A porphyrin-hexa-peri-hexabenzocoronene-porphyrin triad: Synthesis, photophysical properties and performance in a photovoltaic device

Article abstract of DOI:10.1039/c0jm00311e

Hexa-peri-hexabenzocoronene (HBC) is a discotic polycyclic aromatic hydrocarbon with good charge transport characteristics while the light harvesting property of porphyrins is well-known. In order to take advantage of the properties of both materials, a p

Full text of DOI:10.1039/c0jm00311e

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
A porphyrin-hexa-peri-hexabenzocoronene-porphyrin triad: synthesis,
photophysical properties and performance in a photovoltaic device†
a
Wallace W. H. Wong, Tony Khoury,b Doojin Vak,a Chao Yan,a David J. Jones,a Maxwell J. Crossleyb
*
and Andrew B. Holmesa
Received 5th February 2010, Accepted 13th May 2010
DOI: 10.1039/c0jm00311e
Hexa-peri-hexabenzocoronene (HBC) is a discotic polycyclic aromatic hydrocarbon with good charge
transport characteristics while the light harvesting property of porphyrins is well-known. In order to
take advantage of the properties of both materials, a porphyrin-hexa-peri-hexabenzocoronene-
porphyrin triad 6 was synthesised for the first time. In photoluminescence studies, the emission from the
higher energy HBC unit was completely quenched by the porphyrin unit indicating efficient energy
transfer. This novel hybrid material 6 was tested in bulk heterojunction photovoltaic devices reaching
a power conversion efficiency of 1.2%.
Introduction
Hexa-peri-hexabenzocoronene (HBC) is a discotic polycyclic
aromatic molecule consisting of thirteen fused six membered
rings.1,2 HBC and its derivatives have been shown to self-
assemble into columnar structures giving rise to ordered
morphology in films.3,4 The self-association property has brought
a certain amount of interest in the use of HBC-based materials as
semi-conductors in organic electronic applications. Charge
mobility of 5 ꢀ 10ꢁ3 cm2/Vs was achieved in organic field effect
transistor (OFET) devices using a dodecyl-substituted HBC
molecule.5 Organic photovoltaics (OPV) consisting of alkyl-
substituted HBC materials have also been reported.6–10 However,
most HBC derivatives rely on alkyl chains at the periphery for
solubility, which limits the potential for further functionaliza-
tion.1,11–13 Recently, we reported the synthesis and optoelectronic
properties of highly-soluble easily-functionalized HBC building
blocks carrying conjugated substituents.14 The primary challenge
Fig. 1 Structures of fluorenyl hexa-peri-hexabenzocoronene (FHBC) 1
in the synthesis of these systems is the incorporation of conju-
and quinoxalinoporphyrin 2.
gated substituents as well as alkyl groups for solubility. The 9,9-
dioctylfluorenyl hexa-peri-hexabenzocoronene (FHBC) moiety
has emerged as a material with excellent solubility and the
potential for further derivatization.14 FHBC core molecule 1 has
shown exceptional performance in OFETs as well as bulk het-
erojunction OPVs (Fig. 1).15 One strategy to further improve the
OPV performance is to broaden the absorption profile of the
FHBC material. We have recently demonstrated this strategy by
attaching dendritic oligothiophenes of varying sizes to the
periphery of the FHBC core.16
incorporation of metals and by functionalization of the substit-
uents on their periphery. A number of porphyrin derivatives have
been designed for artificial photosynthesis where they can act as
light harvesters, initiators of charge separation and/or charge
carriers.18,19 While porphyrins have been successfully employed
as organic dyes in dye-sensitized solar cells (DSSC),20 the use of
porphyrins in bulk heterojunction (BHJ) OPVs has been less
successful. The challenge in the incorporation of porphyrins in
BHJs is to provide a solid state morphology that allows the
efficient transport of charges to the electrodes after the photo-
excitation and charge separation events.21–23 A recent study has
shown the possibility to use porphyrins as the electron donor
material in a BHJ OPV.24 The formation of nano-scale crystalline
porphyrin domains enables efficient charge separation and
transport. In this study, a quinoxalinoporphyrin 2 will be
combined with the FHBC core 1 (Fig. 1). It is envisaged that this
hybrid material will carry the light harvesting properties of the
porphyrin while the self-assembling behaviour of the FHBC is
maintained.
Porphyrins are the primary light harvesting molecules in
photosynthesis.17 They have absorption that extends well into the
red region of the visible spectrum and have high extinction
coefficients. The properties of porphyrins can be tuned easily by
aSchool of Chemistry, Bio21 Institute, University of Melbourne, 30
Flemington Road, Parkville, Victoria, 3010, Australia. E-mail:
wwhwong@unimelb.edu.au; Fax: +613 83442384; Tel: +613 83442386
bSchool of Chemistry, The University of Sydney, NSW 2006, Australia
† Electronic supplementary information (ESI) available: The 1H and 13
C
NMR spectra of new compounds are provided. See DOI:
10.1039/c0jm00311e
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Synthesis and characterisation
The FHBC-porphyrin hybrid material 6 was prepared by Suzuki
coupling between the FHBC boronic acid pinacol ester deriva-
tive 4 and the bromo-quinoxalinoporphyrin 5 (Scheme 1).
