Electronic tuning of nickel-based bis(dicarbollide) redox shuttles in dye-sensitized solar cells

Article abstract of DOI:10.1002/anie.201002181

Redox B ess: Rational design of a new series of boron-functionalized Ni^III/Ni^IV-bis (dicarbollide) clusters results in a family of robust and tunable redox shuttles (see diagram; EDG and EWG denote electron-donating and -withdrawing groups, respectively). This offers a means to rationally control the redox properties in dye-sensitized solar cells (DSCs), leading to exceptionally high open-circuit voltages.

Full text of DOI:10.1002/anie.201002181

Angewandte
Chemie
DOI: 10.1002/anie.201002181
Redox Shuttles
Electronic Tuning of Nickel-Based Bis(dicarbollide) Redox Shuttles in
Dye-Sensitized Solar Cells**
Alexander M. Spokoyny, Tina C. Li, Omar K. Farha, Charles W. Machan, Chunxing She,
Charlotte L. Stern, Tobin J. Marks,* Joseph T. Hupp,* and Chad A. Mirkin*
Dye-sensitized solar cells (DSCs) are potential next-gener-
ation solar electricity sources since they are inexpensive, easy
to fabricate, and can be relatively efficient.[1] Because of this
promise, there has been extensive research aimed at under-
standing and optimizing the performance of these devices by
systematically altering many of their key components.[2]
Indeed, these studies have involved the design of new
molecular dyes to increase light absorption,[3] evaluation of
alternative semiconducting photoanodes to improve charge
collection,[4] and use of protective layers at the semiconduc-
tor/redox electrolyte interface to suppress dark currents
arising from electron interception.[5] However, less attention
has been devoted towards the influence of the DSC redox
shuttle and how the chemical and physical properties of such
shuttles affect overall device performance. Despite identify-
ing shuttles with redox potentials comparable to the com-
monly used anionic IÀ/I3À redox couple, researchers have been
NiIV bis(dicarbollide) was previously shown to be a fast redox
shuttle.[8] With this new class of complexes, the redox shuttle
potential can be tuned over a 200 mV range, overlapping the
IÀ/I3 redox couple. Therefore, the properties of these
À
complexes can be studied in the context of next-generation
DSCs.
In addition to a noncorrosive nature, the NiIII/NiIV
bis(dicarbollide) system also has many favorable properties
versus other metallocene systems, including fast mass-trans-
port, fast dye regeneration, and attractive electron transfer
kinetics.[8] Notably, the system is stable with respect to the
electrode metals (Ag, Au, and Cu) that are commonly used in
commercial DSC modules. To efficiently use the NiIII/NiIV
bis(dicarbollide) system as a DSC shuttle, new chemistry for
modifying the bis(dicarbollide) cage is required. Hawthorne
and co-workers pioneered the chemistry of icosahedral-based
dicarbollide molecules and developed several important
derivatization routes through the metallacarborane CH
vertices and the B(8) positions (for numbering see Figure S2
in the Supporting Information).[9] However, these strategies
lead to equatorially functionalized bis(dicarbollide) struc-
tures (defined by the two C and three B atoms occupying one
plane),[10] which are sterically encumbered and undergo
irreversible redox processes.[10c]
unable to identify ones with other advantages to the IÀ/I3
À
system.[6] However, the strongly corrosive nature of IÀ/I3À is a
fundamental limitation to the further optimization of DSC
architectures, and significantly restricts the choice of dye and
electrode materials.[7]
Herein, we report a promising new class of noncorrosive
B(9)/(B12)-modified NiIII/NiIV bis(dicarbollide) complexes
derived from the systematic functionalization of the NiIII/
NiIV bis(dicarbollide) framework with electron-donating and
electron-withdrawing groups. Importantly, the parent NiIII/
There are now a variety of synthetic strategies for
preparing functionalized carborane molecules and materi-
als.[11] In an effort to reduce steric encumberance and to create
Ni bis(dicarbollides) with programmable redox properties, we
developed a new synthetic strategy for constructing bis(di-
carbollide) species from the B(9)-functionalized derivatives
of the parent carborane. The approach allows realization of
targets originally recognized as important by Hawthorne,[11f]
and involves five high-yield steps, starting from commercially
available o-carborane 1 (Scheme 1). Following known liter-
ature methods,[12a] the B(9) position of o-carborane 1 can be
iodinated in 95% yield to afford 9-I-carborane 2, which can
be subsequently arylated following the procedure of Zakhar-
kin and co-workers to afford 3a–f in good yields (see pp. S2–
S4 in the Supporting Information).[12b] Compounds 3a–f
undergo deboronation in ethanolic solution overnight, quan-
titatively yielding nido-carborane derivatives, isolated as
N[Me3H]+ salts 4a–f. These compounds were characterized
[*] A. M. Spokoyny,[+] Prof. O. K. Farha, C. W. Machan, C. L. Stern,
Prof. J. T. Hupp, Prof. C. A. Mirkin
Department of Chemistry and the International Institute for Nano-
technology, Northwestern University
2145 Sheridan Road, Evanston, IL 60208 (USA)
E-mail: j-hupp@northwestern.edu
chadnano@northwestern.edu
T. C. Li,[+] Dr. C. She, Prof. T. J. Marks, Prof. J. T. Hupp
Department of Chemistry and Argonne-Northwestern Solar Energy
Research Center (ANSER), Northwestern University
2145 Sheridan Road, Evanston, IL 60208 (USA)
E-mail: t-marks@northwestern.edu
[+] These authors contributed equally to this work.
