Germaindacenodithiophene based low band gap polymers for organic solar cells

Article abstract of DOI:10.1039/c2cc17996b

We report the first synthesis of a fused germaindacenodithiophene monomer and its polymerisation with 2,1,3-benzothiadiazole by Suzuki polycondensation. The resulting polymer, PGeTPTBT, is semicrystalline, despite the presence of four bulky 2-ethylhexyl g

Full text of DOI:10.1039/c2cc17996b

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Cite this: Chem. Commun., 2012, 48, 2955–2957
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COMMUNICATION
Germaindacenodithiophene based low band gap polymers for organic
solar cellsw
a
a
Zhuping Fei,z Raja Shahid Ashraf,z Zhenggang Huang,a Jeremy Smith,b R. Joseph Kline,c
Pasquale D’Angelo,b Thomas D. Anthopoulos,b James R. Durrant,a Iain McCullocha and
Martin Heeney*a
Received 21st December 2011, Accepted 23rd January 2012
DOI: 10.1039/c2cc17996b
We report the first synthesis of a fused germaindacenodithiophene
monomer and its polymerisation with 2,1,3-benzothiadiazole by
Suzuki polycondensation. The resulting polymer, PGeTPTBT, is
semicrystalline, despite the presence of four bulky 2-ethylhexyl
groups. Blends with P70CBM afford solar cells with efficiencies
of 5.02%.
of applications including bulk heterojunction solar cells. The
improved crystallinity has been rationalised on the basis of the
longer C–Si bond length compared to the C–C bond, which
changes the geometry of the fused ring allowing stronger p–p
interactions to occur.8
As part of our investigations into ladder type polymers we
have recently shown that alternating copolymers of indaceno-
dithiophene (IDT)11 and silaindacenodithiophene (SiIDT)3
with benzothiadiazole exhibit promising photovoltaic and
FET properties. In an effort to enhance polymer crystallinity
and performance further, we were interested to investigate the
affects of introducing bridging Ge atoms into the system. Here
we report the synthesis of a novel germaindacenodithiophene
monomer and its co-polymerisation with an electron accepting
2,1,3-benzothiadiazole-4,7-bis(boronic acid pinacol ester) by
Suzuki polycondensation reaction. We show the resulting polymer
PGeTPTBT is semicrystalline and demonstrates promising solar
cell device performance.
There has been tremendous progress in recent years in the
development of low band gap conjugated polymers for use in
photovoltaic devices or field effect transistors.1 One promising
class of polymer is the so-called ladder type, in which linked
aromatic units such as thiophene or benzene are forced to be
co-planar by the use of bridging heteroatoms. In addition to
serving as a point of attachment for solubilising side chains,
the bridging atom also plays an important role in modifying
the electronic properties of the system. Systems containing
bridging atoms from group 14 (C, Si and to a lesser extent Ge)
have been the subject of much interest, partly due to the fact
that two solubilising side chains can be attached to the bridge,
facilitating solubility and self-assembly.2–6 Moreover it has
been demonstrated that the electronic properties of the polymer
backbone itself can be modified by changing the bridging
heteroatom. Thus changing C to Si can result in stabilisation
of the LUMO by interaction of the low lying s* orbital of the
silicon atoms with the p* orbital of the conjugated system.7
The choice of bridging heteroatom has also been shown to
have a significant effect on the crystallinity of the polymer. For
example in comparing the benzothiadiazole co-polymers of a
2,20-bithiophene bridged with either C, Si or Ge containing two
branched ethylhexyl groups, it was found that the C bridged
co-polymer was amorphous whereas the Si8,9 and Ge5,10 bridged
co-polymers were semicrystalline. The crystalline polymers
demonstrated higher charge carrier mobilities ideal for a number
The synthesis of PGeTPTBT is shown in Scheme 1. Here
bulky 2-ethylhexyl side chains were employed as the bridging groups
in order to enhance the polymer solubility and processability. The
synthesis of the key intermediate 2 has been reported by Jen et al.6
Here we adopted another route for the synthesis of 2 via the
lithiation of 13 with lithium diisopropylamide (LDA) and
subsequent quenching with chlorotrimethylsilane (TMSCl) to
afford 2 in a yield of 85%. The bridged monomer 3 was
prepared by the lithiation of 2 with four equivalents of n-BuLi
a Dept. Chemistry and Centre for Plastic Electronics, Imperial College
London, London, SW7 2AZ, UK. E-mail: m.heeney@imperial.ac.uk
b Dept. Physics & Centre for Plastic Electronics, Imperial College, UK
c Polymers Division, National Institute of Standards and Technology
(NIST), Gaithersburg, Maryland 20899, United States
w Electronic supplementary information (ESI) available: Monomer
and polymer synthesis, FET transfer and output plots. See DOI:
10.1039/c2cc17996b
z Both authors have equal contribution.
Scheme 1 Synthetic route to PGeTPTBT.
c
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Chem. Commun., 2012, 48, 2955–2957 2955