Compound 4 was prepared from previously reported FHBC
derivative 3 under palladium catalysed borylation conditions
while the bromo-quinoxalinoporphyrin 5 was prepared by
reaction of the 2,3-dioxo-porphyrin with 1,2-diamino-4-bromo-
benzene.25 The FHBC-zinc porphyrin hybrid 6-Zn was similarly
prepared by Suzuki coupling between compound 4 and zinc
porphyrin 5-Zn. Zinc metallation of porphyrin 5 was achieved by
dissolving zinc acetate in a chloroform–methanol solution of the
porphyrin followed by heating at reflux to give zinc(II) bromo-
quinoxalioporphyrin 5-Zn in 99% yield. Both FHBC-porphyrin
hybrids 6 and 6-Zn were purified by size exclusion chromatog-
raphy. The synthesis of the quinoxalinoporphyrins 2 and 2-Zn
required for comparison studies is reported elsewhere (Fig. 1).26
Evidence for the self-association behaviour of FHBC-
1
porphyrin hybrid 6 was observed in the H NMR spectrum in
CDCl3 (Fig. 2). The resonances assigned to the protons on the
FHBC unit shift up-field with increasing concentration. This is
indicative of p–p stacking of the discotic FHBC unit. In
contrast, the shift in the resonances assigned to the porphyrin
unit (b-pyrrolic H) is relatively small. This is probably due to the
fact that the bulky 3,5-di-tert-butylphenyl groups are preventing
close aggregation of the porphyrin units. These observations are
in good agreement with other FHBC systems reported previously
where the FHBC moieties were shown to p–p stack in a stag-
gered arrangement.15,16 In this way, the FHBC units are in close
contact with each other while the porphyrin units are relatively
separated.
Fig. 2 Concentration dependent 1H NMR spectra of FHBC-porphyrin
hybrid 6 (CDCl3 at 20 ꢂC). Assignment of the spectra was primarily based
on comparison with spectra of known starting materials.
FHBC and porphyrin units (Fig. 3a). This suggests that the
electronic interaction between the FHBC and porphyrin
moieties is weak. Similarly, the UV-Vis spectrum of the
FHBC–Zn porphyrin hybrid 6-Zn also matches the spectra of
the FHBC core 1 and the zinc porphyrin 2-Zn (Fig. 3b).
Interestingly, there are differences in the UV-Vis spectrum of
the FHBC-porphyrin hybrids in solution and in solid state
(Fig. 3c and d). For compound 6 and 6-Zn, the absorption
maxima at 366 nm in solution can be assigned as the
The UV-Vis spectrum of the FHBC-porphyrin hybrid 6 is
simply a super-position of the individual spectrum of the
Scheme 1 Synthesis of FHBC-quinoxalinoporphyrin hybrid.
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Fig. 3 UV-Vis spectra of (a) FHBC core compound 1, quinoxalinoporphyrin 2 and FHBC-porphyrin hybrid 6, and (b) compound 1, zinc porphyrin 2-
Zn and FHBC-zinc porphyrin hybrid 6-Zn in CH2Cl2 solution (10ꢁ6 M), and (c & d) a comparison between normalized solution and thin film UV-Vis
spectra of compounds 6 and 6-Zn.
absorption maxima of the FHBC core. This absorption band is
significantly broadened for both compounds in solid state
which can be attributed to aggregation behaviour of the FHBC
core. On the other hand, the absorption bands belonging to the
porphyrin moiety (Soret and Q bands) remain at similar
wavelengths (Table 1). It is likely that the bulky di-tert-butyl-
phenyl substituents on the porphyrins are preventing aggrega-
tion of the porphyrin units. The HOMO–LUMO energy gap of
these materials can be obtained from the absorption onset of
their UV-Vis spectra (Table 1). The HOMO–LUMO gap of the
FHBC core is 2.87 eV while the HOMO–LUMO gap of the
porphyrin derivatives are all of the order of 2 eV. This
compares favorably with the values obtained from electro-
chemical experiments (vide infra).
FHBC core 1 and quinoxalinoporphyrin 2 (1 : 2 mole ratio)
shows a strong emission from the FHBC molecule and a rela-
tively weak emission from the porphyrin (Fig. 4a). This
comparison confirms that efficient energy transfer is only
occurring for the FHBC-porphyrin hybrid where the FHBC and
porphyrin units are linked covalently. Detailed theoretical
calculations and ultra-fast laser spectroscopic studies are
underway to elucidate the kinetics of these energy transfer
processes in the FHBC-porphyrin hybrid system as they play an
important role in determining device performance.
In solid state, the PL spectra of hybrid 6 as well as the FHBC 1/
quinoxalinoporphyrin 2 blend only show the porphyrin emission
(Fig. 4b) with the FHBC emission completely quenched
(Fig. 4b). The proximity of the FHBC and porphyrin molecules
in the blend film allows the energy transfer from the FHBC
molecule to the porphyrin even with no covalent linkage between
the two units. Similar observations were recorded for the FHBC–
Zn porphyrin system (Fig. 4c and d). By blending these FHBC-
porphyrin electron donors with an electron acceptor (PC61BM),
all thin film PL was quenched. This indicates efficient charge
transfer from the donors to the acceptor and suggests that
these blends may be suitable for bulk heterojunction solar cell
applications.