[**] J.T.H. and T.J.M. gratefully acknowledge the support of BP Solar.
T.J.M. also acknowledges the support of the Energy Frontier
Research Center (DE-SC0001059) at the ANSER Center of North-
western University. J.T.H. also thanks the U.S. DOE (Grant No. DE-
FG02-87ER13808) for funding. C.A.M. acknowledges the DOE
Office (Award No. DE-SC0000989) for support via the NU Non-
equilibrium Energy Research Center. C.A.M. is also grateful for
support from the U.S. ARO.
1
by H, 11B, 13C{1H} NMR spectroscopies, elemental analysis
(4a–f), high-resolution ESI-MS (4a–f), and single-crystal X-
ray analysis (3a–f),[13] all of which confirm their structural
formulations (see pp. S5–S12 in the Supporting Information).
Reaction of 4a–f with aqueous excess of NiCl2·6H2O in
concentrated NaOH solution (40%) results in transient NiII
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002181.
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Communications
Scheme 1. 1) I2, CH2Cl2, 6 h, 95%; 2) RMgBr, diethyl ether, 10 mol%
[Pd(PPh3)2Cl2], RT, 3 d, 67–75%; 3) EtOK, EtOH, reflux, 12 h, 98%;
4) 40% aq. NaOH, aq. NiCl2·6H2O, then air, 65–80% (oxidation) in
situ aq. FeCl3, 5 min (reduction) MeOH, NaBH4, 10 min, 95%.
species, which immediately oxidize upon air exposure to form
the corresponding NiIII complexes 5a–f.
Compounds 5a–f are purified by oxidizing further to the
NiIV analogues using aqueous FeCl3, and passing the resulting
mixture through a silica gel plug, which affords pure 6a–f.
These NiIV compounds can be rapidly and quantitatively
reduced back to the corresponding NiIII complexes with
NaBH4 in methanol, and isolated as tetrabutylammonium
(TBA+) salts. Although compounds 5a–f are paramagnetic,
Figure 1. a) Diffraction-determined structures of 6a–f with atoms
drawn with 50% thermal ellipsoid probabilities. Only one stereoisomer
in the unit cell is shown. All solvents and hydrogen atoms are omitted
for clarity. C gray, B beige, O red, F yellow, Cl purple, Ni green.
b) Crystal lattice packing of 6 f (left) and 6a (right).
1
their H and 13C NMR spectra clearly show the presence of
the aryl functional groups (see pp. S40–S43 in the Supporting
Information). Furthermore, the least soluble 6a–f isomers
were characterized by single-crystal X-ray diffraction (see
Figure 1 and pp. S23–S33 in the Supporting Information).[9e,13]
In the case of complexes 6a, 6b, and 6e in which each
dicarbollide cage is functionalized at the B(9) position, the
meso species readily crystallize, whereas in the case of 6c, 6d,
and 6 f, dd(ll) stereoisomers are obtained (dicarbollide cages
functionalized at two isomeric boron vertices, B(9) and
B(12)). In all cases, a cisoid geometry of the two dicarbollide
cages is observed in the diffraction analyses, with dicarbollide
carbons arranged in a staggered conformation. The carbon–
carbon bond distances in 6a–f show little variation (average
trends. In all cases, an absorption band in the visible region
of the spectrum is observed, which blue-shifts from 6a to 6 f
(Figure 2B). This band sensitivity is not observed in the case
of the 5a–f series, both in solid and solution (Figure 2A).
