ing.8 Triplet state formation and, potentially, triplet energy
migration may thus occur in the compact HBC core as-
semblies. If the triplet state energy is such that it precludes
charge separation processes, then triplet state formation could
be a limitation on the efficiency of photovoltaic devices since
it would represent a potential loss of extracted current.
The HOMO energy levels of the FHBC cores 1 and 2 and
dendrimers 30 and 31 were measured using electrochemical
techniques. Cyclic voltammograms of these compounds were
-
recorded in toluene solution with 0.1 M Bu4N+BF4 as
electrolyte. Both onsets of oxidation for FHBC cores 1 and
2 are at 1.0 V vs ferrocene/ferrocenium, while the oxidation
onsets for dendrimers 30 and 31 are at -0.1 V. This means
the HOMO levels of the FHBC cores and the dendrimers
are -5.8 and -4.7 eV, respectively. The optical band gaps
of all three dendrimers obtained from their thin film UV-
vis spectra are approximately 2.6 eV. These energy levels
confirm that the FHBC dendrimers are an appropriate match
with an electron acceptor, such as [6,6]-phenyl-C61-butyric
acid methyl ester (C60 PCBM) with LUMO at -3.7 eV and
HOMO at -6.1 eV, for use in organic solar cells.9
Figure 3
. IPCE spectra of devices containing dendrimer 30 with
C60 PCBM (black line) and C70 PCBM (red line). Device structure
is ITO/PEDOT:PSS (38 nm)/dendrimer 30:PCBM 1:2 (40-60 nm)/
Ca (20 nm)/Al (100 nm).
In preliminary device studies, bulk heterojunction solar
cells with a device structure of ITO/PEDOT:PSS (38 nm)/
active layer (40-60 nm)/Ca (20 nm)/Al (100 nm) were
fabricated. The devices were tested with an Oriel solar
simulator fitted with a 1000 W Xe lamp filtered to give an
output of 100 mW/cm2 at AM 1.5. The active layer of the
device consists of a blend of one of the dendrimers and C60
PCBM in ratios of 1:2 or 1:4. The performance of the devices
with the three dendrimers is similar reaching Voc ) 0.64 V,
Jsc ) 0.68 mA/cm2, fill factor ) 0.30, and power conversion
efficiency ) 0.13% (see Supporting Information for all
device data). It is worthy of note that no annealing was
carried out on any of the devices, and this is the subject of
ongoing studies. Devices consisting of the dendrimers and
the C70 analogue of C60 PCBM were also fabricated. It has
been shown that C70 can provide better device performance
because of its superior optical absorption profile.10 In a
and under low power conditions (1 mW/cm2) to prevent
device degradation. All devices measured in this study are
stable under a simulated solar spectrum of 100 mW/cm2 at
AM 1.5.
In summary, a series of easily functionalized fluorenyl-
hexabenzocoronene (FHBC) compounds have been synthe-
sized in high yield at a gram scale. Triarylamine oligomers
were attached to these FHBC moieties via Buchwald-Hartwig
coupling leading to a series of electron-donating dendritic
materials. In association with electron-accepting fullerene
derivatives, bulk heterojunction solar cells were fabricated
with external quantum efficiencies over 5%. The assembly
of these FHBC compounds into columnar structures in thin
films are currently under investigation. The effect of thermal
or solvent annealing on solar cell performance is also under
investigation. Further studies are underway to extend the
series of FHBC compounds by attaching different organic
electronic materials to their periphery.
dendrimer-fullerene blend ratio of 1:2, devices with Voc
)
0.66 V, Jsc ) 1.0 mA/cm2, fill factor ) 0.34, and power
conversion efficiency ) 0.22% were measured. A compari-
son of the IPCE spectra of the C60 and C70 devices clearly
shows the contribution of C70 to the photocurrent (Figure
3). The performance of these solar cell devices are either
better than or comparable to that reported in the literature
for devices containing HBC derivatives.11 In previous reports,
the devices were typically tested using monochromatic light
Acknowledgment. We thank the Australian Research
Council (ARC, DP0451189, DP0877325), the Victorian
Government Department of Primary Industries (ETIS), the
Australian Government Department of Innovation, Industry,
Science and Research (ICOSC, CG10059), the Common-
wealth Scientific and Industrial Research Organisation
(CSIRO), the Victorian Endowment for Science, Knowledge
and Innovation (VESKI), University of Melbourne and
DAAD/Go8 exchange scheme for generous financial support.
S.A.H. acknowledges EPSRC, BP-Solar, ESF-SONS2 pro-
gram, and the Royal Society for financial support.
(8) El Hamaoui, B.; Laquai, F.; Baluschev, S.; Wu, J.; Mu¨llen, K. Synth.
Met. 2006, 156, 1182.
(9) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater.
2001, 11, 15.
(10) Wienk, M. M.; Kroon, J. M.; Verhees, W. J.; Knol, J.; Hummelen,
J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem., Int. Ed. 2003, 42,
3371.
(11) (a) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.;
Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (b) Schmidtke,
J. P.; Friend, R. H.; Kastler, M.; Mu¨llen, K. J. Chem. Phys. 2006, 124,
174704. (c) Li, J.; Kastler, M.; Pisula, W.; Robertson, J. W. F.; Wasserfallen,
D.; Grimsdale, A. C.; Wu, J.; Mu¨llen, K. AdV. Funct. Mater. 2007, 17,
2528. (d) Jung, J.; Rybak, A.; Slazak, A.; Bialecki, S.; Miskiewicz, P.;
Glowacki, I.; Ulanski, J.; Rosselli, S.; Yasuda, A.; Nelles, G.; Tomovic´,
Z.; Watson, M. D.; Mu¨llen, K. Synth. Met. 2005, 155, 150.
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
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Org. Lett., Vol. 11, No. 4, 2009