The normalized photoluminescence (PL) spectra of FHBC 1
and FHBC-porphyrin hybrid 6 in CH2Cl2 solution are shown in
Fig. 4a (excitation wavelength ¼ 365 nm). The emission
maximum of the FHBC compound 1 in solution is centered at
491 nm. The solution PL spectrum of FHBC-porphyrin hybrid 6
mainly consists of the emission from the porphyrin moiety (657
amd 728 nm) with very weak emission from the FHBC core. This
is indicative of energy transfer from the FHBC core to the
peripheral porphyrin units. The solution PL of a mixture of
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Table 1 Photophysical properties of FHBC-porphyrin hybrids
labsmax/nm
lPLmax/nm
Solutiona
Filmc
Onset of absorption/nma
Egopt/eVa b
Solutiona
Filmc
,
Compounds
1
2
364 (5.6)
433 (2.8)
367
—
432
618
2.87
2.01
491
656
725
617
673
657
728
621
530
652
724
d
d
—
e
2-Zn
6
410 (0.9)
442 (1.1)
366 (2.1)
439 (4.6)
366 (2.9)
421 (2.8)
448 (2.3)
628
620
632
1.97
2.00
1.96
—
369
440
369
417
452
689
729
e
—
6-Zn
674
a
Measured in CH2Cl2 solution with extinction coefficient (ꢀ105 Mꢁ1 cmꢁ1) shown in brackets. b Determined from the onset of absorption in solution.
c
Spin-coated from chlorobenzene solution. d Thin film UV-Vis absorption spectra were not recorded for porphyrin compounds 2 and 2-Zn due to poor
film forming properties on glass. e Thin film PL spectra of 2-Zn and 6-Zn showed very weak and broad emission between 500 and 800 nm.
The HOMO and LUMO energy levels of the materials in this
study were measured using cyclic voltammetry (Fig. 5). All
porphyrin derivatives show quasi-reversible oxidation and
reduction processes in solution (1 ꢀ 10ꢁ3 M in chlorobenzene with
FHBC-porphyrin hybrid 6 was very similar to that of porphyrin
compound 2 (Fig. 5a and c). FHBC-porphyrin hybrid 6 has
slightly deeper HOMO and LUMO energy levels than porphyrin
2 and the HOMO–LUMO gap of both compounds are close to 1.9
eV (Table 2). The Zn porphyrin derivatives 6-Zn and 2-Zn also
exhibited similar electrochemical properties. In this case, the
HOMO and LUMO energy levels are essentially the same, within
experimental error, and the HOMO–LUMO gaps are both 1.8 eV.
ꢁ
1 M Bu4N+BF4 electrolyte). The HOMO and LUMO energy
levels of the compounds were determined from the onset of the
oxidation and reduction processes, respectively. Ferrocene was
used as the reference. The electrochemical behaviour of the
Fig. 4 Normalized photoluminescence spectra of (a) compounds 1, 6 and a mixture of 1 and 2 (1 : 2 mole ratio) in CH2Cl2 solution excited at 365 nm;
(b) compounds 1, 6 and a mixture of 1 and 2 (1 : 2 mole ratio) in thin films excited at 365 nm; (c) compounds 1, 6-Zn and a mixture of 1 and 2-Zn in
CH2Cl2 solution excited at 365 nm (*The weak emission at 730 nm can be attributed to the 2nd order excitation wavelength (365 nm) in the experiment);
and (d) compounds 2 and 2-Zn excited at 425 nm.
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Fig. 5 Cyclic voltammograms of (a) quinoxalinoporphyrin 2; (b) zinc porphyrin 2-Zn; (c) FHBC-porphyrin hybrid 6; and (d) FHBC-zinc porphyrin
hybrid 6-Zn in chlorobenzene (1 ꢀ 10ꢁ3 M) with 1 M Bu4N+BF4ꢁ as electrolyte.
Table 2 Electrochemical data of FHBC-prophyrin hybridsa
Eonsetox/V EHOMO/eVb Eonsetred/V ELUMO/eVb Egap/eV
1
2
0.22
0.56
ꢁ5.03
ꢁ5.36
ꢁ5.12
ꢁ5.53
ꢁ5.07
ꢁ2.40
ꢁ1.33
ꢁ1.46
ꢁ1.15
ꢁ1.53
ꢁ2.40
ꢁ3.47
ꢁ3.34
ꢁ3.65
ꢁ3.27
2.63
1.89
1.78
1.88
1.80
2-Zn 0.32
0.73
6-Zn 0.27
6
a
Measured in chlorobenzene (1 ꢀ 10ꢁ3 M) with 1 M Bu4N+BF4 as
ꢁ
b
electrolyte, 295 K, scan rate ¼ 100 mV sꢁ1, versus Fc/Fc+. Energy
level determined from E ¼ ꢁ(Eonset + 4.80) (eV).
Fig. 6 Energy level diagram of FHBC 1, porphyrins 2 and 2-Zn and
FHBC-porphyrin hybrids 6 and 6-Zn and PC61BM. The data were
derived from electrochemical experiments.
These electrochemical energy gap values compare favourably
with the optical energy gap values shown earlier. The energy levels
of all organic semi-conductors in this study are shown in Fig. 6.