DFT calculations on the optimized structures of 6a and 6e
(both are meso isomers, providing a good comparative basis)
À
C C 1.63 Æ 0.03 ꢀ), regardless of the substituents. This is in
contrast to dicarbollides with substituents directly on the
equatorial C atoms, which exhibit relatively large variations in
[10g]
À
C C distances.
Although the Ni–bis(dicarbollide) complexes 6a–f appear
structurally similar, they differ markedly in color, suggesting
significant electronic structure differences. In particular, a
unique crystal packing arrangement is observed for 6 f, where
the NiIV center does not interact with the phenyl moieties
(Figure 1B). This observation may reflect electron density
redistribution within the molecule induced by the highly
polarizing CF3 groups. Compound 6a is dark blue while 6b
and 6c are dark red, while compounds 6d and 6e are dark
orange, and 6 f is light orange, similar to the unfunctionalized
NiIV–bis(dicarbollide). The optical properties of 6a–f in
dichloroethane solution reflect the observed solid state
Figure 2. UV/Vis spectra of dicarbollide complexes: a) 5a–f and b) 6a–
f and their corresponding solutions in dichloroethane.
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suggest that the visible excitation arises from intramolecular
electron transfer from the filled orbitals of the electron-rich
aryl substituent to the lowest unoccupied orbital of the
electron-poor bis(dicarbollide) cage (see pp. S12–S19 in the
Supporting Information). These results imply that the bis(di-
carbollide) p-system is conjugated with the aryl functional
groups attached through the B atoms of 6a–f, thereby
providing a basis for tuning the redox potentials through the
aryl substituent electron-donating and -withdrawing groups.
The electrochemistry of 6a–f was studied by cyclic
voltammetry, and similar to the unfunctionalized Ni–bis(di-
carbollide), all derivatives undergo quasi-reversible one-
electron processes. The E1/2 values measured for each metal-
lacarborane derivative correlate well with the electron-
donating or -withdrawing nature of the aryl substituents
(see pp. S20–S21 in the Supporting Information). Plotting
these potentials versus the corresponding Hammett constants
reveals a linear relationship,[14] indicating that one can
deliberately tune the electrochemical potential of these
complexes through choice of substituents (Figure 3). This
by 6, and thus accounting for the significantly larger DSC Voc
values for the Ni–bis(dicarbollide) redox shuttle.[8] Insulating
barrier layers deposited at the semiconductor/electrolyte
interface have also been used to enhance DSC photovoltaic
performance by hindering electron interception by fast
shuttles.[5] As shown in Table 1, photovoltage enhancements
Table 1: DSC open-circuit voltages for various redox electrolytes with 0
and 1 ALD cycle of Al2O3 (1 cycle=1.1 ꢀ on average).
Shuttle
Eredox vs.
0 Cycles[a,b] [mV]
1 Cycle[a,c] [mV]
SCE [mV]
5a/6a
5b/6b
5c/6c
5d/6d
5e/6e
5 f/6 f
125
140
160
200
230
305
690
710
640
710
740
770
760
710
720
790
810
850
[a] Voc uncertainties are within 3%, averaging over three different cells.
[b] DSC photovoltages with no ALD modification. [c] DSC photovoltages
with 1 ALD cycle of Al2O3.
are observed following atomic layer deposition (ALD) of
only ca. 1.1 ꢀ of Al2O3 on the TiO2 photoanode. Most
impressively, 5 f/6 f affords a DSC Voc of 850 mV, significantly
higher than that typically observed in liquid IÀ/I3 solar
À
cells.[17]
A major attraction of the present redox shuttle system is
the ability to tune the DSC photovoltage by derivatization of
the carborane moiety. As the potential difference between the
TiO2 quasi-Fermi level and the redox shuttle potential is
increased, the Voc must necessarily increase. However, the
complex interplay of various DSC kinetic processes such as
dye regeneration, electron recombination, electron-transfer
at the counterelectrode, and shuttle transport through the
electrolyte and framework, make predicting overall photo-
voltaic performance non-straightforward.[18] Nevertheless,
Marcus normal-region behavior is observed here with the
electron-withdrawing derivatives, where there is Voc enhance-
ment in going to more positive shuttle potentials (Table 1),
where the concentration of 6 is held constant to minimize the
varying effects of electron capture from the TiO2 conduction
band. Note, however, that with the electron-donating aryl
substituents, Voc is considerably greater than predicted. In
fact, 5b/6b-based DSCs exhibit extraordinary Voc values of
710 mV, and even with TiO2 photoanode ALD modification,
no noticeable increase in photovoltage is observed. By
monitoring the TiO2 conduction band electrons at 980 nm[19]
with nanosecond transient-absorption spectroscopy (see
p. S22 in the Supporting Information), it is noted that the
fast (t1/2 ꢀ 5 ms) electron interception dynamics appear similar
for all the dicarbollide derivatives, with no noticeable trends
correlating with aryl Ni–bis(dicarbollide) chemical modifica-
tion. Attempts to evaluate the longer timescale dynamics, by
the open-circuit photovoltage decay method, are unfortu-
nately inconclusive.
