Donor-acceptor polymers based on multi-fused heptacyclic structures: Synthesis, characterization and photovoltaic applications

Article abstract of DOI:10.1039/c003040f

We report here two novel 2,7-fluorene- and 2,7-carbazole-based conjugated polymers PFDCTBT and PCDCTBT containing ladder-type heptacyclic structures with forced planarity. PCDTBT shows excellent solubility, low band gap and high hole mobility, leading to a power conversion efficiency of 3.7%.

Full text of DOI:10.1039/c003040f

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Donor–acceptor polymers based on multi-fused heptacyclic structures:
synthesis, characterization and photovoltaic applicationsw
Jhong-Sian Wu, Yen-Ju Cheng,* Martin Dubosc, Chao-Hsiang Hsieh, Chin-Yen Chang and
Chain-Shu Hsu*
Received 12th February 2010, Accepted 1st April 2010
First published as an Advance Article on the web 14th April 2010
DOI: 10.1039/c003040f
We report here two novel 2,7-fluorene- and 2,7-carbazole-based
conjugated polymers PFDCTBT and PCDCTBT containing
ladder-type heptacyclic structures with forced planarity.
PCDTBT shows excellent solubility, low band gap and high
hole mobility, leading to a power conversion efficiency of 3.7%.
effective way to reduce the band gap.8 Moreover, coplanar
geometries and rigid structures can suppress the rotational
disorder around interannular single bonds and lower the
reorganization energy, which in turn enhances the intrinsic
charge mobility.9 However, too strong inter-chain p–p stacking
interactions arising from the high degree of coplanarity tend to
make the polyaromatic conjugated polymers insoluble and
unprocessable. In this communication, we wish to report two
novel D–A copolymers poly(fluorene-dicyclopentathiophene-
alt-benzothiadiazole) (PFDCTBT) and poly(carbazole-dicyclo-
pentathiophene-alt-benzothiadiazole) (PCDCTBT). Both polymers
were designed based on the skeletons of the well-known
PFDTBT and PCDTBT polymers in order to fully take
advantage of their excellent properties. The structural unique-
ness of PFDCTBT and PCDCTBT is that the 3-positions of
two outer thiophenes are covalently tied with the 3,6-position
of central fluorene or carbazole cores by a carbon bridge,
forming two cyclopentadienyl (CP) rings embedded in a multi-
fused heptacyclic structure (Scheme 1). Two additional substi-
tuents attached at the CP rings allow for tailoring the
intermolecular interactions without causing twisting between
the adjacent units, making the resulting polymers highly
soluble. A preliminary test of the photovoltaic performance
based on these polymers shows promise for solar cell
applications.
Over the past few years, tremendous research effort has been
made on all-solution processed polymer solar cells (PSCs) in
order to realize low-cost, light-weight, large-area and flexible
photovoltaic devices. PSCs based on the concept of bulk
heterojunction (BHJ) are the most widely adopted device
architecture to ensure maximum internal donor–acceptor
(D–A) interfacial area for efficient charge separation.1 To
achieve high efficiency of PSCs, the most critical challenge at
the molecular level is to develop p-type conjugated polymers
that simultaneously possess (1) sufficient solubility to guarantee
solution processability and miscibility with an n-type material,
(2) low band gap (LBG) for strong and broad absorption
spectrum to capture more solar photons and (3) high hole
mobility for efficient charge transport. The most effective
approach to produce a LBG polymer is to incorporate electron-
rich donor and electron-deficient acceptor segments along the
conjugated polymer backbone.2 Tricyclic 2,7-fluorene3 and
2,7-carbazole4 units have emerged as promising electron-rich
building blocks to construct D–A polymers because their
derivatives are shown to have deep-lying HOMO energy levels
and good hole-transporting properties which are crucial pre-
requisites to achieve high open-circuit voltages (Voc) and short
circuit currents (Jsc), respectively. Besides, the electron-rich
thiophene unit is the most important key element ubiquitously
incorporated into D–A conjugated polymers to adjust the
optical and electronic properties to optimal levels.5 For instance,
The synthesis of the monomers M1 and M2 is depicted in
Scheme 2. 2,7-Diboronic esters fluorene 1a and carbazole 1b
were reacted with ethyl 2-bromothiophene-3-carboxylate 2 by
Suzuki coupling to obtain compounds 3a and 3b respectively.
Double nucleophilic addition of freshly prepared 4-(2-ethyl-
hexyloxy)phenyl magnesium bromide 4 to the ester groups
of 3 led to the formation of benzylic alcohol 5 which was
subjected to intramolecular annulation through acid-mediated
alternating
copolymers
poly(2,7-fluorene-alt-dithienyl-
benzothiadiazole) (PFDTBT)6 and poly(2,7-carbazole-alt-
dithienylbenzothiadiazole) (PCDTBT)7 have been proven to
be a promising class of p-type photoactive materials for
application in PSCs (Scheme 1). In addition to the D–A
strategy, forced planarization by covalently fastening adjacent
aromatic units in the polymer backbone strengthens the
parallel p-orbital interactions to elongate effective conjugation
length and facilitate p-electron delocalization, providing an
Department of Applied Chemistry, National Chiao Tung University,
1001 Ta Hsueh Road, Hsin-Chu, 30010 Taiwan.
E-mail: yjcheng@mail.nctu.edu.tw, cshsu@mail.nctu.edu.tw
w Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/c003040f
Scheme 1 Chemical structures of non-fused PFDTBT, PCDTBT
polymers and fused PFDCTBT, PCDCTBT polymers.
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Chem. Commun., 2010, 46, 3259–3261 | 3259