All donor materials have appropriate energy levels for application
in bulk heterojunction solar cells with [6,6]-phenyl-C61-butyric
acid methyl ester (PC61BM) as the electron acceptor.
characterized. The ratio of donor and acceptor materials was
optimized at 1 : 2 w/w while the active layer thickness was typi-
cally between 60 and 70 nm. TiOx layers were introduced
following literature procedures.27 The devices were tested in air
and, in general, all devices showed good diode-like behaviour in
the dark and photovoltaic effects under simulated AM 1.5 G
illumination.
Photovoltaic devices
In preliminary experiments, BHJ OPV devices containing
FHBC-porphyrin 6 and PC61BM in the active layer were fabri-
cated and tested. A power conversion efficiency (PCE) of 0.3%
Bulk heterojunction (BHJ) photovoltaic devices with the struc-
ture ITO|PEDOT : PSS|active layer|TiOx/Al were fabricated and
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Fig. 7 BHJ solar cell characteristics of devices containing blends of 1, 6 and PC61BM; (a) Voc; (b) Jsc; (c) fill factor and (d) power conversion efficiency.
The weight ratio of the donor materials (1 and 6) to acceptor material (PC61BM) was kept at 1 : 2.
was obtained (Fig. 7). The performance of these devices was
lower than that of devices containing FHBC 1 and PC61BM even
though the porphyrin moiety in 6 is absorbing at a broader range
of wavelengths of light (Fig. 7 and 3a). Device parameters,
including short-circuit current (Jsc), open-circuit voltage (Voc),
and fill factor (FF) were lower for the device fabricated using
compound 6 compared with that using FHBC 1 (Fig. 7). The
most telling parameter is the fill factor where the FHBC 1 devices
had an average FF of 0.45 and the FHBC-porphyrin 6 device had
an average FF of 0.27. This indicates that the charge transport in
FHBC 1 devices is much more efficient than in FHBC-porphyrin
6 devices and suggests a difference in the solid state morphology
of the active layer blend films. In a previous study, we have
shown that the self-assembly of FHBC 1 into ordered structures
is an important factor in determining BHJ OPV device perfor-
mance as this material characteristic promotes better charge
transport in thin films.15 As such, one of the reasons for lower
device performance for compound 6 could be the steric bulk of
the di-tert-butylphenyl solubilized porphyrin unit disrupting the
assembly of ordered structures in solid state. With this in mind, it
is envisaged that the incorporation of FHBC-porphyrin 6 as
a dopant into a FHBC 1/PC61BM blend (1 : 2 weight ratio) could
be advantageous for broadening spectral absorption and main-
taining efficient charge transport in the BHJ OPV devices.
Fig. 7 shows the device characteristics of a set of BHJ devices
with different ratios of FHBC-porphyrin hybrid 6 in FHBC 1/
PC61BM blends. As the FHBC-porphyrin hybrid was introduced
into the BHJ, the current density of the device increases (Fig. 7b).
This fits with the fact that the active layer of the device is
absorbing more light as the ratio of the FHBC-porphyrin hybrid
increases. However, at around 0.25 weight ratio of 6 to 1, the
current density began to decrease (Fig. 7b). Although the active
layer may be absorbing more light with increasing proportion of
6, the steric bulk of 6 is likely to change the morphology of the
blend film. This can lead directly to the decrease in charge
transport ability of the blend film and hence the decrease in
photocurrent density measured. This hypothesis is also sup-
ported by the trend in the fill factor (Fig. 7c). The fill factor for
the devices started at around 0.45 without compound 6 in the
active layer. The fill factor remained at approximately the same
level until 0.2 weight ratio of 6 was added after which the fill
factor decreased to around 0.25 for the 6/PC61BM device. Voc of
over 0.9 V was recorded for all devices except for the two devices
containing the highest ratios of 6 (Fig. 7a). Given the trends in
current density and fill factor, a maximum power conversion
efficiency of 1.3% was recorded for the device containing 0.2
weight ratio of 6 to 1 (Fig. 7d). The spectral response of devices
containing various donor blends is shown in Fig. 8. All blends
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Fig. 8 Spectral response of devices containing various amounts of
FHBC-porphyrin hybrid 6 and FHBC core 1 as the donor materials. The
weight ratio of the donor materials (1 and 6) to acceptor material
(PC61BM) was kept at 1 : 2.
Fig. 9 Current density to voltage plots of devices containing different
blends of donor materials. The porphyrin content was maintained at 16%
of porphyrin units to FHBC core 1.
converted light into current in the 300 nm to 500 nm range. This
corresponds to the UV-Vis absorption of the FHBC and
fullerene components of the devices, with a very weak contri-
bution from the porphyrin (Fig. 3 and 8). A more detailed
analysis by calculating the internal quantum efficiencies of the
devices is required to quantify the contribution of each active
layer component to the photocurrent generated.22 The incident
photon to electron conversion efficiency (IPCE) maximum
improved from 30% to nearly 45% with up to 25% by weight of
FHBC-porphyrin 6 added to the FHBC core 1 (Fig. 8). The
improvement in IPCE cannot be attributed only to the increase
in light absorption by the FHBC-porphyrin 6 in the blend. This
increase in IPCE between 300 and 450 nm is likely to be the result
of morphology changes in the active layer caused by the addition
of the FHBC-porphyrin additive. Further addition of FHBC-
porphyrin 6 caused a decrease in IPCE. This is probably a result
of inefficient transport of charges through the active layer of the
device stemming from further morphology changes.
