Light harvesting tetrafullerene nanoarray for organic solar cells

Article abstract of DOI:10.1039/b510234k

A light absorbing π-conjugated oligomer-tetrafullerene nanoarray has been synthesized and its photophysical study reveals the presence of an intramolecular energy transfer. A phototovoltaic device fabricated from this nanoarray and poly(3-hexylthiophene)

Full text of DOI:10.1039/b510234k

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COMMUNICATION
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Light harvesting tetrafullerene nanoarray for organic solar cells{{
Carmen M. Atienza,a Gustavo Ferna´ndez,a Luis Sa´nchez,a Nazario Mart´ın,*a Ineˆs Sa´ Dantas,b
Martijn M. Wienk,b Rene´ A. J. Janssen,*b G. M. Aminur Rahmanc and Dirk M. Guldi*c
Received (in Cambridge, UK) 20th July 2005, Accepted 22nd November 2005
First published as an Advance Article on the web 9th December 2005
DOI: 10.1039/b510234k
close proximity of the electroactive units, these molecular
donor–acceptor systems generally provide a high efficiency for
photoinduced charge generation and can be tailored to exhibit
long radical ion state pair lifetimes. Molecular PV devices bear
close resemblance to bulk-heterojunction polymer solar cells, in
which a mixture of a p-conjugated polymer, acting as p-type
material, and a soluble C60 derivative as n-type material,
constitutes the photoactive layer. These bulk-heterojunction solar
cells give power conversion efficiencies of up to 4.4%.3,4 To ensure
efficient charge transport, the fullerene content in most bulk
heterojunctions is high, often 4 : 1 by weight with respect to the
polymer. In this respect, one drawback of the molecular PV
systems employed so far is the relatively low fullerene content,2
usually one donor moiety per fullerene, leading to poor charge
transport. To resolve this issue, we set out to prepare conjugated
oligomers with four fullerene units linked by a single p-conjugated
core. In this communication we report the synthesis and
photophysical properties of a new tetrafullerene nanoarray, in
which four C60’s are covalently attached to a p-conjugated light-
absorbing oligomeric central core. We address the use of this
nanoarray in photovoltaic devices.
A light absorbing p-conjugated oligomer–tetrafullerene nano-
array has been synthesized and its photophysical study reveals
the presence of an intramolecular energy transfer. A photo-
tovoltaic device fabricated from this nanoarray and poly(3-
hexylthiophene) shows an external quantum efficiency of 15%
at 500 nm.
A challenging task in organic electronics is the integration of single
organic structures and materials into devices that exhibit a wide
range of functions (i.e., signal processing, data storage, etc.).1
To function independently such integrated devices may use
molecular photovoltaics (PV) as a source of energy. A particularly
interesting strategy towards molecular PV is covalently attaching
electron donor moieties to C60 and incorporating these molecular
systems into a PV device.2 Because of the covalent bond and
aDepartamento de Qu´ımica Orga´nica, Facultad de Ciencias Qu´ımicas,
Universidad Complutense, E-28040 Madrid, Spain.
E-mail: nazmar@quim.ucm.es; Fax: +34 91 3944103;
Tel: +34 913944227
bEindhoven University of Technology; Chemical Engineering and
Chemistry, 5600 MB Eindhoven, The Netherlands.
E-mail: R.A.J.Janssen@tue.nl; Fax: +31402451036; Tel: +31 402473597
cNotre Dame Radiation Laboratory, University of Notre Dame, Notre
Dame, IN 46556, USA
The multistep synthetic procedure that gives nanoarray 5 starts
from the previously reported diformyl functionalized oligomer 1
(Scheme 1).5 1,1-Dibromo-1-alkene 2 was readily prepared by
reacting 1 with CBr4 and PPh3 according to Corey’s procedure.6 A
Sonogashira cross-coupling reaction7 between 2 and 38 leads to the
{ Electronic supplementary information (ESI) available: Experimental
section, Table S-1 and Fig. S-1 to S-6. See DOI: 10.1039/b510234k
{ We wish to dedicate this work to Professor Siegfried Schneider on the
occasion of his 65th birthday.
Scheme 1 Synthesis of light harvesting multifullerenic nanocomposition 5.
514 | Chem. Commun., 2006, 514–516
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Fig. 1 UV-Vis spectra of compounds 2, 4 and 5 in CH2Cl2.