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at ꢀ78 1C, followed by addition of the dibromo-di-(2-ethylhexyl)
germane. Purification of the resultant compound was complicated
by the tendency of the trimethylsilyl groups to protodesilylate,
especially during flash chromatography. Therefore the crude
product was rapidly filtered through silica, and the resultant
mixture containing germaindacenodithiophene with 2, 1 or 0
trimethylsilyl groups was brominated directly with NBS to afford
3 in a yield of 33%.
wider than IDTBT (1.7 eV), but narrower than the Si bridged
polymer SiIDTBT (1.8 eV) with n-octyl sidechains.
The ordering of spin cast thin films of PGeTPTBT was
investigated by grazing incidence wide angle X-ray scattering
(GIWAXS). The 2D WAXS patterns of PGeTPTBT films
both before and after annealing at 140 1C, are shown in Fig. 2,
with the 1D out-of-plane and in-plane profiles shown in
Fig. S2.w In contrast to the carbon analogue IDT-BT which
lacks any obvious crystallinity,11 the results clearly indicate
that the Ge polymer forms semi-crystalline thin films, despite
the inclusion of the four bulky ethylhexyl groups. Two orders
of (h00) scatterings are apparent in the out-of-plane direction,
indicating the formation of lamellar like sheets of polymer
with a d-spacing of 14.5 A before annealing, and 14.0 A after
annealing. The arcing apparent in the 2D profiles implies that
the lamellar sheets are not well aligned with respect to the
substrate, but have a relatively broad distribution of orientations.
The p–p stacking distance, as measured by the in-plane profile is
3.93 A. Annealing resulted in a modest improvement of film
crystallinity of about 30%. More quantitative information about
changes in crystallinity and orientation were obtained from pole
figures, focussing on the distribution of the (100) peak as a
function of o, the angle between crystallite and the substrate
normal (Fig. S3).12 We find that increase in crystallinity is mainly
driven by an increase in the crystallinity of the misorientated
domains (those not aligned normal to the substrate), whereas the
crystallinity of the aligned domains is relatively unchanged.
In order to investigate the influence of the germanium
bridge on charge carrier mobility, field effect transistors were
fabricated in the top gate, bottom contact configuration using
Cytop as the gate dielectric (Fig. S1). The polymer exhibited
as-cast saturated and linear mobilities of 0.013 and 0.005 cm2/Vs.
Annealing the film did not result in any significant device
improvement, in agreement with the XRD results which
suggest little change in the crystallinity of the polymers in
In our investigations of silaindacenodithiophene we found
that the monomer was unstable under the basic conditions
required for Suzuki cross-coupling, which necessitated the use of
Stille polymerisation with stannylated silaindacenodithiophene.
Unfortunately the tin monomer was difficult to purify due to
its tendency to readily protodestannylate, especially during
chromatography. Thus only moderate molecular weights could
be obtained for the polymer. Encouraged by our recent finding
that dithienogermoles were stable to the basic conditions of
Suzuki coupling,5 we found that 3 could be readily polymerised
under Suzuki conditions. We attribute the higher stability in base
to the reduced polarisation of the C–Ge bond over the C–Si bond.
Polymerisation with 2,1,3-benzothiadiazole-4,7-bis(boronic acid
pinacol ester) was carried out in a biphasic system (toluene/
aqueous Na2CO3) with Pd(PPh3)4 as the catalyst and aliquot
336 as the phase transfer catalyst. After precipitation and solvent
extraction to remove lower weight oligomers and catalyst
residues, PGeTPTBT was obtained as a purple solid in a
typical yield of 82%. As a result of the four branched alkyl
chains, the polymer exhibited good solubility in common
organic solvents such as chloroform, THF and chlorobenzene.
The number average molecular weight (Mn) of PGeTPTBT by
GPC in hot chlorobenzene (80 1C) was found to be 32 KDa
with a PDI of 2.3.
The UV-vis absorption spectra of PGeTPTBT in dilute
chlorobenzene (CB) and as a thin film are shown in Fig. 1. In
solution the polymer exhibits an absorption maximum at 630 nm,
whereas in the solid the polymer shows a main absorption peak at
644 nm with a shoulder at 596 nm. The 14 nm red-shift of the
main absorption peak and the broadening of the absorption from
solution to solid state indicate some aggregation in the solid state.
This is in contrast to the C-bridged polymer (IDTBT) with
identical branched side chains, in which the solution and thin film
spectra are very similar with a maximum at 660 nm,11 suggesting
that the introduction of Ge results in an increase in solid state
aggregation. The absorption onset in solid state is 712 nm,
corresponding to an optical band gap of 1.74 eV, which is slightly
Fig. 1 UV-Vis spectra of PGeTPTBT in dilute chlorobenzene
Fig. 2 GIXD detector images of (top) as-cast thin film of
PGeTPTBT and (bottom) after annealing at 140 1C.
(black line) and in thin film (red line).
c
2956 Chem. Commun., 2012, 48, 2955–2957
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solar cells without thermal annealing. We are currently
investigating the use of additives and co-solvents in an effort
to further increase device efficiency.
This work was supported by the Dutch Polymer Institute
(grant 678). Portions of this research were carried out at the
Stanford Synchrotron Radiation Lightsource, a Directorate
of SLAC National Accelerator Laboratory and an Office of
Science User Facility operated for the U.S. Department of
Energy Office of Science by Stanford University. We thank
Michael F Toney for help with the GIWAXS measurements.
Fig. 3 (a) J-V curves and (b) EQE curves of polymer solar cell based
on different blend ratio of PGeTPTBT : PC71BM.
the transport direction. These values are slightly higher than
those measured for the SiIDTBT bridged polymer containing
n-octyl sidechains in the same device configuration. However
they are significantly lower than analogous IDT-BT polymer,
despite the lack of any obvious crystallinity in IDT-BT. This
surprising result may indicate that transport in PGeTPTBT is
limited by grain boundaries and misaligned domains, and that
optimisation of the coating and annealing conditions may
result in further performance enhancements.
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