Table 3 Bulk heterojunction OPV characteristics for devices containing
various blends of 1, 2, 2-Zn, 6 and 6-Zn. The donor–acceptor ratio was
kept at 1 : 2 by weight while the ratio of porphyrin to FHBC units in the
blends was adjusted to 0.16. The data presented are averages of at least
five devices and the standard deviation is less than 10%
% of additive in FHBC 1
Donor Porphyrin
additive Weight Moles unit
Jsc/
Voc/V mA cmꢁ2 FF
PCE
(%)
None
6
2
6-Zn
2-Zn
—
20
15
20
15
—
8
16
8
—
16
16
16
16
0.93
0.94
0.83
0.80
0.76
1.87
2.91
2.70
2.89
2.03
0.45 0.78
0.44 1.20
0.37 0.83
0.38 0.87
0.34 0.53
16
hand, the incorporation of porphyrin 2 into the FHBC 1/
PC61BM bulk heterojunction may interrupt desirable ordered
morphology and adversely affect device performance. The zinc
porphyrin 2-Zn and the FHBC-zinc porphyrin hybrid 6-Zn were
also used as dye additives in FHBC 1/PC61BM bulk hetero-
junction OPVs. As in the case of the free-base porphyrin deriv-
atives, the FHBC-zinc porphyrin hybrid 6-Zn outperformed the
zinc porphyrin 2-Zn as an additive in the FHBC 1/PC61BM bulk
heterojunction devices (Table 3). It is interesting to note that the
free-base porphyrin additives functioned better than the zinc
porphyrin additives. The reason behind this observation is
unclear at this stage and will be the subject of a further study.
During synthesis and purification, the zinc porphyrin derivatives
appear to be less stable than the free-base porphyrin derivatives.
Metallated porphyrins have been known to de-metallate in
solution and/or in acidic media. This may be a factor affecting
their device performance as device fabrication involves solution
processing and exposure to acidic environments.28
The performance of BHJ devices containing FHBC 1 with
a well defined mole ratio of porphyrin additives and PC61BM
were compared (Fig. 9 and Table 3). The ratio of FHBC 1 to
porphyrin additive was chosen to match the ratio of the best
performing blend device between FHBC-porphyrin hybrid 6 and
FHBC 1 at 0.2 wt. ratio as observed in Fig. 6. A weight ratio of
0.2 between compounds 6 and 1 is equivalent to a ratio of 0.16
between porphyrin units and FHBC units in the blends (Table 3).
The incorporation of porphyrin 2 into the FHBC 1/PC61BM
bulk heterojunction device did not significantly enhance the
overall efficiency of the device. Although there is an increase in
current density compared with the FHBC 1/PC61BM bulk het-
erojunction, it is accompanied by a decrease in Voc and fill factor,
leading to comparable power conversion efficiencies (PCE 0.8%).
This is in contrast to devices containing the FHBC-porphyrin
hybrid 6 where there is an increase in Jsc while similar levels for
Voc and FF are maintained. This result can be rationalized in
the following way. With the porphyrin covalently attached to
the FHBC core, the FHBC-porphyrin hybrid 6 will have the
tendency to reside in the donor FHBC domain of the bulk het-
erojunction. This allows more efficient charge separation and
transport after photo-excitation of the active layer. On the other
Thin film morphology
In preliminary studies, morphology of blend films was investi-
gated using atomic force microscopy (AFM). The surface
morphology of thin films was examined using tapping mode
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Fig. 10 Morphology of blend films on silicon substrate spin coated from chlorobenzene as imaged by tapping mode AFM: (a) FHBC 1/PC61BM 1 : 2
weight ratio; (b) FHBC 1/FHBC-porphyrin hybrid 6/PC61BM 10 : 2 : 24 weight ratio; and (c) FHBC 1/quinoxalinoporphyrin 2/PC61BM 10 : 1.5 : 23
weight ratio. The images (1 ꢀ 1 mm) display the surface topography (height in nm).
AFM. The samples were prepared by spin coating the material of
interest on silicon substrate (25 mg mLꢁ1 in chlorobenzene, 2000
Experimental
All reactions were performed using anhydrous solvent under an
rpm). The tapping mode AFM images of the thin films of blends
inert atmosphere unless stated otherwise. Silica gel (Merck 9385
of FHBC 1, FHBC-porphyrin hybrid
6
and quinox-
Kieselgel 60) was used for flash chromatography. Thin layer
alinoporphyrin 2 with PC61BM are shown in Fig. 10. Nano-scale
phase separation was observed in all three blend films. The blend
of 1 and PC61BM film gave the roughest surface topography (0–
10 nm roughness) with domain sizes of ꢃ100 nm (Fig. 10a). The
blend films became smoother (<3 nm roughness) with the addi-
tion of the porphyrin additives 2 and 6 (Fig. 10b and c). Inter-
estingly, the addition of the FHBC-porphyrin hybrid 6 caused
greater phase separation while the addition of the quinox-
alinoporphyrin 2 appeared to give smaller phase domains. These
differences in film morphology are likely to influence device
performance. However, it is premature to discuss correlations
between morphology and device performance for this system at
this stage, as further experiments are required. X-Ray scattering
experiments on blend films are currently under investigation to
determine the degree of ordered structures in these blends.