Fig. 2 Spectral response of a glass/ITO/PEDOT : PSS/P3HT : 5/LiF/Al
photovoltaic device.
highly light-absorbing tetraaldehyde 4 in 72% yield. This synthetic
strategy has been proven by Kim et al. as a very useful method to
tune the absorption and emission properties of p-conjugated
oligomers.9 Finally, Prato’s 1,3-dipolar cycloaddition reaction
between the azomethine ylide, generated in situ by reacting
N-octylglycine and 4, and one of the double bonds of C60, yielded
5 as a diastereomeric mixture.10 All mentioned compounds have
been fully characterized through a variety of spectroscopic
techniques (See Electronic Supplementary Information (ESI{)).
When comparing the electronic spectra of 2 and 4, a
bathochromic shift of the p–p* absorption band into the visible
region of the solar spectrum (Fig. 1) is observed as a consequence
of the increased length of the p-system. By attaching four C60 units
to the central oligomeric moiety in 5 the conjugation length is only
slightly reduced as evidenced from a small hypsochromic shift
(Fig. 1). However, 5 stills shows an appreciable absorption over
the entire visible region up to y700 nm.
The redox properties of 4, 5 and C60, were determined by cyclic
and differential pulse voltammetry (Table S-1, Fig. S-1, ESI{). In
nanoarray 5, as many as eight redox processes—three of them
corresponding to the oxidation of the central oligomeric core and
the remaining ones due to the reduction of the C60 units and the
central p-conjugated moiety—are seen. The presence of both
electron donor (p-conjugated oligomer) and C60 moieties in
ensemble 5 account for the amphoteric redox behavior observed.
As expected, four quasi-reversible waves appear corresponding to
the fullerene cages (20.88, 21.27, 21.81 and 22.26 V vs Ag/Ag+
in ODCB/MeCN) similarly to other related multifullerene
conjugates.11 A diagnostic wave at 21.50 V can also be seen for
the reduction of the oligomer fragment.
efficient electron acceptor in combination with P3HT as an
electron donor.
To explain the absence of PV activity in pure 5 and to
elucidate the role of the various electroactive components in its
photophysical behavior, a series of experiments—steady-state/
time-resolved fluorescence and transient absorption spectro-
scopy—were performed. Considering the strongly fluorescing
features of p-conjugated oligomers, we tested the fluorescence of
compound 4 as a reference system and compared that with our
newly synthesized nanoarray 5. Regardless of solvent polarity we
find fluorescence quantum yields of 0.55 ¡ 0.02 for 4 and a
fluorescence lifetime of 1.14 ns (in CHCl3). Upon investigating
compound 5—see Fig. 3—several important aspects should be
emphasized. First, we see strongly reduced fluorescence quantum
yields (i.e., toluene: 4.3 6 1024; THF: 3.8 6 1024; o-dichloro-
benzene: 2.9 6 1024). Second, the fluorescence is dual, that is,
besides the oligomer relevant features in the 400–700 nm domain,
we see in the 650–850 nm region additional features. Third, the
650–850 nm characteristics bear close resemblance with that seen
for the fullerene fluorescence in N-methylfulleropyrrolidines.5
The PV properties of 5 were evaluated by spin casting a thin
layer of the pure material and of a 1 : 1 blend with poly(3-
hexylthiophene) (P3HT) onto a PEDOT : PSS covered ITO layer
on a glass substrate and completing the device with an LiF/Al back
contact. Pure 5 gave no significant PV effect, but in combination
with P3HT the devices showed a short-circuit current of
1.2 mA cm22, an open circuit voltage of 0.65 V, and a fill factor
of 0.17 under illumination with a white-light wolfram-halogen
lamp (75 mW cm22) (Fig. S-2, see ESI{). The external quantum
efficiency of the devices reached 0.15 electrons per photon
at 500 nm (Fig. 2). This demonstrates that 5 acts as an
Fig. 3 Room temperature fluorescence spectra of 4 (dot) and 5 in
various solvents (solid)—see labels—recorded with solutions that exhibit
optical absorptions at the 415 nm excitation wavelength of 0.5. The
fluorescence spectrum of 4 is reduced by a factor of 500.
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Fourth, determining the quantum yield of the fullerene fluores-
cence (i.e., 6.0 6 1024 in toluene and o-dichlorobenzene) shows
that its formation is quantitative in 5, despite the exclusive
excitation of the oligomer fragment.
electron donor OPE unit and electron acceptor fullerene moieties.
