much higher
than that obtained with the reference electrolytes [Co(dtb-
which in turn downshifts the quasi-Fermi level, UFn (i.e., eV =
UFn − UF,redox).
oc
CONCLUSION
■
In conclusion, this work has shown the great potential of Cu
complexes that can be coupled with one of the most efficient
classes of sensitizers, Zn2+ porphyrins in transparent solar cells.
The new [Cu(2-mesityl-1,10-phenanthroline)2]+ 1/2 couple
is particularly efficient when combined with D1, reaching a
remarkable 75% increase of PCE with respect to the 5/6 Cu
reference thanks to a better diffusional behavior. Removal of
the electron releasing methyl groups from the positions 4,7 of
the phenanthroline leads to an improvement of cell efficiency
2+/3+
−
−
bpy)3]
2
(2.9%) and comparable to I /I (3.4%), assuring a
3
00 mV increase of V . The lower short-circuit photocurrent,
oc
/2+
jsc, of 1/2-El cannot be attributed either to a poorer dye
regeneration (Figure S10) or to any mass transport limitations
as confirmed by dark-to-light chronoamperometric study
(
Figure S11), which records plateau current immediately after
the light was switched off without occurrence of any significant
peak current attributable to a diffusion-limited process, at least
at the sampling rate (0.1 s) adopted for the acquisition of the
i−t curves. It could be attributed to competitive light harvesting
of 1/2-El with D1, as indicated by a relative 7−10% lower
conversion efficiency at ca. 460 nm, with respect to the
due to a quite high V . Also, as a result of a more oxidizing
oc
2
+/3+
−
U
, 1/2-El neatly overcomes [Co(dtb-bpy) ]
I3 reference couples. Remarkably, Cu complexes are efficient
electron donors toward D1, reaching dye regeneration
and I /
F,redox
3
−
+
−
−
2
+
−
reference electrolytes (Cobalt-El and I /I -El) having a
3
constants up to 2-fold and 4-fold faster than Co and I , in
equimolar conditions, respectively (Table S3). Thus, the
combination of β-substituted tetraaryl Zn2+ porphyrin dyes
with copper-based redox mediators represents a novel route to
low cost, environmentally friendly DSSCs. This first report
opens the way to new promising prospects through which
optimization of electrolyte composition, copper complex
solubility, photoanode architecture, and exploitation of other
weaker absorption at this wavelength (Figure 1). On the
+
other hand. the Q-band photoconversion, where Cu MLCT
absorption is negligible, is nearly superimposable in 1/2-El and
in the references. In any case, this lower light harvesting
efficiency is more than counterbalanced by the Fermi energy of
1
/2-El (UF,redox = −4.70 eV, Table 3) with respect to both
− −
Cobalt-El and I /I -El, resulting in a better V
which,
3
OC
2+
together with the higher FF, leads to the best PCE value.
The best electrolyte 1/2-El was finally tested with the D2
Zn2 porphyrin. Comparison with D1 is reported in Figures 3
Zn -porphyrins will surely result in a net improvement of
DSSCs efficiencies and could be of interest also for DSSCs
+
45
operating under indoor lighting.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02323.
Figures and tables related to the characterization of dyes
and electrolytes; and electrochemical, photoelectrochem-
ical, and transient absorption spectroscopy measure-
ments (PDF)
AUTHOR INFORMATION
Corresponding Authors
E-mail: dominique.roberto@unimi.it (D.R.).
E-mail: maddalena.pizzotti@unimi.it (M.P.).
E-mail: stefano.caramori@unife.it (S.C.).
■
Figure 3. Polarization curves under 100 mW cm−2 AM 1.5G
illumination (solid lines) and in the dark (dashed lines) of D2-
sensitized DSSCs, filled with 1/2-El. For the sake of comparison, the
D1-sensitized cell is also reported.
*
*
*
ORCID
Gabriele Di Carlo: 0000-0002-8782-7945
Alessio Orbelli Biroli: 0000-0002-5463-1444
Dominique Roberto: 0000-0001-9659-4760
Francesca Tessore: 0000-0002-6691-0256
and S12. The constantly lower IPCE (Figure S12) together
with the lower j points toward a sluggish electron injection,
sc
+
notwithstanding the comparable energy of the D*|D excited
+
+
Author Contributions
state (E (D*|D = −1.82 and −1.81 V vs Fc |Fc for D1 and
1/2
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
D2, respectively; Table S2). This behavior can reasonably be
explained by an energy increase of the conduction band edge
with D2, leading to a lower driving force for the electron
injection, attributable to the different nature of the anchoring
group, being the electrolyte exactly the same.
Notes
The authors declare no competing financial interest.
The 40 mV mismatch in Voc (Figure S13) derived after
ACKNOWLEDGMENTS
simply shifting upward the jE curve of D2 until j matches that
■
sc
of D1 (so virtually equaling charge injection fluxes) can roughly
Regione Lombardia and Fondazione Cariplo are gratefully
acknowledged for financial support and for the use of
instrumentation purchased through the SmartMatLab Centre
project (2014). A.O.B. thanks Regione Lombardia (Accordo
44
coincide with the conduction band shift, being dark currents
quite superimposable. Moreover the lower Voc for the D2
photoanode is a direct consequence of the decreased injection
G
DOI: 10.1021/acs.inorgchem.7b02323
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Quadro Regione Lombardia-CNR 2016, project: “I-ZEB Verso
Edifici Intelligenti a Energia Zero per la crescita della citta
intelligente”) for financial support. Unife acknowledges POR-
FESR HPSolar, PRIN 2010-2011, and SolarChem Consortium
for support.
open circuit voltage of porphyrin-sensitised TiO
2
solar cells. Chem.
Commun. 2008, 4741−4743.
̀
(18) Griffith, M. J.; Sunahara, K.; Wagner, P.; Wagner, K.; Wallace,
G. G.; Officer, D. L.; Furube, A.; Katoh, R.; Mori, S.; Mozer, A. J.
Porphyrins for dye-sensitised solar cells: new insights into efficiency-
determining electron transfer steps. Chem. Commun. 2012, 48, 4145−
4
162.
DEDICATION
(19) Chang, Y.-C.; Wang, C.-L.; Pan, T.-Y.; Hong, S.-H.; Lan, C.-M.;
■
Kuo, H.-H.; Lo, C.-F.; Hsu, H.-Y.; Lin, C.-Y.; Diau, E.W.-G. A strategy
to design highly efficient porphyrin sensitizers for dye-sensitized solar
cells. Chem. Commun. 2011, 47, 8910−8912.
The strong belief of Professor Renato Ugo in the potential of
coupling porphyrin dyes with copper electrolytes for DSSCs led
to this paper that we dedicate to him on the occasion of his
(20) Ripolles-Sanchis, T.; Guo, B.-C.; Wu, H.-P.; Pan, T.-Y.; Lee, H.-
7
9th birthday.
W.; Raga, S. R.; Fabregat-Santiago, F.; Bisquert, J.; Yeh, C.-Y.; Diau,
E.W.-G. Design and characterization of alkoxy-wrapped push−pull
porphyrins for dye-sensitized solar cells. Chem. Commun. 2012, 48,
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