chromatography was performed on Merck Kieselgel 60 silica gel
1
on glass (0.25 mm thick). H and 13C NMR spectroscopy were
carried out using either the Varian Inova-400 (400 MHz), the
Varian Inova-500 (500 MHz) or the Bruker AMX 400 (400
MHz) instruments. Mass spectra were recorded with a MALDI-
TOF MS Bruker Reflex 2 (DCTB as matrix). IR spectra were
obtained on a Perkin Elmer Spectrum One FT-IR spectrometer
while UV-vis spectra were recorded using a Cary 50 UV-vis
spectrometer. Photoluminescence was measured with a Varian
Cary Eclipse fluorimeter. Melting points were determined on
€
a Buchi 510 melting point apparatus. Elemental analyses were
obtained on a Perkin-Elmer EA 2400 (for C H) and commercially
through CMAS, Victoria. FHBC compound 1 and quinoxalino-
porphyrins 2, 2-Zn and 5 have been reported previously.14,25,26
All other compounds and reagents are commercially available.
FHBC boronic acid pinacol ester derivative 4
Conclusions
Compound 1 (500 mg, 0.32 mmol), Pd(dppf)Cl2 (5 mg), potas-
sium acetate (500 mg) and bis(pinacolborolane) (500 mg) were
placed in a Schlenk tube and warmed to 60 ꢂC under high
vacuum for 1 h. At this stage, sublimation of the bis(pincolbor-
olane) was observed. Freshly distilled DMF (10 mL) was added
and the reaction was heated at 80 ꢂC for 14 h. The reaction was
cooled and MeOH (10 mL) was added. The resulting yellow
precipitate was collected and washed with MeOH (3 ꢀ 50 mL). A
yellow powder (450 mg, 91% yield) was isolated after drying in
vacuum. m.p. >250 ꢂC. Elemental analysis: calcd. for
A fluorenyl hexa-peri-hexabenzocoronene (FHBC)-porphyrin
hybrid material was synthesised for the first time. Photo-
luminescence studies showed quenching of the FHBC fluores-
cence by the porphyrin units indicating efficient energy transfer.
The performance of bulk heterojunction (BHJ) photovoltaic
devices with the FHBC-porphyrin hybrid 6 as the donor material
and fullerene derivative (PC61BM) as the acceptor material
was modest (PCE ¼ 0.3%) compared with the FHBC 1 device
(PCE ¼ 0.8%). Interestingly, BHJ devices containing ternary
blends of 1, 6 and PC61BM performed better than blends of
6/PC61BM or 1/PC61BM. A maximum efficiency of 1.2% was
achieved with a 1/6/PC61BM blend ratio of 5 : 1 : 12 by weight.
The spectral response of the devices indicate a small contribution
to the photocurrent by the porphyrin units. As such, the
improvement in device performance in the ternary blends is likely
to be a result of changes in active layer film morphology.
Qunioxalino-porphyrin 2 and metallated derivative 2-Zn and
6-Zn were also tested in devices and showed a decrease in
performance compared with FHBC-prophyrin hybrid 6. Studies
are in progress to probe the changes in thin film morphology in
ternary blends.
C
112H120B2O4, C 86.7, H 7.8, B 1.4, O 4.1; found C 86.5, H 7.7.
FT-IR (neat, cmꢁ1): 2928, 2855, 1610, 1355, 1147, 815, 759, 740.
1H NMR (500 MHz, 2.5 mM, CDCl3, 20 ꢂC, d): 0.81 (t, J 7 Hz,
12H, –CH3), 1.07 (m, 8H, –CH2–), 1.23 (m, 40H, –CH2–), 1.52 (s,
24H, pinacol-CH3), 2.34 (m, 8H, –CH2–), 7.35 (br m, 4H, HBC–
H), 7.73 (d, J 7 Hz, 2H, ArH), 7.89–8.03 (m, 18H, ArH), 8.19 (s,
4H, HBC–H), 8.40 (s, 4H, HBC–H). 13C NMR (125 MHz,
CDCl3, 20 ꢂC, d): 14.2, 22.7, 24.3, 25.1, 29.4, 29.5, 30.3, 31.9,
40.5, 55.5, 83.8, 118.3, 118.5, 119.0, 119.4, 120.2, 120.8, 122.0,
122.4, 123.2, 124.8, 126.8, 128.5, 128.9, 129.1, 134.1, 137.4, 140.2,
141.4, 144.1, 150.4, 152.2. HR-MALDI-TOF (m/z): Found [M]+
1549.9392, C112H120B2O4 requires 1549.9404.
7012 | J. Mater. Chem., 2010, 20, 7005–7014
This journal is ª The Royal Society of Chemistry 2010

View Article Online
FHBC-porphyrin hybrid 6
32.1, 32.2, 35.0, 35.1 (2), 41.0, 55.8, 119.0 (2), 119.4, 120.5, 120.6
(2), 120.7 (2), 120.8, 120.9 (2), 122.0, 122.8, 123.6, 124.8, 125.3,
127.3, 128.2 (2), 128.9, 129.0 (2), 130.8, 131.5 (3), 131.6 (2), 131.7,
132.4, 139.2, 140.6, 141.1, 141.4, 141.6, 141.7, 141.8, 141.9, 142.0,
148.6, 148.9, 149.0, 149.2, 150.0, 150.6, 151.2, 152.3, 152.5, 152.6.
MALDI-TOF (m/z): 3749.7 ([M + H]+ requires 3749.1).
