Ambipolar Hexa-peri-hexabenzocoronene-fullerene hybrid materials

Article abstract of DOI:10.1021/ol102166m

A new class of self-assembling hexa-peri-hexbenzocoronene (HBC)-fullerene hybrid materials has been synthesized and characterized. Photoluminescence experiments indicate that energy transfer processes can be tuned in these donor-acceptor systems by varying the length and nature of the linker group. In preliminary device testing, ambipolar charge transport behavior is observed in organic field effect transistors, while single active component organic photovoltaic devices consisting of these materials achieved a maximum external quantum efficiency of 30%.

Full text of DOI:10.1021/ol102166m

ORGANIC
LETTERS
2010
Vol. 12, No. 21
5000-5003
Ambipolar Hexa-peri-hexabenzocoronene-
Fullerene Hybrid Materials
Wallace W. H. Wong,*,† Doojin Vak,† Th. Birendra Singh,‡ Shijie Ren,† Chao Yan,†
David J. Jones,† Irving I. Liaw,† Robert N. Lamb,† and Andrew B. Holmes†
School of Chemistry, UniVersity of Melbourne, Victoria 3010, Australia, and
CSIRO Molecular and Health Technologies, Ian Wark Laboratory,
Clayton South, Victoria 3169, Australia
wwhwong@unimelb.edu.au
Received September 10, 2010
ABSTRACT
A new class of self-assembling hexa-peri-hexbenzocoronene (HBC)-fullerene hybrid materials has been synthesized and characterized.
Photoluminescence experiments indicate that energy transfer processes can be tuned in these donor-acceptor systems by varying the length and
nature of the linker group. In preliminary device testing, ambipolar charge transport behavior is observed in organic field effect transistors, while
single active component organic photovoltaic devices consisting of these materials achieved a maximum external quantum efficiency of 30%.
The arrangement of electron donor (D) and electron
acceptor (A) materials in heterojunction organic photo-
voltaic (OPV) devices is a determining factor in device
performance. One approach is to use block copolymers
to achieve nanoscale morphology. In these systems, D and
A blocks are covalently linked and the blocks phase
separate according to the size of the D and A blocks.
Several groups have demonstrated this concept,1-3 and
the benchmark for the power conversion efficiency of these
OPV devices is at 1.7%.4 Small molecule or oligomeric D-A
materials are also potential candidates in single component
OPV devices. Several examples of donor-acceptor dyad
molecules have been tested in OPV devices.5-11 The best
oligomeric system, with efficiency up to 1.5%, showed
assembly of nanoscale D-A morphology containing highly
ordered domains.12,13
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79, 47
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† University of Melbourne.
.
‡ CSIRO Molecular and Health Technologies.
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10.1021/ol102166m  2010 American Chemical Society
Published on Web 10/04/2010

Scheme 1. Synthesis of Fullerene-Hexa-peri-hexabenzocoronene-Fullerene Triads 1 and 2
Scheme 2. Synthesis of Hexa-peri-hexabenzocoronene-Fullerene Dyad 3
Hexa-peri-hexabenzocoronene (HBC) is a planar aro-
matic molecule and has been shown to self-assemble into
columnar14-17 and nanotube11,18 structures in solution and
in solid state. Recently, we reported the synthesis and
optoelectronic properties of a highly soluble functional HBC
building block carrying conjugated substituents.19-21 The
9,9-dioctylfluorenyl hexa-peri-hexabenzocoronene (FHBC)
moiety has emerged as a material with excellent solubility
and the potential for further derivatization. In this study, we
report the synthesis, characterization, and device properties
of a series of FHBC-fullerene D-A materials. By taking
advantage of the self-assembling properties of the FHBC
core, there is potential to achieve an ordered D-A morphol-
ogy that is beneficial for OPV device performance.
The strategy for the synthesis of the ambipolar hexa-peri-
hexabenzocoronene-fullerene materials is based on the highly
soluble FHBC core unit. In all cases, the fullerene moieties are
connected to the FHBC derivatives in the last step of the
synthesis sequence through the well-known 1,3-dipolar cy-
cloaddition of azomethine ylids to the fullerene (Schemes 1 and
2). One interesting observation is that initial attempts to connect
the FHBC boronic acid pinacol ester derivative 4 and fullerene
derivative 5 using palladium-catalyzed carbon-carbon bond
formation proved unsuccessful (Scheme 1). Very few examples
of palladium-catalyzed coupling involving fullerene derivatives
can be found in the literature.22-28 The experimental observa-
tions in this study suggest the fullerene derivatives are easily
reduced by palladium(0) species. This redox reaction probably
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Org. Lett., Vol. 12, No. 21, 2010
5001