Although energy transfer processes between C60 and the central
p-conjugated oligomer12 prevents an efficient photoconversion
performance, the combination of 5 and P3HT led to external
quantum efficiencies of 15%. Work is currently in progress
towards the preparation of new nanoarrays and nanoensembles
with better donor abilities and light-absorbing properties. This is
expected to improve their applicability as active components in
solar cells.
To gather mechanistic evidence for the fullerene fluorescence we
inspected the excitation spectrum of the 650–850 nm features.
Since, the resulting spectrum matches the ground state spectrum of
the oligomer (Fig. S-3, see ESI{), we postulate a quantitative
transfer of singlet excited state energy from the high lying oligomer
singlet state (2.45 eV) to the lower lying fullerene singlet (1.76 eV).
Undisputable evidence for the reaction sequence con-
sidered above came from time-resolved transient absorption
Financial support by the MCyT of Spain and Comunidad de
Madrid (Projects BQU2002-00855 and HSE/MAT0633-04), the
Dutch Polymer Institute (DPI#324) and the EU (RTN network
‘‘WONDERFULL’’ and ‘‘CASSIUS CLAYS’’), SFB 583, DFG
(GU 517/4-1), FCI, and the office of Basic Energy Sciences of the
U. S. Department of Energy (NDRL 4638), are gratefully
acknowledged. We thank the CAIs of the UCM. C. M. A. and
G. F. thank MCyT and MEC for a research grant.
measurements. Singlet and triplet features of
4
and
N-methylfulleropyrrolidines were described recently.5 Important
for the current work are only the singlet–singlet characteristics that
include maxima at around 600 and 880 nm, for compound 4 and
N-methylfulleropyrrolidines, respectively. Upon 387 nm femtose-
cond excitation of the oligomer–fullerene conjugate, where the
oligomer absorption is dominant, led initially to the singlet excited
absorption of the oligomer fragment with a strong maximum at
875 nm. In contrast to the reference, which shows a fairly stable
singlet–singlet signature, 5 reveals an ultrafast singlet deactivation
(Fig. S-4 and S-5, see ESI{). Hereafter, only the fullerene singlet–
singlet transitions are observed at 5 ps (Fig. S-4, see ESI{) with a
maximum around 890 nm. Again, a singlet energy transfer is
responsible for this observation. The kinetics of the energy transfer
can be (i) estimated from the y1000-fold fluorescence quenching
in 5 and the 1.14 ns fluorescence lifetime of 4 to be extremely fast,
i.e. on the order of 1012 s21 (1 ps) and (ii) determined from the
femtosecond experiments (Fig. S-5, see ESI{) as 8.3 ¡ 0.5 6
1011 s21. Singlet excited N-methylfulleropyrrolidines are known to
form the corresponding triplet state with near unit quantum yield
via the intersystem crossing dynamics (i.e., 6.6 6 108 s21). The
same process occurs in the nanoarray 5—Fig. S-6{ shows, for
example, the triplet–triplet transition in the 550–950 region, as
detected at the conclusion of the femtosecond experiments (i.e.,
1.5 ns). When extending the time-resolved measurements into the
nanosecond and microsecond time windows the same triplet
features were found—see Fig. S-6{—with an overall quantum
yield of 0.98. Under our experimental conditions the oxygen-
sensitive fullerene triplet decays with a lifetime of about 20 ms and
reinstates hereby—in the absence of molecular oxygen—the singlet
ground state of 5.
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This photophysical relaxation and the PV effects of pure 5 and
its blend with P3HT are confirmed by photoinduced absorption
experiments on thin solid-state films recorded at 80 K (Fig. S-7, see
ESI{). Under these conditions, pure 5 shows the broad featureless
spectrum of the fullerene triplet state and a vibronically resolved
photoluminescence at 1.68 and 1.52 eV. No photoinduced charges
are observed, consistent with the absence of a PV effect. In
contrast, a mixture of P3HT and 5, measured under identical
conditions shows the clear features of P3HT radical cations at
y0.5 and 1.3 eV. Importantly, the spectrum of the P3HT : 5 blend
differs considerably from that of neat P3HT, which is dominated
by a triplet band at 1.06 eV and that of 5.
In summary, a visible light-absorbing nanoarray—integrating
four C60 moieties and a single p-conjugated oligomer—has been
synthesized from bisaldehyde 1. The redox studies reveal an
amphoteric redox behavior stemming from the presence of both
516 | Chem. Commun., 2006, 514–516
This journal is ß The Royal Society of Chemistry 2006