Compound 4 (120 mg, 0.08 mmol), bromoquinoxalinoporphyrin
5 (200 mg, 0.16 mmol) and Pd(PPh3)4 (1 mg) were dissolved in
degassed toluene (5 mL). Tetraethylammonium hydroxide
solution (20% wt. in water, 2 mL) was thoroughly degassed
and added to the reaction mixture. The resulting solution was
heated at 90 ꢂC for 14 h and the product was extracted into
toluene. The toluene solution was dried over Na2SO4 and filtered
through a plug of silica. After the removal of toluene, the
resulting residue was purified by size exclusion chromatogra-
phy (Bio-Rad, Bio-Beads S-X1, THF) and a brownish purple
solid (150 mg, 68% yield) was obtained after precipitation
from methanol. m.p. >250 ꢂC. Elemental analysis: calcd. for
{5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)-60-
bromoquinoxalino[2,3–b0]porphyrinato}zinc(II) – Zinc(II) Bromo-
quinoxalinoporphyrin 5-Zn
A
solution of 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-60-
bromoquinoxalino[2,3–b0]porphyrin 5 (400 mg, 0.321 mmol) and
zinc(II) acetate dihydrate (735 mg, 3.349 mmol) in chloroform (40
mL) and methanol (20 mL) was heated at reflux for 30 min. The
solvent was removed under vacuum and the residue purified by
column chromatography over silica (dichloromethane/light
petroleum; 1 : 1). The front running green band was collected
and the solvent removed to give {5,10,15,20-tetrakis(3,5-di-tert-
butylphenyl)-60-bromoquinoxalino[2,3–b0]porphyrinato}zinc(II)
5-Zn (419 mg, 99%) as a dark green microcrystalline solid, mp >
C264H286N12, C 87.4, H 8.0, N 4.6; found C 86.5, H 8.1, N 4.1.
FT-IR (neat, cmꢁ1): 2959, 2925, 2859, 1593, 1470, 1363, 1248,
801, 759. UV-Vis: lmax, nm (3, ꢀ 105 Mꢁ1 cmꢁ1) ¼ 366 (2.1), 439
(4.6). 1H NMR (500 MHz, CDCl3, 3 mM, 20 ꢂC, d): ꢁ2.43 (s, 4H,
porphyrin NH), 0.81 (br m, 12H, octyl-CH3), 1.28–1.41 (br m,
48H, –CH2–), 1.54 (s, 72H, t-butyl), 1.56 (s, 36H, t-butyl), 1.63
(s, 36H, t-butyl), 2.49–2.62 (m, 8H, –CH2–), 7.69 (br, 4H,
HBC–H), 7.83 (m, 4H, ArH), 7.98–8.16 (m, 34H, ArH), 8.31
(br s, 4H, ArH), 8.40 (br, 4H, HBC–H), 8.58 (br, 4H, HBC–H),
8.76 (br, 4H, HBC–H), 8.82 (s, 4H, ArH), 9.03 (br m, 4H,
porphyrin-H), 9.12 (br m, 2H, porphyrin-H), 9.17 (br m, 2H,
porphyrin-H). 13C NMR (125 MHz, CDCl3, 20 ꢂC, d): 14.2, 21.5,
22.8, 24.6, 29.5, 29.7, 29.8, 30.7, 31.7(2), 31.8, 31.9, 32.0 (2), 32.2,
35.0, 35.1 (2), 35.2, 41.1, 55.8, 118.3, 118.8, 119.4, 120.7, 120.9,
121.1, 122.1, 122.8, 123.6, 125.3, 126.5, 127.1, 127.8, 128.2, 128.5,
129.0, 129.3, 129.6, 130.9, 132.4, 132.6, 134.2, 137.8, 138.2, 139.4,
139.6, 139.7, 140.4, 141.2, 141.9, 146.1, 146.2, 148.8, 149.0, 149.1,
152.1, 152.3, 152.8, 153.4, 155.0. HR-MALDI-TOF (m/z): found
[M + H]+ 3625.4400, C264H286N12 requires 3625.2821.
ꢂ
300 C. An analytically pure sample was obtained by recrystal-
lisation from a chloroform–methanol solution. (HR-ESI-FT/
ICR Found: [M + H]+ 1305.5979. C82H94BrN6Zn requires
1305.6009). nmax (CHCl3): 2962s, 2932s, 2901s, 2870m, 1589s,
1543w, 1474m, 1427w, 1358w, 1296w, 1250w, 1157w, 1126w,
1003w cmꢁ1. lmax (CHCl3): 342 (log 3 4.51), 361sh (4.41), 422
(5.32), 451 (5.08), 540 (3.93), 575 (4.32), 620 (4.22), 801 (2.70) nm.
1H NMR (400 MHz, CDCl3): d 1.47 (18 H, s, t-butyl H); 1.49 (18
H, s, t-butyl H); 1.53 (36 H, s, t-butyl H); 7.75 (1 H, d, J 9.0 Hz,
quinoxaline H); 7.79 (2 H, t, J 1.8 Hz, Hp); 7.85 (1 H, d of d, J 8.9,
J 2.2 Hz, quinoxaline H); 7.91 (1 H, t, J 1.8 Hz, Hp); 7.94–7.95 (5
H, d overlapped with t, Ho, Hp); 8.06 (1 H, d, J 2.1 Hz, qui-
noxaline H); 8.09 (4 H, d, J 1.8 Hz, Ho); 8.91 (2 H, s, b-pyrrolic
H); 9.00 (2 H, d, J 4.7 Hz, b-pyrrolic H); 9.03–9.05 (2 H, m, b-
pyrrolic H). Mass spectrum (ESI) (m/z): 1305.7 ([M + H]+
requires 1305.6).