Table 1. Thermal, Optical, and Redox Data of the FHBC-Fullerene Compounds
max
λabs
(nm)
Tdeg (°C)a
Tg (°C)b
solutionc
film
Eox
(V)d
HOMO (eV)e
Ered
(V)d
LUMO (eV)f
Eg (eV)g
onset
onset
compounds
1
385
129
370 (2.7)
420 (0.9
370 (2.2)
419 (0.6)
365 (2.6)
372
372
372
0.67
0.51
0.54
-5.47
(-5.41)
-5.31
(-5.39)
-5.34
-1.00
-1.00
-1.06
-3.80
1.67
2
3
359
406
125
127
-3.80
-3.74
1.51
1.60
(-5.38)
a Degradation temperature (Tdeg) observed from TGA corresponding to 5% weight loss at 10 °C/min under nitrogen flow. b Glass transition temperature
c
(Tg) from DSC at 10 °C/min under nitrogen flow. Measured in o-dichlorobenzene solution and extinction coefficient ( × 105 M-1 cm-1) in brackets.
d Cyclic voltammograms measured in o-dichlorobenzene, 1 × 10-3 M, Bu4NPF6 (0.1 M), 295 K, scan rate ) 100 mV s-1, versus Fc/Fc+. e Determined from
EHOMO ) -(Eoxonset + 4.80) (eV), data in brackets measured by photoelectron spectroscopy in air.29,30 f Determined from ELUMO ) -(Eredonset + 4.80) (eV).
g Energy gap (Eg) ) LUMO - HOMO.
maintains the Pd species in the Pd(II) state, and consequently, the
Pd catalyst is deactivated. Further studies will be required to
ascertain the mechanism of this catalyst deactivation.
The synthesis of FHBC-fullerene hybrids 1 was achieved
by first coupling the FHBC boronic acid pinacol ester derivative
4 with 7-bromofluorene-2-carboxaldehyde 6 (Scheme 1). Con-
densation of aldehydes 7 with N- methylglycine gave the
azomethine ylid intermediate which reacted with C60 to give
FHBC-fullerene hybrid 1. The FHBC-fullerene derivative 2,
with a flexible tetraethylene glycol linker between the FHBC
and fullerene moieties, was similarly obtained (Scheme 1).
Starting from the monosubstituted FHBC boronic acid pinacol
ester derivative 10, FHBC-fullerene dyad 3 was obtained in
good yield after coupling with aldehyde 6 followed by 1,3-
dipolar cycloaddition to C60 (Scheme 2). Typically, an excess
of C60 was added to the 1,3-dipolar cycloaddition reactions in
order to drive the reaction to completion. The relatively low
molecular weight C60 can be easily separated from the desired
products by size exclusion chromatography. The identity and
purity of all new compounds were verified by a range of
standard analytical techniques (see Supporting Information).
All three FHBC-fullerene hybrid materials 1, 2, and 3
showed good thermal stability in thermal gravimetric analysis
(TGA) experiments (Table 1). In differential scanning calorim-
etry (DSC) experiments, a weak transition at around 125 °C
was observed for the three compounds and can be assigned to
the rearrangement of the alkyl side chains on the fluorene
moieties (Table 1). All three FHBC-fullerene compounds have
their absorption maximum at ∼370 nm and can be assigned to
the absorption band of the FHBC unit (Figure 1). The solution
PL spectra of the compounds (10-4 M in o-dichlorobenzene)
were recorded using 370 nm excitation (Figure 1). The relative
PL intensities observed are in agreement with the different
spatial separation between the donor FHBC and acceptor
fullerene units in compounds 1, 2, and 3. For compound 1, two
fullerene units are separated from the HBC core by two fluorene
units and showed very weak fluorescence from the HBC unit.
On the other hand, compound 2 has a tetra(ethylene glycol)
linker between the HBC and the fullerene, while compound 3
has a four-fluorene spacer. In these cases, the emission from
the HBC unit is relatively strong compared with that of
compound 1 at the same solution concentration. Time-resolved
Figure 1. UV-vis absorption and photoluminescence (PL) spectra
of FHBC-fullerene compounds in o-dichlorobenzene solution.
spectroscopic experiments are now in progress to determine the
energy transfer mechanisms involved. Photoluminescence was
not observed in solid state samples of the compounds (films
with 50 nm thickness). This indicates close packing of the
FHBC-fullerene molecules in the solid state with quenching
of the FHBC fluorescence by the fullerene moieties. In
electrochemical experiments, the FHBC-fullerene compounds
showed reversible reductions typical of fullerene derivatives
(Table 1). The oxidation of these materials closely matches that
of the parent FHBC compound (see Supporting Information
for cyclic voltammograms).
From previous studies, it is well-known that the FHBC
moiety aggregates strongly in solution and assembles into
ordered structures in the solid state.20,21 Similar behavior was
also observed for the ambipolar FHBC-fullerene hybrid
materials. In solution NMR spectroscopic studies, the
resonances assigned to the FHBC protons shift significantly
upfield with increasing concentration of the hybrid materials.
Figure 2 shows the concentration-dependent 1H NMR
spectrum of compound 3 (see Supporting Information for
1H NMR spectrum of 1 and 2). Protons on the periphery of
cyclic aromatic systems usually appear downfield because
of the ring current effect. The upfield shift of the FHBC
(29) Kane, E. O. Phys. ReV. 1962, 127, 131.
(30) Kirihata, H.; Uda, M. ReV. Sci. Instrum. 1981, 52, 68.
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Org. Lett., Vol. 12, No. 21, 2010