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and 1.66 eV (746 nm) for PCDCTBT. These results suggest
that the donating strength of carbazole is stronger than that of
the fluorene moiety, shifting the ICT band of PCDCTBT to
lower energy. It should be emphasized that PFDCTBT and
PCDCTBT have more red-shifted absorption spectra and
smaller band gaps in comparison with their corresponding
opt
non-fused PFDTBT (Eg
= 1.87 eV) and PCDTBT
opt
(Eg = 1.88 eV) analogues, demonstrating that the electron
coupling between the rigidified donor and the acceptor units is
enhanced. Because two 4-(2-ethylhexyloxy)phenyl moieties
substituted at the carbon of CP rings may dilute strong
intermolecular p–p interactions, the profiles of absorp-
tion spectra of PFDCTBT and PCDCTBT are essentially
unchanged with slight broadening of the bands and red shift
of the band edges from the solution state to the solid state.
This result implies the amorphous nature of the polymers
which can be further confirmed by no obvious thermal transi-
tion in the differential scanning calorimetry measurements
(Fig. S2, ESIw). It is worth noting that the intensities of the
shorter wavelength bands of both polymers in the solid state
are apparently stronger than those in the solution state, which
also suggests that the rigid and coplanar heptacyclic units can
enhance their light absorption ability in the solid state.
Scheme 2 Synthetic route of the M1 and M2 monomers leading to
the targeted polymers PFDCTBT and PCDCTBT.
Friedel–Crafts reaction to furnish the fused heptacyclic arene
6. Because of the strong nitrogen-directing effect of the
carbazole to facilitate the regioselective cyclization, 6b was
obtained in nearly quantitative yield of 97% in the presence of
AcOH, whereas using stronger acids H2SO4/AcOH was required
to generate 6a in a moderate yield of 52%. Carbazole-based 6b
can be efficiently lithiated by t-butyllithium through deproto-
nation followed by reacting with trimethyltin chloride to
afford the distannyl M2. However, fluorene-based 6a was
brominated to yield compound 7 which was then lithiated to
obtain distannyl M1. M1 and M2 were copolymerized with
the acceptor 4,7-dibromo-2,1,3-benzothiadiazole 8 by Stille
coupling to give PFDCTBT (Mn = 8.5 kDa, PDI = 1.89) and
PCDCTBT (Mn = 8.2 kDa, PDI = 1.59), respectively.
Thanks to the branched 2-ethylhexyloxy side chains as the
solubilizing groups, both polymers show excellent solubilities
in common organic solvents, such as chloroform, toluene,
dichlorobenzene, overcoming the planarity–solubility tradeoff
to facilitate their processability.
Cyclic voltammetry (CV) was employed to examine the
electrochemical properties (Fig. 2). Both polymers showed
a stable and reversible p-doping/n-doping process in the
cathodic and anodic scans. The HOMO levels being estimated
to be À5.32 eV for PFDCTBT and À5.38 eV for PCDCTBT
are in an ideal range to assure better air-stability and greater
attainable Voc in the final device.10 The LUMO energy levels
are approximately located at À3.55 eV for PFDCTBT and
À3.61 eV for PCDCTBT, which are positioned 0.2–0.3 eV
above the LUMO level of the PC71BM acceptor (3.8 eV) to
ensure energetically favorable electron transfer.11 This can be
unambiguously evidenced by the complete photoluminescence
quenching in the film of the PFDCTBT/PC71BM and
PCDCTBT/PC71BM (1 : 2, w/w) blends (Fig. S3, ESIw).
Despite their amorphous nature, PFDCTBT and
PCDCTBT show good hole transporting properties due to their
rigid and coplanar structures. Based on the space-charge-limited
current (SCLC) method, PFDCTBT and PCDCTBT exhibited
high hole mobilities within the same order of magnitude
(2.5 Â 10À4 cm2/Vs and 1 Â 10À4 cm2/Vs, respectively). On
the basis of ITO/PEDOT:PSS/polymer:PC71BM(1 : 2, w/w)/
Ca/Al configuration, bulk heterojunction solar cells were
fabricated and characterized under simulated 100 mW cmÀ2
AM 1.5 G illumination. The current density–voltage charac-
teristics of the devices are shown in Fig. 3. Without extensive
The thermal stability was analyzed by thermogravimetric
analysis (TGA). PFDCTBT and PCDCTBT exhibited suffi-
ciently high decomposition temperatures (Td) of 410 1C and
434 1C, respectively, indicating that carbazole-based
PCDCTBT is thermally more stable than fluorene-based
PFDCTBT (Fig. S1, ESIw).
Both PFDCTBT and PCDCTBT exhibited two charac-
teristic bands in the absorption spectra (Fig. 1). The shorter
wavelength absorbance comes from the p–p* transition of the
heptacyclic units, while the lower energy band is attributed to
the intramolecular charge transfer (ICT) between the electron-
rich and the benzothiadiazole segments. Compared to
PFDCTBT showing the absorption maxima at 411 nm and
580 nm in the thin film, PCDCTBT exhibited a similar
absorption maximum at 412 nm but a bathochromic shift of
the ICT band at 607 nm. In addition, the optical band gaps
(Egopt) deduced from the onset of absorption in the solid
state are determined to be 1.76 eV (703 nm) for PFDCTBT
Fig. 1 Normalized absorption spectra of PFDCTBT (a) and PCDCTBT
(b) in toluene solution and the solid state.
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PCEs of 2.8% and 3.7%, respectively. We anticipate that
further improvement of device performance is highly achiev-
able through carefully optimizing the processing conditions
which are ongoing in our laboratory.
This work is supported by the National Science Council and
‘‘Aim for the Top University Plan’’ of the National Chiao
Tung University and Ministry of Education, Taiwan.
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PSS/polymer:PC71BM/Ca/Al devices under illumination of AM
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3 Current density–voltage characteristics of ITO/PEDOT:
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 3259–3261 | 3261