FHBC–Zn-porphyrin hybrid 6-Zn
Compound 4 (120 mg, 0.08 mmol), bromoquinoxalino Zn-
porphyrin 5-Zn (210 mg, 0.16 mmol) and Pd(PPh3)4 (1 mg) were
dissolved in degassed THF (10 mL). An aqueous solution of
NaHCO3 (100 mg in 2 mL) was thoroughly degassed and added
to the reaction mixture. The resulting solution was heated at
Photovoltaic device fabrication and testing
Poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate)
ꢂ
60 C for 14 h and the product was extracted into toluene. The
(PEDOT:PSS, Baytron P AI 4083) was spin coated (5000 rpm)
on patterned ITO glass which was washed by detergent, deion-
ized water, methanol, acetone and 2-propanol in ultrasonication
bath and UV/ozone treated. The films were baked at 150 ꢂC for 5
min in air. Solution of FHBC compounds and PC61BM (25 mg
mLꢁ1) were prepared in chlorobenzene separately. The solution
was stirred at 60 ꢂC for 2 h and cooled down to room tempera-
ture before they were mixed together. Blend solutions (donor–
acceptor 1 : 2) were made by mixing appropriate volumes of the
solutions. The resulting solutions were stirred for 30 min and spin
coated (1500 rpm) on the PEDOT:PSS films. TiOx precursor
solution (1 : 200 in methanol) was deposited on the active layer
by spin coating (2000 rpm) to form ꢃ10 nm of TiOx layer. The
films were exposed to air for about 20 min at room temperature
for hydrolysis or baked at 150 ꢂC for 15 s (total air exposure time
was about 10 min). The films were transferred to a metal evap-
oration chamber and aluminium (100 nm) was deposited through
a shadow mask (active area was 0.20 cm2) at approximately 1 ꢀ
toluene solution was dried over Na2SO4 and filtered through
a plug of silica. After the removal of toluene, the resulting residue
was purified by size exclusion chromatography (Bio-Rad,
Bio-Beads S-X1, THF) and a greenish purple solid (100 mg,
35% yield) was obtained after precipitation from methanol.
m.p. >250 ꢂC. Elemental analysis: calcd. for C264H282N12Zn2,
C 84.5, H 7.6, N 4.5, Zn 3.5; found C 82.4, H 7.8, N 4.1. FT-IR
(neat, cmꢁ1): 2961, 2923, 2854, 1591, 1467, 1259, 1088, 1016, 796.
UV-Vis: lmax, nm (3, ꢀ 105 Mꢁ1 cmꢁ1) ¼ 366 (2.9), 421 (2.8), 448
1
ꢂ
(2.3). H NMR (500 MHz, CDCl3, 20 C, d): 0.81 (br m, 12H,
octyl-CH3), 1.27–1.41 (br m, 48H, –CH2–), 1.56 (br s, 108H,
t-butyl), 1.64 (s, 36H, t-butyl), 2.45–2.61 (m, 8H, –CH2–), 7.47–
7.59 (br m, 8H, Ar–H), 7.83 (m, 4H, ArH), 8.01–8.14 (m, 34H,
ArH), 8.39–8.58 (br s, 16H, ArH), 8.95 (s, 4H, ArH), 9.06 (br m,
4H, porphyrin-H), 9.12 (br m, 2H, porphyrin-H), 9.16 (br m, 2H,
porphyrin-H). 13C NMR (125 MHz, CDCl3, 20 ꢂC, d): 14.1, 22.7,
24.5, 29.5 (2), 29.6, 29.7, 30.4, 30.6, 31.4, 31.4, 31.5 (2), 31.8, 31.9,
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 7005–7014 | 7013

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10ꢁ6 torr. Film thickness was determined by Veeco Dektak 150 +
Surface Profiler. The thickness of the photoactive layers was
optimised for each of the donor–acceptor blends and was typi-
cally between 60 and 70 nm. The solar cells were illuminated at
100 mW cmꢁ2 using 1 kW Oriel solar simulator with an AM 1.5G
filter in air and J–V curves were measured using a Keithley 2400
source measurement unit. For accurate measurements, the light
intensity was calibrated using a reference silicon solar cell
(PVmeasurements Inc.) certified by the National Renewable
Energy Laboratory. Spectral response was measured with
a Keithley 2400 source meter, using monochromatic light from
a Xe lamp in combination with a monochromator (Oriel,
Cornerstone 130). A calibrated Si cell was used as a reference.
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Acknowledgements
We thank the Australian Research Council (DP0451189,
DP0877325, DP0208776), the Commonwealth Scientific and
Industrial Research Organisation (CSIRO), the Victorian
Government Department of Primary Industries (VICOSC
SERD) and International Science Linkage Project CG 100059
(DIISR, Australia) for generous financial support.
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7014 | J. Mater. Chem., 2010, 20, 7005–7014
This journal is ª The Royal Society of Chemistry 2010