Figure 3. Redox potential of NiIII/NiIV derivatives versus Hammett
constants.
study is the first example of systematic electronic tuning of
transition metal–bis(dicarbollides) using the type of novel B-
arylation chemistry reported herein.
The electrochemical dependance of electrochemical prop-
erties on carborane functionalization was explored for CoII/
CoIII–bis(dicarbollides) through monosubstitution at the B(8)
positions with alkyl and aryl groups by Teixidor and co-
workers.[15a] Most recently the same group reported redox
tunability of cobalt-based clusters by sequential chlorination
at multiple boron vertices.[15b] More importantly, the control
reported herein allows direct probing of the effect of varying
Ni–bis(dicarbollide) redox potential on DSC response. As an
example, modifying the aryl substituent with electron-with-
drawing CF3 groups raises E1/2 of the bis(dicarbollide) to over
300 mV vs. SCE, affording an impressive DSC open-circuit
voltage (Voc) of 770 mV for the 5 f/6 f couple under 1 sun
illumination (see the Supporting Information for details of
DSC fabrication). In contrast, ferrocene/ferrocenium based
DSCs yield only Voc = 200 mV under the same conditions.[16]
We attribute this significant Voc enhancement to the large
rotational barrier of the dicarbollide ligands required to
interconvert 5 and 6, thereby retarding electron interception
Differences in the electron-donating or -withdrawing
substituents effect are apparent in the DSC J–V curves,
where cells based on more electron-rich dicarbollides have fill
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Communications
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factors and power conversion efficiencies superior to the
electron-poor dicarbollides (Figure SI5). This may be a
consequence of more favorable electrolyte–dye interactions
involving the electron-donating aryl methoxy and tolyl
groups. In fact, the 5b/6b couple has lower solubility and, in
contrast to 5a/6a, does not have potentially coordinating
groups. In the cases of the 5a/6a and 5b/6b redox couples,
DSC performance is optimized at only 1.8 mm of complex 6.
However, for 5c/6c, 5d/6d, 5e/6e, and 5 f/6 f, doubling the
concentration of 6 increases fill factors and overall DSC
performance. The polar electron-withdrawing groups may
have a greater chemical interaction with the dye, resulting in a
higher local shuttle concentration near the TiO2 photoanode
(due to slower diffusion). With backside-illumination through
the platinum counterelectrode, fill factors and device effi-
ciencies increase for the electrolytes having low molar
absorptivities (see Figure S5 in the Supporting Information).
In general, device power conversion efficiencies ranging from
0.7 to 2.0% (for J–V curves see Figure S5 in the Supporting
Information) are achieved with the present Ni–bis(dicarbol-
lide) shuttles. Future work will focus on optimizing electrolyte
concentrations for these derivatives.
In conclusion, we have demonstrated that through chem-
ical functionalization of the Ni–bis(dicarbollide) moiety in the
B(9/12) positions, the NiIII/NiIV redox pair potential can be
systematically tuned, and these derivatives can be readily
synthesized on multigram scales. To date, limited probing of
the importance of the DSC redox shuttle chemical character-
istics have been carried out, and to create useful DSC systems,
major advances must be achieved in optimizing the electro-
lyte–dye interaction.[20] The new NiIII/NIV–bis(dicarbollide)
systems lay the groundwork for such optimization, and
although the entire scope is not yet explored, the chemistry
reported herein offers an avenue to create diverse DSC redox
shuttles, which will yield valuable fundamental information
concerning device function. Through functional groups that
modulate redox shuttle polarity, solubility, and binding
properties, it should now be possible to identify structures
that maximize DSC kinetics and performance.
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Published online: June 28, 2010
Keywords: bis(dicarbollide)s · carboranes ·
.
dye-sensitized solar cells · redox shuttles · solar cells
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