Table 2. OFET and OPV Device Performance
OFET
OPVa
µhole
,
µelectron
,
Jsc,
mA/cm2
cm2/(Vs)
cm2/(Vs)
Voc, V
FF
η, %
1
2
3
2.1 × 10-7
4.4 × 10-7
3.4 × 10-6
4.7 × 10-7
0.7 × 10-7
2.0 × 10-6
0.80
0.37
0.87
0.90
0.78
0.80
0.21
0.22
0.31
0.15
0.06
0.22
a The data shown are average values over several devices.
layer|TiOx/Al [ITO, indium tin oxide; PEDOT:PSS, poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate)] were fabricated
and characterized. The active layer of FHBC-fullerene hybrid
material was deposited from o-dichlorobenzene solution by spin
coating giving films of ∼50 nm. TiOx layers were introduced
by following literature procedures.31 In general, all devices
showed good diode-like behavior in the dark and photovoltaic
effects under simulated AM 1.5 G illumination. Table 2
summarizes the device performance of the various solar cells,
and the following characteristic parameters are given: short-
circuit currents (Jsc), open-circuit voltages (Voc), fill factors (FF),
and power conversion efficiencies (η). The current density to
voltage curves and external quantum efficiency (EQE) spectra
of selected devices can be found in Supporting Information.
Although the power conversion efficiencies of these devices
are rather modest, a maximum EQE of 30% was recorded for
the device containing compound 3 at 390 nm. This value is an
order of magitude higher than that reported for a previous
HBC-fullerene system.11 Given the thickness of the active
layers (∼50 nm) and the short wavelength absorption of these
materials, there is potential for improvement in this class of
materials. One of the major loss mechanisms in efficiency is
thought to be charge recombination, particularly for FHBC-
fullerene derivative 1. Time-resolved spectroscopic studies are
currently underway to ascertain the rates of charge separation
and recombination in these systems. In addition, our group is
actively pursuing further structural variations in these FHBC-
fullerene systems in order to improve spectral absorption as
well as solid state D-A morphology.
Figure 2.
1H NMR spectrum of FHBC-fullerene dyad 3 at various
concentrations in CDCl3.
proton resonances with increasing concentration is indicative
of π-π stacking of the FHBC core. Interestingly, there is little
change in the resonances belonging to protons of the fluorene
and fullerene units on these hybrid compounds. This suggests
a strong π-π interaction of the FHBC cores with the peripheral
fullerene units arranged in a staggered manner. The solid-state
structure of the FHBC-fullerene compounds were examined
using X-ray diffraction experiments. Films of the compounds
were deposited onto ultrathin silicon substrate, and two-
dimensional X-ray scattering patterns were recorded (see
Supporting Information). The two-dimensional wide-angle
X-ray scattering (2D-WAXS) was carried out at SAXS/WAXS
beamline in the Australian Synchrotron with wavenumber of
0.62A. The scattering patterns revealed that the π-π stacking
distances for compounds 1, 2, and 3 are 0.351, 0.358, and 0.341
nm, respectively. This is consistent with our previous reports
on FHBC materials.21 The minor variations in π-π stacking
distance is probably a result of the steric hindrance of the
different fullerene substituents in the three compounds.
The charge transport properties of the FHBC-fullerene
hybrid compounds were examined by incorporating these
materials into organic field effect transistors (OFET). Bottom
gate bottom contact OFETs were fabricated by depositing
the organic semiconductor from o-dichlorobenzene solution
by spin coating onto a lithographically patterned substrate
(see Supporting Information for details). A summary of
charge mobility values for the OFET devices is presented
in Table 2 (see Supporting Information for current-voltage
data). It is clear that the OFET devices show ambipolar
Acknowledgment. We thank the Australian Research
Council (ARC) for Discovery Projects (DP0451189,
DP0877325), the Victorian Government (DPI-VICOSC),
the Commonwealth Scientific and Industrial Research
Organisation (CSIRO), and the University of Melbourne
for generous financial support. The X-ray experiments
were undertaken on the SAXS/WAXS beamline at the
Australian Synchrotron, Victoria, Australia.
behavior with reasonable hole (µhole) and electron (µelectron
)
Supporting Information Available: Experimental pro-
cedures, full spectroscopic data for all new compounds, and
device data. This material is available free of charge via the
Internet at http://pubs.acs.org.
mobilities that are comparable to those of a HBC-fullerene
system previous reported.11 More importantly, the hole and
electron mobilities in each of the devices containing com-
pounds 1, 2, and 3 are well balanced.
Given such encouraging OFET data, a preliminary attempt
was made to use these ambipolar materials as the single active
component in heterojunction organic photovoltaic (OPV)
devices. OPV devices with the structure ITO|PEDOT:PSS|active
OL102166M
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